Gravitational and shear-associated pressure gradients in the abdomen

The abdomen has been variously characterized as a hydrostatic system, in which pressures exhibit a gravitational gradient and pressure fluctuations are spatially uniform, and as a compartment, in which pressure gradients are not simply gravitational and pressure fluctuations differ markedly from place to place. To characterize the pressures acting on the ventral abdominal wall, we used saline-filled catheters and air-filled balloons in anesthetized dogs in various body positions during spontaneous breathing and mechanical ventilation. Pressures were measured in the stomach and at multiple sites next to the abdominal wall. Under most circumstances, measurements next to the abdominal wall exhibited a hydrostatic gravitational gradient of approximately 0.89 cmH2O/cm height and pressure fluctuations were spatially homogeneous. Deviations from this hydrostatic behavior were seen when abdominal pressures were compared with gastric pressures, when measurements were made with a balloon catheter, and when the lower abdomen was constricted with a binder. Analysis of these and previously published data suggests that the abdomen does, at times, behave like a hydraulic system but can deviate from simple hydrostatic behavior to the extent that shape-stable abdominal viscera are deformed.

Influence of waveform and analysis technique on lung and chest wall properties

To test an approach for measuring respiratory system resistance (R) and elastance (E) during non-sinusoidal forcing, we measured airway and esophageal pressures and flow at the trachea of 9 anesthetized-paralyzed dogs during sinusoidal forcing (SF) and 4 types of non-sinusoidal forcings at 0.15 and 0.6 Hz and 300 ml tidal volume. During SF, calculations of E and R of the lungs, chest wall or total system from discrete Fourier transform (DFT) and two other widely used methods (multiple regression and volume-pressure loop analysis) did not differ from each other (P > 0.05). During forcing with sinusoidal or step inspiration with passive expiration (inspiratory to expiratory ratio, I/E, = 1:1), Es from any analysis method were within 10% of values during SF. Although Rs of the lungs, chest wall or total system were not affected by waveform shape with DFT (P > 0.05), the other analysis methods gave values for R during non-SF that differed (P < 0.05) from those during SF by up to 77%. If I/E was changed to 1:2, with or without an added 10% inspiratory pause, values for E and R differed least from values during SF if DFT was used. During severe pulmonary edema induced by infusion of oleic acid in the right atrium, results for lung properties were similar to controls, despite large increases in E and R of the lungs. We conclude that E and R of the lungs and chest wall can be measured by DFT using nonsinusoidal forcing waveforms available on most clinical ventilators, incurring only modest error.

Endotracheal tube and tracheobronchial obstruction as causes of hypoventilation with high inspiratory pressures

Two cases of difficult ventilation are presented, the first caused by endotracheal tube obstruction with nasal turbinate, and the second caused by tracheobronchial obstruction with blood clots. The clinical presentation in each case was characterized by extreme difficulty in ventilating and severe hypercapnia despite vigorous ventilatory efforts with either a mechanical ventilator or resuscitator bag. A simple manipulation of the endotracheal tube cuff helped to differentiate between increased impedance caused by endotracheal tube obstruction as opposed to increased respiratory system impedance beyond the tip of tube. In the second patient, in whom even a short interruption of ventilation was poorly tolerated, simultaneous rigid bronchoscopy (for removal of intratracheal masses) and ventilation via endotracheal tube were successfully performed.

Effect of pulmonary edema on transfer of gas from acinus to airways

We directly measured the effect of progressive pulmonary edema on gas transfer from the acinus by injecting 133Xe dissolved in saline through a pulmonary artery catheter into an acinar region with occluded blood flow and measuring 133Xe washout by gamma scintillation scanning. We measured washout in six anesthetized paralyzed dogs during mechanical ventilation with O2 before and after injection of 0.6 mg/kg of oleic acid into the right atrium, which induces severe pulmonary edema within 2 h. Changes in the elastance and resistance of the lung were also calculated from measurements of airway flow, airway pressure, and esophageal pressure. Before injection of oleic acid, the monoexponential rate constant for 133Xe washout was 3.6 +/- 1.4 (SE) min-1; from this we estimated that the rate of gas transfer of 133Xe from the acini was 1.0 l/min. The rate constant decreased gradually after the injection and was correlated with increases in elastance and resistance (r = -0.66) and decreases in alveolar PO2 (r = 0.71). At 2 h after injection, the rate constant (1.2 +/- 0.8 min-1) was lower than control (P < 0.01), and the rate of gas transfer of 133Xe from the acini was < 0.32 l/min. We conclude that resistance in the acini is increased during pulmonary edema and that it is correlated, in the oleic acid model, with changes in overall lung mechanical properties.

Transfer of gas from the acinus during continuous flow and intermittent positive pressure ventilation

We used a technique of measuring Xenon133 washout (XeW) from the alveolar space to evaluate transfer of gas from the acinus (Mackenzie et al., J. Appl. Physiol. 68: 2013-2018, 1990) during 2 min of apnea, 2 min of tracheal insufflation with oxygen (TRIO) and 90 sec of intermittent positive pressure ventilation (IPPV) in 6 anesthetized and paralyzed dogs. Xenon133 dissolved in saline was injected into an occluded acinar region through a pulmonary artery catheter, and XeW was measured by gamma scintillation scanning. With this technique, XeW during apnea represents the contribution of cardiogenic oscillations in regional flow. The XeW rate constant (min-1 +/- SE) was 0.37 +/- 0.03 during apnea. This was not different (P > 0.05) with TRIO (0.29 +/- 0.04). With IPPV, the rate constant increased to 3.49 +/- 0.39, faster than with either apnea or TRIO (P < 0.001). We conclude that: (1) TRIO does not increase convective gas transfer from the acini compared to apnea; and (2) transfer of gas out of the acini due to cardiogenic oscillations is a very small portion of the total gas eliminated during IPPV.

Lung and chest wall mechanical properties before and after cardiac surgery with cardiopulmonary bypass

From measurements of airway and esophageal pressures and flow, we calculated the elastance and resistance of the total respiratory system (Ers and Rrs), chest wall (Ecw and Rcw), and lungs (EL and RL) in 11 anesthetized-paralyzed patients immediately before cardiac surgery with cardiopulmonary bypass and immediately after chest closure at the end of surgery. Measurements were made during mechanical ventilation in the frequency and tidal volume ranges of normal breathing. Before surgery, frequency and tidal volume dependences of the elastances and resistances were similar to those previously measured in awake seated subjects (Am. Rev. Respir. Dis. 145: 110-113, 1992). After surgery, Ers and Rrs increased as a result of increases in EL and RL (P < 0.05), whereas Ecw and Rcw did not change (P > 0.05). EL and RL exhibited nonlinearities (i.e., decreases with increasing tidal volume) that were not seen before surgery, and RL showed a greater dependence on frequency than before surgery. The changes in RL or EL after surgery were not correlated with the duration of surgery or cardiopulmonary bypass time (P > 0.05). We conclude that 1) frequency and tidal volume dependences of respiratory system properties are not affected by anesthesia, paralysis, and the supine posture, 2) open-chest surgery with cardiopulmonary bypass does not affect the mechanical properties of the chest, and 3) cardiac surgery involving cardiopulmonary bypass causes changes in the mechanical behavior of the lung that are generally consistent with those caused by pulmonary edema induced by oleic acid (J. Appl. Physiol. 73: 1040-1046, 1992) and decreases in lung volume.

Alternative model of respiratory tissue viscoplasticity

Respiratory tissue impedance exhibits both tidal volume and frequency dependences in the ranges of normal breathing. Hildebrandt argued that these indicate tissue viscoplasticity and offered a model in support of his argument consisting of viscoelastic and plastoelastic compartments, both mechanically in parallel (J. Appl. Physiol. 28: 365-372, 1970). Although the model appears to be qualitatively consistent with oscillatory behavior of a wide variety of respiratory tissues, it yields only moderately good quantitative correspondences despite a relatively large number of parameters, eight. One reason may be the model topology, which implies that rate-dependent and amplitude-dependent processes are decoupled. This is contrary to observed behavior. In this study we offer a model in which viscoelastic and plastoelastic compartments are mechanically coupled through a serial arrangement. The total number of parameters in the model is four. Using a least squares technique, we fitted this model to impedance data of chest wall, healthy lungs, and edematous lungs, all measured in vivo. We found that the model could account for the major, as well as the more subtle, features of the chest wall data with fewer parameters and fewer ad hoc assumptions than Hildebrandt's model. Although it lacks anatomic specifics, the model suggests that the observed chest wall behavior may stem from the actin-myosin cross-bridge kinetics. It also seems applicable to lung tissue, although the requirements for the plastoelastic compartment are less certain. In the case of edematous lungs, the applicability of the model is difficult to establish.

Effect of lung volume on lung resistance and elastance in awake subjects measured during sinusoidal forcing

BACKGROUND

Although lung volume may be changed by certain procedures during anesthesia and mechanical ventilation, dependence of the dynamic mechanical properties of the lungs on lung volume are not clear. Based on studies in dogs, the authors hypothesized that changes in lung mechanics caused by anesthesia in healthy humans could be accounted for by immediate changes in lung volume and that lung resistance will not be decreased by positive end-expiratory airway pressure if tidal volume and respiratory frequency are in the normal ranges.

METHODS

Lung resistance and dynamic lung elastance were measured in six healthy, relaxed, seated subjects during sinusoidal volume oscillations at the mouth (5 mL/kg; 0.4 Hz) delivered at mean airway pressure from -9 to +25 cmH2O. Changes in lung volume from functional residual capacity were measured with inductance plethysmographic belts.

RESULTS

Decreases in mean mean airway pressure that caused decreases in lung volume from functional residual capacity comparable to those typically observed during anesthesia were associated with significant increases in both dynamic lung elastance and lung resistance. Increases in mean mean airway pressure that caused increases in lung volume from functional residual capacity did not increase lung resistance and increased dynamic lung elastance only above about 15 cmH2O.

CONCLUSIONS

Increases in dynamic lung elastance and lung resistance with anesthesia can be explained by the accompanying, acute decreases in lung volume, although other factors may be involved. Increasing lung volume by increasing mean airway pressure with positive end-expiratory pressure will decrease lung resistance only if the original lung volume is low compared to awake, seated functional residual capacity.

Effects of mean airway pressure and tidal volume on lung and chest wall mechanics in the dog

Dependencies of the dynamic mechanical properties of the respiratory system on mean airway pressure (Paw) and the effects of tidal volume (VT) are not completely clear. We measured resistance and dynamic elastance of the total respiratory system (Rrs and Ers), lungs (RL and EL), and chest wall (Rcw and Ecw) in six healthy anesthetized paralyzed dogs during sinusoidal volume oscillations at the trachea (50-300 ml; 0.4 Hz) delivered at mean Paw from -9 to +23 cmH2O. Changes in end-expiratory lung volume, estimated with inductance plethysmographic belts, showed a typical sigmoidal relationship to mean Paw. Each dog showed the same dependencies of mechanical properties on mean Paw and VT. All elastances and resistances were minimal between 5 and 10 cmH2O mean Paw. All elastances, Rrs, and RL increased greatly with decreasing Paw below 5 cmH2O. Ers and EL increased above 10 cmH2O. Ecw, Ers, Rcw, and Rrs decreased slightly with increasing VT, but RL and EL were independent of VT. We conclude that 1) respiratory system impedance is minimal at the normal mean lung volume of supine anesthetized paralyzed dogs; 2) the dependency of RL on lung volume above functional residual capacity is dependent on VT and respiratory frequency; and 3) chest wall, but not lung, mechanical behavior is nonlinear (i.e., VT dependent) at any given lung volume.

Effect of posture on lung and regional chest wall mechanics

BACKGROUND

Little is known about the extent to which changes in postures in clinical situations affect respiratory mechanics, even in humans with healthy respiratory systems. This study tested the hypothesis that posture has only small effects on overall respiratory system mechanics in healthy subjects, despite changes in parts of the respiratory system in some postures.

METHODS

Measurements were made of airway flow, airway and esophageal pressures, and rib cage and abdominal volume displacements (with inductance plethysmography) of awake, healthy subjects, relaxed at functional residual capacity, during external forcing at 0.2 Hz with a tidal volume of 8-10 ml/kg. From these measurements, discrete Fourier transform was used to calculate elastances (E) and resistances (R) of the total respiratory system, lungs, total chest wall, and compartments of the chest wall (rib cage, diaphragm-abdomen, and belly wall). Measurements were made while the subjects were in nine different postures: in six of these, the torso was straight; in three, the torso was bent or twisted.

RESULTS

Although changes in mechanics of parts of the respiratory system were evident in certain postures, overall respiratory mechanics were not greatly affected by posture. Changing from sitting to supine decreased E and R of the diaphragm-abdomen about 50% (P < .05), but total chest wall E and R changed only slightly. Lung E increased 24% (P < .05), but total respiratory E did not change (P < .05). Lung and total respiratory R increased 40-50% (P < .05) with this same change in posture. As long as the torso was straight, however, changes in orientation of 30 degrees from the horizontal or a shift to lateral posture resulted in only minor changes in the variables measured. Postures in which the torso was twisted or bent increased E of the total chest wall 20-30% compared to supine (P < .05), due to increases in E of one or more compartments. Respiratory system E also increased, at most 14%. Although lung R decreased 30-45% (P < .05) in these postures compared to supine with a straight torso, chest wall and total respiratory R generally were unchanged.

CONCLUSIONS

Changes in respiratory system mechanics over a wide range of postures that may be encountered clinically are relatively small in healthy awake subjects due to adaptability of total chest wall mechanical behavior.

Changes in functional residual capacity and regional diaphragm lengths after upper abdominal surgery in anesthetized dogs

The respiratory performance of the diaphragm may be altered by changes in mechanical or neural factors, or both, induced by upper abdominal surgery. We conducted this study to examine the effects of upper abdominal surgery on postoperative respiratory function. We studied resting lengths of four diaphragm regions, three in the costal and one in the crural diaphragm, with biplane video-roentgenography in six dogs immediately after upper abdominal surgery and up to 30 days postoperatively. Functional residual capacity was 16.7% smaller immediately after surgery compared with values obtained in the same animals after 30 days. Simultaneously measured resting lengths of each of the diaphragm regions immediately after surgery were longer, on average by 8.3%, than 30 days postoperatively. During the postoperative course, resting diaphragm lengths gradually and uniformly decreased as functional residual capacity increased. Phrenic nerve stimulation in four other dogs immediately after identical surgery resulted in large diaphragm shortening (from 42% to 55%), indicating that neither the diaphragm nor phrenic nerves were injured by the surgical manipulation. We hypothesize that respiratory dysfunction after upper abdominal surgery may be, at least in part, attributed to a decreased central drive for breathing caused by activation of the afferent limb of an inhibitory reflex owing to stretching of the diaphragm.

Effects of cardiac oscillations on acinar gas mixing during pulmonary edema

We used a previously reported technique (Mackenzie et al., J. Appl. Physiol. 68: 2013-2018, 1990) to measure the effects of severe pulmonary edema on acinar cardiogenic gas mixing in anesthetized dogs. We also tested how increases in lung volume affected gas mixing in healthy lungs and during pulmonary edema. Cardiogenic gas mixing was evaluated by measurement of the rate of washout of xenon133 injected into an occluded pulmonary artery during apnea. The rate constant of xenon133 washout was 0.40 min-1 (+/- 0.06 SE) in the healthy lung at functional residual capacity. It decreased (P < 0.05) to 0.08 min-1 (+/- 0.03) when lung volume was raised 500 ml. Pulmonary edema was induced by injection of oleic acid (0.06 mg.kg-1) into the right atrium over a 4-min period; clinical signs of severe pulmonary edema were present after 90 min. The rate constant for xenon133 washout (0.07 +/- 0.03 min-1) was less than in the healthy lung (P < 0.05), and was not changed after lung volume was increased (P > 0.05). We conclude that, in the presence of severe pulmonary edema: (1) acinar resistance is increased and/or magnitude of cardiogenic oscillations is decreased; and (2) salutary effects of increased lung volume are not due to enhancement of cardiogenic gas mixing.

Effects of analysis method and forcing waveform on measurement of respiratory mechanics

The respiratory system has been shown to exhibit nonlinear mechanical properties in the frequency (f) range of normal breathing, manifested by tidal volume (Vt) dependence. Calculations of respiratory system resistance (R) and elastance (E) from pressure-flow measurements during external forcing at a given f may be ambiguous, especially if non-sinusoidal forcing waveforms are used. We evaluated the degree to which R and E depended upon: (1) analysis method (Fourier transform, multiple regression and pressure-volume loop analysis) and; (2) shape of the forcing waveform (sinusoidal, quasi-sinusoidal and step). We measured pressure and flow at the mouth of 5 healthy, awake subjects, relaxed at functional residual capacity, during forcing with the three different waveforms in the normal range of f (0.2-0.6 Hz) and Vt (250-750 ml). During sinusoidal forcing, E and R were not affected by analysis method (P greater than 0.2). With Fourier transform and multiple regression, E was not affected by waveform shape (P greater than 0.05); with loop analysis, E was slightly (less than 10%) higher during quasi-sinusoidal and step forcing than during the sine (P less than 0.05). R was least affected by waveform shape with Fourier transform. We conclude that, in the f and Vt range of normal breathing: (1) respiratory system impedance is 'quasi-linear,' i.e. despite dependencies of R and E on Vt, non-linearities are not large enough to restrict interpretation of R and E at a given f and Vt; (2) it may be possible to measure R and E using non-sinusoidal forcing waveforms available on most clinical ventilators, incurring only modest error.

Lung and chest wall impedances in the dog in normal range of breathing: effects of pulmonary edema

We evaluated the effect of pulmonary edema on the frequency (f) and tidal volume (VT) dependences of respiratory system mechanical properties in the normal ranges of breathing. We measured resistance and elastance of the lungs (RL and EL) and chest wall of four anesthetized-paralyzed dogs during sinusoidal volume oscillations at the trachea (50-300 ml, 0.2-2 Hz), delivered at a constant mean airway pressure. Measurements were made before and after severe pulmonary edema was produced by injection of 0.06 ml/kg oleic acid into the right atrium. Chest wall properties were not changed by the injection. Before oleic acid, EL increased slightly with increasing f in each dog but was independent of VT. RL decreased slightly and was independent of VT from 0.2 to 0.4 Hz, but above 0.4 Hz it tended to increase with increasing flow, presumably due to the airway contribution. After oleic acid injection, EL and RL increased greatly. Large negative dependences of EL on VT and of RL on f were also evident, so that EL and RL after oleic acid changed two- and fivefold, respectively, within the ranges of f and VT studied. We conclude that severe pulmonary edema changes lung properties so as to make behavior VT dependent (i.e., nonlinear) and very frequency dependent in the normal range of breathing.

Continuous endobronchial insufflation during internal mammary artery harvest

Endobronchial insufflation of oxygen offers possible advantages over conventional ventilation modes in some clinical situations in which nonmovement of the chest may be desirable; however, endobronchial insufflation of oxygen has yet to be used during thoracic surgery in humans. Furthermore, the physiologic mechanisms underlying gas exchange during endobronchial insufflation of oxygen are unclear. This study assessed endobronchial insufflation of oxygen at 45 L/min in 11 patients with an open chest during internal mammary artery harvest. Cardiorespiratory function was measured at baseline during conventional mechanical ventilation and at 5-min intervals during the study period of 20-30 min. In all patients, clinically acceptable gas exchange was achieved, although PaCO2 increased from 32 +/- 3.2 to 44 +/- 7.5 mm Hg (mean +/- SD) at 5 min, but thereafter was unchanged (P greater than 0.1). Cardiac output, vascular pressures, and heart rate were unchanged, although pHa decreased. Surgical access for internal mammary artery harvesting was improved. No mucosal damage or complications occurred. During endobronchial insufflation of oxygen, efficacy of gas exchange and body weight were not correlated, but both subject height and age were correlated with high PaO2 and low PaCO2. We conclude that (a) endobronchial insufflation of oxygen can be used in patients with an open chest; (b) the efficacy of endobronchial insufflation of oxygen is probably improved by increased lung size and by collateral ventilation; and (c) cardiogenic gas mixing contributes little to gas exchange during endobronchial insufflation of oxygen.

Effect of tidal volume on respiratory system elastance and resistance during anesthesia and paralysis

Recent studies have shown that the mechanical properties of the respiratory system at normal breathing frequency in awake humans depend on tidal volume. Few measurements of respiratory system properties during anesthesia have accounted for this dependence. From measurements of airway pressure, flow and esophageal pressure, we calculated elastances and resistances of the total respiratory system (Ers and Rrs), chest wall (Ecw and Rcw), and lungs (El and Rl) in supine human volunteers during quasisinusoidal volume forcing in a normal range of breathing (250 to 800 ml) at normal breathing frequency (0.2 Hz). Measurements were made (1) with subjects awake and voluntarily relaxed; (2) after isoflurane-N2O anesthesia (end-tidal isoflurane concentration 0.3 to 0.5%); and (3) after complete muscle paralysis with vecuronium. In all conditions, Ers, Ecw, El, Rrs, and Rcw decreased at 800 ml tidal volume compared with 250 ml; Rl showed a similar decrease in awake measurements only. Compared with awake measurements, each elastance tended to increase after anesthesia, but only the increase in Ers was significant. Compared with anesthesia, there was no effect of paralysis on any measurement. We conclude that (1) tidal volume dependence of respiratory system properties in the normal range of breathing occurs in the absence of muscle activity; (2) anesthesia increases Ers and (3) respiratory muscle activity appears to be inhibited by isoflurane-N2O anesthesia at end-tidal isoflurane concentration of 0.3 to 0.5% during normocapnia.

Lung, chest wall, and total respiratory system resistances and elastances in the normal range of breathing

We measured total respiratory system and lung and chest wall resistances (Rrs, Rl, and Rcw) and elastances (Ers, El, and Ecw) in awake, relaxed human subjects during sinusoidal volume forcing at the mouth from 0.2 to 0.6 Hz with tidal volumes (VT) of 6 to 18% VC at constant mean airway pressure. In addition, we repeated measurements with the lowest VT at a lower airway pressure and therefore at a lower mean lung volume (Vl). Rrs and Rcw decreased with increasing respiratory frequency (f) and VT, but Rl was independent of f and VT. All resistances were higher at the lower Vl. Ers and Ecw increased with increasing f and decreased with increasing VT. El increased slightly with increasing f but was not affected by VT. All elastances tended to increase at the lower Vl. We conclude that in the normal range of breathing amplitude and frequency, (1) lung properties are nearly constant if mean lung volume does not change, and (2) f and VT dependencies of total respiratory system properties are caused by the chest wall.

Lung and chest wall impedances in the dog: effects of frequency and tidal volume

Dependences of the mechanical properties of the respiratory system on frequency (f) and tidal volume (VT) in the normal ranges of breathing are not clear. We measured, simultaneously and in vivo, resistance and elastance of the total respiratory system (Rrs and Ers), lungs (RL and EL), and chest wall (Rcw and Ecw) of five healthy anesthetized paralyzed dogs during sinusoidal volume oscillations at the trachea (50-300 ml, 0.2-2 Hz) delivered at a constant mean lung volume. Each dog showed the same f and VT dependences. The Ers and Ecw increased with increasing f to 1 Hz and decreased with increasing VT up to 200 ml. Although EL increased slightly with increasing f, it was independent of VT. The Rcw decreased from 0.2 to 2 Hz at all VT and decreased with increasing VT. Although the RL decreased from 0.2 to 0.6 Hz and was independent of VT, at higher f RL tended to increase with increasing f and VT (i.e., as peak flow increased). Finally, the f and VT dependences of Rrs were similar to those of Rcw below 0.6 Hz but mirrored RL at higher f. These data capture the competing influences of airflow nonlinearities vs. tissue nonlinearities on f and VT dependence of the lung, chest wall, and total respiratory system. More specifically, we conclude that 1) VT dependences in Ers and Rrs below 0.6 Hz are due to nonlinearities in chest wall properties, 2) above 0.6 Hz, the flow dependence of airways resistance dominates RL and Rrs, and 3) lung tissue behavior is linear in the normal range of breathing.

Efficacy of tracheal insufflation of oxygen during oleic acid-induced pulmonary edema

STUDY OBJECTIVES

To determine whether tracheal insufflation of oxygen (TRIO) might be useful in field resuscitation of casualties with lung dysfunction.

DESIGN

Physiological measurements of cardiac and respiratory function were compared before and after oleic acid lung injury.

SETTING AND PARTICIPANTS

Beagles were studied in a laboratory.

INTERVENTIONS

Oleic acid (0.06 mL/kg) was injected over four minutes into the central venous port of a pulmonary artery catheter. Measurements were made during 30 minutes of TRIO before and after acute lung injury.

MEASUREMENTS

Hemodynamic and respiratory measurements, including intravascular pressures, heart rate, cardiac output, blood gases, respiratory system compliance, and O2 consumption were recorded during conventional mechanical ventilation and TRIO.

RESULTS

Before acute lung injury, PaO2 (mean +/- SD) increased (P less than .05) from 96 +/- 7.4 (13 +/- 1.0 kPa) during conventional mechanical ventilation to 360 +/- 123 mm Hg (48 +/- 16.4 kPa) after TRIO. PaCO2 (mean +/- SD) increased (P less than .05) from 39.5 +/- 1.1 (5.3 +/- 0.1 kPa) to 102 +/- 27.4 mm Hg (13.6 +/- 3.6 kPa). Arterial and mixed venous pH values decreased in proportion to PCO2. After acute lung injury, compliance decreased. PAO2 decreased (P less than .05) to 58 +/- 8.4 mm Hg (7.7 +/- 1.1 kPa) during conventional mechanical ventilation and increased (P less than .05) to 84 +/- 19.6 mm Hg (11.2 +/- 2.6 kPa) after 30 minutes of TRIO.

CONCLUSION

Despite poor gas exchange after acute lung injury, TRIO maintained adequate oxygenation and may be useful for emergency ventilation even when pulmonary edema complicates resuscitation.

Manual resuscitators and spontaneous ventilation--an evaluation

BACKGROUND AND METHODS

Although it is useful in certain clinical situations for manual resuscitator units to be used with spontaneously ventilating patients, there are few data regarding their performance in these settings. We measured the percent-delivered oxygen from 13 adult manual resuscitator units during simulated spontaneous ventilation in the range of respiratory frequency, tidal volume, and oxygen supply in which manual resuscitator units might be used with patients. We also measured the resistive pressure developed during simulated ventilation and at constant inspiratory flow of 50 L/min.

RESULTS

Oxygen supply, tidal volume, minute ventilation, and reservoir volume all influenced percent-delivered oxygen, but the most important determinant of percent-delivered oxygen was valve design. Valves incorporating a "disc" element to prevent air entrainment from the expiratory port gave the most efficient oxygen delivery, while "duck-bill" valves did not reliably prevent air entrainment. Only two of the manual resuscitator units tested developed high resistive pressure.

CONCLUSION

Reliable administration of high percent-delivered oxygen to spontaneously ventilating patients, while retaining the capability to manually ventilate them, is best achieved by a manual resuscitator unit with a valve of low resistance, incorporating a disc to prevent air entrainment. We recommend that manufacturers indicate on the product information sheet the degree (and confidence limits) to which their manual resuscitator unit presents resistance and delivers oxygen to a spontaneously ventilating subject.

Respiratory system mechanical behavior in the chicken

We evaluated whether the avian respiratory system displays the same fundamental mechanical behavior during external forcing as found in mammals. We measured airway flow and pressures in the trachea, air sacs and thoracoabdominal cavity in 4 anesthetized-paralyzed roosters during sinusoidal volume oscillations at the trachea in the normal range of euthermic breathing frequency, f(0.2 to 1.0 Hz), and tidal volume, VT (10-50 ml). From the pressure and flow waveforms, we calculated resistance (R) and elastance (E) of the total respiratory system and its major compartments (lungs, air sacs and chest wall). E of the chest wall was minimum (147 cmH2O.L-1 +/- 7 SE) at 0.2 Hz-50 ml and was consistently, slightly lower than E of the total respiratory system over the entire range studied. Both elastances showed the same dependence on f and VT, increasing slightly with increasing f and decreasing with increasing VT. R of the chest wall was maximum (35.6 cmH2O.L- 1.sec-1 +/- 2.2 SE) at 0.2 Hz-10 ml and decreased with increasing f and VT, although the VT effect diminished at the higher f. E and R of the air sacs were much smaller than those of the chest wall, but showed similar f and VT dependencies. R of the lungs, due to resistance of the airways, was minimum (6.8 cmH2O.L-1.sec-1 +/- 1.5 SE) at 0.2 Hz-10 ml and increased with both f and VT. Total respiratory R reflected R of the air sacs and chest wall at low f and R of the lungs at high f. The f and VT dependencies of E and R in the chicken were strikingly similar to those measured in various types of mammalian respiratory tissues (Stamenović et al. (1990) J. Appl. Physiol. 69: 973-988. We conclude that, despite important anatomical differences between species, avian and mammalian respiratory tissues exhibit fundamentally similar mechanical behavior.

Automated real-time data acquisition and analysis of cardiorespiratory function

Microcomputer generation of an automated record without complexity or operator intervention is desirable in many circumstances. We developed a microcomputer system specifically designed for simplified automated collection of cardiorespiratory data in research and clinical environments. We tested the system during possible extreme clinical conditions by comparison with a patient simulator. Ranges used were heart rate of 35-182 beats per minute, systemic blood pressures of 65-147 mmHg and venous blood pressures of 14-37 mmHg, all with superimposed respiratory variation of 0-24 mmHg. We also tested multiple electrocardiographic dysrhythmias. The results showed that there were no clinically relevant differences in vascular pressures, heart rate, and other variables between computer processed and simulator values. Manually and computer recorded physiological variables were compared to simulator values and the results show that computer values were more accurate. The system was used routinely in 21 animal research experiments over a 4 month period employing a total of 270 collection periods. The file system integrity was tested and found to be satisfactory, even during power failures. Unlike other data collection systems this one (1) requires little or no operator intervention and training, (2) has been rigorously tested for accuracy using a wide variety of extreme patient conditions, (3) has had computer derived values measured against a standardized reference, (4) is reliable against external sources of computer failure, and (5) has screen and printout presentations with quick and easily understandable formats.

Mechanical coupling of the rib cage, abdomen, and diaphragm through their area of apposition

Although volumetric displacements of the chest wall are often analyzed in terms of two independent parallel pathways (rib cage and abdomen), Loring and Mead have argued that these pathways are not mechanically independent (J. Appl. Physiol. 53: 756-760, 1982). Because of its apposition with the diaphragm, the rib cage is exposed to two distinct pressure differences, one of which depends on abdominal pressure. Using the analysis of Loring and Mead as a point of departure, we developed a complementary analysis in which mechanical coupling of the rib cage, abdomen, and diaphragm is modeled by a linear translational transformer. This model has the advantage that it possesses a precise electrical analogue. Pressure differences and compartmental displacements are related by the transformation ratio (n), which is the mechanical advantage of abdominal over pleural pressure changes in displacing the rib cage. In the limiting case of very high lung volume, n----0 and the pathways uncouple. In the limit of very small lung volume, n----infinity and the pathways remain coupled; both rib cage and abdomen are driven by abdominal pressure alone, in accord with the Goldman-Mead hypothesis. A good fit was obtained between the model and the previously reported data for the human chest wall from 0.5 to 4 Hz (J. Appl. Physiol. 66:350-359, 1989). The model was then used to estimate rib cage, diaphragm, and abdominal elastance, resistance, and inertance. The abdomen was a high-elastance high-inertance highly damped compartment, and the rib cage a low-elastance low-inertance more lightly damped compartment. Our estimate that n = 1.9 is consistent with the findings of Loring and Mead and suggests substantial pathway coupling.