cmH2O Pressure Research Articles

Breathing pattern and workload during automatic tube compensation, pressure support and T-piece trials in weaning patients

Background and objective: Automatic tube compensation has been designed as a new ventilatory mode to compensate for the non-linear resistance of the endotracheal tube. The study investigated the effects of automatic tube compensation compared with breathing through a T-piece or pressure support during a trial of spontaneous breathing used for weaning patients from mechanical ventilation of the lungs. Methods: Twelve patients were studied who were ready for weaning after prolonged mechanical ventilation (10.2 ± 8.4 days) due to acute respiratory failure. Patients with chronic obstructive pulmonary disease were excluded. Thirty minutes of automatic tube compensation were compared with 30 min periods of 7 cmH2O pressure support and T-piece breathing. Breathing patterns and workload indices were measured at the end of each study period. Results: During T-piece breathing, the peak inspiratory flow rate (0.65 ± 0.20 L s−1) and minute ventilation (8.9 ± 2.7 L min−1) were lower than during either pressure support (peak inspiratory flow rate 0.81 ± 0.25 L s−1; minute ventilation 10.2 ± 2.3 L min−1, respectively) or automatic tube compensation (peak inspiratory flow rate 0.75 ± 0.26 L s−1; minute ventilation 10.8 ± 2.7 L min−1). The pressure-time product as well as patients' work of breathing were comparable during automatic tube compensation (pressure-time product 214.5 ± 104.6 cmH2O s−1 min−1, patient work of breathing 1.1 ± 0.4 J L−1) and T-piece breathing (pressure-time product 208.3 ± 121.6 cmH2O s−1 min−1, patient work of breathing 1.1 ± 0.4 J L−1), whereas pressure support resulted in a significant decrease in workload indices (pressure-time product 121.2 ± 64.1 cmH2O s−1 min−1, patient work of breathing 0.7 ± 0.4 J L−1). Conclusions: In weaning from mechanical lung ventilation, patients' work of breathing during spontaneous breathing trials is clearly reduced by the application of pressure support 7 cmH2O, whereas the workload during automatic tube compensation corresponded closely to the values during trials of breathing through a T-piece.

Ventilator treatment in the Nordic countries. A multicenter survey.

A 1-day point prevalence study was performed in the Nordic countries to identify ventilator-treatment strategies in the region. On 30 May 30 2001 all mechanically ventilated patients in 27 intensive care units (ICUs) were registered via the internet. The results are shown as medians (25th, 75th percentile). One hundred and eight patients were included (69% male) with new simplified acute physiology score (SAPS) 48 (37,57) and 4.5 d (2,11) of ventilator treatment. The most frequent indication for ventilator treatment was acute respiratory failure (73%). Airway management was by endotracheal tube (64%), tracheostomy (32%) and facial mask (4%). Pressure regulated ventilator modes were used in 86% of the patients and spontaneous triggering was allowed in 75%. The tidal volume was 7 ml/kg (6,9), peak inspiratory pressure 22 cmH2O (18,26) and positive end-expiratory pressure (PEEP) 6 cmH2O (6,9). FiO2 was 40% (35,50), SaO2 97% (95-98), PaO2 11 kPa (10,13), PaCO2 5.4 kPa (4.7,6.3), pH 7.43 (7.38,7.47) and BE 2.0 mmol/l (- 0.5,5). The PaO2/FiO2 ratio was 220 mmHg (166,283). The peak inspiratory pressure (r=0.37), mean airway pressure (r=0.36), PEEP (r=0.33), tidal volume (r=0.22) and SAPS score (r=0.19) were identified as independent variables in relation to the PaO2/FiO2 ratio. The vast majority of patients were ventilated with pressure-regulated modes. Tidal volume was well below what has been considered conventional in recent large trials. Correlations between the parameters of gas exchange, respiratory mechanics, ventilator settings and physiological status of the patients was poor. It appears that blood gas values are the main tool used to steer ventilator treatment. These results may help to design future interventional studies of ventilator treatment.

Effects of sustained pressure application on compliance and blood gases in healthy porcine lungs.

Short periods of sustained increase in airway pressures (Press(up)) are believed to re-open lung areas that collapsed upon induction of anaesthesia. Recruitment of alveolar surface is usually assessed in terms of changes in the pressure-volume (PV) curve. The purpose of this study was to analyse PV-curves before and after a Press(up) and to ascertain whether such changes are compatible with the concept of recruitment of lung volume. During ketamine anaesthesia, 12 healthy piglets were subjected to a Press(up) with end-expiratory pressure (PEEP) of 12 cmH2O and end-inspiratory pressure of 40 cmH2O. Before and after Press(up), PV-curves were obtained from a slow insufflation of 630 ml at zero PEEP (ZEEP). Compliance was non-linear both before and after Press(up) increasing up to 300 ml and sharply decreasing thereafter. After Press(up), the entire compliance curve was shifted to a higher absolute level. Up to 100 ml and a pressure level corresponding to the lower inflection point on the PV-curve (LIP), compliance was higher before Press(up). No effects on blood gases could be observed. If the similar shape of the compliance curve corresponds to a similar chain of re-opening and overdistension events, this would imply that all volume gained by Press(up) is lost within 10 min, without explaining the higher absolute compliance following Press(up). We speculate that a) re-opening of rapidly collapsing small airways determines the initial compliance increase; b) the lower compliance after Press(up) until LIP indicates reduced intratidal re-opening of lung regions; and c) changes in bronchomotor tone induced by Press(up) raise the absolute compliance, with a similar scenario of alveolar and small airway recruitment now taking place but at different degrees of airway stiffness.

Does intraprostatic vasopressin prevent the transurethral resection syndrome?

To determine whether intraprostatic vasopressin (IPVP) prevents the transurethral resection (TUR) syndrome during prostatectomy. The study comprised 36 consecutive patients (mean age 68 years) with prostates clinically assessed as >/= 20 g who underwent standard transurethral prostatectomy (TURP). Ten units of vasopressin in 0.5 mL were diluted with 9.5 mL isotonic saline and injected into the prostate transrectally before TURP. Blood samples were taken before and immediately after TURP to measure serum sodium concentration and free haemoglobin levels. The TURP irrigant used was cooled, boiled water maintained at 70-80 cmH2O pressure during resection. Twenty patients had alcohol added to the irrigant and their breath alcohol assessed at 10-min intervals during TURP. All patients had their pulse rate, blood pressure and sensorium monitored continuously. Extreme care was taken to avoid and/or identify capsular damage during resection. The mean weight of tissue resected was 36 g and the mean resection time 24 min. There was no significant change in clinical variables during TURP. In 19 patients the breath alcohol changes were insignificant. Changes in free haemoglobin were not significant, but the levels decreased after TURP in four patients, caused by the dilution consequent on the infusion of 800-1000 mL isotonic saline during surgery. Serum sodium concentrations showed only insignificant decreases, except in one patient whose breath alcohol suggested the absorption of 500 mL of irrigant. This patient's serum sodium concentration decreased by 9 mmol/L; 1 L of 5% dextrose was infused during the procedure and capsular damage was recognized early during TURP. Insignificant volumes of irrigant entered the circulation of the patients during TURP with water irrigation and IPVP. The greatest risk factor for fluid entry during TURP is capsular damage. IPVP decreases bleeding and therefore improves visibility, so allowing the early identification of capsular damage. IPVP seems therefore to be of help during TURP by decreasing bleeding and allowing insignificant volumes of irrigant to enter the vasoconstricted vessels; it appears to prevent the TUR syndrome.

Correção da Incontinência Urinária de Esforço com Sling: Resultados Iniciais

Purpose: to analyze the surgical results after slings with vaginal wall, performed by the Urogynecology and Vaginal Surgery Sector of UNIFESP/EPM, for the treatment of incontinent women with hypermobility of the bladder neck, who show great risk of surgery failure with other techniques or in those with intrinsic sphincteric deficiency (ISD) and, also, surgery recurrence. Methods: we studied 21 patients submitted to surgery in order to correct urinary incontinence by the vaginal wall sling technique, in the period from December 1997 to February 1999, with postoperative follow-up which varied between 1 and 14 months (average 8.2). The mean age of patients was 56 years (39 to 77 years), 15 (71.4%) were in menopause and 6 (28.6%) in menacme. All patients were evaluated before the surgery through medical interview, physical examination, ultrasound and urodynamic study, the grade of urinary loss being high in 66.7% and moderate in 33.3% of the patients. All patients showed hypermobility of the bladder neck (more than 10 mm) and 12 patients had previous surgery to correct the urinary incontinence. Regarding the urodinamic study, the patients manifested urinary loss with maximum pressure of urethral closure (MPUC) varying from 20 to 124 cmH2O (average 55.2) and Valsalva leak point pressure (VLPP) varying from 18 to 128 cmH2O (average 60.3). The indications of surgery were: ISD (11 patients ¾ 52.4%), obesity (5 patients ¾ 23.8%), ISD and obesity (2 patients ¾ 9.5%), surgery recurrence (2 patients ¾ 9.5) and ISD and first grade womb prolapse (1 patient ¾ 4.8%). Results: as complications, 6 patients (28.6%) showed temporary urinary retention after surgery, 1 patient (4.8%) infection in the urinary tract, 1 patient (4.8%) presence of polypropylene suture in the vagina, 1 patient (4.8%) infection of the surgery wound, 4 patients (19%) developed urgency/incontinence, 1 (4.8%) urgency and 1 (4.8%) difficulty in urinating (high postvoiding residue). The grade of the patients' satisfaction was satisfactory, with 15 patients (71.4%) referring cure, 3 patients (14.3%) improvement, in 2 patients (9.5%) the urinary loss remained unchanged and in 1 patient (4.8%) the urinary loss got worse. Conclusions: the vaginal wall sling surgery is efficient for the treatment of specific cases of stress urinary incontinence, emphasizing intrinsic urethral sphincteric incompetence, surgery recurrence and predisposing factors to failure of other techniques.

Use of recruitment maneuvers and high positive end-expiratory pressure in a patient with acute respiratory distress syndrome

To present the use of a novel high-pressure recruitment maneuver followed by high levels of positive end-expiratory pressure in a patient with the acute respiratory distress syndrome (ARDS). Observations in one patient. The medical intensive care unit at a tertiary care university teaching hospital. A 32-yr-old woman with severe ARDS secondary to streptococcal sepsis. The patient had severe gas exchange abnormalities because of acute lung injury and marked lung collapse. Attempts to optimize recruitment based on the inflation pressure-volume (PV) curve were not sufficient to avoid dependent lung collapse. We used a recruitment maneuver using 40 cm H2O of positive end-expiratory pressure (PEEP) and 20 cm H2O of pressure controlled ventilation above PEEP for 2 mins to successfully recruit the lung. The recruitment was maintained with 25 cm H2O of PEEP, which was much higher than the PEEP predicted by the lower inflection point (P(Flex)) of the PV curve. Recruitment was assessed by improvements in oxygenation and by computed tomography of the chest. With the recruitment maneuvers, the patient had a dramatic improvement in gas exchange and we were able to demonstrate nearly complete recruitment of the lung by computed tomography. A PV curve was measured that demonstrated a P(Flex) of 16-18 cm H2O. Accumulating data suggest that the maximization and maintenance of lung recruitment may reduce lung parenchymal injury from positive pressure ventilation in ARDS. We demonstrate that in this case PEEP alone was not adequate to recruit the injured lung and that a high-pressure recruitment maneuver was required. After recruitment, high-level PEEP was needed to prevent derecruitment and this level of PEEP was not adequately predicted by the P(Flex) of the PV curve.

Effects of biphasic positive airway pressure in patients with chronic obstructive pulmonary disease

Biphasic positive airway pressure (BiPAP) is a ventilatory mode in which two pressure levels (higher (Phigh) and lower (Plow)) acting as continuous positive airway pressure (CPAP) alternate at preset time intervals. BiPAP combines pressure-controlled ventilation with unrestricted spontaneous breathing. BiPAP has not yet been evaluated in patients with chronic obstructive pulmonary disease (COPD). Therefore, the effects of BiPAP (15 cmH2O Phigh and 5 cmH2O Plow) pressure support (PS; 15 cmH2O and positive end-expiratory pressure (PEEP) 5 cmH2O) and CPAP (5 cmH2O) on respiratory mechanics in COPD patients were compared. Twenty-one COPD patients were supported in randomized order with BiPAP, PS and CPAP. Pressure-time product (PTP), work of breathing (WOB), change in oesophageal pressure (deltaPoes), mouth occlusion pressure (P0.1), intrinsic PEEP (PEEPi), tension time index (TTI), respiratory frequency, and tidal volume (VT) were measured. During BiPAP, the COPD patients showed a significantly higher PTP, WOB, deltaPoes, P0.1, TTI and PEEPi than during PS. Comparing the Plow phases of BiPAP and CPAP, the breaths during the Plow phases of BiPAP had a lower VT and a greater WOB and PTP due to a higher PEEPi than on CPAP alone. In conclusion, biphasic positive airway pressure carries the risk of increased work of breathing in spontaneously breathing chronic obstructive pulmonary disease patients. Pressure support is superior for reducing their respiratory muscle effort.

Effects of noninvasive positive pressure ventilatory support in non-COPD patients with acute respiratory insufficiency after early extubation.

To investigate the effects of noninvasive positive pressure ventilation (NPPV) on pulmonary gas exchange, breathing pattern, intrapulmonary shunt fraction, oxygen consumption, and resting energy expenditure in patients with persistent acute respiratory failure but without chronic obstructive pulmonary disease (COPD) after early extubation. Prospective study. Multidisciplinary intensive care unit of a university hospital. 15 patients after prolonged mechanical ventilation (> 72 h) with acute respiratory insufficiency after early extubation. Criteria for early extubation were arterial oxygen tension (PaO2) > or = 40 mm Hg (fractional inspired oxygen 0.21), arterial carbon dioxide tension (PaCO2) < or = 55 mm Hg, pH > 7.32, respiratory rate < or = 40 breaths per min, tidal volume (VT) > or = 3 ml/kg, rapid shallow breathing index < or = 190 and negative inspiratory force > or = 20 cmH2O. After extubation, two modes of NPPV were applied [continuous positive airway pressure (CPAP) of 5 cmH2O and pressure support ventilation (PSV) with 15 cmH2O pressure support]. Oxygenation and ventilatory parameters improved during both modes of NPPV (p < 0.05): increase in PaO2 of 11 mm Hg during CPAP and 21 mm Hg during PSV; decrease in intrapulmonary shunt fraction of 7% during CPAP and 12% during PSV; increase in tidal volume of 1 ml/kg during CPAP and 4 ml/kg during PSV; decrease in respiratory rate 6 breaths/min during CPAP and 9 breaths/min during PSV. Oxygen consumption (15% during CPAP, 22% during PSV) and resting energy expenditure (12% during CPAP, 20% during PSV) were reduced (p < 0.05). PaCO2 decreased, whereas minute ventilation and pH increased during PSV (p < 0.05). The median duration of NPPV was 2 days. Two patients had to be reintubated. In non-COPD patients with persistent acute respiratory failure after early extubation, NPPV improved pulmonary gas exchange and breathing pattern, decreased intrapulmonary shunt fraction, and reduced the work of breathing.

Effect of CPAP during cardiopulmonary bypass on postoperative lung function. An experimental study.

Respiratory failure secondary to cardiopulmonary bypass (CPB) remains a major complication after cardiac surgery. We tested the hypothesis that post-CPB lung function impairment can be prevented by continuous positive airway pressure (CPAP) applied during the CPB. In 6 pigs, CPAP with 5 cmH2O pressure was applied during CPB. Six other pigs served as control, i.e. the lungs were open to the atmosphere during CPB. After median sternotomy, the right atrial appendage as well as the ascending aorta were cannulated. The total CPB duration was 90 min with 45 min cardioplegic arrest. Ventilation-perfusion distribution was measured with the multiple inert gas elimination technique and atelectasis by CT-scanning. Large atelectasis appeared after CPB, corresponding to 14.5% +/- 5.5 (percent of the total lung area) in the CPAP group and 18.7% +/- 5.2 in the controls (P = 0.20). Intrapulmonary shunt increased and PaO2 decreased after the CPB in both groups. We conclude that in this pig model post-CPB atelectasis is not effectively prevented by CPAP applied during CPB.

Glucocorticoid regulation of surfactant components in immature lambs

To assess effects of dose and duration of glucocorticoid exposure on maturation of the fetal lung, we administered single or multiple doses of betamethasone (0.5 mg/kg im) to pregnant sheep for 2 or 21 days before preterm delivery at 125 days of gestation. Lung function (compliance, lung volume at 40 cmH2O pressure, and ventilatory efficiency index) was increased after two to four weekly doses of glucocorticoid (2.5- to 4-fold increase) and after 48 h of exposure (1.4- to 2.3-fold). Total protein of lavage fluid decreased similarly with three doses, four doses, and 48 h of treatment. In lambs with long-term exposure to betamethasone, there was a similar, dose-dependent increase in concentrations of saturated phosphatidylcholine and surfactant proteins A (SP-A) and B (SP-B) (maximal 2- to 3-fold in tissue and 10- to 15-fold in lavage fluid). Levels of SP-A and SP-B were closely correlated in lavage fluid. In animals treated for 48 h, only tissue SP-B was increased (2.7-fold). We conclude that 48 h of glucocorticoid treatment improves lung function in the premature lamb without a detectable increase in lavage surfactant components and that longer exposure to antenatal glucocorticoid increases surfactant lipid and proteins in a coordinated fashion. The enhanced response with repetitive dosing indicates that the process of glucocorticoid-induced lung maturation is either reversible and/or gestational age dependent.

Elastance of the pleural space: a predictor for the outcome of pleurodesis in patients with malignant pleural effusion.

In patients who have malignant pleural effusion, it is difficult to diagnose trapped lung before thoracoscopy, thoracostomy, and drainage of the effusion. Predicting the outcome of pleurodesis in patients who have malignant pleural effusion without trapped lung is also difficult. To investigate the factors that may be useful in diagnosing trapped lung and predicting the outcome of pleurodesis. Prospective cohort study. University medical center. 65 patients with symptomatic malignant pleural effusion. A chest tube was inserted for drainage of the effusion, and pleurodesis was done using bleomycin as the sclerosant. The pH and glucose levels of the effusion and the elastance of the pleural space (defined as the decline in pleural fluid pressure in cm H2O after removal of 500 mL of effusion) were measured. The outcome of pleurodesis was evaluated 1 month after the chest tube was removed. Patients with an elastance of 19 cm H2O or more had a higher incidence of trapped lung (11 of 14 patients) than did those with an elastance less than 19 cm H2O (3 of 51 patients) (P < 0.001). None of the 14 patients with an elastance of 19 cm H2O or more and none of the 14 patients with a trapped lung had successful pleurodesis. Forty-two of 43 patients with an elastance less than 19 cm H2O who did not have a trapped lung had successful pleurodesis. Elastance seemed to be the best predictor for trapped lung and outcome of pleurodesis, although outcome was also correlated with pH and glucose levels of the effusion. Low-dose bleomycin (30 mg) is as effective as the usual dose of bleomycin (60 mg) for pleurodesis. In patients with symptomatic malignant pleural effusion, measurement of the elastance of the pleural space is a simple and effective method for the diagnosis of trapped lung and prediction of the outcome of chemical pleurodesis with bleomycin.

Effect of inflation on interstitial cuff and pressure in liquid-filled rabbit lung

Degassed rabbit lungs were inflated to 15 cmH 2O pressure with 3% albumin solution. To study cuff growth, lungs were frozen in liquid N 2 at several (20–120 min) inflation times. Cuff-to-vessel area ratio measured from frozen lung pieces increased with time reaching a maximum value (0.5–0.9) by 1 h. Time constants (to) of cuff growth were similar to those (28 min) of the interstitial pressure (Pi) response measured by micropuncture at the lung hilum. Pi response was slower with saline (to = 84 min) than with albumin. Compared to saline, positively charged protamine sulphate increased the Pi response (to = 44 min). Time constants for cuff growth and Pi response were smaller at 15 cmH 2O than at 5 cmH 2O inflation pressure (30 vs. 60–120 min). Electrical analog models indicated a doubling of interstitial resistance with a four-to eight-fold decrease in interstitial specific compliance at the higher inflation pressure, the latter attributed to nonlinear elastic behavior of lung parenchyma.

Terbutaline stimulates alveolar fluid resorption in hyperoxic lung injury.

Alveolar fluid resorption occurs by active epithelial sodium transport and is accelerated by terbutaline in healthy lungs. We investigated the effect of terbutaline on the rate of alveolar fluid resorption from rat lungs injured by hyperoxia. Rats exposed to > 95% O2 for 60 h, sufficient to increase wet-to-dry lung weight and cause alveolar edema, were compared with air-breathing control rats. After anesthesia, the animals breathed 100% O2 for 10 min through a tracheostomy. Ringer solution was instilled into the alveoli, and the steady-state rate of volume resorbed at 6 cmH2O pressure was measured via a pipette attached to the tracheostomy tubing. Ringer solution in some animals contained terbutaline (10(-3) M), ouabain (10(-3) M), or both. Normoxic animals resorbed 49 +/- 6 microliters.kg-1.min-1; ouabain reduced this by 39%, whereas terbutaline increased the rate by 75%. The effect of terbutaline was blocked by ouabain. Hyperoxic animals absorbed 78 +/- 9 microliters.kg-1.min-1; ouabain reduced this by 44%. Terbutaline increased the rate by a mean of 39 microliters.kg-1.min-1, similar to the absolute effect seen in the normoxic group, and this was blocked by ouabain. Terbutaline accelerates fluid resorption from both normal and injured rat lungs via its effects on active sodium transport.

A Case of Severe Persistent Unilateral Lung Hemorrhage Successfully Treated with Differential Lung Ventilation and High Continous Positive Airway Pressure

We reported on a case with left lung hemorrhage which was unresponsive to conventional therapy but successfully treated with differential lung ventilation using high contnvous positive air way pressure (CPAP).A 22 yr-old woman with anorexia nervosa developed organ failure and unilateral lung hemorrhage. Coagulants and fresh frozen plasma were given and intratracheal injections of adrenaline and thrombin through a bronchofiberscope were tried without success. A double-lumen endotracheal tube was intubated and differential ventilation was started to prevent blood from pouring from the left lung into the right. High continuous positive airway pressure of 20cm H2O was applied to the left lung. The right lung was ventilated with 400ml of tidal volume, 15cm H2O of pressure support ventilation, 15·min-1 of synchronized intermittent mandatory ventilation and 7cm H2O of positive end expiratory pressure. Differential lung ventilation using high CPAP on the left lung was continued for 40hrs. The lung hemorrhage was stopped completely without segmential pneumonia.

Simultaneous Estimation of Filtration Variables in Isolated Rat Lungs in Zone 3 Conditions.

Estimation of filtration variables (filtration coefficient, perimicrovascular pressure, and reflection coefficient) is important for evaluating pulmonary microvascular permeability to fluids, especially when the lungs are inflamed, injured, or being preserved for transplantation. Here we report a new method for estimating filtration variables simultaneously in isolated rat lung lobes in zone 3 conditions (pulmonary arterial pressure > pulmonary venous pressure > alveolar pressure). We used Krebs-Henseleit solution containing 6% bovine serum albumin for the perfusate and maintained perfusion using a constant-pressure circuit system. Pulmonary venous and alveolar pressures were kept at 2.5 and 2.0 cmH2O, respectively, and pulmonary arterial pressure was set so that lung weight did not change. Then we increased both pulmonary arterial and venous pressures by 3 cm H2O simultaneously, and recorded the weight gain. Next, we diluted the perfusate with Krebs-Henseleit solution to about half the original concentration, and recorded the weight gain. Using the Starling equation, the values we obtained for the filtration variables were filtration coefficient, 5.3 mg. min-1.cmH2O-1.g-1; perimicrovascular pressure, 1.1 cmH2O; reflection coefficient, 0.54. These values agree with values from previous reports. Since these 3 filtration variables are interrelated, this method for simultaneous measurement is more accurate than independent measurements are. The advantages of this method are that it does not require direct measurement of interstitial pressure or collection of lymph fluid. What we need to calculate these filtration variables are the initial filtration rates and the albumin osmotic pressures for perfusate.

Bench evaluation: Three face-shield CPR barrier devices: Respir Care 1995; 40/6: 618–623

UNLABELLED Due to the fear of disease transmission, the practice of mouth-to-mouth (M-M) rescue breathing is rarely performed; to address this concern, many types of CPR barrier devices have been developed. These include bag-valve-mask devices, mouth-to-mask devices, and face shields (FS). The purpose of this study was to measure the volumes delivered during mouth-to-face shield (M-FS) breathing, to measure the back pressure and calculate the resistance to flow through their 1-way valves, and to test for backward leak of gas through the valves. METHODS Three FS brands were evaluated: Kiss of Life (KOL), MicroSHIELD (Micro) and Res-Cue Key (RCK). Volume delivered during M-M and M-FS breathing was evaluated by 10 rescuers who used the devices while performing rescue breathing on a CPR mannequin. Back pressure was measured and resistance calculated by directing airflow through the 1-way valves. Backward leak was evaluated by measuring the O2 concentration at the rescuer side of the valve while 100% O2 was directed toward the patient side of the valve. Differences among the brands were evaluated using analysis of variance. RESULTS The mean (SD) values for volumes in L were: M-M 1.00 (0.25), Micro 0.77 (0.20), RCK 0.64 (0.10), and KOL 0.24 (0.11). Mean values for back pressure in cm H2O at 50 L/min were Micro 16.7 (1.29), KOL 7.22 (0.13), and RCK 2.15 (0.16). Significant backward leak only occurred with RCK. CONCLUSION Not one of the FSs tested met all of the requirements suggested by the American Heart Association and by the International Standards Organization.

Pressure support ventilation during isoflurane anaesthesia

We have studied the respiratory effects of 5 and 10 cmH2O pressure support ventilation during anaesthesia with 1.5% end-tidal concentration of isoflurane in nine healthy, spontaneously breathing, adult patients. Some of the patients demonstrated an irregular respiratory pattern with periods of apnoea and we therefore went on to study a further seven patients with a continuous 500 s recording of airflow. Pressure support ventilation augmented mean (SD) tidal volume from 212 (56) ml to 360 (88) ml at 5 cmH2O and to 509 (108) ml at 10 cmH2O (n = 16, p < 0.05). Mean (SD) respiratory rate decreased from 26 (6) min-1 to 22 (6) min-1 at 5 cmH2O and 17 (5) min-1 at 10 cmH2O pressure support (n = 16, p < 0.05). Mean (SD) inspiratory work of breathing decreased from 1.77 (0.70) J. min-1 to 0.31 (0.36) J.min-1 at 5 cmH2O and 0.16 (0.26) J.min-1 at 10 cmH2O pressure support ventilation (n = 9, p < 0.05). Analysis of the respiratory rhythm in the second group of seven patients revealed an oscillating respiratory pattern in four patients at 5 cmH2O and six of the seven patients at 10 cmH2O pressure support ventilation. The metabolic advantage of the decreased work of breathing during pressure support ventilation during anaesthesia is unlikely to balance the disadvantage of an oscillating respiratory rhythm.

Pulmonary inflammatory cell response to sustained endotoxin administration

We have developed a model of human sepsis in sheep. Twenty-four hours after continuous infusion of Escherichia coli endotoxin (lipopolysaccharide) (10 ng.kg-1.min-1) was begun, pulmonary transvascular fluid flux was almost five times the baseline values, cardiac output was nearly doubled, and mean arterial pressure was reduced by approximately 20 mmHg. At this time, the animals were killed and their lungs were fixed by endotracheal installation of 2.5% glutaraldehyde at 25 cmH2O pressure. Morphometry was performed by point counting, and data were expressed as relative volume density. Pulmonary edema and congestion were observed in sheep receiving lipopolysaccharide, whereas sham controls appeared normal. There was an increase in interstitial volume density. There was a significant increase (P < 0.01) in volume density of the pulmonary intravasculature (180%), interstitial macrophages (270%), and mast cells (240%). The volume densities of intravascular and interstitial polymorphonuclear neutrophils also showed a small insignificant increase.

Effects of pressure and duration of hyperkalemic infusions on endothelial function

Infusions of crystalloid hyperkalemic cardioplegic solutions (CHCSs) are known to impair endotheliumdependent coronary relaxation. This impairment might also be influenced by high perfusion pressure and duration of CHCS infusion. To verify this hypothesis, we designed experiments to study the influence of pressure and duration of CHCS infusion as modulating factors in CHCS-related endothelial impairment. Isolated hearts of Sprague-Dawley rats were studied in a Langendorff apparatus for coronary endothelial function. Hearts (n = 6) were exposed to four different CHCSs containing 12, 24, 40, or 100 mmol/L of potassium chloride (KCl). Endothelial and smooth muscle functions were respectively tested by infusion of 5-hydroxytryptamine (1 × 10−6 mol/L) and sodium nitroprusside (1 × 10−5 mol/L) before and after CHCS perfusion. In group I (n = 24), 37 °C CHCSs were perfused at 80 cm H2O of pressure for 30 minutes. In group II (n = 24), the same CHCSs were perfused at 160 cm H2O for 30 minutes. In group III (n = 18), CHCSs containing 24, 40, and 100 mmol/L of KCl were infused at 160 cm H2O for 10 minutes. In all groups, response to sodium nitroprusside was unaltered by CHCS infusion, indicating that smooth muscle function was preserved. However, in group II, 5-hydroxytryptamine-induced vasodilatation was significantly impaired in hearts perfused with CHCS containing 24 mmol/L of KCl or more, suggesting endothelial damage. This study demonstrates that, in addition to KCl concentration, pressure and duration of infusion are two major determinants in CHCS-mediated endothelial damage.

Hydraulic conductivity of basement membrane with computed values for fiber radius and void volume ratio.

The basement membrane contributes to resistance to ultrafiltration of the capillary wall, but efforts to conduct studies of this material have been hindered by difficulty in obtaining homogenous sources and by compression from high filtration pressures. We utilized Matrigel in a conductivity chamber and showed that it reproducibly yielded a specific hydraulic conductivity of (2.247 +/- 0.328) x 10(-14) cm2, which is within 21% of that obtained for porcine glomerular basement membrane by Robinson and Walton (Microvasc. Res. 38: 36-48, 1989). No compression or hysteresis was observed over 5-25 cmH2O pressure difference. Depending on whether data were fit to drag or hydraulic radius fibermatrix models, fiber radius was 0.626-0.696 nm, and void volume ratio varied between 0.826 and 0.846, the former agreeing with the major components of Matrigel. Ready availability of Matrigel, its uniform hydraulic conductivity, and its fit to fibermatrix theory make it ideal to study changes in basement membrane during physiological and pathological changes. Moreover, results of such studies are likely applicable to endothelial barriers, whose luminal and intercellular surfaces are lined with fibrillar materials.