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International audience; Alongside ultrasonic visualisation, measurement of injection pressure is an effective tool for reducing the risk of intraneural injection during peripheral nerve block. The aim of this study was to compare injection pressure profiles when measured along the injection line with the pressure measured directly at the needle tip using different rates of injection. A syringe pump delivered a 5-ml injection of saline into silicone gel at three different speeds (5 ml.min-1 , 10 ml.min-1 and 15 ml.min-1 ). Fibreoptic pressure sensors recorded real-time pressure profiles of the injection pressure directly at the needle tip and along the injection line. A total of 15 injections were successfully performed, five for each injection rate, totalling 30 recorded pressure profiles. More rapid rates of injection caused peak pressure measured in-line to increase, whereas pressure measured at the needle tip remained constant (mean (SD) pressure in-line 30.76 (3.45) kPa vs. 72.25 (1.55) kPa and mean (SD) pressure at needle tip 19.92 (1.22) kPa vs. 20.93 (2.66) kPa at 5 ml.min-1 and 15 ml.min-1 , respectively). Injection pressure profiles showed that in-line pressure measurement failed to record precise real-time pressure changes occurring at the needle tip (mean (95%CI) pressure difference 10.8 (6.98-14.70) kPa vs. 51.2 (47.52-54.89) kPa for in-line and needle-tip measures, respectively). We conclude that, in order to accurately monitor the true injection pressure generated, independent from operator and injection parameters, measurement at the needle tip is necessary, as injection pressure measured along the injection line is an unreliable surrogate.

Although target-controlled infusion has been in use for more than two decades, its benefits are being obscured by anomalies in clinical practice caused by a number of important problems. These include: a variety of pharmacokinetic models available in open target-controlled infusion systems, which often confuse the user; the extrapolation of anthropomorphic data which provokes anomalous adjustments of dosing by such systems; and the uncertainty of regulatory requirements for the application of target-controlled infusion which causes uncontrolled exploitation of drugs and pharmacokinetic models in target-controlled infusion devices. Comparison of performance of pharmacokinetic models is complex and mostly inconclusive. However, a specific behaviour of a model in a target-controlled infusion system that is neither intended nor supported by scientific data can be considered an artefact or anomaly. Several of these anomalies can be identified in the current commercially available target-controlled infusion systems and are discussed in this review.

The aim of our in-vitro study was to assess the impact of infusion set characteristics on the accuracy of morphine doses in patient-controlled analgesia. Two infusion sets differing in conception and dead-space volume were assessed: a standard set and a low dead-space volume Y-set. The patient-controlled analgesia programme parameters were as follows: bolus equal to 1 ml at 100 ml.h(-1) ; lockout intervals equal to 5 and 10 min; and carrier fluid flow rate equal to 10 and 50 ml.h(-1) . Morphine concentration was determined by an ultraviolet spectrophotometric method. The morphine doses were significantly different from one set to the other during bolus and lockout intervals, whatever the patient-controlled analgesia programme. The average doses were approximately 1.3-6.0 times higher with the low dead-space volume Y-set during bolus. Our study underlines the impact of infusion set characteristics on the accuracy of morphine patient-controlled analgesia doses.

Summary When providing total intravenous anaesthesia, careful selection of end-points is required in titrating dose to effect during induction. Although propofol and remifentanil have predominantly different pharmacodynamic effects, they are seen to interact in achieving loss of consciousness and analgesia. To highlight these differences, we performed a double-blind, randomised controlled trial, comparing one group of patients receiving propofol alone (n = 42) with another group receiving remifentanil plus propofol (n = 46) as a target-controlled infusion of remifentanil (Minto; 3 ng.ml−1). Propofol was also titrated using a target-controlled infusion (Marsh effect model) to produce loss of response to tactile and vocal stimuli, and subsequently to loss of response to pain. The effect-site concentration of propofol at which 50% of patients lost tactile/verbal response was 2.9 μg.ml−1 in the propofol only group and 2.4 μg.ml−1 in the remifentanil with propofol group. In contrast, loss of pain response occurred at 4.4 μg.ml−1 in the propofol group, and 2.7 μg.ml−1 in the remifentanil with propofol group, with correspondingly lower bispectral index values. Judicious use of analgesia in total intravenous anaesthesia can have a propofol-sparing effect and potentially minimise the suppression of brain electrical activity.

Target controlled infusion (TCI) devices can be associated with significant safety concerns when used during magnetic resonance imaging (MRI). We tested the safety and compatibility of newer TCI systems in a 3-Tesla MRI environment. Two Asena PK and two Agilia TCI pumps were used to administer TCI propofol (at target blood concentrations of 0.5 and 6.0 mu g.ml(-1)) using the Marsh model under magnetic fields of up to 50 G with a T2-weighted sequence. We assessed the devices for projectile risk, accuracy of drug delivery, alarm function and effects on MR image quality. Both devices did not demonstrate any significant deflection at the tested field strengths, and performed within acceptable limits (cumulative error in total delivered volume

Ambulatory local anaesthetic delivery systems are often limited by a short effective duration of infusion. Prolonging nerve blockade by substituting a new pump as recommended by the manufacturers, represents a substantial consumable item cost ($US 300-500). We therefore evaluated the flow delivery performance of 31 single model elastomeric devices (all with a 2 ml.h(-1) background and 5 ml every hour bolus capability) that had been filled, used in clinical practice and then refilled in the laboratory. For the second infusion, there was a pattern of over-infusion (< 10 ml.h(-1)) in the first hour; however, all pumps depreciated to < 150% of predicted by the second hour. The subsequent performance of all pumps was not only within safe limits, but also predominantly within the range (background infusion +/- 15%, bolus +10/-20%) specified by the manufacturer for primary infusion. We conclude that this elastomeric regional anaesthesia pump design performs satisfactorily after having been refilled following a single previous use.

A modified patient-controlled analgesia pump provided doses of propofol 3 mg or midazolam 0.1 mg in 0.3 ml, over 5.4 s, with no lockout, during transvaginal oocyte retrieval. Alfentanil 0.2 mg was administered at three points during the procedure, and on request. Patients were randomly assigned to receive either propofol (25 patients) or midazolam (22 patients). The mean age, weight, duration of procedure and dose of alfentanil were similar in both groups. Onset of sedation with propofol or midazolam took 70.6 (SD 22.4) and 106.3 (50.7) s respectively. Mean doses over the first 5 min were midazolam 2.7 (1.2) mg, and propofol 54 (18) mg. Thereafter requirements decreased: midazolam 0.065 (0.065) mg.min-1, propofol 2.1 (1.3) mg.min-1. All patients successfully completed the procedure; none required additional sedation. P-deletion, reaction time, and critical flicker fusion tests revealed similar depression in both groups immediately postoperatively. After 30 min the p-deletion and critical flicker fusion scores were still impaired in the midazolam, but not in the propofol, group.

A critically ill septic patient on haemofiltration for acute renal failure suffered sudden circulatory and respiratory collapse. The cause of the collapse was traced to an interaction between mechanical devices (syringe driver infusion pumps and haemofiltration equipment) connected to two central venous catheters.

Summary Target-controlled infusion systems have been shown to result in the administration of larger doses of propofol, which may result in delayed emergence and recovery from anaesthesia. The aim of this study was to investigate if this was due to a difference in the depth of hypnosis (using the bispectral index monitoring) between the manual and target controlled systems of administration. Fifty unpremedicated patients undergoing elective surgery were randomly allocated to have their anaesthesia maintained with manual or target-controlled propofol infusion schemes. In both groups, the rate of propofol administration was adjusted according to standard clinical criteria while bispectral index scores were recorded by an observer not involved in the delivery of anaesthesia. The total dose of propofol used was higher in the target controlled group (mean 9.9 [standard deviation 1.6] compared with 8.1 [1.0] mg.kg−1.h−1 in the manual group [p

Animal studies have shown that injection pressures > 75 kPa indicate probable intrafascicular needle tip position. This study describes the flow/pressure characteristics of seven common needle systems. A syringe pump delivered flow rates of 5, 6.67, 10, 13.3, 15 and 20 ml.min(-1) through these needle systems, while keeping the needle tips open to atmosphere. A pressure transducer connected between the syringe and needle provided a real-time graphical display for analysis. Mean plateau pressures increased linearly with flow and with decreasing needle diameter (2.7-92 kPa). Flow rates > 17 ml.min(-1) and needle sizes 22 G and smaller produced mean plateau pressures > 75 kPa. Pressure monitors upstream from the needle may produce false-positive alarms at high flow rates due to needle resistance, and unreliable readings due to non-laminar flow. We recommend injection rates ≤ 15 ml.min(-1) (0.25 ml.s(-1) ) to reduce the effect of factors upstream from the needle tip as a cause of high pressure readings.

Summary We prospectively audited the bacterial contamination of re-useable analgesia infusion pumps. In a one-month period, 112 samples from the handset and keypads of our analgesia infusion pumps were cultured for bacterial contamination. Forty-five percent of handset swabs and 46% of keypad swabs grew bacteria; the commonest organism being coagulase-negative staphylococcus. An additional cleaning step using 70% isopropyl alcohol wipes was introduced and the contamination rate was re-audited in 100 samples. The contamination rate was reduced to 6% of handset swabs and 4% of keypad swabs. A high initial rate of bacterial contamination of re-useable analgesia infusion pumps was significantly reduced by the implementation of a simple, additional cleaning procedure.

Alarms are key components of peri-operative monitoring devices, but a high false-alarm rate may lead to desensitisation and neglect. The objective of this study was to quantify the number of alarms and assess the value of these alarms during moderate-risk surgery. For this purpose, we analysed documentation of anaesthesia workstations during 38 surgical procedures. Alarms were classified on technical validity and clinical relevance. The median (IQR [range]) alarm density per procedure was 20.8 (14.5-34.2 [3.7-85.6]) alarms.h⁻¹ (1 alarm every 2.9 min) and increased during induction and emergence of anaesthesia, with up to one alarm per 0.99 min during these periods (p < 0.001). Sixty-four per cent of all alarms were clinically irrelevant, whereas 5% of all alarms required immediate intervention. The positive predictive value of an alarm during induction and emergence was 20% (95% CI 16-24%) and 11% (95% CI 8-14%), respectively. This study shows that peri-operative alarms are frequently irrelevant, with a low predictive value for an emerging event requiring clinical intervention.

Summary The Baxter Intermate LV250 infusor is a large capacity, high flow mechanical infusor, designed to deliver viscous solutions and suspensions. The objective of the laboratory study was to evaluate the LV250 infusor when used with either 0.9% sodium chloride solution or an emulsion, propofol 1%. Following this, the LV250 infusor with the patient-control module was assessed in the clinical setting as to its suitability for use in patient-controlled sedation with a propofol and lignocaine mixture. Between reservoir volumes of 250 and 55 ml, the L V250 infusor was found to infuse a propofol 1% and lignocaine mixture at a lower flow compared with the sodium chloride solution, 170ml.h-1 and 260ml.h-1 respectively. The lower flow with the propofol mixture may be a result of its higher viscosity. All patients using the LV250 infusor for patient-controlled sedation found it easy to use and stated that they were able to achieve their desired level of sedation. All patients maintained arterial oxygen saturations above 95%. The LV250 infusor performed reliably and with little inter-device variation, and with the patient-control module could be used as an alternative to electronic devices for providing patient-controlled sedation. Compared with electronic infusors, its advantages include compactness and simplicity of design.

This study evaluated the effect of limiting maximal infusion-pump flow rate on suppression of remifentanil-induced cough during target-controlled infusion. Two hundred and ten patients were randomly assigned to receive remifentanil at an effect-site concentration of 4.0 ng.ml(-1) with maximal flow rate limited to 100 (group R(100)), 200 (group R(200)), or 1200 ml.h(-1) (group R(1200)). The number of episodes of cough were recorded and graded as mild (1-2), moderate (3-4), or severe (5 or more). The incidence of cough was 2.9% in group R(100), 5.7% in group R(200) and 25.7% in group R(1200). Patients in group R(100) and R(200) had a significantly lower incidence of cough than those in group R(1200) (p0.05). Zero, two and five patients coughed a moderate amount in groups R(100), R(200) and group R(1200), respectively (p0.05). Limiting maximal infusion rate during remifentanil TCI suppressed remifentanil-induced cough.

Dual-lumen haemodiafiltration catheters enable continuous renal replacement therapy in the critically ill and are often co-located with central venous catheters used to infuse drugs. The extent to which infusions are immediately aspirated by an adjacent haemodiafiltration catheter remains unknown. A bench model was constructed to evaluate this effect. A central venous catheter and a haemodiafiltration catheter were inserted into a simulated central vein and flow generated using centrifugal pumps within the simulated vein and haemodiafiltration circuit. Ink was used as a visual tracer and creatinine solution as a quantifiable tracer. Tracers were completely aspirated by the haemodiafiltration catheter unless the infusion was at least 1 cm downstream to the arterial port. No tracer was aspirated from catheters infusing at least 2 cm downstream. Orientation of side ports did not affect tracer elimination. Co-location of central venous and haemodiafiltration catheters may lead to complete aspiration of infusions into the haemodiafilter with resultant drug under-dosing.

Anaesthetic equipment plays a central role in anaesthetic practice but brings the potential for malfunction or misuse. We aimed to explore the national picture by reviewing patient safety incidents relating to anaesthetic equipment from the National Reporting and Learning System for England and Wales between 2006 and 2008. We searched the database using the system's own classification and by scrutinising the free text of relevant incidents. There were 1029 relevant incidents. Of these, 410 (39.8%) concerned patient monitoring, most commonly screen failure during anaesthesia, failure of one modality or failure to transfer data automatically from anaesthetic room to operating theatre. Problems relating to ventilators made up 185 (17.9%) of the reports. Sudden failures during anaesthesia accounted for 142 (13.8%) of these, with a further 10 cases (0.9%) where malfunction caused a sustained or increasing positive pressure in the patient's airway. Leaks made up 99 (9.6%) of incidents and 53 (5.2%) of incidents arose from the use of infusion pumps. Most (89%) of the incidents caused no patient harm; only 30 (2.9%) were judged to have led to moderate or severe harm. Although equipment was often faulty, user error or unfamiliarity also played a part. A large variety of causes led to a relatively small number of clinical scenarios, that anaesthetists should be ready, both individually and organisationally, to manage even when the cause is not apparent. We make recommendations for enhancing patient safety with respect to equipment. You can respond to this article at http://www.anaesthesiacorrespondence.com.

Summary We present the results of a laboratory study of the loss of isoflurane from a to-and-fro system, ventilated with oxygen, using a standard ventilator connected to the system via a long corrugated tube (the trunk) in place of the reservoir bag. No conventional fresh gas supply is used and isoflurane is injected as a liquid directly into the soda lime canister. The loss of isoflurane from the system due to mixing in the trunk was generally less than that lost by absorption into rubber and plastic system components. Glass syringes used for injection of liquid isoflurane were found to be a potential source of much greater leaks. The results showed that a to-and-fro system ventilated with oxygen via an open trunk functions as a virtually completely closed anaesthetic breathing system.

Summary The accuracy of a computer-controlled infusion of midazolam, based on previously published pharmacokinetic parameters, was tested prospectively in 12 adult female patients undergoing general anaesthesia. Anaesthesia consisted of an initial bolus followed by an exponetially decreasing infusion of midazolam given according to body weight, fentanyl, nitrous oxide and vecuronium. Venous blood samples were taken at 15 min-intervals throughout the procedures and for 1–2 h postoperatively. The bias of the model was – 5.1% (95% C1 –11.3 to 1.2%) and precision 24.8% (95%CI 20.9 to 28.6%). Least squares regression analysis decreased the bias to—2.8% but did not alter precision. Retrospective fitting of an alternative set of published parameters for midazolam resulted in significant deterioration of the model. The precision was similar to that found in past studies of intravenous anaesthetic agents. Further improvement in the accuracy of midazolam infusion awaits improved understanding of the causes of pharmocokinetic variability.

Summary A system was developed to test the accuracy of patient-controlled analgesia devices in situations simulating clinical use. Bolus requests are made automatically at predetermined intervals, and the infusate delivered is measured and recorded without the need for operator presence. To ensure clinical relevance, the bolus request times used in this study corresponded to a pattern typical of those requested by patients on the ward. Graseby, Abbott Provider 5500 and IVAC patient-controlled analgesia devices were tested and found to deliver reasonably accurately over a 24 h period. However, when an infusion was started in an unprimed system or after a period of no bolus requests in a bolus-only mode the Graseby and IVAC machines under-delivered. This system provides a means of testing patient-controlled analgesia devices operating in any delivery mode.

Summary An automatically pressurised infusor system, the Alton Dean infusor, has been compared with two types (cloth and plastic) of commonly used pneumatically pressurised bags. All the infusors had reasonably accurate pressure gauges but pressure could only be consistently maintained with the cloth infusors or with the Alton Dean infusors when connected to a compressed air supply. Sequential fast infusions were possible with all infusors, but simultaneous infusions could be limited in number when the pressure infusor was connected directly to pipeline gas. One of the Alton Dean pressure infusors tested had a leak in the pressurisation system, and the pressure adjustment valves were difficult to manipulate. These may require modification.

The quality of sedation and postoperative recovery have been assessed for intra-operative sedation provided by either patient-controlled sedation with propofol or a standard method using divided doses of midazolam and fentanyl, in 40 ASA 1 day surgery patients undergoing extraction of third molar teeth under local analgesia. Patient-controlled sedation with propofol produced sedation no deeper than full eyelid closure with prompt response to verbal command, but deeper levels were seen in three patients in the midazolam and fentanyl group. Patient satisfaction was higher in the patient-controlled sedation propofol group for both subjective intra-operative feelings (p less than 0.01) and willingness to have the procedure again in the same manner (p less than 0.05). Amnesia was more limited to intra-operative events (rather than extending into the postoperative period) in the patient-controlled sedation propofol group (p less than 0.05). Drug dose was correlated with duration of procedure and surgical difficulty in the patient-controlled sedation propofol group but not in the midazolam and fentanyl group. Postoperative testing included a new computerised test, the FAST index, which indicated a dose-dependent reduction in cognitive function in the midazolam and fentanyl group, which persisted until the time of discharge. Changes in cognitive function in the patient-controlled sedation propofol group in the same postoperative interval were significantly less and not related to propofol dose.

Summary The Graseby patient-controlled analgesia system was evaluated in the laboratory and in clinical use. The problems encountered with eight examples used to treat 510 patients are reported. Laboratory performance revealed the unit to be accurate at infusion volumes of 1 and 2 ml.

Target controlled infusion (TCI) pumps function using a programme based on a pharmacokinetic/pharmacodynamic model. We compared the Marsh and Schnider models to find out which better correlates with the clinically observed effect of propofol as assessed by the Observer Assessment of Alertness/Sedation (OAAS) score and the Bispectral index. We assessed the sedation score and Bispectral index score in 40 un-premedicated patients undergoing surgical procedures under spinal anaesthesia with propofol sedation to a target concentration of 2 microg.ml(-1). Half of the patients received TCI propofol driven by the Schnider model in effect site control, the other half were sedated with TCI propofol driven by the Marsh model in plasma control. We calculated the effect site concentration predicted by both models for all the patients. Changes in the sedation score and Bispectral index correlated better with the Marsh than with the Schnider effect site prediction in both study groups.

Pressurised infusion devices may have only limited capability to detect and remove air during pressurised infusions. In order to assess pressure infusion systems with regard to their actual air elimination capabilities four disposable pressure infusion systems and fluid warmers were investigated: The Level 1 (L-1), Ranger (RA), Gymar (GY), and the Warmflo (WF). Different volumes of air were injected proximal to the heat exchanger and the remaining amount of air that was delivered at the end of the tubing was measured during pressurised infusions. Elimination of the injected air (100-200 ml) was superior by the RA system when compared to L-1 (p0.01). The GY and WF systems failed to eliminate the injected air. In conclusion, air elimination was best performed by the RA system. In terms of the risk of air embolism during pressurised infusions, improvements in air elimination of the investigated devices are still necessary.

This multicentre, non-comparative study investigated the range of target blood propofol concentrations required to sedate 122 adult intensive care patients when propofol was administered using Diprifusor target-controlled infusion systems together with opioid analgesia. Depth of sedation was assessed with a modified Ramsay score and the target blood propofol setting was adjusted to achieve the sedation desired for each patient. A desired level of sedation was achieved for 84% of the sedation period. In postcardiac surgery patients the median time-weighted average propofol target setting was 1.34 microg.ml(-1) (10th - 90th percentiles: 0.79-1.93 microg.ml(-1)). Values in brain injured and general ICU patients were 0.98 (10th - 90th percentiles: 0.60-2.55) microg.ml(-1) and 0.42 (10th - 90th percentiles: 0.16-1.19) microg.ml(-1), respectively. Measured propofol concentrations were generally close to values predicted by the Diprifusor system. Target settings in the range of 0.2-2.0 microg.ml(-1) are proposed for propofol sedation in this setting with titration as required in individual patients.

Patient-maintained sedation is a mode of patient-controlled sedation during which propofol is administered using a target-controlled infusion, with patient demand increasing the target concentration. A system tested previously for safety in our institution resulted in oversedation. Aiming to improve safety, we modified the system by increasing the lockout period to 4 min,reducing the starting concentration to 0.5 microg x ml(-1) and the increments on demand to 0.1 microg x ml(-1). As in the previous study, healthy volunteers attempted to render themselves unconscious by frequently pressing the demand button. To assess effects on memory, volunteers were given keywords to remember every 15 min. The maximum target concentration reached varied between 1.0 and 2.5 microg x ml(-1). No volunteers lost consciousness, however, one volunteer had a brief period of apnoea and oxygen desaturation. The Cp50 for loss of memory for words was 1.26 microg x ml(-1). Although this version represents an improvement, we conclude that the system is not yet completely suitable for use without anaesthetic supervision.

The aim of this study was to determine whether infusion sets containing antisyphon devices increased the time to initial flow from syringe drivers. The antisyphon devices assessed were those manufactured by B Braun, Wescott and Vygon. Each device was placed between a 50-ml syringe and a spiral extension set and primed with saline. A fourth syringe and spiral extension set acted as a control. The infusion sets were placed in four identical syringe drivers and started simultaneously. The time from pressing the start button until the initial flow for each infusion set (start-up time) was recorded. The test was conducted 15 times each at 2 ml.h-1, 10 ml. h-1 and 50 ml.h-1. At 2 ml.h-1 the start-up time was significantly longer with all the antisyphon sets compared with the control (p0. 0001). At higher infusion rates the differences between the antisyphon sets and the control were less pronounced. Clinicians who use syringe driver infusions should be aware of this delay between the activation of the infusion pump and the onset of flow and take steps to prevent it.

A patient who required pleurectomy had a 30% pneumothorax when she was presented for anaesthesia. She had refused to have this drained, and it had not responded to conservative management. She was anaesthetised using a computer-controlled propofol infusion system, without the use of nitrous oxide, and a chest drain was inserted before the institution of positive pressure ventilation. This technique reduces the hazards associated with general anaesthesia in the presence of an undrained pneumothorax. It may be a safe alternative method of induction of anaesthesia in other conditions in which positive pressure ventilation must be avoided, such as bronchopleural fistula.

We have observed in vitro the distribution of flow from 10 identical multihole epidural catheters during continuous infusion with four different infusion pumps. The pumps chosen were the B Braun Perfusor Secura FT syringe driver and three volumetric infusion pumps utilising different pumping mechanisms (Dekra 3000 BL, Graseby 500 and CADD-Prizm). These pumps infused 0.9% saline through each catheter at 5 ml.h-1, 15 ml.h-1, 50 ml.h-1 and 99 ml.h-1 for 3 min. The number of holes through which flow occurred and the catheter hole where flow predominated during each test were recorded. The pressure waveform generated during each infusion was displayed and the peak pressure recorded. In 38 of the 160 tests (24%) the largest proportion of flow was seen at the hole closest to the catheter tip. The CADD pump generated multihole flow during significantly more tests (p0.0001) than the other pumps and produced significantly higher driving pressures (p0.001) at all infusion rates compared with the Graseby and Perfusor pumps. The CADD was the only pump to produce flow from all three holes of the catheter at 5 ml.h-1.

This paper describes practical aspects relating to the safe use of target controlled infusion systems in anaesthesia. Consideration is given to the correct use of syringes and infusion lines for any target controlled infusion system. The importance of appropriate connections, minimising infusion line dead space and the avoidance of syphoning are emphasised. The first two commercially available infusion pumps to incorporate the 'Diprifusor' module for the administration of propofol by target controlled infusion are the Graseby 3500 (Graseby Medical Ltd) and the Vial Master TCI (Fresenius Vial SA, originally developed by Becton Dickinson). Particular features of these systems are discussed. Finally, the practical consequences of possible misuse of infusion systems incorporating pharmacokinetic models are considered.