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Whether to repair nonsevere tricuspid regurgitation (TR) during surgery for ischemic mitral valve regurgitation (IMR) remains uncertain.The goal of this study was to investigate the incidence, predictors, and clinical significance of TR progression and presence of ≥moderate TR after IMR surgery.Patients (n = 492) with untreated nonsevere TR within 2 prospectively randomized IMR trials were included. Key outcomes were TR progression (either progression by ≥2 grades, surgery for TR, or severe TR at 2 years) and presence of ≥moderate TR at 2 years.Patients' mean age was 66 ± 10 years (67% male), and TR distribution was 60% ≤trace, 31% mild, and 9% moderate. Among 2-year survivors, TR progression occurred in 20 (6%) of 325 patients. Baseline tricuspid annular diameter (TAD) was not predictive of TR progression. At 2 years, 37 (11%) of 323 patients had ≥moderate TR. Baseline TR grade, indexed TAD, and surgical ablation for atrial fibrillation were independent predictors of ≥moderate TR. However, TAD alone had poor discrimination (area under the curve, ≤0.65). Presence of ≥moderate TR at 2 years was higher in patients with MR recurrence (20% vs. 9%; p = 0.02) and a permanent pacemaker/defibrillator (19% vs. 9%; p = 0.01). Clinical event rates (composite of ≥1 New York Heart Association functional class increase, heart failure hospitalization, mitral valve surgery, and stroke) were higher in patients with TR progression (55% vs. 23%; p = 0.003) and ≥moderate TR at 2 years (38% vs. 22%; p = 0.04).After IMR surgery, progression of unrepaired nonsevere TR is uncommon. Baseline TAD is not predictive of TR progression and is poorly discriminative of ≥moderate TR at 2 years. TR progression and presence of ≥moderate TR are associated with clinical events. (Comparing the Effectiveness of a Mitral Valve Repair Procedure in Combination With Coronary Artery Bypass Grafting [CABG] Versus CABG Alone in People With Moderate Ischemic Mitral Regurgitation, NCT00806988; Comparing the Effectiveness of Repairing Versus Replacing the Heart's Mitral Valve in People With Severe Chronic Ischemic Mitral Regurgitation, NCT00807040).

Fully understanding the genetic factors involved in Autism Spectrum Disorder (ASD) requires whole-genome sequencing (WGS), which theoretically allows the detection of all types of genetic variants. With the aim of generating an unprecedented resource for resolving the genomic architecture underlying ASD, we analyzed genome sequences and phenotypic data from 5,100 individuals with ASD and 6,212 additional parents and siblings (total n=11,312) in the Autism Speaks MSSNG Project, as well as additional individuals from other WGS cohorts. WGS data and autism phenotyping were based on high-quality short-read sequencing (>30x coverage) and clinically accepted diagnostic measures for ASD, respectively. For initial discovery of ASD-associated genes, we used exonic sequence-level variants from MSSNG as well as whole-exome sequencing-based ASD data from SPARK and the Autism Sequencing Consortium (>18,000 trios plus additional cases and controls), identifying 135 ASD-associated protein-coding genes with false discovery rate SCN2A and a nuclear mitochondrial insertion impacting SYNGAP1. Polygenic risk scores did not differ between children with ASD in multiplex families versus simplex, and rare, damaging recessive events were significantly depleted in multiplex families, collectively suggesting that rare, dominant variation plays a predominant role in multiplex ASD. Our study provides a guidebook for exploring genotype-phenotype correlations in the 15-20% of ASD families who carry ASD-associated rare variants, as well as an entry point to the larger and more diverse studies that will be required to dissect the etiology in the >80% of the ASD population that remains idiopathic. All data resulting from this study are available to the medical genomics research community in an open but protected manner.

Fully understanding autism spectrum disorder (ASD) genetics requires whole-genome sequencing (WGS). We present the latest release of the Autism Speaks MSSNG resource, which includes WGS data from 5,100 individuals with ASD and 6,212 non-ASD parents and siblings (total n = 11,312). Examining a wide variety of genetic variants in MSSNG and the Simons Simplex Collection (SSC; n = 9,205), we identified ASD-associated rare variants in 718/5,100 individuals with ASD from MSSNG (14.1%) and 350/2,419 from SSC (14.5%). Considering genomic architecture, 52% were nuclear sequence-level variants, 46% were nuclear structural variants (including copy-number variants, inversions, large insertions, uniparental isodisomies, and tandem repeat expansions), and 2% were mitochondrial variants. Our study provides a guidebook for exploring genotype-phenotype correlations in families who carry ASD-associated rare variants and serves as an entry point to the expanded studies required to dissect the etiology in the ∼85% of the ASD population that remain idiopathic.

Vibrio vulnificus, a gram-negative bacterium, is the leading cause of seafood-borne illnesses and mortality in the United States. Previous studies have identified metabolites 2-C-methylerythritol 4-phosphate (MEP) as being essential for V. vulnificus growth and function. It was shown that 1-deoxy-D-xylulose-5-phosphate reductoisomerase (Dxr) is a critical enzyme in the viability of V. vulnificus, and many other bacteria, as it catalyzes the rearrangement of 1-deoxy-D-xylulose-5-phosphate (Dxp) to 2-C-methylerythritol 4-phosphate (MEP) within the MEP pathway, found in plants and bacteria. The MEP pathway produces the isoprenoids, isopentenyl diphosphate and dimethylallyl pyrophosphate. In this study, we produced and structurally characterized V. vulnificus Dxr. The enzyme forms a dimeric assembly and contains a metal ion in the active site. Protein produced in Escherichia coli co-purifies with Mg2+ ions, however the Mg2+ cations may be substituted with Mn2+, as both of these metals may be utilized by Dxrs. These findings will provide a basis for the design of Dxr inhibitors that may find application as antimicrobial compounds.

To determine the frequency and risk factors for non-home discharge (NHD) and its association with clinical outcomes and quality of life (QOL) at 1 year following cardiac surgery in patients with ischemic mitral regurgitation (IMR).Discharge disposition was evaluated in 552 patients enrolled in trials of severe or moderate IMR. Patient and in-hospital factors associated with NHD were identified using logistic regression. Subsequently, association of NHD with 1-year mortality, serious adverse events (SAEs), and QOL was assessed.NHD was observed in 30% (154/522) with 25% (n = 71/289) in moderate and 36% (n = 83/233) in patients with severe IMR (unadjusted P = .006), a difference not significant after including age (5-year change: adjusted odds ratio [adjOR], 1.52; 95% confidence interval [CI], 1.35-1.72; P .001), diabetes (adjOR, 1.94; 95% CI, 1.27-2.94; P = .002), and previous heart failure (adjOR, 1.64; 95% CI, 1.06-2.52; P = .03). Odds of NHD were increased for patients with postoperative SAEs (adjOR, 1.85; 95% CI, 1.19-2.86; P = .01) but not based on type of cardiac surgery. Greater rates of death and SAEs were observed in NHD patients at 1 year: adjusted hazard ratio, 4.29 (95% CI, 2.14-8.59; P .001) and adjusted rate ratio, 1.45 (95% CI, 1.03-2.02; P = .03), respectively. QOL did not differ significantly between groups.NHD is common following surgery for IMR, influenced by older age, diabetes, previous heart failure, and postoperative SAEs. These patients may be at greater risk of death and subsequent SAEs after discharge. Discussion of NHD with patients may have important implications for decision-making and guiding expectations following cardiac surgery.

Artificial breathing systems to help humans survive extreme environments are used over a range of ambient pressures, using various gases of different volumetric concentrations. These activities include anaesthesia and intensive care activity, high-altitude mountaineering, firefighting, aerospace extravehicular space activity and underwater diving operations. A circle breathing system is one in which the exhaled carbon dioxide is absorbed by an alkali substance and the remaining unused gases are recirculated, usually for the sake of economy and environment. This allows the flow of the fresh gas to be considerably reduced, thereby saving on fresh-gas supply. Circle systems are often used in the circumstances cited above, although not always at low fresh-gas flows. The circle system used in anaesthesia and intensive care has the least engineering demands made on it, although it is used on patients who are highly vulnerable; it usually provides a mixture of air and oxygen, and perhaps a breathable anaesthetic gas, all at sea-level pressure. Mountaineering and firefighting applications involve an extreme earthbound environment, with the user undergoing extreme physical work. The astronaut’s spacesuit and life support system contains a high-flow circle system, the breathing gases themselves pressurising the suit as well as providing respiratory life support and thermal comfort; the gas provided is pure oxygen at about a third of sea-level atmosphere. There are numerous varieties of breathing systems for diving, including a circle system, often for clandestine naval activity; the gases used are a combination of oxygen, nitrogen and helium, to minimise the possibility of decompression sickness, nitrogen narcosis and oxygen toxicity and must be provided at a varying pressure and concentration appropriate to depth.

ObjectivesWith large numbers of COVID-19 patients requiring mechanical ventilation and ventilators possibly being in short supply, in extremis two patients may have to share one ventilator. Careful matching of patient ventilation requirements is necessary. However, good matching is difficult to achieve as lung characteristics can have a wide range and may vary over time. Adding flow restriction to the flow path between ventilator and patient gives the opportunity to control the airway pressure and hence flow and volume individually for each patient. This study aimed to create and validate a simple model for calculating required flow restriction.Design/SettingExperimental/Secondary CareMethodsWe created a simple linear resistance-compliance model, termed the BathRC model, of the ventilator tubing system and lung allowing direct calculation of the relationships between pressures, volumes, and required flow restriction. Experimental measurements were made for parameter determination and validation using a clinical ventilator connected to two test lungs. For validation, differing amounts of restriction were introduced into the ventilator circuit.ResultsThe BathRC model was able to predict tidal lung volumes with a mean error of 4% (min:1.2%, max:9.3%).ConclusionWe present a simple model validated model that can be used to estimate required flow restriction for dual patient ventilation. The BathRC model is freely available; this tool is provided to demonstrate that flow restriction can be readily estimated.ARTICLE SUMMARYStrengths and limitations of this studyData collected on a clinical systemSimple model, easy to implementValidation performedLimited to 2 types of restrictors testedData from only one ventilator type

Objective The Cardiothoracic Surgical Trials Network reported that left ventricular reverse remodeling at 2 years did not differ between patients with moderate ischemic mitral regurgitation randomized to coronary artery bypass grafting plus mitral valve repair (n = 150) or coronary artery bypass grafting alone (n = 151). To address health resource use implications, we compared costs and quality-adjusted survival. Methods We used individual patient data from the Cardiothoracic Surgical Trials Network trial on survival, hospitalizations, quality of life, and US hospitalization costs to estimate cumulative costs and quality-adjusted life years. A microsimulation model was developed to extrapolate to 10 years. Bootstrap and deterministic sensitivity analyses were performed to address uncertainty. Results In-hospital costs were $59,745 for coronary artery bypass grafting plus mitral valve repair versus $51,326 for coronary artery bypass grafting alone (difference $8419; 95% uncertainty interval, 2259-18,757). Two-year costs were $81,263 versus $67,341 (difference 13,922 [2370 to 28,888]), and quality-adjusted life years were 1.35 versus 1.30 (difference 0.05; −0.04 to 0.14), resulting in an incremental cost-effectiveness ratio of $308,343/quality-adjusted life year for coronary artery bypass grafting plus mitral valve repair. At 10 years, its costs remained higher ($107,733 vs $88,583, difference 19,150 [−3866 to 56,826]) and quality-adjusted life years showed no difference (−0.92 to 0.87), with 5.08 versus 5.08. The likelihood that coronary artery bypass grafting plus mitral valve repair would be considered cost-effective at 10 years based on a cost-effectiveness threshold of $100K/quality-adjusted life year did not exceed 37%. Only when this procedure reduces the death rate by a relative 5% will the incremental cost-effectiveness ratio fall below $100K/quality-adjusted life year. Conclusions: The addition of mitral valve repair to coronary artery bypass grafting for patients with moderate ischemic mitral regurgitation is unlikely to be cost-effective. Only if late mortality benefits can be demonstrated will it meet commonly used cost-effectiveness criteria.

Artificial breathing systems to help humans survive extreme environments are used over a range of ambient pressures, using various gases of different volumetric concentrations. These activities include anaesthesia and intensive care activity, high-altitude mountaineering, firefighting, aerospace extravehicular space activity and underwater diving operations. A circle breathing system is one in which the exhaled carbon dioxide is absorbed by an alkali substance and the remaining unused gases are recirculated, usually for the sake of economy and environment. This allows the flow of the fresh gas to be considerably reduced, thereby saving on fresh-gas supply. Circle systems are often used in the circumstances cited above, although not always at low fresh-gas flows. The circle system used in anaesthesia and intensive care has the least engineering demands made on it, although it is used on patients who are highly vulnerable; it usually provides a mixture of air and oxygen, and perhaps a breathable anaesthetic gas, all at sea-level pressure. Mountaineering and firefighting applications involve an extreme earthbound environment, with the user undergoing extreme physical work. The astronaut's spacesuit and life support system contains a high-flow circle system, the breathing gases themselves pressurising the suit as well as providing respiratory life support and thermal comfort; the gas provided is pure oxygen at about a third of sea-level atmosphere. There are numerous varieties of breathing systems for diving, including a circle system, often for clandestine naval activity; the gases used are a combination of oxygen, nitrogen and helium, to minimise the possibility of decompression sickness, nitrogen narcosis and oxygen toxicity and must be provided at a varying pressure and concentration appropriate to depth.

Chapter 25 introduced some basic generic principles applicable to many measurement and monitoring techniques. Chapter 43 introduces those principles not covered in Chapter 25 and discusses in detail the clinical applications and limitations of the many monitoring techniques available to the modern clinical anaesthetist. It starts with non-invasive blood pressure measurement, including clinical and automated techniques. This is followed by techniques of direct blood pressure measurement, noting that transducers and calibration have been discussed in Chapter 25. This is followed by electrocardiography. There then follows a section on the different methods of measuring cardiac output, including the pulmonary artery catheter, the application of ultrasound in echocardiography, pulse contour analysis (LiDCO™ and PiCCO™), and transthoracic electrical impedance. Pulse oximetry is then discussed in some detail. Depth of anaesthesia monitoring is then described, starting with the electroencephalogram and its application in BIS™ monitors, the use of evoked potentials, and entropy. There then follow sections on gas pressure measurement in cylinders and in breathing systems, followed by gas volume and flow measurement, including the rotameter, spirometry, and the pneumotachograph, and the measurement of lung dead space and functional residual capacity using body plethysmography and dilution techniques. The final section is on respiratory gas analysis, starting with light refractometry as the standard against which other techniques are compared, infrared spectroscopy, mass spectrometry, and Raman spectroscopy (the principles of these techniques having been introduced in Chapter 25), piezoelectric and paramagnetic analysers, polarography and fuel cells, and blood gas analysis.

This chapter introduces some basic physical principles that contribute to the function of various monitors used in anaesthetic practice. Topics include biological signal processing, operational amplifiers, including single-ended amplifiers, and the benefit of patient-isolated differential amplifiers; it includes filtering, digital processing, and electrodes. The generic principles of transducers are introduced, including resistive, capacitive, and inductance strain gauges used in transducers, photoelectric, piezoelectric, and chemical transducers, calibration of transducers, and the significance of resonance and damping in measurement systems. Since both are widely used in monitoring systems, there is an introduction to spectroscopy and magnetism.

This chapter covers the basic science of physics relevant to anaesthetic practice. Equipment and measurement devices are covered elsewhere. Starting with fundamentals, atomic structure is introduced, followed by dimensions and units as used in science. Basic mechanics are then discussed, focusing on mass and density, force, pressure, energy, and power. The concept of linearity, hysteresis, and frequency response in physical systems is then introduced, using relevant examples, which are easy to understand. Laminar and turbulent fluid flow is then described, using flow measurement devices as applications of this theory. The concept of pressure and its measurement is then discussed in some detail, including partial pressure. Starting with the kinetic theory of gases, heat and temperature are described, along with the gas laws, critical temperature, sublimation, latent heat, vapour pressure and vaporization illustrated by the function of anaesthetic vaporizers, humidity, solubility, diffusion, osmosis, and osmotic pressure. Ultrasound and its medical applications are discussed in some detail, including Doppler and its use to measure flow. This is followed by an introduction to lasers and their medical uses. The final subject covered is electricity, starting with concepts of charge and current, voltage, energy, and power, and the role of magnetism. This is followed by a discussion of electrical circuits and the rules governing them, and bridge circuits used in measurement. The function of capacitors and inductors is then introduced, and alternating current and transformers are described.

The purpose of this study was to identify joint angular kinematics that corresponds to shooting accuracy in the stationary ice hockey wrist shot. Twenty-four subjects participated in this study, each performing 10 successful shots on four shooting targets. An eight-camera infra-red motion capture system (240 Hz), along with passive reflective markers, was used to record motion of the joints, hockey stick, and puck throughout the performance of the wrist shot. A multiple regression analysis was carried out to examine whole-body kinematic variables with accuracy scores as the dependent variable. Significant accuracy predictors were identified in the lower limbs, torso and upper limbs. Interpretation of the kinematics suggests that characteristics such as a better stability of the base of support, momentum cancellation, proper trunk orientation and a more dynamic control of the lead arm throughout the wrist shot movement are presented as predictors for the accuracy outcome. These findings are substantial as they not only provide a framework for further analysis of motor control strategies using tools for accurate projection of objects, but more tangibly they may provide a comprehensive evidence-based guide to coaches and athletes for planned training to improve performance.

Medical gases used in anaesthesia and intensive care include oxygen, nitrous oxide, medical air, entonox, carbon dioxide, heliox and nitric oxide. Breathable gases administered to patients are stored either in bulk outside the hospital or in cylinders within the hospital. Medical gases are then distributed throughout the hospital via a pipeline network.

Objective: The aim of this study is to investigate the influence of Emergency Thoracotomy (ET) on mortality in a group of patients suffering from severe thoracic trauma requiring Helicopter Emergency Medical Service (HEMS) transfer to hospital. This is not clearly defined especially when thoracotomy takes place in the pre-hospital setting. Methods: A retrospective review of 670 consecutive patients with severe thoracic trauma, transferred to The Royal London Hospital by HEMS between November 1994 and December 2002. ET (on scene, in the Accident and Emergency (A&E) department or in the operating theatre) was performed in 53 patients (7.7%). Both univariate and multivariate analyses were performed to evaluate ET as an independent predictor of mortality. Results: There were 510 males and 160 females with a mean Injury Severity Score (ISS) of 35.12 ^ 17.5. Univariate analysis identified ET to be a predictor of mortality (OR ¼ 0.15, 95% CI ¼ 0.07 ‐0.30). However, with multivariate analysis, ET was not found to be an independent predictor of mortality (OR ¼ 1.93, 95% CI ¼ 0.61‐ 6.1). The independent predictors of mortality identified were: age . 60 years (OR 5.57, 95% CI 2.19 ‐ 14.16), Glasgow Coma Score ,8 at the scene (OR ¼ 7.4, 95% CI ¼ 3.15 ‐ 17.46), ISS . 25 (OR 5.3, 95% CI ¼ 1.64 ‐ 17.11), need for intubation at the scene (OR ¼ 2.80, 95% CI ¼ 1.022 ‐ 7.69), oxygen saturation in A&E (, 89%) (OR ¼ 2.39, 95% CI ¼ 1.13 ‐ 5.05), haemothorax (OR ¼ 3.30, 95% CI ¼ 1.53 ‐7.13) and bilateral injury (OR ¼ 3.1, 95% CI ¼ 1.51‐ 6.61). Conclusions: Our study has shown that when confounding variables are accounted for, ET is not a predictor of mortality following severe chest trauma. This implies that in a well-selected group of patients it may be a significant and life-saving procedure. q 2004 Elsevier B.V. All rights reserved.

OBJECTIVES We sought to assess the relationship between completeness of revascularization and adverse events at one year in the ARTS (Arterial Revascularization Therapies Study) trial. BACKGROUND There is uncertainty to what extent degree of completeness of revascularization, using up-to-date techniques, influences medium-term outcome. METHODS After consensus between surgeon and cardiologist regarding the potential for equivalence in the completeness of revascularization, 1,205 patients with multivessel disease were randomly assigned to either bypass surgery or stent implantation. All baseline and procedural angiograms and surgical case-record forms were centrally assessed for completeness of revascularization. RESULTS Of 1,205 patients randomized, 1,172 underwent the assigned treatment. Complete data for review were available in 1,143 patients (97.5%). Complete revascularization was achieved in 84.1% of the surgically treated patients and 70.5% of the angioplasty patients (p < 0.001). After one year, the stented angioplasty patients with incomplete revascularization showed a significantly lower event-free survival than stented patients with complete revascularization (i.e., freedom from death, myocardial infarction, cerebrovascular accident and repeat revascularization) (69.4% vs. 76.6%; p < 0.05). This difference was due to a higher incidence of subsequent bypass procedures (10.0% vs. 2.0%; p < 0.05). Conversely, at one year, bypass surgery patients with incomplete revascularization showed only a marginally lower event-free survival rate than those with complete revascularization (87.8% vs. 89.9%). CONCLUSIONS Complete revascularization was more frequently accomplished by bypass surgery than by stent implantation. One year after bypass, there was no significant difference in event-free survival between surgically treated patients with complete revascularization and those with incomplete revascularization, but patients randomized to stenting with incomplete revascularization had a greater need for subsequent bypass surgery.

Ratings by an expert panel of the appropriateness of treatments may offer better guidance for clinical practice than the variable decisions of individual clinicians, yet there have been no prospective studies of clinical outcomes. We compared the clinical outcomes of patients treated medically after angiography with those of patients who underwent revascularization, within groups defined by ratings of the degree of appropriateness of revascularization in varying clinical circumstances.This was a prospective study of consecutive patients undergoing coronary angiography at three London hospitals. Before patients were recruited, a nine-member expert panel rated the appropriateness of percutaneous transluminal coronary angioplasty (PTCA) and coronary-artery bypass grafting (CABG) on a nine-point scale (with 1 denoting highly inappropriate and 9 denoting highly appropriate) for specific clinical indications. These ratings were then applied to a population of patients with coronary artery disease. However, the patients were treated without regard to the ratings. A total of 2552 patients were followed for a median of 30 months after angiography.Of 908 patients with indications for which PTCA was rated appropriate (score, 7 to 9), 34 percent were treated medically; these patients were more likely to have angina at follow-up than those who underwent PTCA (odds ratio, 1.97; 95 percent confidence interval, 1.29 to 3.00). Of 1353 patients with indications for which CABG was considered appropriate, 26 percent were treated medically; they were more likely than those who underwent CABG to die or have a nonfatal myocardial infarction--the composite primary outcome (hazard ratio, 4.08; 95 percent confidence interval, 2.82 to 5.93)--and to have angina (odds ratio, 3.03; 95 percent confidence interval, 2.08 to 4.42). Furthermore, there was a graded relation between rating and outcome over the entire scale of appropriateness (P for linear trend=0.002).On the basis of the ratings of the expert panel, we identified substantial underuse of coronary revascularization among patients who were considered appropriate candidates for these procedures. Underuse was associated with adverse clinical outcomes.

Background: There is no clear consensus as to the correct screening procedure to identify patients undergoing cardiac surgery and who are at greatest risk of stroke because of the presence of significant carotid artery stenosis. Such screening is important because some patients benefit from combined carotid and cardiac surgery and, regardless of this, the information gained puts the cardiac surgeon in a position to provide an accurate assessment of surgical risk. Our objective was to examine current clinical practice of carotid artery investigation prior to urgent cardiac surgery and to review this illustrative practice in the context of the world literature. Hypothesis: The study aimed to establish that current typical practice for screening cardiac surgical patients for carotid artery disease is illogical according to the evidence in the world literature. Methods: The study consisted of a retrospective assessment of all patients undergoing urgent cardiac surgery and a Medline-derived literature review, and included all patients undergoing urgent cardiac surgery at a tertiary cardiothoracic center between January 1 and December 31, 1997. Results: Of 529 patients undergoing urgent cardiac surgery, 44 (8%) were screened preoperatively by duplex Doppler ultrasonography for carotid disease. The indications for screening were asymptomatic carotid bruit in 24 patients, history of stroke or transient ischemic attack (TIA) in 12 patients, and neither stroke, TIA, or bruit in 7 patients. The tests were requested either by the attending cardiologists or by the cardiac surgeon to whom they were referred. One patient had already been diagnosed as having carotid artery disease in the past. Thirteen patients underwent additional carotid investigations. Eleven patients were demonstrated to have internal carotid artery stenosis ≥ 60% and 3 patients underwent combined cardiac and carotid surgery. Review of the literature revealed the following groups to be at increased risk of future stroke unrelated to surgery, and of postoperative stroke: those with a history of stroke or TIA, those with carotid bruits, and, of importance, all patients with significant carotid stenosis. Recent data suggest that symptomatic patients and the elderly are at greatest risk. Conclusions: Only 8% of patients undergoing urgent cardiac surgery in a 1-year period were screened for carotid artery disease. We suggest that screening should definitely be performed in all patients with a history of stroke or TIA, all patients with a bruit, and all patients aged > 65 years. The literature suggests, however, that significant reductions in stroke rate could be achieved by screening the whole cardiac surgical population, although there is a paucity of data that are specifically pertinent to this patient subgroup. Further data are therefore required for the construction of a scientifically valid and medicolegally sound policy.

Background Previous studies investigating the appropriateness of invasive management of coronary disease had not reported the internal consistency of their ratings and may now be out of date. The aim of this study was to measure the influence of clinical factors on contemporary ratings of the appropriateness of coronary angiography, percutaneous transluminal coronary angioplasty (PTCA) and coronary artery bypass graft (CABG) in the Appropriateness of Coronary Revascularisation (ACRE) study. Methods The Delphi-RAND technique was used, in which an expert panel (four cardiologists, three cardiothoracic surgeons, a general physician and a general practitioner), meeting in 1995, rated mutually exclusive indications (n o 2178 for angiography, n o 995 for PTCA and n o 984 for CABG). The main outcome measures were the appropriateness category (inappropriate, uncertain or appropriate) for each of the three procedures and treatment preference. Results For revascularization, the strongest determinant of inappropriateness was coronary anatomy. The odds ratio (OR) for inappropriate PTCA was 10.6 (95 per cent confidence interval (CI) 4.8‐23.5) for the effect of left main stem or threevessel disease versus single-vessel disease, and for CABG it was 0.06 (95 per cent CI 0.03‐0.15). The number of diseased vessels was strongly related to preference for medical, PTCA or CABG treatment (p for linear trend

The insertion of a suction catheter or a bronchoscope down an endotracheal tube increases the resistance to gas flow down the tube. The extent to which this occurs depends on the relative diameters of the endotracheal tube and the coaxially introduced catheter. This study utilises a laboratory model to quantify this effect, using a steady flow down an annulus between two tubes whose long axes lie co-axially. Two diameters of an endotracheal tube were modelled to represent flow down adult and neonatal endotracheal tubes; these were of internal diameter (do) 6.3 mm and 3.2 mm, and of length (L) 555 mm. A steady flow of air was generated to pass through the model ‘endotracheal’ tube. Flowrates were calculated to give Re of approximately 5000 for the larger endotracheal tube, and of approximately 1300 for the smaller. These values correspond to clinically appropriate flowrates in adult and neonatal patients, respectively. The pressure drop ΔPo down the endotracheal tube was measured initially without any obstruction, using a calibrated pressure transducer. Catheters of diameter (di) 0.8 mm, 1.6 mm, and 3.2 mm were introduced into the larger diameter endotracheal tube, while catheters of 0.8 mm and 1.6 mm were introduced into the smaller one, and flow was restored to its original value. The pressure drops ΔP down the endotracheal tubes were measured with the catheters introduced a length ‘x’ into the tube, to x = L/2 and to x = L. Results are compared with a theoretical calculation on the basis of laminar flow for concentric tubes. If a sampling tube or suction catheter is used down the length of an infant’s endotracheal tube, the results show that for most values of do/di, there is a significant rise in ΔP/ΔPo. Where a flexible bronchoscope is used down an endotracheal tube or a telescope down a rigid bronchoscope, the value of ΔP/ΔPo may also increase unacceptably where do/di is low. The results show that for equal do/di, and equal values of x, ΔP/ΔPo are lower for higher values of Re than for lower; and that for lower values of Re there is a more rapid increase in ΔP/ΔPo as x increases, than for higher Re, especially at low values of do/di. This result quantifiably confirms clinical experience; that care must be taken in introducing a catheter down a neonatal endotracheal tube. Deviation of these results from the theoretical calculation is less for the smaller Reynolds numbers and smaller values of do/di, because under these conditions the flow is more likely to be laminar, with a greater degree of concentricity.

OBJECTIVE--To compare the cardioprotective efficacy of cold crystalloid cardioplegia and intermittent ischaemic arrest in patients undergoing elective coronary artery surgery. DESIGN--Prospective randomised trial. SETTING--London teaching hospital. SUBJECTS--20 patients with at least moderately good left ventricular function undergoing elective coronary artery surgery by one experienced surgeon and needing at least two bypass grafts. INTERVENTIONS--Patients were randomised to cold crystalloid cardioplegia or intermittent ischaemic arrest. MAIN OUTCOME MEASURES--The primary determinant of the efficacy of myocardial protection was serial measurement (before and at 1, 6, 24, and 72 hours after the end of cardiopulmonary bypass) of cardiac troponin T (cTnT), a highly sensitive and specific marker of myocardial damage. RESULTS--There was no significant difference in age, ejection fraction, number of grafts, bypass times, or cross clamp times between the two groups. One patient in the cardioplegia group had a perioperative infarct and was excluded from further study. In both groups there was a significant increase in cTnT, with peak concentrations being reached 6 hours after the end of cardiopulmonary bypass and remaining significantly high at 72 hours. At 6 hours the median (75% interquartile range) concentrations of cTnT were similar in both groups (1.8 (1.0-3.6) micrograms/l for cardioplegia v 1.9 (1.0-3.5) micrograms/l for intermittent ischaemic arrest). CONCLUSION--This trial shows that intermittent ischaemic arrest, even without systemic cooling or venting of the left ventricle, provides a similar level of myocardial protection to cardioplegia in patients with moderate left ventricular function and short ischaemic times.

OBJECTIVE--To examine the cardioprotective efficacy of allopurinol in patients undergoing elective coronary artery surgery. DESIGN--Prospective randomised trial. SETTING--London teaching hospital. PATIENTS--Twenty patients with at least moderately good left ventricular function undergoing elective coronary artery surgery and requiring at least two bypass grafts. INTERVENTIONS--Patients were randomised to receive allopurinol (1200 mg in two divided doses) or to act as controls. MAIN OUTCOME MEASURE--The primary determinant of the efficacy of myocardial protection was serial measurement (preoperatively and subsequently at one, six, 24, and 72 hours after the end of cardiopulmonary bypass) of cardiac troponin T (cTnT) a highly sensitive and specific marker of myocardial damage. Additional evidence was provided by serial measurement of the MB-isoenzyme of creatine kinase (CK-MB) and myoglobin, ECG changes, and clinical outcome. RESULTS--There was no significant difference in age, ejection fraction, number of grafts, bypass times, or cross clamp times between the two groups. In both groups there was a highly significant (p < 0.01) rise in cTnT, CK-MB, and myoglobin. Peak concentrations were reached between one (CK-MB and myoglobin) and six hours (cTnT) after the end of cardiopulmonary bypass. At 72 hours cTnT concentrations were six times higher than baseline concentrations whereas CK-MB and myoglobin were approximately double baseline concentrations. There was no significant difference in cTnT, CK-MB, or myoglobin between the allopurinol and control groups at any time. There was no diagnostic ECG evidence of perioperative infarction in any patient. CONCLUSION--Unlike previous reports this study did not show that allopurinol had a cardioprotective effect in patients with good left ventricular function undergoing elective coronary artery surgery.

Many infusions are given by gravity assisted, drip sets that give a flowrate dependent on the height of the reservoir above the patient, the length of the tubing, the bore of the IV cannula, the density and viscosity of the fluid being delivered, and the patient’s venous pressure. However there is an increasing tendency to use programmable volumetric intravenous pumps and syringe drivers to deliver intravenous anaesthesia, fluids, patient controlled analgesia, epidural infusions and other drugs. Not only are they programmable, but they can also be adjusted to give desired flowrates or volumes. Some infusion devices are powered only by gravity, but the flowrate is controlled by a photoelectric drip rate detector in conjunction with a microprocessor controlled drip occlusion device. Other infusion devices use a stepper motor to control the rate of infusion. A stepper motor is designed so that the rotation is by a fixed amount per supplied electrical pulse, independent of the mechanical load it is carrying. The pulses are controlled by a microprocessor in the pump and the rate of infusion is dependent on the stepper motor’s output. Syringe drivers are designed to use a range of syringe sizes and some require special delivery tubing. The flow is a continuous, pulsatile flow and accuracy is 2–5%. Some syringe drivers are driven by clockwork motors, others by a battery powered motor that is intermittently on and off, depending on required flowrate. The driving mechanism is usually by a screw threaded rod connected to the syringe plunger. Other syringe drivers use a stepper motor connected to the screw threaded rod. Care should be taken not to position the syringe driver above the patient’s venous cannula or the syringe may siphon a drug additional to that programmed on the driver, by virtue of the weight of the column of fluid in the tubing above the patient. Care should also be taken to avoid any bubbles in the syringe reaching the patient. Modern syringe drivers are usually sufficiently accurate over the desired range of infusion [Stokes et al. 1990]. However, there may be a delay before the drug is delivered to the patient as the parts attached to the syringe take up slack [O’Kelly et al. 1992].

The World Federation of Societies of Anaesthesiology (WFSA) adopted standards relating to the safe practice of anaesthesia in 1992 and such standards had already been proposed by a number of countries in order to cut the morbidity due to anaesthesia itself. In the modern era it is easy to forget that historically anaesthesia and surgery did indeed have associated morbidity and mortality and there was very little assistance from technology to monitor patients. The evolution of these standards is based on two main requirements of monitoring. The first is to record anticipated deviations from normal values, which require accurate measurement to ensure patient safety. The second is to warn of unexpected, life-threatening events that, by definition, occur without warning, and could affect the fit, young patient as easily as the old and infirm. All international standards stress the importance of the continual presence of a fully trained and accredited anaesthetic person, and one Australian study demonstrated that many mishaps occur in the absence of such a person [Runciman 1988]. This applies to general and regional anaesthesia, sedation and recovery. Because perceptions of safety and standards vary throughout the world, despite the presence of an International Standards Organisation, debate about the minimum requirements for monitoring continue. Central to the maintenance of these standards is the quality of persons entering the specialty, the quality of training programmes, and the continuing education of specialists throughout a professional lifetime [Sykes 1992]. It is difficult to determine with certainty the effect that additional technological monitoring has on safety. One clear example is the inability of the trained human eye to detect cyanosis, this human failure occurring maximally at 81–85% oxygen saturation. Clearly, the pulse oximeter has improved the quality of cyanosis detection. Numerous studies all over the world have shown that mortality due to anaesthesia itself fell significantly between the 1950s and the 1980s, by which time extensive technological monitoring was being introduced, and training programmes had been very much improved. Utting [1987] reviewed 750 cases of death and cerebral damage reported to the British General Medical Council between 1970 and 1982 that were thought to be the result of errors in technique.

This chapter covers the background physics that is not otherwise covered in the relevant chapters associated with the equipment discussed. It is a useful introduction to some loosely related, but widely applicable concepts. Any measurement made by the anaesthetist can be simplified and made easier to understand if it is represented by its basic dimensions. Dimensions are the basic components of equations and are independent of the units used. For example a common term in physics and medicine is velocity, which has the dimensions of length per unit time, written dimensionally as [L][T]−1. This means that, independent of the measurement system used, the measurement of velocity requires that the numerical value of a length be divided by the numerical value of a time. Equations or graphical axes can be predicted and their validity can be checked by dimensional analysis. Each side of an equation can be represented in basic dimensions and both sides should balance. The dimensions needed to describe most events are mass [M], length [L], time [T], and temperature [θ]. An example of dimensional analysis is described later in this chapter but first units must be discussed. All measurements need to have their correct unit and symbol attached to them. Equations have a unique language and the syntax must be correct for communication within the international community. In the past many different systems of units were used; one was Imperial, another c.g.s. The Systéme International (SI) of units was established in 1960 and is now the recognised system of measurement communication. For completeness the base quantities are shown and some useful derived physical quantities are shown in Table 3.1 In the SI system the combination of basic units involves multiplication and division but multiplication is shown as a space and division is shown as a negative superscript. For example, velocity, in metres per second, is m s−1. Prefixes to the name of each unit are usually in multiples of 103 and 10−3. There are a few non-SI units that are still used in medicine, (not just anaesthesia) and these seem resilient to change. One is the millimetre of mercury (mmHg) for intravascular pressures (100 mmHg = 13.3 kPa), and total and partial gas pressures.

A blood gas machine has electrodes to measure pH, pCO2 and pO2 and often measures Hb and some biochemistry as well [King et al. 2000]. Derived values from such a device include O2 saturation, O2 content, bicarbonate, base excess and total CO2. This is the Clarke electrode described in the previous section on gas analysers and is suitable for both respiratory and blood O2 analysis. A pH unit has been defined in Chapter 1 as. In words, this can be described as ‘the negative logarithm, to base ten, of the hydrogen ion concentration’. The physical principle on which the pH electrode is based depends on the fact that when a membrane separates two solutions of different [H+], a potential difference exists across the membrane. In a pH electrode, such a membrane is usually made of glass and the development of a potential difference between the two solutions is thought to be due to the migration of H+ into the glass matrix. If one solution consists of a standard [H+], the pH of the other solution can be estimated by measurement of the potential difference between them. The glass membrane used is selectively permeable to H+. No current flows in this device, which does not wear out, in contrast to the Clark electrode, in which current does flow and that does need periodic replacement. The pH measurement system is shown diagrammatically in Figure 17.1. It consists of two half cells. In one half it has an Ag/AgCl electrode and in the other a Hg/HgCl2 (calomel) electrode. Each electrode maintains a fixed electrical potential. The Ag/AgCl electrode is surrounded by a buffer solution of known pH, surrounded by the pH sensitive glass. Outside the glass membrane is the test solution, usually blood, whose pH is to be measured. It is the potential difference across the glass, between these two solutions, which is variable. The blood or other solution is separated from the calomel electrode by a porous plug and a potassium chloride salt bridge to minimise KCl diffusion. The potential difference across the system is about 60 mV per unit of pH change at 37◦C.

This chapter covers the processing and application of electrical signals from the body, in particularly the electroencephalogram (EEG), the electrocardiogram (ECG), and the electromyogram (EMG). The EEG and ECG will be considered in their monitoring capacity. The EMG will be discussed along with simulation and neuromuscular blockade and monitoring. The electrocardiogram (ECG) is a surface reflection of the propagation of electrical depolarisation and repolarisation over the various contractile chambers of the heart. Depolarisation is the trigger for releasing the stored contractile energy in the cardiac muscle. Each chamber also produces electrical action and polarising recovery potentials associated with the mechanical contribution of the recovery. The ECG can be divided into two major components: one associated with the propagation of excitation and recovery of the atria; the other with these events occurring in the ventricles. Excitation of the atria gives rise to the P wave, after which the atrial contractions propel blood into the ventricles. An atrial recovery wave exists, but it is rarely seen, as it is obscured by ventricular excitation, which is signalled by the QRS wave. During the later part of the QRS wave, ventricular contraction commences. Recovery of the ventricles is preceded by the T wave. The ECG labels, i.e. PQRST, are shown in Figure 18.1. To localise the direction of excitation and recovery of the heart chambers (and also to estimate the extent of cardiac injury), a variety of electrode arrangements can be used. The electrodes (which are normally disposable silver–silver chloride as described in Chapter 5) are positioned on easily located anatomical landmarks such as the right arm (RA), the left arm (LA), and the left leg (LL), with the right leg usually providing the reference or common. The standard (1, II, III), augmented (aVR, aVL, aVF) and precordial (V) leads are routinely recorded by electrocardiographers. It is possible to locate the direction of excitation and recovery by considering that the direction of the event (excitation or recovery) is at right angles to the isoelectric lead (i.e. the lead with equal forces in the positive and negative). This can be demonstrated by forming an equilateral triangle (Einthoven’s triangle) such as in Figure 18.2(a).

The physics of pressure, flow and the gas laws have been discussed in Chapter 7 in relation to the behaviour of gas and vapour. This section will focus on the physical principles of the measurement of gas pressure, volume and flow. Unlike a liquid, a gas is compressible and the relationship between pressure, volume and flow depends on the resistance to gas flow (or impedance if there is a frequency dependence between pressure and flow in alternating flow, see Chapter 4 for the electrical analogy of this) in conduits (bronchi, anaesthetic tubing); it also depends on the compliance of structures being filled and emptied (alveoli, reservoir bags, tubing or bellows). Normal breathing occurs by muscular expansion of the thorax, thus lowering the intrathoracic pressure, allowing air or anaesthetic gas to flow towards the alveoli down a pressure gradient from atmospheric pressure. When positive pressure ventilation occurs, gas is ‘pushed’ under pressure into the alveoli. Depending on the exact relationship between the ventilator and the lungs, different relationships exist between airway pressure (rather than alveolar pressure, which cannot easily be measured) and gas flow and volume. Gas pressure measurement devices were traditionally in the form of an aneroid barometer, a hollow metal bellows calibrated for pressure and temperature, which contracts when the external pressure on it increases, and expands when it decreases. The movement is linked to a pointer and indicator dial. It is often more convenient to make the device in the shape of part of a circular section, but the principle is the same. This is what the Bourdon gauge, which commonly measures pressure in gas cylinders, looks like. The detection of movement of the diaphragm of an aneroid barometer can take several forms. The movement can either be linked via a direct mechanical linkage to a pointer, or diaphragm movement can be linked to a capacitative or inductive element in an electrical circuit, such as a Wheatstone bridge. Airway pressure during spontaneous breathing or artificial ventilation is low. The preferred units of measurement are cm H2O and the range of values is between −20 and +20 cmH2O. The aneroid barometer to measure this will therefore be of light construction, using thin copper for the bellows material.

Training and education using simulation has been used extensively in many high risk industries including aviation, nuclear power, military and rail. Repeated exposure to simulated crises and events has meant that, for example, airline crews are well prepared to face a rare disaster when it happens in real life. The use of simulation and simulators in medicine, to train and educate healthcare professionals has gained increasing attention in recent years and many simulation centres have now been set up in the UK. The Bristol Medical Simulation Centre, which opened in 1997, was the first training centre of its kind in the UK. There are now over 70 similar centres in the UK and many more with manikins in simpler settings, and hundreds of centres throughout the world [Department of Health 2010]. These offer a similar concept to that which the high risk industries use, where training for medical emergencies using sophisticated manikins are used in realistic medical settings, and task trainers are used to teach, for example, practical surgical skills. Many potential accidents in medicine are due to human error and communication problems [(Kohn et al. 1999, Department of Health 2009)]. Simulators can help train teams to function optimally using human factors style teaching. Simulation could also be a practical solution to several current educational issues. These include the challenges faced by educational institutions in securing clinical placements, the decrease in social acceptance of trainees learning on patients, the drive to maximise patient safety, and the dramatic decrease in training time being available to junior doctors due to the reduction in hours through the European Working Time Directive. The simulations centres consist of a number of different designated rooms. Simulated operations and team training can be carried out in the operating room. This room is made as close as possible to the modern operating room. It contains real equipment such as ventilators, defibrillators, patient monitors, trolleys and drip stands. A control room is next to the operating room, with a one way viewing window. This is where the manikin is controlled and where the simulation training is viewed and video recorded.

Ultrasound has many uses in areas of medicine associated with anaesthesia. It is used for imaging, visualisation of needle and catheter positioning, therapy and, together with the Doppler effect, for measurement of flow velocity. Real-time information can be obtained with ease, and with the low energies used, diagnostic equipment exposures are not thought to be a safety issue, either for the operator or the patient. Ultrasound is a form of mechanical energy that consists of high frequency vibrations at frequencies above human hearing range (> 20 kHz) and up to frequencies in the tens of MHz range. The frequencies used are dependent on issues such as the penetration and resolution required. It is thought that low intensity ultrasound passes through living tissue without altering tissue function. Higher energy can produce heating and cavitation, both of which can alter cell function. Ultrasound is generated by electrically inducing a deformation in a piezoelectric crystal, which compresses and decompresses the medium to which it is coupled at a rate equal to the frequency of the driving voltage. The pressure changes travel through the medium in a longitudinal direction and the distance between the points of maximum pressure, or compression, is known as the wavelength. Figure 10.2 shows the relationship between the period of the wave and the wavelength. The length of this distance is dependent on the elasticity (compressibility) and the density of the medium, and the delay between the movement of adjacent particles in the medium. As shown in the figure, the wavelength (λ) and the transmission frequency (f) are related to the propagation velocity c by c = f λ. The magnitude of the wave is the difference between the maximum and minimum pressure values. The wave propagates by movement of particles: it cannot travel in a vacuum and it does not ionise the medium through which it travels. The propagation of an ultrasound wave is not constant throughout the body. Various parts allow the passage of the wave at different velocities. Also the wave is attenuated differently by the various tissue types. For example, in soft tissues the ultrasound wave has a velocity of between 1460 and 1630 m s−1 whereas in bone it is 2700–4100 m s−1.

Graphs are used to represent pictorially or clarify a relationship between two variables, say x and y, or t and function (t). If x and y are related by the linear equation y = mx + c, then the graphical relationship is a straight line as in Figure 1.1, where m is the slope, a constant value for a straight line and c is the value of y when x = 0. Figure 1.2 shows the relationship when m and c take negative values. Note that if c = 0, the line passes through the origin, 0, of the x–y axes. Examples of other linear relationships that can be represented in this way include the following (the bracketed symbols are the variables equivalent to x and y above): v = u + a t; (variables are t and v), where u is the starting velocity of an object that is subjected to acceleration a for time t, after which its velocity is v.

This chapter will provide background to enable the reader to understand basic statistics and be able then to follow more complex statistical ideas. Although statistics is more than the mere analysis of data, it is a subject largely about data, so this will be discussed first. Data can be categorical or numerical, and in these two classifications there are various different types of data. This is the allocation of the individual to one of two categories. Often these relate to the presence or absence of some attribute. These data also have many other names such as binary, dichotomous and attribute data. Examples of such categorisations for patients include: ◆ Male/Female ◆ Smoker/Non-smoker ◆ Anaesthetist/Surgeon ◆ Married/Single. Each of these can be only be one or the other – they could be coded ‘1’ or ‘0’ to be binary (or on, off). For example male = 0, female = 1, or vice versa. Many classifications require more than two categories, such as: blood group, type of doctor, country of birth. Also the two categories, such as described previously, might be expanded into several categories. For example the married/single could be expanded to: married/single/divorced/separated/ widowed. This sort of data is called nominal data where there are several categories, but with no logical order. When there is a natural order (such as in seniority), the data are then called ordinal data. For example, anaesthetists could be divided into: ‘Foundation year 1’, ‘Foundation year 2’, ‘speciality doctor’, consultants’, ‘senior consultants’ and ‘clinical directors’. Ordinal data can be reduced to two categories, with possibly a considerable loss of information (e.g. ‘senior doctors’, ‘junior doctors’). Discrete numerical data are where the observation takes exact numerical values. Counts or events are discrete values. For example: number of children, number of ectopic beats in a time period and so on. Continuous (or analogue) data are usually obtained by some form of measurement. Examples are body temperature, blood pressure, height and weight. These values have an infinite number of possibilities, depending on the measurement interval, and variation. Although there are infinite possibilities, measurement systems usually round the continuous data up, or down, to discrete values. Blood pressure is often rounded up to the nearest 5 mmHg, for example.

This chapter continues the discussion of electricity but looks at the effect of connecting components together and briefly looks at the operational amplifier and active circuits (circuits discussed in the previous chapter have been passive ones, which involve no electronic circuits). It will then describe how the circuits can be used to process biological signals. If a resistor, an inductor and a capacitor are joined as in Figure 5.1, the magnitude of the voltage across the resistor (Vout) will vary as the input frequency, Vin, changes, because of the properties of the capacitor and inductor. The resistive action of inductor and capacitor oppose each other, and at a certain resonant frequency (r in the figure), the total AC resistance (impedance) will be at a minimum. The graph of reactance against frequency demonstrates this, and shows the minimum reactance at the resonant frequency. As the LC and R are forming a voltage divider, the voltage across R will be maximum at this frequency. At other input frequencies, the output voltage will be low. This simple circuit forms the basics of the passive band-pass filter, where the filter passes, or lets through, a certain band of frequencies (around the resonant frequency in this circuit). Frequencies lower or higher than the band-pass will be attenuated. Normally operational amplifiers (discussed in the next section) or digital filters are more effective and are used to achieve the same effect. Resonance is important for the understanding of the behaviour of transducers and this is discussed in Chapter 12. If a square wave is applied to the resistor capacitor network as shown in Figure 5.2, the capacitor charges up on the rising edge of the input with a time constant equal to the product of the resistance (Ω) and the capacitance (F), RC, and the output voltage will be Vouput = Vinput (1 − e−t /RC ). When the square wave is in the off state, or zero volts, the voltage will fall exponentially with the same time constant. It can be seen from the diagram that, with the appropriate values of R and C, the output from the network is a filtered version of the input.

In Europe and other advanced medical communities, medical gases are generally supplied by pipeline, with cylinders available as back up. Large hospitals usually have oxygen supplied and stored in liquid form, since one volume of it provides 840 volumes of gaseous oxygen at 15◦C. It is stored in a secure Vacuum Insulated Evaporator (VIE) on the hospital site. The arrangement is shown in Figure 22.1. The VIE consists of an insulated container, the inner layer of which is made of stainless steel, the outer of which is made of carbon steel. The liquid oxygen is stored in the inner container at about−160◦C (lower than the critical temperature of−118◦C) at a pressure of between 700 and 1200 kPa. There is a vapour withdrawal line at the top of the VIE, from which oxygen vapour can go via a restrictor to a superheater, where the gas is heated towards ambient temperature. Where demand exceeds supply from this route, there is also a liquid withdrawal line from the bottom of the VIE, from which liquid oxygen can be withdrawn; the liquid can be made to join the vapour line downstream of the restrictor and pass either through the superheater or back to the top of the VIE. The liquid can also be made to pass through an evaporator before joining the vapour line. After passing through the superheater, the oxygen vapour is passed through a series of pressure regulators to drop the pressure down to the distribution pipeline pressure of 410 kPa. It should be remembered that no insulation is perfect and there is a pressure relief valve on top of the VIE in case lack of demand and gradual temperature rise results in a pressure build up in the container. There is a filling port and there is usually considerable wastage in filling the VIE; the delivery hose needs to be cooled to below the critical temperature, using the tanker liquid oxygen itself to cool the delivery pipe. The whole VIE device is mounted on a hinged weighing scale and is situated outside the hospital building, protected by a caged enclosure, which also houses two banks of reserve cylinders.

In discussing humidity in the preceding chapter, the concept of equilibrium between water and its vapour has been introduced as a thermodynamic concept. The concept of vaporisation of other liquids such as volatile anaesthetic agents follows on naturally from that, but first of all it will be worth taking a detour through a discussion on solubility of gases and vapours in their own and other liquids [Davis 2003]. To maintain simplicity in the discussion on humidity, no mention was made of the presence of air or other gas above the surface of the water, only the water vapour. Depending on the solubility of the gas in the liquid, a variable amount of the gas dissolves in the liquid, whether that be air in water or carbon dioxide in blood. As will be discussed in the section on vaporisation, some molecules of gas enter the liquid and some leave it, depending on their individual kinetic energies, until equilibrium is reached. If the pressure inside the container with the gas or vapour and liquid is increased, then the partial pressure of the gas above the liquid surface increases; this increases the population density of gas molecules, resulting in more of the gas molecules dissolving in the liquid. Henry’s Law states that for a fixed temperature the solubility of a gas in a liquid is proportional to its partial pressure in equilibrium with the liquid. Note the condition of constant temperature because, in addition, solubility decreases with increased temperature. This occurs because an increase in the thermal energy of the dissolved gas molecules increases the partial pressure of the gas and encourages it to come out of solution (see below on vaporisation). Thus gas bubbles are more apparent in liquids that are heated. A historical clinical example of the relevance of ambient pressure and nitrogen solubility in body tissues is in decompression sickness associated with tunnel workers. Modern examples include underwater diving and, to a lesser extent, aviators and space walking astronauts. Nitrogen is a compressible gas and goes into solution in body tissue spaces under compression if the miner, tunnel worker, or diver is breathing ambient air.

Blood pressure measurement occurs either non-invasively or invasively, and usually refers to systemic arterial pressure measurement, but can also refer to systemic venous or pulmonary arterial pressure measurement. In 1733 the Reverend Stephen Hales was the first person to measure the blood pressure in vivo in unanaesthetised horses by direct cannulation of the carotid and femoral arteries. In doing so he observed the pulsatile nature of flow in the circulation. In 1828 Poiseuille developed the mercury manometer, and used it to measure blood pressure in a dog. The mercury manometer has, of course, become the standard technique against which other techniques are compared. The earliest numerical information on blood pressure measurement came from direct rather than indirect measurement in 1856 by Faivre, using Poiseuille’s device. However, in the last part of the nineteenth century, non-invasive measurement techniques were developed. In 1903, Codman and Cushing introduced the concept of routine intraoperative blood pressure measurement, which at the time was a revolutionary concept. Nowadays it is a fundamental part of minimal monitoring criteria. There are several techniques of non-invasive BP (NIBP) measurement, all of which function by occluding the pulse in a limb with a proximal cuff, then detecting its onset again distally, on lowering the cuff pressure. Detection methods include palpation, auscultation, plethysmography, oscillotonometry and oscillometry. Accuracy of all non-invasive techniques depends on cuff size in relation to the limb concerned, and over which artery the cuff is placed. Such techniques of NIBP measurement are necessarily intermittent. Much discussion has taken place on the accuracy of these devices, and the accuracy of diastolic pressure measurements needs improving, and there are ideas proposed for new non-invasive devices [Tooley and Magee 2009]. In the absence of a stethoscope, this technique is simple and reliable. After inflating the cuff on the upper arm to a pressure of above that of systolic, the cuff is then deflated while palpating the brachial artery and the systolic pressure is measured with a mercury column at first detection of the pulse. A study by van Bergen [1954] showed that BP can be underestimated by this method by up to 25% at 120 mmHg.

Domestic ‘mains’ systems in the UK use AC at 50 Hz. In the USA the systems use 60 Hz. These frequencies are used as they are efficient frequencies for transmission from power generation to the users and minimise the effect of leakage currents due to capacitance, which is discussed later in this chapter. The mains is initially generated in a power station and the power generated there (volts × amps) is enough to supply a number of hospitals and consumers. The voltage and the potential to do work must be transmitted to the user and this is usually achieved with overhead pylons, or sometimes by underground cables. Both types of cable will be designed to carry current, but the higher the current, the greater power lost to heat (I 2R). It is desirable to keep the current as low as possible to reduce this transmission heat loss, and this can be done by making the voltage as high as possible. For a given power, (V × I) there can be a high V and low I or vice versa. The transmission voltage is normally greater than 11 kV. It arrives at a domestic substation and is transformed down to (in the UK) 230 V RMS by a transformer. At the substation, as illustrated in Figure 6.1, one connection of the transformer is firmly bound to earth at what is called the star point, and this forms the start of what is called the neutral lead. The earth connection forms a vital part of electrical safety. The connection on the other side of the transformer is called the live lead and this is at 230 V RMS. These two leads are taken to the individual outlets or mains sockets, the live carrying the voltage to the load, and the neutral lead carrying the return current back to the source of the supply. The earth connection of the mains socket is connected back to the star point separately, although sometimes in older installations this can be earthed locally. In this way only one of the socket points is live, and the other is at near zero potential.

In continuing the concepts of energy and work, heat is a form of energy, and heat and work are related by the laws of thermodynamics. The first law states that the amount of mechanical work achievable out of a thermodynamic system going through a cyclic process is proportional to the amount of heat put into it, which is intuitive. The second law states that the amount of work gained from the system is always less than the amount of heat put into it; in other words there is some energy wasted, which is also believable. Heating a substance gives its constituent molecules increased kinetic energy, which can be put to mechanical use, for example in driving a turbine. Such an increase in energy either raises its temperature or changes its state. The increasing energy causes increasing velocity of movement of molecules and eventually breaks intermolecular bonds, giving the molecules more freedom of movement. As characteristic temperatures are reached, e.g. melting point or boiling point, the energy increase goes into changing the substance’s state into a more energetic one, e.g. solid to liquid, or liquid to gas, without any associated temperature change. Loss of heat from a substance involves the reverse processes. The gas laws have their origin in this concept of the kinetic theory of gases, and are an inherent part of consideration of the effects of heating and cooling, compression and expansion of gases. Pressure has been defined in Chapter 7, but to understand the concept, one that is taken for granted, it is necessary to start with the idea of molecules of matter undergoing a change in momentum (the physical definition of momentum being mass × velocity, see Chapter 3). It is unnecessary here to deduce the following equation, but the proof can be found in any physics or engineering text [Macintosh et al. 1987; Dorrington 1989; Hill 1976]).

The pulmonary artery catheter was the mainstay of clinical cardiac output measurement for many years, but because of its relatively invasive nature and the lack of improvement of clinical outcome with its use, it is now seldom used in a modern clinical environment. Any perceived accuracy of the technique is now considered unnecessary in the face of the risks of its use, and with the introduction of newer non-invasive techniques. Nevertheless, it is worth describing, partly because of its historical interest, and partly because of the technologies involved. A catheter passed into the right atrium from an easily accessible central vein can be passed through the right ventricle and out into the pulmonary arterial tree while the vascular waveforms are visualised. Figure 13.1 shows the waveforms as they appear to the user. A small balloon at the tip of the catheter allows it to be flow directed and wedged in a pulmonary arterial vessel. At this point the pulsatile waveform is lost and the tip of the catheter is looking ahead, down the pulmonary arterial tree towards the left atrium, a system with a relatively low pressure drop from one end to the other, the flow in that vessel having been brought temporarily to a standstill. Thus the pulmonary artery occlusion pressure (PAOP) or pulmonary capillary wedge pressure (PCWP) can be considered a reasonably accurate representation of left atrial pressure or left ventricular filling pressure. This assumes that there is no pulmonary vascular disease, such as pulmonary hypertension, or mitral valve disease, in which case PAOP would not be an accurate representation of left atrial pressure. If the catheter is placed in the apical region of the pulmonary vascular tree, the excess of the alveolar pressure in inspiration over pulmonary capillary pressure becomes significant, and the latter is a less accurate reflection of left atrial pressure. The balloon should not be over-inflated for fear of rupturing the pulmonary artery, and this is one of its perceived risks that has led to less usage. Once the measurement has been made, the balloon should be deflated so that the pulmonary arterial waveform is once again visible, if necessary withdrawing the catheter a bit to achieve this; failure to do so would result in regional lack of perfusion and may result in ischaemia.

The most important interface between the breathing system and the patient’s lungs is an airway management device (AMD). Post-operatively it can be considered to be a means of delivering oxygen enriched air to the patient. Intraoperatively it is intended to secure the patient’s airway, which might otherwise obstruct due to deep anaesthesia, to provide a reasonably gas tight seal to ensure accurate delivery of anaesthetic gases and, if necessary, to protect the lungs against aspiration of gastric contents. Postoperatively, the AMD can be nasal prongs or a variable performance mask, whose efficiencies may not be predictable [Wagstaff et al. 2007]. Intraoperatively it might be an artificial airway with a facemask, a supraglottic airway of one of the many types now available or an endotracheal tube (ETT). A supraglottic airway is one that sits in the pharynx or larynx above the vocal cords and these days is usually a laryngeal mask airway (LMA) of the numerous types now available, a cuffed oropharyngeal airway (COPA), or a Combitube. The LMA types available consist of: the classical LMA; the flexible (reinforced) LMA with a flexible tube to the breathing system; the ‘Proseal’, which has a gastric drainage tube as well as a gas transport tube; the intubating LMA, a device with a rigid right angled tube that acts as a ventilation conduit in the usual way, but through which an endotracheal tube may also be blindly introduced into the trachea; the ‘I-gel’ which has a gastric and a respiratory port as does the Proseal, but is less bulky, and whose bowl does not require inflation with air, but is filled with a gel that expands with body heat to form a seal. These days, almost all devices are made of material that excludes latex, but care should be taken to ensure this is indeed the case when there is a latex sensitive patient. Depending on the exact surgical and anaesthetic circumstances, the anaesthetist’s experience and equipment availability, a choice is made between these devices to secure the airway for a given operation. Additionally, there are other devices available to assist in securing the airway, such as the laryngoscope, the fibre optic bronchoscope and the cricothyrotomy tube.

The pulse oximeter is a device for non-invasive, continuous measurement of oxygen saturation. As such it is arguably one of the most important intraoperative monitors at the disposal of anaesthetists, and efforts are being made to make pulse oximeters available at all operating locations throughout the world [Walker et al. 2009]. Although the device measures oxygen saturation of arterial blood, which is the physiological end point of interest, it is not a replacement for monitoring all the events which may lead to hypoxaemia; in other words it does not replace an oxygen analyser at the common gas outlet of the anaesthetic machine. Depending on the site of the probe, usually ear lobe or finger, there is a variable delay between the onset of a causative hypoxaemic event and detection of hypoxaemia by the pulse oximeter, the delay being longer the more peripherally placed is the probe. Appropriate size and design of the probe for accuracy and safety in children is important [Howell et al. 1993] and finger probes are more accurate but slower to respond than ear probes [Webb et al. 1991]. Forehead reflectance probes have been used with good results [Casati et al. 2007]. It is also true that the human eye is notoriously bad at detecting cyanosis in the range of saturations 81–85%. For additional information on Monitoring Principles see Chapter 11. It is clear, however, that in a hierarchy of monitors for anaesthesia, the pulse oximeter is indispensable. A pulse oximeter uses two separate technologies: one is plethysmography, where reproduction of the pulsatile waveform takes place; the other is spectroscopy, where absorption of light of specific wavelengths by body tissues occurs and is analysed. The spectroscopic aspects depend on the laws of Beer and Lambert, which can be combined to state that the amount of light absorbed by a substance is proportional to the thickness of the substance sample (the path length of the light) and the concentration of the substance.

There are, and have been, many monitors designed to monitor depth of anaesthesia and to give an indication of awareness during surgery, which use electrical signals obtained from the human body. Some have been designed as just research devices, some have been available commercially, but have been withdrawn, and some are still available. Most, but not all, are based on the spontaneous EEG and the AER. Some have been designed to use properties of the ECG. Although useful, all of the discussed monitors have some shortcomings, and not all are 100% sensitive and specific to discriminate between consciousness and unconsciousness, and none correlate exactly with clinical states and levels of anaesthesia. The design of the commercial monitor, the Cerebral Function Monitor (CFM) was based on simple time domain measures already discussed [Maynard et al. 1969]. The CFM took the EEG from a single pair of parietal electrodes. The signal was amplified and passed through a band-pass filter and differentiator, which had the effect of accentuating the gain of the higher end of the 2–15 Hz pass band. The output of this specialised filter was integrated to produce a voltage output, which varied with time. It was plotted on a logarithmic scale. The trace on the paper gave an indication of the power of the EEG and the width of the line gave an indication of the signal’s variability. A schematic of an example of a CFM trace is shown in Figure 19.1(a). The CFM although useful did have its problems [Sechzer 1977]. When used to monitor depth of anaesthesia, the machine was shown to be unreliable, especially when using inhalational agents. The response is biphasic, as has already been discussed in chapter 18. Also burst suppression, as already discussed, is smoothed out by the action of the filtering in the CFM, so effectively the burst suppression can artificially elevate the readings producing a paradoxical rise in cerebral function [Sinha 2007] The machine was further developed into the Cerebral Function Analysing Monitor (CFAM)[Maynard 1984]. This machine produced two chart recorder outputs, as shown in Figure 19.1. There was a chart similar to the CFM trace, and also a chart that produced frequency domain data consisting of the EEG displayed as traditional EEG frequency bands.

As discussed in Chapter 4, when a voltage is applied across a conductor, a current will flow. If the voltage is applied across the body via suitable electrodes the body becomes part of the circuit and a current will also flow, the magnitude depending on the properties of the tissues in its path, particularly the resistance. This current can cause heating or other physiological effects, depending on the frequency of the driving voltage. The effects of the domestic mains current flowing through the body was discussed in Chapter 6, but different effects occur as the frequency of the voltage is increased. As the frequency goes up, the heating increases but the tissue stimulation decreases and, at frequencies above 100 kHz (i.e. radio frequencies), the effect is entirely heating. This heating effect in the body by electric current is called diathermy, but the location, concentration and how this heat is used is dependant on the electrode design and the current concentration or current density at any point in the circuit. For a certain applied voltage, the average current throughout the circuit will be the same. The current density is the current per unit area, and so if the material in which the current passes is smaller, the heating effect increases. The resistance of the material is proportional to its size, so as the material becomes smaller then its resistance gets larger. The heating power is the product of the current squared and the resistance (power = I2 × R). Surgical diathermy (or electrosurgery) is where either one or both of the electrodes are very small, and it is used to cut and coagulate tissue. The smaller electrode can be made into a pointed surgical tool and localised heating will occur at the tip of the instrument. The smaller and more pointed the instrument is, the greater the current density will be at the tip. This electrode is classified as the active or live one. The current densities around this electrode can be as much as 10 A cm−2, and the total heating power typically around 200 W.

This device was invented by Sir Ivan Magill and Sir Robert Macintosh to visualise the vocal cords to aid intubation. The curved blade of the Macintosh laryngoscope is still popular as the standard and its design has been reshaped in recent years to reduce the biomechanical forces on the teeth [Bucx et al. 1997, Bucx et al. 1994]. The straight blade of the Magill laryngoscope can make the view easier under some circumstances, as do modern variants [Henderson 1997]. The handle has a battery and a light source in it, and the light is transmitted by a fibre optic cable to the tip of the blade, which is usually at right angles to the handle. There are many different shaped blades to suit different circumstances, particularly to visualise the anatomically different airway of the neonate. Other variants to aid difficult intubations include the Polio laryngoscope, in which the angle between the handle and the blade is an obtuse one and the McCoy laryngoscope, which has a lever to manipulate the tip of the laryngoscope blade to improve the view of the vocal cords [McCoy et al. 1993]; some of these devices are shown in Figure 24.1. In recent years a new range of laryngoscopes has been introduced with fibre optic systems that allow indirect visualisation of the vocal cords via an eyepiece or a small video screen. These devices clearly improve access to the airway where there would otherwise be a laryngoscopic view with a high Cormack-Lehane score. They currently include the McGrath [Ray et al. 2009], the Glidescope, the Airtraq [Lange et al. 2009] (shown in Figures 24.2(a) and (b)), the C-MAC [McElwain et al. 2010 and others] and testing to date has been on manikins. As with all new devices, their efficacy depends on the skill of the user; one study has demonstrated greater skill with the familiar Macintosh laryngoscope than with the newer ones [Powell et al. 2009]. With the emergence of Jakob–Creutzfeldt disease and the recognition of the infectious risk of prions from tonsillar tissue on laryngoscope blades, a range of disposable paediatric blades has been produced, with variable quality of mechanical flexibility and light quality [Goodwin et al. 2006].

A good knowledge of physics, measurement, and equipment is essential for practising anaesthetists. However, these subjects do present considerable problems because few anaesthetists have any background in physics. This book explains the physical principles and applications of physics in anaesthesia, and the statistical methods that anaesthetists are required to understand. The book includes sections on equipment and safety in anaesthesia, and electrical safety. Fully updated for the second edition, the book also includes new chapters on the depth of anaesthesia and on electrical safety. The contents are based on the syllabus of the FRCA exam in mathematics, physics, clinical measurement, monitoring, and equipment. The book starts with mathematics, statistics and physics background, not only to enhance the understanding for what follows in the book, but also because these basic sciences are fundamental to many other aspects of medical science. Areas, which in the authors' teaching experience, candidates have found particularly troublesome, such as electricity and electrical safety, are discussed in some detail in order to clarify.