George M. Barnas

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 increases 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. These changes should be considered in patients where increases in impedance may be critical, such as in obese patients and those with pulmonary disease. (Anesth Analg 1995;81:744-50)

Assessment of time-domain analyses for estimation of low-frequency respiratory mechanical properties and impedance spectra

Time-domain estimation has been invoked for tracking of respiratory mechanical properties using primarily a simple single-compartment model containing a series resistance (Rrs) and elastance (Ers). However, owing to the viscoelastic properties of respiratory tissues,Rrs andErs exhibit frequency dependence below 2 Hz. The goal of this study was to investigate the bias and statistical accuracy of various time-domain approaches with respect to model properties, as well as the estimated impedance spectra. Particular emphasis was placed on establishing the tracking capability using a standard step ventilation. A simulation study compared continuous-timeversus discrete-time approaches for both the single-compartment and two-compartment models. Data were acquired in four healthy humans and two dogs before and after induced severe pulmonary edema while applying sinusoidal and standard ventilator forcing.Rrs andErs were estimated either by the standard Fast Fourier Transform (FFT) approach or by a time-domain least square estimation. Results show that the continuous-time model form produced the least bias and smallest parameter uncertainty for a single-compartment analysis and is quite amenable for reliable on-line tracking. The discrete-time approach exhibits large uncertainty and bias, particularly with increasing noise in the flow data. In humans, the time-domain approach produced smooth estimates ofRrs andErs spectra, but they were statistically unreliable at the lower frequencies. In dogs, both the FFT and time-domain analysis produced reliable and stable estimates forRrs orErs spectra for frequencies out to 2 Hz in all conditions. Nevertheless, obtaining stable on-line parameter estimates for the two-compartment viscoelastic models remained difficult. We conclude that time-domain analysis of respiratory mechanics should invoke a continuous-time model form.

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° 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 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. (Anesth Analg 1996;82:501-5)

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.

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.

Manual resuscitators and spontaneous ventilation--an evaluation.

Background and MethodsAlthough 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. ResultsOxygen 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. ConclusionReliable 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.

Tracheal insufflation of oxygen at low flow: capabilities and limitations.

Tracheal insufflation of oxygen (TRIO) may provide temporary oxygenation for patients or sustain life in apneic mass casualties when conventional ventilatory techniques are not available or feasible. Logistically, minimum flows of TRIO (&OV0312;min) are desirable for field use and to reduce barotrauma should airway obstruction occur. We carried out a feasibility study to determine the efficacy of &OV0312;min of TRIO delivered within 1 cm of the carina, in nine anesthetized and paralyzed dogs. Minimum flows of TRIO for these dogs of average weight (12 kg) was 91 mL/min. In six of the dogs &OV0312;min TRIO was continued and provided oxygenation for an average of 1.5 h compatible with subsequent resuscitation with conventional ventilation. However, Paco2 levels increased to mean values of 256 mm Hg in the 90 min. To determine what the effect of increased gas mixing was on gas exchange, we repeated &OV0312;min TRIO for 10 min in six of the dogs with and without high frequency oscillations superimposed on the TRIO flow. The oscillations (60 mL at 16.3 Hz) increased carbon dioxide excretion but significantly impaired oxygenation. In completely apneic animals, TRIO at low flow delivered by cricothyroidotomy may be useful as an emergency procedure when upper airway obstruction limits the use of other airway management techniques. However, enhancement of gas mixing during low-flow TRIO impairs oxygenation, so that higher flows would be required when respiratory efforts occur.

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.

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.