Acetazolamide plus low-dose dexamethasone is better than acetazolamide alone to ameliorate symptoms of acute mountain sickness.

METHODS: In a double-blind study, we compared the efficacy of a combination of sustained-release acetazolamide and low-dose dexamethasone and acetazolamide alone for prophylaxis against acute mountain sickness (AMS) caused by rapid ascent to high altitude. Before ascent, 13 subjects were randomly assigned to receive a combination of one sustained-release acetazolamide capsule (500 mg) in the afternoon and 4 mg dexamethasone every 12 h, or a combination of the same dose of acetazolamide once daily and a placebo every 12 h. Days 1 and 2 were spent at 3698 m (La Paz, Bolivia), while days 3 and 4 were spent at 5334 m (Mount Chaclataya, Bolivia). Ascent was by 2 h motor vehicle ride. Heart rates, peripheral oxygen saturations and a modified score derived from the Environmental Symptom Questionnaire (modified-ESQ) were measured on each day. In addition, weighted averages of the cerebral (AMS-C) and respiratory (AMS-R) symptoms were calculated for days 3 and 4. RESULTS: Heart rate and modified-ESQ scores increased on days 3 and 4 compared with the other days in the acetazolamide/placebo group only (p < 0.05). Oxygen saturations decreased in both groups on days 3 and 4 (p < 0.05), but the decrease was greater in the acetazolamide/placebo group (p < 0.05). AMS-C and AMS-R scores rose above the suggested thresholds for indication of AMS on days 3 and 4 in the acetazolamide/placebo group only (p < 0.05). CONCLUSION: We conclude that this combination of sustained-release acetazolamide once daily and low-dose dexamethasone twice daily is more effective in ameliorating the symptoms of AMS than azetazolamide alone at the ascent that was studied.

Acute effects of bilateral lung volume reduction surgery on lung and chest wall mechanical properties.

STUDY OBJECTIVES: To characterize acute changes in the dynamic, passive mechanical properties of the lungs and chest wall, elastance (E) and resistance (R), caused by lung volume reduction surgery (LVRS). DESIGN: Prospective data collection. PATIENTS: Nine anesthetized/paralyzed patients with severe emphysema. INTERVENTIONS: Bilateral LVRS. MEASUREMENTS AND RESULTS: From measurements of airway and esophageal pressures and flow during mechanical ventilation throughout the physiologic range of breathing frequency (f) and tidal volume (VT), E and R of the total respiratory system (Ers and Rrs), lungs (EL and RL), and chest wall (Ecw and Rcw) immediately before and after LVRS were calculated. After surgery, Ers, EL, Rrs, and RL were all greatly increased at each combination off and VT (p<0.05). Ecw and Rcw showed no consistent changes (p>0.05). The increases in EL were greatest in those patients with the lowest residual volumes, highest FEV1 values, and highest maximum voluntary ventilations measured 3 months preoperatively (p<0.05); the increases in RL were greatest in those patients with the lowest preoperative residual volumes (p<0.05). The largest increases in RL were in those patients with the largest decreases in residual volume and total lung capacity, measured 3 months postoperatively, caused by LVRS (p<0.05). CONCLUSION: Acute effects of LVRS are large increases in lung elastic tension and resistance; these increases need to be considered in immediate postoperative care, and can be predicted roughly from results of preoperative pulmonary function tests.

Effects of split torso positioning and laparoscopic surgery for donor nephrectomy on respiratory mechanics.

STUDY OBJECTIVE: To test whether split torso positioning, abdominal insufflation, and other procedures performed during laparoscopic nephrectomy would affect mechanical impedances to inflation [i.e., elastance (E) and resistance (R) of the total respiratory system (Ers, and Rrs), lungs (EL and RL), and chest wall (Ecw and Rcw)] differently from previously studied laparoscopic procedures. DESIGN: Unblinded study, each patient serving as own control. SETTING: University hospital. PATIENTS: 12 ASA physical status I and II patients scheduled for laparoscopic donor nephrectomy, all without cardiopulmonary disease. INTERVENTIONS: Patients were anesthetized and paralyzed, tracheally intubated and mechanically ventilated at 10, 20, and 30 breaths/minute and at tidal volumes of 250, 500, and 800 ml. Measurements were made in the following positions: supine, split torso, abdominal insufflation (Pab = 15 mmHg), and supine after deflation. MEASUREMENTS AND MAIN RESULTS: Airway flow and pressure and esophageal pressure were measured. Discrete Fourier transformation was used to calculate E and R. These were analyzed with repeated measures, linear multiple regression with accepted level of significance at p < 0.05. Ers, Ecw, and Rcw increased (p < 0.05) while EL decreased (p < 0.05) when patients changed from supine to split torso. During Pab = 15 mmHg, Ers, Ecw, and Rcw increased further and Rrs and RL increased (p < 0.05). Following abdominal deflation, Ecw and Ers remained elevated (p < 0.05). The changes in Ecw caused by laparoscopy and surgery were greater than we have previously measured in other laparoscopic procedures, while the changes in EL were less. CONCLUSIONS: Laparoscopic nephrectomy affects lung and chest wall mechanical properties differently from other laparoscopic procedures. This finding could be due to the split torso positioning, and the effects of abdominal swelling on the chest wall caused by administration of more perioperative fluids with laparoscopic nephrectomy.

Lung mechanical behavior during one-lung ventilation.

OBJECTIVE: Switching from two-lung to one-lung ventilation would be expected to have large effects on lung mechanical properties, and these effects may depend on tidal volume and respiratory frequency. These changes in lung mechanics with one-lung ventilation may be similar to pulmonary edema. Deteriorating lung mechanics during pulmonary edema have been attributed to a loss of ventilated lung units. Therefore, changes in lung mechanics caused by one-lung ventilation were measured and compared with those previously seen during pulmonary edema. DESIGN: Prospective study. SETTING: Research laboratory. INTERVENTIONS: After induction of anesthesia, beagle dogs' tracheas were intubated with an endotracheal tube with a bronchial blocker (Univent System Corp, Tokyo, Japan) to apply one-lung ventilation. The proper position of the bronchial blocker during one-lung ventilation was confirmed with a fiberoptic bronchoscope. MEASUREMENTS AND MAIN RESULTS: Lung elastance (EL) and resistance (RL) were calculated from measurements of airway pressure, esophageal pressure, and airway flow in five anesthetized, paralyzed dogs during sinusoidal forcing at a constant mean airway pressure of 10 cmH2O in a wide range of breathing frequencies (0.2 to 1.0 Hz in intervals of 0.2) and tidal volumes (50, 100, 200, and to 300 mL). Measurements were made before and after the left mainstem bronchus was occluded with the bronchial blocker. During ventilation of both lungs, EL and RL depended relatively little on frequency, and both EL and RL were independent of tidal volume. During one-lung ventilation, EL doubled and, at most frequencies, RL increased; frequency dependences were not increased, and no dependence on tidal volume was observed. CONCLUSIONS: The lack of tidal volume dependence in EL and lack of large-frequency dependence in RL during one-lung ventilation are inconsistent with changes induced by severe pulmonary edema. Although decreases in ventilatable lung volume may contribute to increases in lung elastance, other characteristics of mechanical behavior during one-lung ventilation differ from those of pulmonary edema; therefore, other additional mechanisms must be involved in determining lung mechanical properties during severe pulmonary edema.

Chest wall and lung mechanics during acute hemorrhage in anesthetized dogs.

OBJECTIVES: In trauma and in surgical patients, respiratory mechanics may change because of many factors, including the hypotension induced by hemorrhage. The effects of acute hemorrhage on elastic and resistive characteristics of the respiratory system were studied. DESIGN: Prospective study. SETTING: Anesthesia research laboratory. INTERVENTIONS: Acute hemorrhagic shock was induced in 24 supine anesthetized/paralyzed, mechanically ventilated dogs by blood withdrawal over a 12-minute period to decrease systolic arterial pressure to 50 mmHg; additional blood was subsequently withdrawn to maintain this pressure for 2 hours. Total respiratory system dynamic compliance and resistance and lung and chest wall compliances and resistances were measured. MEASUREMENTS AND MAIN RESULTS: Total respiratory system dynamic compliance decreased from control (0.03 +/- 0.002 L/cmH2O) by the first 10 minutes of shock (p < 0.05) and was 9.8 +/- 2% lower than control 2 hours after the induction of shock because of decreases in both lung (9.6 +/- 3%) and chest wall (7.7 +/- 3%) compliances. Total respiratory resistance increased 12.8 +/- 3% from control (3.08 +/- 0.19 cmH2O/L/s) after 2 hours of shock (p < 0.05) because of an increase in chest wall resistance (21.6 +/- 8%, p < 0.05). Pulmonary resistance was not significantly increased (p > 0.05). In six control dogs, prepared similarly but not hemorrhaged, chest wall compliance and resistance did not change, but lung compliance gradually decreased by 17.8% during 150 minutes of anesthesia/paralysis. Lung resistance increased only after 100 minutes (p < 0.05). CONCLUSIONS: (1) Hemorrhagic shock caused slight changes in the chest wall, but effects on lung mechanics were a consequence of prolonged mechanical ventilation during anesthesia/paralysis, and (2) changes in respiratory mechanics caused by hemorrhagic shock are small and, unless other deleterious factors are present, would probably have little clinical significance.

Effects of PEEP on respiratory mechanics are tidal volume and frequency dependent.

How the effects of frequency, tidal volume (VT) and PEEP interact to determine the mechanical properties of the respiratory system is unclear. Airway flow and airway and esophageal pressures were measured in ten intubated, anesthetized/paralyzed patients during mechanical ventilation at 10-30 breaths/min and VT of 250-800 ml. From these measurements, Fourier transformation was used to calculate elastance (E) and resistance (R) of the total respiratory system (subscript rs), lungs (subscript L) and chest wall (subscript cw) at 5, 10 and 0 cm PEEP. As PEEP increased from 0-5 cmH2O, all elastances and resistances decreased (P < 0.05). Increasing PEEP to 10 cmH2O decreased EL, Rrs, and RL further (P < 0.05). The changes in Ers, EL, Rrs and RL caused by PEEP were less (P < 0.05) as VT increased, while changes in Rrs, RL and Ers were less (P < 0.05) as frequency increased. VT dependences in Ers and Rrs were enhanced (P < 0.05) at 0 cmH2O PEEP. The ratio of EL to chest wall elastance was not affected by PEEP (P > 0.05), but increased (P < 0.05) with increasing VT at 5 and 10 cmH2O PEEP. We conclude that it is critical to standardize ventilatory parameters when comparing groups of patients or testing clinical intervention efficacy and that the differential effects on the lungs and chest wall must be considered in optimizing the application of PEEP.

Respiratory impedances and acinar gas transfer in a canine model for emphysema.

We examined how the changes in the acini caused by emphysema affected gas transfer out of the acinus (Taci) and lung and chest wall mechanical properties. Measurements were taken from five dogs before and 3 mo after induction of severe bilateral emphysema by exposure to papain aerosol (170-350 mg/dose) for 4 consecutive wk. With the dogs anesthetized, paralyzed, and mechanically ventilated at 0.2 Hz and 20 ml/kg, we measured Taci by the rate of washout of 133Xe from an area of the lung with occluded blood flow. Measurements were repeated at positive end-expiratory pressures (PEEP) of 10, 5, 15, 0, and 20 cmH2O. We also measured dynamic elastances and resistances of the lungs (EL and RL, respectively) and chest wall at the different PEEP and during sinusoidal forcing in the normal range of breathing frequency and tidal volume. After final measurements, tissue sections from five randomly selected areas of the lung each showed indications of emphysema. Taci during emphysema was similar to that in control dogs. EL decreased by approximately 50% during emphysema (P < 0.05) but did not change its dependence on frequency or tidal volume. RL did not change (P > 0.05) at the lowest frequency studied (0.2 Hz), but in some dogs it increased compared with control at the higher frequencies. Chest wall properties were not changed by emphysema (P > 0.05). We suggest that although large changes in acinar structure and EL occur during uncomplicated bilateral emphysema, secondary complications must be present to cause several of the characteristic dysfunctions seen in patients with emphysema.

Response time studies of a new, portable mass spectrometer.

OBJECTIVE: Mass spectrometers are frequently used by anesthesiologists perioperatively to monitor patients' respiratory function and levels of inhaled anesthetics. Due to size, complexity and expense, they are typically used in a time-sharing manner which degrades their performance. We assessed the accuracy of the Random Access Mass Spectrometer (RAMS), Marquette Electronics) which is small enough to be dedicated to a single patient. METHODS: We compared the 10-90% rise times for O2, CO2, N2O and isoflurane for the RAMS with different catheter configurations to those of a MedSpect mass spectrometer (Allegheny International Medical Technology) operating under ideal conditions. For CO2 the lag of the RAMS relative to the MedSpect was also measured. Next, perioperative conditions were stimulated by ventilating anesthetized dogs with a variety of inhalatory gases and ventilatory parameters, and the interchangeability of the two devices was assessed. RESULTS: When fitted with a catheter with minimal dead space the MedSpect had rise times of 0.11-0.12 sec while the RAMS had rise times of 0.07-0.12 sec and a delay of 0.19 sec compared to the MedSpect. The rise times and delay of the RAMS increased when using a larger catheter and water trap. Although there were statistically significant differences in some values for inhaled and end-tidal gases under simulated perioperative conditions, particularly at the higher frequencies, these differences were small and for most purposes not clinically significant. CONCLUSIONS: Our results demonstrate that the RAMS configured for clinical conditions performs nearly as well as the MedSpect under ideal conditions. The small differences between the two, confined almost entirely to their end-tidal CO2 values, could be due to differences in instrument calibration, by the larger sampling catheter commonly used in clinical settings, or by a combination of both factors. Therefore the RAMS is sufficiently accurate for clinical use and would alleviate problems associated with time-shared mass spectrometers.

Impact of pleurotomy, continuous positive airway pressure, and fluid balance during cardiopulmonary bypass on lung mechanics and oxygenation.

OBJECTIVE: To determine effects of surgical pleurotomy, continuous positive airway pressure, and fluid balance during cardiopulmonary bypass (CPB) on lung mechanical properties and indices of oxygenation. DESIGN: Prospective, descriptive, and interventional study. SETTING: Cardiothoracic service at a major university referral center. PARTICIPANTS: Eighteen anesthetized-paralyzed patients undergoing elective coronary artery bypass grafting requiring CPB. INTERVENTIONS: During CPB, continuous positive airway pressure (CPAP) was applied to nine patients, in nine others, no CPAP was applied. MEASUREMENTS AND MAIN RESULTS: From measurements of airway and esophageal pressures and flow, lung resistance and elastance were determined before sternotomy and after sternal reapproximation. Measurements were made during forced ventilation over a physiologic range of tidal volumes and frequencies, and frequency and volume dependences of lung resistance and elastance were additionally identified. In all patients, lung resistance and elastance increased after CPB, consistent with models of pulmonary edema. Multiple regression analysis showed that these increases were relatively less in patients with intact pleurae (p < 0.05) or net negative fluid balance (p < 0.05); however, no difference in these increases was noted between patients receiving CPAP and those receiving no CPAP. Increases in lung resistance were positively correlated to net fluid balance, and negatively correlated to frequency and tidal volume (p < 0.05). Increases in lung elastance were positively correlated to tidal volume (p < 0.05). Absolute change in alveolar-arterial oxygen gradient was negatively correlated with net fluid balance, whereas percentage change was positively correlated to changes in lung elastance (p < 0.05). CONCLUSIONS: These findings suggest that pleurotomy before CPB and positive fluid balance during CPB enhance postbypass pulmonary edema and/or atelectasis, as demonstrated by acute changes in respiratory mechanics and indices of oxygenation. Low levels of CPAP applied during CPB did not significantly change either mechanical properties or oxygenation.

Tetrodotoxin infusion: nonventilatory effects and role in toxicity models.

OBJECTIVES: To determine the cardiovascular, autonomic, and neuromuscular effects of an IV infusion of tetrodotoxin (TTX) when ventilation is supported. METHODS: TTX was infused in 18 anesthetized beagles during conventional mechanical ventilation. TTX infusion continued at a rate of 9.3 micrograms/kg/hr until apnea occurred with 1 minute of ventilator disconnection. Measurements included intravascular pressures, heart rate (HR), cardiac output, blood gases, displacements of the rib cage and abdomen, O2 delivery, and responses to train-of-four and tetanic peripheral nerve stimulation. Results are expressed as mean +/- SD. RESULTS: During TTX infusion, all the dogs had discoordinate movements of the rib cage, abdomen, and limbs. Vomiting, urination, defecation, and increased salivation occurred. Nicotinic and muscarinic effects, neuromuscular blockade, and cardiovascular depression were produced by TTX. Apnea occurred in 72.0 +/- 17.0 minutes when a total of 119.0 +/- 17.4 micrograms of TTX was infused. At apnea, decreases in arterial pressure, cardiac index, HR, O2 delivery, and systemic vascular resistance occurred, while pulmonary artery pressure and pulmonary vascular resistance increased. Loss of response to tetanic stimulation was closely correlated with the dose of TTX that produced apnea. CONCLUSION: The clinical symptoms and signs of TTX poisoning are similar to those of anticholinesterase poisons, and TTX dosing as described by this model may serve as a surrogate for organophosphorus poisoning. The model may be useful to determine optimum therapies for TTX poisoning and, since TTX prevents sodium influx into cells, to investigate enhanced survival in animals suffering from ischemia.

Breathing when chest wall muscle are tonically contracted for isometric, non-respiratory tasks.

We have previously shown that regional chest wall impedance increases when the chest wall muscles are tonically contracted to perform isometric, non-respiratory tasks. To test how this affects breathing, we measured respiratory frequency, tidal volume, end-tidal PCO2, electromyographic activity (EMG) at four points on the chest wall surface, and regional displacements across six planes of the chest wall during maintenance of three different postures that necessitated strong tonic respiratory muscle contraction. These postures included a static push-up, a bilateral leg-lift and a partial sit-up. The subjects (n = 8) were able to maintain the postures for 1.5-2.5 min, and strong tonic EMG activity was observed in each posture at all points measured. The rate and depth of breathing and pattern of regional chest wall displacements were variable within the group of subjects and among the three postures. However, minute ventilation increased and end-tidal PCO2 decreased in each subject during each posture (P < 0.05). In six of the eight subjects, transdiaphragmatic pressure (Pdi) was measured during 1 min of the same exercises. The ratio of the breathing fluctuation in Pdi to tidal volume was at least twice as high compared with rest, except for two subjects during the leg-lifts. We conclude that strong tonic contraction of the chest wall muscles impedes, but does not limit, breathing, and that there is no single breathing strategy used during such conditions.

Efficacy of several modes of continuous-flow insufflation for resuscitation of a canine model of acute respiratory arrest.

STUDY OBJECTIVE: To test the efficacy of several modes of continuous-flow insufflation on the maintenance of physiologic parameters in a model of respiratory arrest, and the effect of these modes on neurologic outcome. METHODS: Anesthetized dogs were slowly infused with tetrodotoxin over 75 minutes to the point of respiratory arrest. We used two different modes of continuous-flow insufflation: endobronchial insufflation (EI) of air 3 cm distal to the carina (.25 or 1.0 L.kg-1.min-1); and tracheal insufflation of oxygen (TRIO) 1 cm proximal to the carina (.08 or .2 L.kg-1.min-1). RESULTS: EI at either flow rate provided ventilation sufficient to allow the dogs to recover effective spontaneous breathing and be removed from ventilation after 4 hours. By this time, almost all cardiovascular variables and blood gas values were normal. TRIO at .2 L.kg-1.min-1 also resulted in successful recovery, although Pa02, as well as systemic and pulmonary arterial pressures and vascular resistances, remained increased at the end of the 4-hour period. TRIO at the low flow rate, however, resulted in deterioration of blood gas values and systemic arterial pressure; dogs required conventional mechanical ventilation after 45 minutes of low-flow TRIO. CONCLUSION: EI can be used to maintain oxygenation in acute respiratory arrest when conventional techniques are not feasible; TRIO at .2 L.kg-1.min-1 is also effective.

Effects of Trendelenburg and reverse Trendelenburg postures on lung and chest wall mechanics.

STUDY OBJECTIVE: To test whether the Trendelenburg ("head-down") or reverse Trendelenburg ("head-up") postures change lung and chest wall mechanical properties in a clinical condition. DESIGN: Unblinded study, each patient serving as own control. SETTING: University of Maryland at Baltimore Hospital, Baltimore, Maryland. PATIENTS: 15 patients scheduled for laparoscopic surgery. INTERVENTIONS: Patients were anesthetized and paralyzed, tracheally intubated and mechanically ventilated at 10 to 30 per minute and at a tidal volume of 250 to 800 ml. Measurements were made before surgery in supine, head-up (10 degrees from horizontal) and head-down (15 degrees from horizontal) postures. MEASUREMENTS AND MAIN RESULTS: Airway flow and airway and esophageal pressures were measured. From these measurements, discrete Fourier transformation was used to calculate elastances and resistances of the total respiratory system, lungs, and chest wall. Total respiratory elastance and resistance increased in the head-down posture compared with supine due to increases in lung elastance and resistance (p < 0.05); but chest wall elastance and resistance did not change (p > 0.05). Lung elastance also exhibited a negative dependence on tidal volume while head-down that was not observed in the supine posture. The change in lung elastance compared with supine was positively correlated to body mass index (weight/height2) and negatively correlated to tidal volume. Lung and chest wall elastance and resistance were not affected by shifting from supine to head-up (p > 0.05). CONCLUSIONS: The Trendelenburg posture increases the mechanical impedance of the lung to inflation, probably due to decreases in lung volume. This effect may become clinically relevant in patients predisposed with lung disease and in obese patients.

Respiratory mechanics in the open chest: effects of parietal pleurae.

To understand how the parietal pleurae affect the mechanical behavior of the human respiratory system after the chest wall is opened by median sternotomy, we studied 18 anesthetized/paralyzed patients immediately before coronary artery bypass grafting surgery. Elastances and resistances of the total respiratory system (ETr, Rrs) were calculated from measurements of airway pressure and flow during mechanical ventilation in the frequency and tidal volume ranges of normal breathing. Elastances and resistances of the lungs (EL, RL), chest wall (Ecw, Rcw) were also estimated from measurements of esophageal pressure. Data were collected in the closed chest, after median sternotomy with the parietal pleurae intact and after the left parietal pleura was opened for internal mammary artery harvest. After sternotomy with pleurae intact (n = 14), Ers did not change but Rrs decreased (p < 0.05). Ecw (including the contribution of the pleurae) was higher than in the closed chest (p < 0.05) while EL and RL were lower (p < 0.05); Rcw did not change. Opening the left pleura (n = 10) decreased Ers (p < 0.05), but Rrs did not change. We conclude that the chest wall/pleurae compartment offers significant impedance to lung expansion after sternotomy and rib retraction, unless one pleura is opened.

Changes in lung and chest wall properties with abdominal insufflation of carbon dioxide are immediately reversible.

Previously we have reported that large increases in lung and chest wall elastances as well as lung resistance occur with abdominal insufflation of carbon dioxide during laparoscopic surgery. To examine whether these effects were reversible with abdominal deflation, we calculated lung and chest wall elastances and resistances from measurement of airway flow and pressure and esophageal pressure in 17 anesthetized/paralyzed patients undergoing laparoscopic surgery. Measurements were made immediately prior to abdominal insufflation and after deflation. Lung and chest wall elastances and resistances were not changed from baseline (P > 0.05), although total respiratory elastance remained slightly increased compared to baseline (P < 0.05). The change in total respiratory elastance did not correlate with abdominal insufflation time, surgical site, smoking history, or physical characteristics of the patients. There were no differences in frequency and tidal volume dependences of the elastances and resistances before and after abdominal insufflation (P > 0.5). We conclude that residual changes in respiratory mechanics caused by carbon dioxide insufflation during laparoscopic surgery are minor, and that the reported compromise of respiratory function indicated by pulmonary function tests after laparoscopy does not appear to be due to changes in passive mechanical properties of the lungs or chest wall.

Automated system for detailed measurement of respiratory mechanics.

OBJECTIVE: The mechanical properties of the respiratory system (i.e., elastance and resistance) depend on the frequency, tidal volume, and shape of the flow waveform used for forcing. We developed a system to facilitate accurate measurements of elastance and resistance in laboratory and clinical settings at the frequencies and tidal volumes in the physiologic range of breathing. METHODS: A personal computer (PC) is used to drive a common clinically used ventilator while simultaneously collecting measurements of airway flow, airway pressure, and esophageal pressure from the experimental subject or animal at different frequencies and tidal volumes. Analysis analogous to discrete Fourier transform at the fundamental frequency (i.e., ventilator setting) is used to calculate elastances and resistances of the total respiratory system and its components, the lungs and the chest wall. We have shown that this analysis is independent of the high-frequency harmonics that are present in the waveform from clinical ventilators. RESULTS: The system has been used successfully to make measurements in anesthetized/paralyzed dogs and awake or anesthetized human volunteers in the laboratory, and in anesthetized human volunteers in the laboratory, and in anesthetized humans in the operating room and intensive care unit. Elastances and resistances obtained with this approach are the same as those obtained during more controlled conditions, e.g., sinusoidal forcing. CONCLUSIONS: Accurate, standardized measurements of lung and chest wall properties can be obtained in many settings with relative ease with the system described. These properties, and their frequency and tidal volume dependences in the physiologic range, provide important information to aid in the understanding of changes in respiratory function caused by day-to-day conditions, clinical intervention and pathologies.

Effect of surface forces on oscillatory behavior of lungs.

The effect of alveolar surface tension on lung dynamic behavior was investigated by measuring total lung and tissue impedances in excised rabbit lungs at breathing frequencies of 0.2-0.8 Hz and tidal volumes of 10, 20, and 30 ml before and after lavage with 3-dimethyl siloxane, which provided a constant surface tension of 16 dyn/cm. The lungs were oscillated around the mean deflation pressures of 5 (control) and 8 cmH2O (lavaged), i.e., lung volume of 60% of total lung capacity. The total lung impedance was calculated from measurements of pressure and airflow at the trachea, and tissue impedance was measured by the alveolar capsule technique. The airway contribution was obtained as the difference between total lung and tissue impedances. In the lavaged lungs, dynamic elastance (Edyn) decreased and tissue resistance (Rti) increased relative to the control values over the entire frequency range. Airway resistance increased at the higher flow rates only. The decrease in Edyn could be attributed to the absence of surface film elastance in the lavaged lungs. The increase in airway resistance could be attributed to accentuated flow dependence due to changes in airway geometry and residual lavage liquid. However, the most intriguing result was the increase in Rti in the lavaged lungs. It could be attributed to altered mechanics at the alveolar duct level after lavage. It is concluded that dissipative properties of lung tissue are major determinants of Rti, whereas elastic properties of both tissue and surface film are important determinants of Edyn.

Effects of PEEP on acinar gas transfer in healthy and lung-injured dogs.

We measured cardiorespiratory variables and 133xenon washout from a nonperfused lung region (XeW) in six anesthetized/paralyzed dogs, mechanically ventilated with 60% O2 at different positive end-expiratory pressures (PEEP). XeW in this technique represents directly measured acinar gas transfer (3). Measurements were repeated after induction of lung injury by lavaging the lungs 11 to 13 times with 600 ml saline. In control dogs, lung compliance (CL), alveolar ventilation (Valv), and XeW all decreased with increasing PEEP from 0 to 25 cm H2O (p < 0.05), while lung resistance (RL) did not change. After lavage, CL, Valv, and XeW below 15 cm H2O PEEP were all less than control values (p < 0.05), while RL was higher than control values. As PEEP increased from 0 to 20 cm H2O, Valv and XeW increased, but CL did not change; RL decreased only from 0 to 5 cm H2O. At 20 cm H2O PEEP, Valv and CL were not different from control values (p > 0.05), and XeW was higher than control values (p < 0.05). At estimated alveolar volumes above 400 ml, values for XeW before and after lavage were similar. We conclude that, during severe lung injury: (1) increasing PEEP to moderate levels will increase acinar gas transfer but, after a certain lung volume is reached, further increases in PEEP will have effects similar to the healthy condition; (2) overall mechanical properties of the lung do not reflect the responses to PEEP of the lung periphery.

The effects of increased abdominal pressure on lung and chest wall mechanics during laparoscopic surgery.

We tested the hypothesis that increases in pressure in the abdomen (Pab) exerted by CO2 insufflation during laparoscopy would increase elastance (E) and resistance (R) of both the lungs and chest wall. We measured airway flow and airway and esophageal pressures of 12 anesthetized/paralyzed tracheally intubated patients during mechanical ventilation at 10-30/min and tidal volume of 250-800 mL. From these measurements, we used discrete Fourier transformation to calculate E and R of the lungs and chest wall. Measurements were made at 0, 15, and 25 mm Hg Pab in the 15 degrees head-down (Trendelenburg) posture and at 0 and 15 mm Hg Pab in the 10 degrees head-up (reverse Trendelenburg) posture. Lung and chest wall Es and Rs while head-down increased at Pab = 15 mm Hg, and both Es increased further at Pab = 25 mm Hg (P < 0.05). Both Es and Rs also increased while head-up at Pab = 15 mm Hg (P < 0.05), but increases in lung E and R were less than while head-down (P < 0.05). The increase in lung E and R at Pab = 15 mm Hg in either posture were positively correlated to body weight or body mass index, whereas the increases in chest wall E and R were negatively correlated to the same factors (P < 0.05). Lung and chest wall mechanical impedances increase with increasing Pab; the increases depend on body configuration and are greater while head-down.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxygen transport and cardiovascular effects of resuscitation from severe hemorrhagic shock using hemoglobin solutions.

OBJECTIVE: To test the short-term efficacy of three hemoglobin solutions in restoring cardiac output, intravascular pressures, oxygen transport (DO2), and oxygen consumption (VO2) after resuscitation from severe hemorrhagic shock. DESIGN: Prospective study. SETTING: Research laboratory. SUBJECTS: Beagle dogs. INTERVENTIONS: After anesthesia and instrumentation, hemorrhagic shock was induced for 2 hrs by blood withdrawal to maintain systolic blood pressure at 50 mm Hg. Resuscitation then occurred with one of four different resuscitation fluids. One group of dogs was not resuscitated. Survival rate was monitored for 8 days. MEASUREMENTS AND MAIN RESULTS: In 33 beagle dogs, cardiovascular variables (DO2 and VO2) were compared after resuscitation with 8% stroma-free hemoglobin, 4% or 8% pyridoxalated-hemoglobin-polyoxyethylene conjugate (PHP44 and PHP88, respectively), or autologous whole blood. The dogs were anesthetized, paralyzed, mechanically ventilated (FIO2 of 0.21), and instrumented with arterial and pulmonary artery catheters. An average of 63% of estimated blood volume was removed to maintain systolic blood pressure at 50 mm Hg for 2 hrs. The dogs then were either not resuscitated (n = 4) or resuscitated with 8% stroma-free hemoglobin (n = 7), PHP44 (n = 6), PHP88 (n = 8), or whole blood (n = 8), with a volume equivalent to the withdrawn blood. Cardiovascular variables, DO2, VO2, oxygen extraction ratios, and blood concentrations of lactic acid and catecholamines were determined before, and for < or = 6 hrs after, resuscitation from hemorrhagic shock. Blood smears were microscopically examined. In addition, the survival rate was monitored for 8 days after resuscitation. By 2 hrs of hemorrhagic shock, there was a large decrease in DO2 (p < .05) and an increase in oxygen extraction ratio from 0.27 to 0.70 (p < .05). There was a 3.5-fold increase in lactate concentrations and a 25-fold increase in catecholamine concentrations as compared with preshock values. All dogs not resuscitated died within 1.75 hrs after 2 hrs of shock. After resuscitation with whole blood, all cardiovascular and oxygen transport variables returned to approximately prehemorrhage values and remained so throughout the measurement period. After resuscitation with any hemoglobin solution, DO2 returned transiently to control values. However, recovery of DO2 was short-lived in all hemoglobin solution groups, and, by 4 hrs postresuscitation in all groups, DO2 was less than the DO2 of the dogs receiving whole blood (p < .05). These changes were associated with decreases in total hemoglobin concentrations compared with the values immediately before resuscitation (p < .05). In addition, with resuscitation using the PHP solutions, blood smears demonstrated aggregation of red blood cells and platelets. On day 8 after hemorrhagic shock, the survival rate was 100% for whole blood and PHP44, 86% for 8% stroma-free hemoglobin, and 33% for PHP88. CONCLUSIONS: Resuscitation from severe hemorrhagic shock with 8% stroma-free hemoglobin, PHP44, or PHP88 is equally effective in restoring cardiac index and vascular pressures as using whole blood. However, resuscitation with the three hemoglobin solutions only transiently restored DO2 after hemorrhagic shock. The subsequent reduction of DO2 compared with the DO2 value using whole blood was due mostly to hemodilution. With the two PHP solutions, formation of red blood cell aggregates probably resulted in sequestration of red cell mass and additional loss of oxygen carrying capacity.