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B1PAP=bilevel positive airway pressure; CAH’chromc alveolar hypoventilation; CNEP=continuous negative extrathoracic pressure; CPAP=continuous positive airway pressure; DMD=Duchenne muscular dystrophy; EPAP=expiratory positive airway pressures; EPR electrophrenic respiration; GPB=glossopharyngeal breathing; IAPVintermittent abdominal pressure ventilator, ffAP inspiratory positive airway pressures; IPPBintermittent positive pressure breathing; IPPVintennittent positive pressure ventilation; MI-E=mechanical insufflation-exsufflation; NPBVs=negative pressure body ventilators; PCEF=peak cough expiratory flows; RTIs= respiratory tract infections; VCvital capacity

Esophageal replacement after esophagogastric injury caused by ingestion of lye may require the interposition of a retrosternal ileocolic graft. In this new anatomic situation, the mesenteric circulation supplying the graft is subjected to the intrathoracic pressure surrounding the graft. Thus, mesenteric blood flow supplying the graft may be impaired when intrathoracic pressure is increased during mechanical ventilation. This study was designed to evaluate the effect of increasing intrathoracic pressure by application of a positive end-expiratory pressure on mesenteric blood flow supplying esophageal ileocolic grafts. Eight cases were studied in the immediate postoperative period. Miniaturized implantable Doppler microprobes were sutured to the single artery supplying the graft and connected to an 8 MHz pulsed Doppler flowmeter. Two sets of measurements were successively performed with zero end-expiratory pressure ventilation and after application of a 15 cm water positive end-expiratory pressure. Positive end-expiratory pressure induces mean arterial pressure (-12%); p p p

Recent interest in functional hemodynamic monitoring for the bedside assessment of cardiovascular insufficiency has heightened. Functional hemodynamic monitoring is the assessment of the dynamic interactions of hemodynamic variables in response to a defined perturbation. Accordingly, fluid responsiveness can be predicted in a quantities fashion by measuring as arterial pulse pressure variation and left ventricular stroke volume variation during positive pressure breathing or the change in cardiac output response to a passive leg raising maneuver. However, volume responsiveness, though important, reflects only part of the overall spectrum of functional physiological variables that can be measured to define physiologic state and monitor response to therapy. Dynamic tissue O2 saturation (StO2) responses to complete stop flow conditions, which can be created by measuring hand StO2 and occluding flow with a blood pressure cuff, assesses cardiovascular sufficiency and microcirculatory blood flow distribution. Furthermore, these measures can be made increasingly more sensitive and specific if coupled to other "traditional" measures of organ perfusion, such as blood lactate levels.

To present an updated discussion of those aspects of controlled positive pressure breathing and retained spontaneous regulation of breathing that impact the management of patients whose tissue oxygenation is compromised by acute lung injury.The recent introduction of ventilation techniques geared toward integrating natural breathing rhythms into even the earliest phase of acute respiratory distress syndrome support (e.g., airway pressure release, proportional assist ventilation, and neurally adjusted ventilatory assist), has stimulated a burst of new investigations.Optimizing gas exchange, avoiding lung injury, and preserving respiratory muscle strength and endurance are vital therapeutic objectives for managing acute lung injury. Accordingly, comparing the physiology and consequences of breathing patterns that preserve and eliminate breathing effort has been a theme of persisting investigative interest throughout the several decades over which it has been possible to sustain cardiopulmonary life support outside the operating theater.

Engineering developments allow mankind to explore the flight environment. However, the physical nature of the atmosphere and the mode of operation of aircraft can impose significant challenges to human physiology. On ascent the increasingly rarefied atmosphere can deprive the aviator of an adequate supply of oxygen. They therefore can develop hypoxia, which is both insidious and hazardous, with a significant risk of death. The time required to cause injury or worse varies from many minutes to a few seconds, depending on the altitude concerned. Airborne life-support systems must provide adequate protection to all on board an aircraft, for routine and emergency purposes. The systems themselves must be compatible with human physiological requirements and safe to operate. In addition to the drop in atmospheric pressure on ascent the ambient temperature can fall to damaging levels and therefore cabin conditioning systems must ensure that the internal thermal environment and the cabin pressure are appropriate. Aircrew flying high-performance aircraft may be exposed to increased levels of acceleration, especially radial acceleration and high G imposes physiological challenges for which a range of counter-measures including anti-G trousers and now positive pressure breathing for G have been developed. G-induced loss of consciousness (G-loc) remains a hazard of high-performance flight. Vision, hearing, and balance are special senses that may be affected by flight and measures to support these and address some of the issues noted above may require engineering solutions. Aircraft designers need to be aware of the physiological aspects of flight and consider their particular requirements throughout the design of an aircraft. Keywords: altitude; hypoxia; life support; DCI; acceleration; G-loc; thermal; vision

Rapid right ventricular pacing is safe, effective, and established method to provide balloon stability during balloon aortic valvuloplasty (BAV). Controlled transient respiratory arrest at this point of time may further reduce left ventricular stroke volume, providing an additional benefit to maintain balloon stability. Two groups were studied. Among the 10 patients, five had rapid pacing alone (Group A), while the other five were provided with cessation of positive pressure breathing as well (Group B). The outcomes of BAV in the two groups of patients were studied. One patient in Group A had failed balloon dilatation even after the fourth attempt, while in Group B there were no failures. The peak systolic gradient reduction was higher in Group B (70.05% in comparison to 52.16% of group A). In Group A, five subjects developed aortic regurgitation (grade 2 in four and grade 3 in one, while no grade 3 aortic regurgitation developed in any patient in Group B). Controlled transient respiratory arrest along with rapid ventricular pacing may be effective in maintaining balloon stability and improve the outcome of BAV.

Preoxygenation increases oxygen reserves and duration of apnea without desaturation (DAWD), thus it provides valuable additional time to secure the airway. The purpose of this Continuing Professional Development (CPD) module is to examine the various preoxygenation techniques that have been proposed and to assess their effectiveness in healthy adults and in obese, pregnant, and elderly patients.The effectiveness of preoxygenation techniques can be evaluated by measuring DAWD, i.e., the time for oxygen saturation to decrease to90%. Clinically, preoxygenation is considered adequate when end-tidal oxygen fraction is90%. This is usually achieved with a 3-min tidal volume breathing (TVB) technique. As a rule, asking the patient to take four deep breaths in 30 sec (4 DB 30 sec) yields poorer results. Eight deep breaths in 60 sec (8 DB 60 sec) is equivalent to TVB 3 min. The DAWD is decreased in obese patients, pregnant women, and patients with increased metabolism. Obese patients may benefit from the head-up position and positive pressure breathing. A TVB technique is preferable in the elderly. Failure to preoxygenate is often due to leaks, which commonly occur in edentulous or bearded patients. In cases of difficult preoxygenation, directly applying the circuit to the mouth might be a useful alternative. Supplying extra oxygen in the nasopharynx during apnea might increase DAWD.Since ventilation and tracheal intubation difficulties are unpredictable, this CPD module recommends that all patients be preoxygenated. The TVB 3 min and the 8 DB 60 sec techniques are suitable for most patients; however, the 4 DB 30 sec is inadequate.

Liquid ventilation with oxygenated perfluorocarbon eliminates surface tension due to pulmonary air/fluid interfaces, and improves pulmonary function and gas exchange in surfactant deficiency. In liquid ventilation, perfluorocarbon is oxygenated, purged of CO2, and cycled into and out of the lungs using an investigational device. A new approach, perfluorocarbon-associated gas exchange, uses a conventional ventilator and combines features of liquid ventilation and continuous positive-pressure breathing. In 13 normal piglets, a volume of perfluorocarbon equivalent to the normal functional residual capacity (30 mL/kg) was instilled into the trachea, left in situ, and volume-regulated gas ventilation (FIO2 1.0) was resumed. For 1 hr, perfluorocarbon was continuously bubble-oxygenated within the lungs, where it directly participated in gas exchange.PaO2 and PaCO2 averaged 401 +/- 51 and 40 +/- 4 torr (53.6 +/- 6.8 and 5.3 +/- 0.5 kPa), respectively. Peak airway pressure during perfluorocarbon-associated gas exchange (22 +/- 2 cm H2O at 1 hr) and during continuous, positive-pressure breathing (23 +/- 4 cm H2O) were nearly identical. Venous oxygen saturation and pH were normal (73 +/- 8% and 7.43 +/- 0.05, respectively, at 1 hr).Perfluorocarbon-associated gas exchange was uniformly well tolerated, and its efficiency approached that of continuous positive-pressure breathing. Applications of perfluorocarbon technology to lung disease may not be limited by existing instrumentation.

To evaluate the effects of positive airway pressure on lung mechanics and airway pressure transmission into the intrathoracic space, airway and intrathoracic pressure were recorded during stepwise lung inflation in 17 infants undergoing open heart surgery during the first year of life. Eleven of the 17 patients had cuffed tracheal tubes allowing calculation of lung and chest wall compliance. In 7 of these 11 patients, lung inflation to 2.5 ml/kg above functional residual capacity produced an improvement in initially low lung compliance. This volume increment required elevation of airway pressure by 4-11 cm H2O above ambient. Further lung inflation to 10 ml/kg did not effect an improvement in lung mechanics. In the remaining patients, initial lung compliance was high and remained unchanged throughout the range of lung inflation. Transmission of pressure into the intrathoracic space averaged 47 +/- 9% (mean +/- SD) and ranged from 33% to 61% when airway pressure was 10-15 cm H2O. The results indicate that the use of moderately elevated airway pressure is required to optimize lung distensibility in most infants immediately after open heart surgery. When evaluating circulatory effects of elevated airway pressure, assumption of a 50% pressure transmission is appropriate for clinical purposes. However, the observed wide range of pressure transmission warrants careful hemodynamic monitoring during continuous positive pressure breathing.

Background: Breathing exercises against a resistance during expiration are often used as treatment for patients with chronic obstructive pulmonary disease (COPD). Controversy still exists regarding the clinical application and efficacy. Objectives: The aim of this systematic review was to determine the effects of chest physiotherapy techniques with positive expiratory pressure (PEP) for the prevention and treatment of pulmonary impairment in adults with COPD. Methods: The review was conducted on randomised, controlled clinical trials in which breathing exercises with positive expiratory pressure were compared with other chest physical therapy techniques or with no treatment, in adult patients with COPD. A computer-assisted literature search of available databases from 1970 to January 2008 was performed. Two reviewers extracted data independently and assessed the trials systematically with an instrument for measuring methodological quality. Results: In total, 11 trials met the inclusion criteria, of which 5 reached an adequate level of internal validity. Several kinds of PEP techniques with a diversity of intensities and durations of treatment have been evaluated with different outcome measures and follow-up periods. Benefits of PEP were found in isolated outcome measures in separate studies with a follow-up period Conclusion: Prior to widespread prescription of long-term PEP treatment, more research is required to establish the benefit of the technique in patients with COPD.

BackgroundFluid resuscitation is not only used to prevent acute kidney injury (AKI) but fluid management is also a cornerstone of treatment for patients with established AKI and renal failure. Ultrafiltration removes volume initially from the intravascular compartment inducing a relative degree of hypovolemia. Normal reflex mechanisms attempt to sustain blood pressure constant despite marked changes in blood volume and cardiac output. Thus, compensated shock with a normal blood pressure is a major cause of AKI or exacerbations of AKI during ultrafiltration.MethodsWe undertook a systematic review of the literature using MEDLINE, Google Scholar and PubMed searches. We determined a list of key questions and convened a 2-day consensus conference to develop summary statements via a series of alternating breakout and plenary sessions. In these sessions, we identified supporting evidence and generated clinical practice recommendations and/or directions for future research.ResultsWe defined three aspects of fluid monitoring: i) normal and pathophysiological cardiovascular mechanisms; ii) measures of volume responsiveness and impending cardiovascular collapse during volume removal, and; iii) measured indices of each using non-invasive and minimally invasive continuous and intermittent monitoring techniques. The evidence documents that AKI can occur in the setting of normotensive hypovolemia and that under-resuscitation represents a major cause of both AKI and mortality ion critically ill patients. Traditional measures of intravascular volume and ventricular filling do not predict volume responsiveness whereas dynamic functional hemodynamic markers, such as pulse pressure or stroke volume variation during positive pressure breathing or mean flow changes with passive leg raising are highly predictive of volume responsiveness. Numerous commercially-available devices exist that can acquire these signals.ConclusionsProspective clinical trials using functional hemodynamic markers in the diagnosis and management of AKI and volume status during ultrafiltration need to be performed. More traditional measure of preload be abandoned as marked of volume responsiveness though still useful to assess overall volume status.

KURATA, T., KONDO, T., HAYASHI, Y., OHTA, Y. and YAMABAYASHI, H. Continuous Positive Airway Pressure Breathing in Supine and Upright Postures. Tohoku J. Exp. Med., 1990, 160 (1), 1-9-Continuous positive airway pressure (CPAP) breathing of 10cmH2O was applied to 10 healthy conscious subjects both in supine and 70 degree head-up postures. CPAP increased the end-expiratory lung volume from 2.44±0.66 (S.D.) to 3.07±0.68 liters in supine subjects, and from 3.01±0.57 to 3.64±0.57 liters in head-up subjects. During CPAP, rib cage motion became predominant when compared to abdominal motion in upright posture. Among the respiratory parameters, inspiratory duration was significantly shortened during CPAP in either posture, but was changed less significantly by individual posture change alone. Mean inspiratory flow decreased during CPAP only in upright posture. Abdominal muscles were recruited for expiration during CPAP in either posture, and the EMG recorded from the lower intercostal space developed inspiratory activity during CPAP in head-up posture. All of 10 subjects felt most dyspneic during CPAP with the head-up. We conclude that the mechanism of increase in lung volume during CPAP is mediated by different pathway from that of posture change, and that CPAP in upright posture can alter the respiratory pattern of the subject.

In Part I of this review, we have covered basic concepts regarding cardiorespiratory interactions. Here, we put this theoretical framework to practical use. We describe mechanisms underlying Kussmaul's sign and pulsus paradoxus. We review the literature on the use of respiratory variations of blood pressure to evaluate volume status. We show the possibilities of attaining the latter aim by investigating with ultrasonography how the geometry of great veins fluctuates with respiration. We provide a Guytonian analysis of the effects of PEEP on cardiac output. We terminate with some remarks on the potential of positive pressure breathing to induce acute cor pulmonale, and on the cardiovascular mechanisms that at times may underly the failure to wean a patient from the ventilator.

To determine the short-term effects of non-invasive positive pressure ventilation (PPV) on spontaneous baroreflex sensitivity, we acquired time series of RR interval and beat-to-beat blood pressure in 55 healthy volunteers (mean age 46.5+/-10.5 years), who performed breathing tests on four occasions at frequencies of 12 and 15/min, with application of PPV of 5 mbar, and without positive pressure (control). Using spectral and transfer function analysis, we estimated RR interval variability (HRV) and systolic blood pressure variability (SBPV), as well as the gain (alpha-index) and phase shift (Phi) of the baroreceptor reflex for low- (LF) and high-frequency (HF) bands. Compared to control breathing, PPV at 12 and 15/min led to an increase in mean RR (p

Military medicine has made numerous enduring contributions to the advancement of pulmonary medicine. Acute respiratory distress syndrome was first recognized as a complication in battlefield casualties in World War I and continued to play a significant role in the treatment of casualties through the Vietnam War. Innovative surgeons during World War II devised methods to assist their patients with positive pressure breathing. This concept was later adopted and applied to the development of mechanical ventilation in the late 1940s and early 1950s. The continued treatment of acute respiratory distress syndrome in combat casualties by military physicians has provided a major impetus for advances in modern mechanical ventilation and intensive care unit medicine.

AFRL/HEPG conducted an initial evaluation of the Joint Service Mustang Anti-G Suit (JSMAGS) produced by Mustang Survival, Inc. Performance of the JSMAGS was compared to that of the CSU-13B/P anti-G suit. Four subjects completed centrifuge trials up to +9 Gz using positive pressure breathing provided by the COMBAT EDGE system in combination with each of the two anti-G suits. The tests were conducted within the limits established by a test plan previously approved by the Brooks Institute Review Board. The constraints of that test plan did not permit a determination of the G-endurance afforded by JSMAGS compared to that of the CSU-13B/P. While the small number of subjects in this quick-look effort did not permit any definitive statistics, these initial test results indicate that JSMAGS provides G-protection that is comparable to that provided by the CSU-13B/P and, as indicated by the heart rate data, may require less physical effort to provide that level of protection. Had the test plan limits permitted a longer SACM exposure, endurance values for JSMAGS may have been better due to a decreased physical effort requirement.

The aim of this study is to investigate the upper limit of protection afforded by various protective equipment and techniques that used in combination and used individually. A multi-element, nonlinear mathematical model, which was previously used to produce cardiovascular response to +Gz stress and protection afforded by anti-G suits, was modified by incorporating pulmonary-respiration model, as well as detailed vessel models of abdomen and lower extremities into previous model. Using the model, hemodynamic responses of cardiovascular system to +Gz and to different combination of protection measures were simulated. Simulation results indicate that when appropriate counter-pressure is exerted to lower body, the combination of extended coverage anti-G suit with appropriate positive pressure breathing could provide effortless protection around +9Gz. For higher no-staining protection to +12 Gz, tilt-back seat with angles greater than 55deg should be used. It also suggests that for anti-G techniques that enhance +Gz tolerance by means of elevating intrathoracic pressure or increasing blood volume, such as ECGS, PBG, and AGSM, their combined protection may be expressed as an additive sum of their individual effects. However, the combined protection of tilt-back seat with other anti-G techniques is higher than the simple addition

Continuous positive-pressure ventilation in acute respiratory failure. By Kumar A, Falke KJ, Geffin B, Aldredge CF, Laver MB, Lowentein E, Pontoppidan H. N Engl J Med 1970; 283:1430-6. Reprinted with permission. Continuous positive-pressure ventilation was used in eight patients with severe acute respiratory failure. Cardiac output and lung function were studied during continuous positive-pressure ventilation (mean end-expiratory pressure, 13 cm H2O) and a 30-min interval of intermittent positive-pressure ventilation. Although the mean cardiac index increased from 3.6 to 4.5 l/min per square meter of body surface area, the mean intrapulmonary shunt increased by 9% with changeover to intermittent positive-pressure ventilation. Satisfactory oxygenation was maintained in all patients during continuous positive-pressure ventilation with 50% inspired oxygen or less. With intermittent positive-pressure ventilation, arterial oxygen tension promptly fell by 161 mm of mercury, 79% occurring within 1 min. Prevention of air-space collapse during expiration and an increase in functional residual capacity probably explain improved oxygenation with continuous positive-pressure ventilation. In four patients, subcutaneous emphysema or pneumothorax developed. Weighed against the effects of prolonged hypoxemia, these complications were not severe enough to warrant cessation of continuous positive-pressure ventilation.

Due to more stringent requirements to protect personnel against hazardous gasses, the inspiratory resistance of the present generation of respiratory protective devices tends to increase. Therefore an important question is to what extent inspiratory resistance may increase without giving problems during physical work. In this study the effects of three levels (0.24; 1.4 and 8.3 kPasl-1) of inspiratory resistance were tested on maximal voluntary performance. Nine male subjects performed a graded exercise test on a cycle ergometer with and without these three levels of inspiratory resistance. Oxygen consumption, heart rate, time to exhaustion and external work were measured. The results of these experiments showed that increasing inspiratory resistance led to a reduction of time to exhaustion (TTE) on a graded exercise test(GXT). Without inspiratory resistance the mean TTE was 11.9 min, the three levels of resistance gave the following mean TTE's: 10.7, 7.8 and 2.7 min. This study showed that TTE on a GXT can be predicted when physical fitness (VO 2-max) of the subject and inspiratory resistance are known. The metabolic rate of the subjects was higher with inspiratory resistance, but no differences were found between the three selected inspiratory loads. Other breathing parameters as minute ventilation, tidal volume, expiration time and breathing frequency showed no or minor differences between the inspiratory resistances. The most important conclusion of these experiments is that the overall workload increases due to an increase in inspiratory resistance

Due to more stringent requirements to protect personnel against hazardous gasses, the inspiratory resistance of the present generation of respiratory protective devices tends to increase. Therefore an important question is to what extent inspiratory resistance may increase without giving problems during physical work. In this study the effects of three levels (0.24; 1.4 and 8.3 kPasl-1) of inspiratory resistance were tested on maximal voluntary performance. Nine male subjects performed a graded exercise test on a cycle ergometer with and without these three levels of inspiratory resistance. Oxygen consumption, heart rate, time to exhaustion and external work were measured. The results of these experiments showed that increasing inspiratory resistance led to a reduction of time to exhaustion (TTE) on a graded exercise test(GXT). Without inspiratory resistance the mean TTE was 11.9 min, the three levels of resistance gave the following mean TTE's: 10.7, 7.8 and 2.7 min. This study showed that TTE on a GXT can be predicted when physical fitness (VO 2-max) of the subject and inspiratory resistance are known. The metabolic rate of the subjects was higher with inspiratory resistance, but no differences were found between the three selected inspiratory loads. Other breathing parameters as minute ventilation, tidal volume, expiration time and breathing frequency showed no or minor differences between the inspiratory resistances. The most important conclusion of these experiments is that the overall workload increases due to an increase in inspiratory resistance

Continuous positive pressure breathing consisting of a pressure in the airways above the atmospheric level during spontaneous inspiration and expiration was used in the treatment of pulmonary edema and severe pneumonia even before World War II [1]. Positive endexpiratory airway pressure was also very commonly used in the experimental laboratory in any open chest preparation in order to prevent expiratory lung collapse. An important precondition for the introduction of positive endexpiratory pressure (PEEP) in conjunction with mechanical ventilation was established by the experiments of Cournand et al. in 1948 [2]. They found, however, that, compared to mechanical ventilation with ambient endexpiratory pressure, mechanical ventilation with PEEP was associated with a marked decrease in cardiac output due to reduced venous return of blood to the heart. Possible negative circulatory effects were the major concern in the early phase of clinical application of positive endexpiratory pressure.

We investigated whether improvement in ventilation perfusion (.V(A)/.Q) distribution during mechanical ventilation using biphasic positive airway pressure (BIPAP) with spontaneous breathing may be attributed to an effectively increased transpulmonary pressure (P(TP)) and can also be achieved by increasing P(TP) during controlled ventilation.In 12 pigs with saline lavage-induced lung injury we compared the effects of BIPAP to pressure-controlled ventilation with equal airway pressure (PCV(AW)) or equal transpulmonary pressure (PCV(TP)) on V(A)/.Q distribution assessed by the multiple inert gas elimination technique (MIGET).Animal laboratory study.Intrapulmonary shunt was 33+/-11% during BIPAP, 36+/-10% during PCV(AW) and 33+/-15% during PCV(TP) ( p= n.s.). BIPAP resulted in higher PaO(2) than PCV(AW) (188+/-83 versus 147+/-82 mmHg, p0.05), but not than PCV(TP) (187+/-139 mmHg). Oxygen delivery was significantly higher during BIPAP (530+/-109 ml/min) versus 374+/-113 ml/min during PCV(AW) and 353+/-93 ml/min during PCV(TP) ( p0.005). Tidal volume with PCV(TP) increased to 11.9+/-2.3 ml/kg, compared to 8.5+/-0.8 with BIPAP and 7.6+/-1.4 with PCV(AW) ( p0.001) and cardiac output decreased to 3.5+/-0.6 l/min (BIPAP 4.9+/-0.8 and PCV(AW) 3.9+/-0.8, p0.006).In experimental lung injury, BIPAP with preserved spontaneous breathing was effective in increasing regional P(TP), since pressure-controlled ventilation with the same P(TP) resulted in similar gas exchange effects. However, PCV(TP) caused increased airway pressures and tidal volumes, whereby, with BIPAP, less depression of oxygen delivery and cardiac output were observed. BIPAP could be useful in maintaining pulmonary gas exchange and slightly improving oxygenation without interfering with circulation as strongly as PCV does.

The requirements to maintain a positive pressure with respiratory protection during heavy exercise and the effects on ventilation and feelings of discomfort were investigated. Eight male subjects participated, using the respirator system during rest and exercise at about 80% of their individual maximum power. A blower was used at maximum and medium capacity and at two pressure levels (3 and 15 mbar). Additionally, the mouth pressure was used as a feedback for the blower. The blower decreased the fraction of the breathing cycle with negative pressures from 50% (SD 4%) to 15% (SD 10%) during exercise. Negative pressures occurred at all settings of the blower during exercise. Thus, the currently available commercial blower systems do not supply a sufficient airflow to maintain a positive pressure during heavy exercise. Positive pressure breathing did not affect the ventilation and the circulation. But the oxygen consumption was higher with the blower and respirator than without.

Positive-pressure breathing (PPB), a technique in which air is forced into a pilot by means of a face mask to raise intrathoracic pressure, is examined. It has been shown to lengthen subjects' ability to endure +G/sub z/ acceleration epochs and has been proposed for operational use to increase G-tolerance. However, PPB can cause ear pain, ear and sinus blocks, pneumothorax, and air embolism. All these side-effects are potentially fatal to the pilot. The pressure proposed for operational use, at altitudes that occur in partially pressurized cockpits of operational aircraft, causes greater lung distension than pressure gradients known to cause pulmonary overpressurization in animals and man. Other means of increasing G-tolerance safely are also examined. >

The requirements to maintain a positive pressure with respiratory protection during heavy exercise and the effects on ventilation and feelings of discomfort were investigated. Eight male subjects participated, using the respirator system during rest and exercise at about 80% of their individual maximum power. A blower was used at maximum and medium capacity and at two pressure levels (3 and 15 mbar). Additionally, the mouth pressure was used as a feedback for the blower. The blower decreased the fraction of the breathing cycle with negative pressures from 50% (SD 4%) to 15% (SD 10%) during exercise. Negative pressures occurred at all settings of the blower during exercise. Thus, the currently available commercial blower systems do not supply a suf cient air ow to maintain a positive pressure during heavy exercise. Positive pressure breathing did not affect the ventilation and the circulation. But the oxygen consumption was higher with the blower and respirator than without.

The requirements to maintain a positive pressure with respiratory protection during heavy exercise and the effects on ventilation and feelings of discomfort were investigated. Eight male subjects participated, using the respirator system during rest and exercise at about 80% of their individual maximum power. A blower was used at maximum and medium capacity and at two pressure levels (3 and 15 mbar). Additionally, the mouth pressure was used as a feedback for the blower. The blower decreased the fraction of the breathing cycle with negative pressures from 50% (SD 4%) to 15% (SD 10%) during exercise. Negative pressures occurred at all settings of the blower during exercise. Thus, the currently available commercial blower systems do not supply a suf cient air ow to maintain a positive pressure during heavy exercise. Positive pressure breathing did not affect the ventilation and the circulation. But the oxygen consumption was higher with the blower and respirator than without.

Positive acceleration in the head to toe direction (+Gz) causes a reduction in blood circulation to the head which adversely affects the central nervous system and causes a decrease in vision. It has been established that positive pressure breathing enhances +Gz tolerance by increasing head level blood pressure. Utilizing the NAWC centrifuge, eight subjects outfitted with Navy combat edge (NCE) protective systems were exposed to a series of +Gz rapid onset rate (ROR) profiles and positive pressure breathing schedules. Stroke volume (SV), heart rate (HR), and cardiac output (CO) were measured using a new impedance cardiograph (ICG), the Drexel patented IQ system. Blood pressure (BP), local blood volume using an infrared plethysmograph (IRP), respiratory rate, and peripheral light loss were also measured. These parameters were used to investigate potential alternate positive pressure breathing schedules to enhance the tactical pilots' +Gz tolerance.

The authors present the current status of a loss-of-consciousness detection and recovery system being developed by the US Air Force. It is noted that crucial issue of operational utility of a loss-of-consciousness monitoring system is pilot acceptance. Experience with the auto-save system demonstrated in the AFTI/F-16 has shown that a logic and aircraft-state system could do much to alleviate the loss of pilots and aircraft from the sequelae of gravity-induced loss-of-consciousness. A physiological state variable based system is described, and it is suggested as a natural choice for combination with an auto-save system and would result in enhanced reliability and reduce false alarm incidents, both of which would lead to greater acceptance and reliance on the system. The development of high-G training curricula for USAF pilots enrolled in lead-in fighter training at Hollomon AFB should greatly reduce the risk of GLOC. The development of an assisted positive pressure breathing system, called Combat Edge, will better protect Air Force pilots from long duration, high-G exposures, and will possibly help reduce GLOC statistics. However, neither of these developments, singly or in concert, will totally eradicate the GLOC problem. >

The British Standards Institution 'Guide to implementing an effective respiratory protective device programme' (BS 4275) lists assigned protection factors (APFs) for various types of respiratory protective equipment (RPE). The APFs were allocated either on the basis of available workplace studies data which met set criteria or on the basis of professional judgement that there is equivalence between its operation and that of a device for which an APF is derived from workplace data. However, in many cases no workplace study information exists to support this professional judgement. As an interim measure, pending information based on workplace measurements, the breathing resistance of a range of tight-fitting RPE from negative pressure filtering devices through to self-contained positive pressure breathing apparatus was measured at various breathing rates. The relative inhalation resistances were then compared on the assumption that similar breathing resistance performance is likely to give similar inward leakage on a facepiece and hence similar protection if all other factors, such as fit, etc., are equal. This work indicates that for most devices the allocation of APFs by analogy to other devices seems to be acceptable. However, there appears to be no justification for the allocation of an APF value of 100 to continuous flow compressed air line breathing apparatus. It is recommended that it should be lowered to 40 until there is valid workplace study data to support the current APF of 100. The work provides an informative insight into the relative performance of devices.

Background Diaphragmatic excursion during spontaneous ventilation (SV) in normal supine volunteers is greatest in the dependent regions (bottom). During positive pressure ventilation (PPV) after anesthesia and neuromuscular blockade and depending on tidal volume, the nondependent region (top) undergoes the greatest excursion, or the diaphragm moves uniformly. The purpose of this study was to compare diaphragmatic excursion (during SV and PPV) in patients with chronic obstructive pulmonary disease (COPD) with patients having normal pulmonary function. Methods Twelve COPD patients and 12 normal control subjects were compared. Cross-table diaphragmatic fluoroscopy was performed while patients breathed spontaneously. After anesthetic induction and pharmacologic paralysis and during PPV, diaphragmatic fluoroscopy was repeated. For analytic purposes, the diaphragm was divided into three segments: top, middle, and bottom. Percentage of excursion of each segment during SV and PPV in normal subjects was compared with the percentage of excursion of each segment in patients with COPD. Results There was no significant difference in the pattern of regional diaphragmatic excursion (as a percentage of total excursion)-top, middle, bottom-when comparing COPD patients with control subjects during SV and PPV. In the control subjects, regional diaphragmatic excursion was 16 +/- (5), 33 +/- (5), 51 +/- (4) during SV and 49 +/- (13), 32 +/- (6), 19 +/- (9) during PPV. In COPD patients, regional diaphragmatic excursion was 18 +/- (7), 34 +/- (5), 49 +/- (7) during SV and 47 +/- (10), 32 +/- (6), 21 +/- (9) during PPV. Conclusion Regional diaphragmatic excursion in patients with COPD during SV and PPV is similar to that in persons with normal pulmonary function.

Methanol (CH3OH) is a chemical feedstock of increasing importance as well as a commonly used solvent. In the early 1980s methanol production was introduced at a new petrochemical complex in the Saudi port of Jubail. A case is presented of a consultant supervising tank cleaning prior to methanol loading. He wore positive pressure breathing apparatus but no protective clothing. After 2-3 hours working in the confined space of the tank, he worked on deck and continued to wear his methanol-soaked clothing which eventually dried out. Visual symptoms of acute methanol toxicity presented some 8 hours after exposure. The appropriate treatment (with ethanol provided by the ship bond) was carried out in hospital and the individual recovered completely. Most reported cases of methanol toxicity are social in origin, arising from ingestion. This particular case, though unusual, does present some interesting lessons.

A burn-injured 17-year-old boy suffered from an apical pneumothorax of the right side caused either by placement of a subclavian catheter or by the blast injury. Following this, he developed scrotal emphysema. Air extended along the subcutis of the lateral thoracic and abdominal wall through a lesion in the parietal pleura up to the inguinal canal to the scrotum caused by the positive pressure breathing. After cessation of the PEEP-breathing, there was no recurrence.

The performance of speech recognisers in a combat aircraft is degraded seriously by the extreme physical stresses to which the crew are subjected. This paper describes measurements of first and second formant frequencies of nine vowels from one speaker recorded under high levels of acceleration, with and without positive pressure breathing. Under acceleration alone, F2 is reduced for high front vowels, while F1 remains constant, but for back and mid vowels, F1 reduces with little change in F2. When positive pressure breathing is introduced, nearly all vowels are affected, and the "vowel triangle" on the F1-F2 plane collapses inwards, towards the neutral vowel position. If these changes are found to be consistent between speakers, it is hoped to develop techniques of voice transformation to reverse them and thus improve the performance of speech recognisers in this harsh environment.

The last decade has brought many new insights into the mechanical properties of injured lungs that have profoundly altered the ventilatory management of patients with acute respiratory distress syndrome (ARDS) [1–4]. The pioneering studies by Gattinoni and colleagues [5–11] have revealed a topographical heterogeneity in lung involvement and reinforced the idea that large portions of an injured lung are derecruited (i.e., they do not expand during positive pressure ventilation). This important insight formed the basis for the ‘baby lung concept’ which explains why the lungs of patients with ARDS appear to be particularly prone to injury from overdistension. The baby lung concept emerged along with an appreciation for the varied clinical and morphologic manifestations of baro(volu)-trauma [2, 12–17] and has provided a mechanistic basis for several large ventilator management trials [1, 4, 18]. These trials, however, have yielded conflicting results, emphasizing the many degrees of freedom in ventilator mode and settings. Of particular note, is the variance in positive end-expiratory pressure (PEEP) used in the different studies, even though there is strong experimental evidence that the application of PEEP can protect the lung from ventilator-induced lung injury [12, 14, 19–21]. The protective action of PEEP is commonly attributed to a reduction in tissue shear stresses caused by the repeated opening and collapse of unstable lung units. Using computer tomography (CT), Gattinoni and colleagues concluded that dependent lung units of patients with ARDS are collapsed [7–9]. In arriving at this conclusion, Gattinoni embraced the idea that dependent lung must bear the weight of the superimposed ‘heavy’ edematous lung parenchyma [22] and is thus prone to collapse, and that within-breath recruitment and derecruitment occurs during positive pressure breathing.

Beaumont, Maurice, Damien Lejeune, Henri Marotte, Alain Harf, and Frédéric Lofaso. Effects of chest wall counterpressures on lung mechanics under high levels of CPAP in humans. J. Appl. Physiol. 83(2): 591–598, 1997.—We assessed the respective effects of thoracic (TCP) and abdominal/lower limb (ACP) counterpressures on end-expiratory volume (EEV) and respiratory muscle activity in humans breathing at 40 cmH2O of continuous positive airway pressure (CPAP). Expiratory activity was evaluated on the basis of the inspiratory drop in gastric pressure (ΔPga) from its maximal end-expiratory level, whereas inspiratory activity was evaluated on the basis of the transdiaphragmatic pressure-time product (PTPdi). CPAP induced hyperventilation (+320%) and only a 28% increase in EEV because of a high level of expiratory activity (ΔPga = 24 ± 5 cmH2O), contrasting with a reduction in PTPdi from 17 ± 2 to 9 ± 7 cmH2O ⋅ s−1 ⋅ cycle−1during 0 and 40 cmH2O of CPAP, respectively. When ACP, TCP, or both were added, hyperventilation decreased and PTPdi increased (19 ± 5, 21 ± 5, and 35 ± 7 cmH2O ⋅ s−1 ⋅ cycle−1, respectively), whereas ΔPga decreased (19 ± 6, 9 ± 4, and 2 ± 2 cmH2O, respectively). We concluded that during high-level CPAP, TCP and ACP limit lung hyperinflation and expiratory muscle activity and restore diaphragmatic activity.

Peters, Jochen K., George Lister, Ethan R. Nadel, and Gary W. Mack. Venous and arterial reflex responses to positive-pressure breathing and lower body negative pressure. J. Appl. Physiol. 82(6): 1889–1896, 1997.—We examined the relative importance of arteriolar and venous reflex responses during reductions in cardiac output provoked by conditions that increase [positive end-expiratory pressure (PEEP)] or decrease [lower body negative pressure (LBNP)] peripheral venous filling. Five healthy subjects were exposed to PEEP (10, 15, 20, and 25 cmH2O) and LBNP (−10, −15, −20, and −25 mmHg) to induce progressive but comparable reductions in right atrial transmural pressure (control to minimum): from 5.9 ± 0.4 to 1.8 ± 0.7 and from 6.5 ± 0.6 to 2.0 ± 0.2 mmHg with PEEP and LBNP, respectively. Cardiac output (impedance cardiography) fell less during PEEP than during LBNP (from 3.64 ± 0.21 to 2.81 ± 0.21 and from 3.39 ± 0.21 to 2.14 ± 0.24 l ⋅ min−1 ⋅ m−2with PEEP and LBNP, respectively), and mean arterial pressure increased. We observed sustained increases in forearm vascular resistance (i.e., forearm blood flow by venous occlusion plethysmography) and systemic vascular resistance that were greater during LBNP: from 19.7 ± 2.91 to 27.97 ± 5.46 and from 20.56 ± 2.48 to 50.25 ± 5.86 mmHg ⋅ ml−1 ⋅ 100 ml tissue−1 ⋅ min ( P < 0.05) during PEEP and LBNP, respectively. Venomotor responses (venous pressure in the hemodynamically isolated limb) were always transient, significant only with the greatest reduction in right atrial transmural pressure, and were similar for LBNP and PEEP. Thus arteriolar rather than venous responses are predominant in blood volume mobilization from skin and muscle, and venoconstriction is not intensified with venous engorgement during PEEP.

OBJECTIVES: Sewer workers are used to unpleasant smells, but may be required to investigate unusual ones. Twenty six men were involved in investigation of episodes of such a smell after neighbourhood complaints over several weeks. METHODS: Workers exposed to the smell were investigated by clinical follow up, lung function tests, and measurement of pituitary function. RESULTS: 14 of the 26 developed subacute symptoms including sore throat, cough, chest tightness, breathlessness, thirst, sweating, irritability, and loss of libido. Severity of symptoms seemed to be dose related. Minor symptoms resolved over several weeks but those more seriously affected have shown deteriorating respiratory symptoms and lung function and remain unable to work a year after the incident. In one, evidence of mild cranial diabetes insipidus was found. Analysis of gas from the sewer showed the presence of a mixture of thiols and sulphides, known to be highly odorous and not normally found in sewers. The source remains unknown. CONCLUSIONS: Several of these men seem to have developed delayed airways disease and disturbances of hypothalamic function. Such an outcome has not to our knowledge been described before. Despite the presence of the smell, standard safety gas detection equipment used to ensure the sewer was safe to enter failed to indicate the presence of a hazard. Protection against such incidents can only be provided by the use of positive pressure breathing apparatus.

Objective : To compare the effectiveness of perfluorocarbon-associated gas exchange to volume controlled positive pressure breathing in supporting gas exchange, lung mechanics, and survival in an acute lung injury model. Design : A prospective, randomized study. Setting : A university medical school laboratory approved for animal research. Subjects : Neonatal piglets. Interventions : Eighteen piglets were randomized to receive perfluorocarbon-associated gas exchange with perflubron (n = 10) or volume controlled continuous positive pressure breathing (n = 8) after acute lung injury was induced by oleic acid infusion (0.15 mUkg iv). Measurements and Main Results : Arterial and venous blood gases, hemodynamics, and lung mechanics were measured every 15 mins during a 3-hr study period. All animals developed a metabolic and a respiratory acidosis during the infusion of oleic acid. Following randomization, the volume controlled positive pressure breathing group developed a profound acidosis (p

Increased intrathoracic pressure with positive pressure breathing (PPB) induces renal hypoperfusion and excretion function impairment the mechanism of which may be partially related to reflex sympathetic nerve activation. The consequences of renal denervation on PPB-induced renal impairment are unknown. This study was conducted to evaluate the effects of increasing intrathoracic pressure with positive end-expiratory pressure (PEEP) on renal blood flow (RBF, pulsed Doppler implantable microprobes) and function in 12 kidney transplantation recipients during the immediate post-transplantation period. Three sets of measurements were performed during successively zero end-expiratory pressure (ZEEP), 15 cm H2O PEEP, and back to ZEEP. PEEP ventilation was associated with mean arterial pressure (MAP) and cardiac output (CO) decrease (-12%, p < 0.01; -26%, p < 0.01, respectively). RBF remained constant in the three protocol conditions. PEEP ventilation was associated with a decrease in urinary output (8.5 +/- 5.6 versus 12.9 +/- 8.6 ml/min; p < 0.01), urinary sodium concentration (115 +/- 14 versus 121 +/- 12 mmol/L; p < 0.01) sodium excretion rate (1 +/- 0.7 versus 1.6 +/- 1.1 mmol/min; p < 0.01), and creatinine clearance (17.1 +/- 10 versus 23.2 +/- 13.6 ml/min; p < 0.01). PEEP-induced urinary output decrease was correlated to renal perfusion pressure decrease (r = 0.7, p = 0.016). These results suggest that despite denervation and renal blood flow stability, renal handling of water and salt is perfusion pressure-dependent during PEEP in human renal allograft recipients.

The old saying, `Safety is paramount.' was never more true than it is in the area of ejection safety for high-speed fighter aircraft. The fighter aircraft of today has been designed to endure tremendous structural loading during dogfight or evasive maneuvers. It can fly faster, turn quicker, stay in the air longer (with in-flight refuel) and carry more bombs than its predecessor. Because of human physiological limits, the human has become the weak link in today's fighter aircraft. The fighter pilot must endure and function with peak performance in conditions that are much worse than anything the majority of us will ever encounter. When these conditions reach a point that human endurance is exceeded, devices such as anti-g suits and positive pressure breathing apparatus help the fighter pilot squeeze out that extra percentage of strength necessary to outperform the opponent. As fighter aircraft become more sophisticated, helmet trackers, helmet displays and noise cancellation devices are being added to the helmet. Yet the fighter pilot's helmet must remain lightweight and be aesthetically appealing, while still offering ballistic protection. It must function with existing life support equipment such as the Combined Advanced Technology Enhanced Design g-Ensemble (COMBAT-EDGE). It must not impede the pilot's ability to perform any action necessary to accomplish the planned mission. The helmet must protect the pilot during the harsh environment of ejection. When the pilot's only resort is to pull the handle and initiate the ejection sequence, the helmet becomes his salvation or instant death. This paper discusses the safety concerns relative to the catapult phase of ejecting from a high-speed fighter while wearing an advanced fighter helmet.

Objective: To determine whether oxygenation and lung mechanics are preserved during perfluorocarbon associated gas exchange of 24 hrs duration and after evaporation of perfluorocarbon. Design: Prospective, experimental animal trials. Setting: Animal laboratory in a university setting. Subjects: Ten normal, neonatal piglets weighing 2.5 to 4.5 kg. Interventions: Ten piglets were anesthetized with fentanyl (25 μg/kg/hr), paralyzed with metocurine iodide (0.3 mg/kg) and placed on volume regulated continuous positive pressure breathing instituted at an FIO 2 setting of 1.0, tidal volume of 15 mg/kg, respiratory rate of 25 breaths/min and positive end-expiratory pressure of 4 cm H 2 O. Perfluorocarbon associated gas exchange was initiated by intratracheal instillation of perflouorooctylbromide (30 mL/kg) followed by gas ventilation at the same settings. Evaporative losses were replaced by intratracheal instillation of 2.5 mL/kg/hr of perfluorocarbon. In one group of five piglets, evaporative losses were replaced for 24 hrs until the end of the study. In the other group of five piglets, replacement of perfluorocarbon was discontinued after 2 hrs, although gas ventilation was continued for 24 hrs. Blood gases and lung mechanics were measured in both groups. Histologic evaluation of lungs from both groups of animals was performed. Measurements and Main Results: Airway pressures and blood gases were stable throughout the 24-hr study period in both groups. Airway pressures in the evaporative group increased as evaporation of perfluorocarbon neared completion. There was no hemodynamic deterioration during the 24-hr study period. Histology showed good preservation of lung architecture in both groups. Conclusions: Perfluorocarbon associated gas exchange was safe and effective in normal piglets for a period of 24 hrs. Evaporation of perfluorocarbon and resumption of continuous positive pressure breathing was well tolerated

The interactions between hemodynamic and hormonal modifications during antidiuresis and antinatriuresis induced by positive end-expiratory pressure (PEEP) were studied in six patients under 15 cm H 2 O PEEP before PEEP and after the addition of lower body positive pressure (LBPP) to PEEP (PEEP+LBPP). We measured or calculated the following: cardiac index, systemic arterial, right atrial, pulmonary arterial, and pulmonary artery occlusive pressures; indexed renal blood flow (iodohippurate 131 sodium clearance); total blood volume (chromium 51 radiolabeled RBCs); glomerular filtration rate; urinary output; fractional excretion of sodium (FE Na + ); plasma concentrations of antidiuretic hormone (ADH), plasma renin activity (PRA), norepinephrine and epinephrine; urinary concentration of PGE 2 (PGE2U). Although LBPP application corrected PEEP deleterious effects on systemic and renal hemodynamics, sustained fall in Vu and in FE Na + were observed. Antidiuresis was not due to ADH release. Sympathetic activation and high PRA appeared the main determinants of renal function alterations in PEEP ventilation.

During airway pressure release ventilation (APRV), tidal ventilation occurs between the increased lung volume established by the application of continuous positive airway pressure (CPAP) and the relaxation volume of the respiratory system. Concern has been expressed that release of CPAP may cause unstable alveoli to collapse and not reinflate when airway pressure is restored.To compare pulmonary mechanics and oxygenation in animals with acute lung injury during CPAP with and without APRV.Experimental, subject-controlled, randomized crossover investigation.Anesthesiology research laboratory, University of South Florida College of Medicine Health Sciences Center.Ten pigs of either sex.Acute lung injury was induced with an intravenous infusion of oleic acid (72 micrograms/kg) followed by randomly alternated 60-min trials of CPAP with and without APRV. Continuous positive airway pressure was titrated to produce an arterial oxyhemoglobin saturation of at least 95% (FIO2 = 0.21). Airway pressure release ventilation was arbitrarily cycled to atmospheric pressure 10 times per minute with a release time titrated to coincide with attainment of respiratory system relaxation volume.Cardiac output, arterial and mixed venous pH, blood gas tensions, hemoglobin concentration and oxyhemoglobin saturation, central venous pressure, pulmonary and systemic artery pressures, pulmonary artery occlusion pressure, airway gas flow, airway pressure, and pleural pressure were measured. Tidal volume (VT), dynamic lung compliance, intrapulmonary venous admixture, pulmonary vascular resistance, systemic vascular resistance, oxygen delivery, oxygen consumption, and oxygen extraction ratio were calculated.Central venous infusion of oleic acid reduced PaO2 from 94 +/- 4 mm Hg to 52 +/- 9 mm Hg (mean +/- 1 SD) (p0.001) and dynamic lung compliance from 40 +/- 6 mL/cm H2O to 20 +/- 6 mL/cm H2O (p = 0.002) and increased venous admixture from 13 +/- 3% to 32 +/- 7% (p0.001) in ten swine weighing 33.3 +/- 4.1 kg while they were spontaneously breathing room air. After induction of lung injury, the swine received CPAP (14.7 +/- 3.3 cm H2O) with or without APRV at 10 breaths per minute with a release time of 1.1 +/- 0.2 s. Although mean transpulmonary pressure was significantly greater during CPAP (11.7 +/- 3.3 cm H2O) vs APRV (9.4 +/- 3.8 cm H2O) (p0.001), there were no differences in hemodynamic variables. PaCO2 was decreased and pHa was increased during APRV vs CPAP (p = 0.003 and p = 0.005). PaO2 declined from 83 +/- 4 mm Hg to 79 +/- 4 mm Hg (p = 0.004) during APRV, but arterial oxyhemoglobin saturation (96.6 +/- 1.4% vs 96.9 +/- 1.3%) did not. Intrapulmonary venous admixture (9 +/- 3% vs 11 +/- 5%) and oxygen delivery (469 +/- 67 mL/min vs 479 +/- 66 mL/min) were not altered. After treatment periods and removal of CPAP for 60 min, PaO2 and intrapulmonary venous admixture returned to baseline values.Intrapulmonary venous admixture, arterial oxyhemoglobin saturation, and oxygen delivery were maintained by APRV at levels induced by CPAP despite the presence of unstable alveoli. Decrease in PaO2 was caused by increase in pHa and decrease in PaCO2, not by deterioration of pulmonary function. We conclude that periodic decrease of airway pressure created by APRV does not cause significant deterioration in oxygenation or lung mechanics.

The possibility that the decreased urinary flow during continuous positive-pressure breathing (CPPB) may be a consequence of a reflex mediated via the cardiopulmonary baroreceptors to increase neurohumoral secretion or to change the sympathetic outflow was assessed. Muscle sympathetic nerve activity (MSNA) on the right peroneal nerve, vasoactive hormones, and renal and cardiovascular responses were measured during CPPB (+12 mmHg) in 10 male subjects (22.0 +/- 0.6 yr, 66.8 +/- 1.5 kg body wt). The experiments consisted of a 1-h control, 1 h with CPPB (experimental) or without CPPB (a time control), and a 1-h recovery period. Two blood samples were taken during each period for measurements of arginine vasopressin (AVP), plasma aldosterone (PAldo), plasma renin activity (PRA), norepinephrine, and atrial natriuretic peptide (ANP), and urine was collected hourly for the measurement of urine volume and electrolytes and clearances. MSNA rapidly increased (P < 0.05) at the onset of CPPB, continued to increase during exposure, and rapidly returned to the normal level at recovery. The MSNA changes coincided with increased plasma NE and were concurrent with a reduced (P < 0.05) urine output associated with a reduction of both free water and osmolal clearances, Na+ and osmolal excretions, and creatinine clearance (glomerular filtration rate). AVP and PRA increased (P < 0.05), whereas PAldo and ANP were unchanged. The results are consistent with the concept that increased sympathetic outflow may play a role in the reduction of urinary output and Na+ excretion during unloading of the cardiopulmonary receptors.

OBJECTIVES a) To study the dose response of the calcium-channel-mediated increases in pulmonary vascular resistance with different levels of positive end-expiratory pressure; b) to study the reversibility of the calcium-channel mediated increases in pulmonary vascular resistance after discontinuation of positive end-expiratory pressure; and c) to study the effect of cyclooxygenase and lipoxygenase inhibition on the calcium-channel mediated increases in pulmonary vascular resistance. DESIGN A prospective, multiexperimental, dose response study. SETTING Laboratory setting in a university hospital. SUBJECTS Twenty-three 4- to 10-day-old neonatal lambs. INTERVENTIONS AND MEASUREMENTS Lungs of neonatal lambs were isolated in situ, and perfused at a constant flow rate, and ventilated at a fixed tidal volume and rate. Mean pulmonary arterial pressure responses to the application and discontinuation of four levels (3.7, 7.4, 11, and 14.7 mm Hg) of positive end-expiratory pressure were studied before and after calcium-channel blockade with verapamil (5 mg) (n = 12). In addition, the mean pulmonary arterial pressure response to 11 mm Hg of positive end-expiratory pressure was studied before and after inhibition of cyclooxygenase with indomethacin (10 mg/kg) (n = 6) and lipoxygenase with diethylcarbamazine (100 mg/kg) (n = 5). RESULTS The magnitude of the calcium-channel-dependent mean pulmonary arterial pressure response 4 mins after the application of positive end-expiratory pressure was dose related (2.1, 3.0, 4.1, and 5.5 mm Hg with 3.7, 7.4, 11.0, and 14.7 mmHg positive end-expiratory pressure, respectively) and entirely reversible on discontinuation of positive end-expiratory pressure with a time course of 2 to 4 mins. Neither indomethacin nor diethylcarbamazine affected the pulmonary arterial pressure responses to positive end-expiratory pressure. Airway pressure changes with positive end-expiratory pressure were not affected by verapamil, indomethacin, or diethylcarbamazine. CONCLUSIONS The calcium-channel-mediated pulmonary arterial pressure responses with positive end-expiratory pressure, applied during continuous positive pressure breathing, occur even at low levels of positive end-expiratory pressure, are dose dependent, and are not abolished by treatment with indomethacin or diethylcarbamazine.