Contributions of diffusion jet flow and cardiac activity to regional ventilation in CFV.

The magnitude and regional distribution of local gas transport during constant-flow ventilation (CFV) were quantified by imaging the washout of nitrogen 13 (13NN) from anesthetized and paralyzed mongrel dogs with positron emission tomography. Equal jet flows, through two 2-mm-ID bronchial catheters 1 cm distal to the carina, were adjusted to provide eucapnic CFV (total flow = 57.6 ml.s-1.kg-1). Basal, midheart, and apical transverse sections were studied in supine and prone anesthetized dogs. The ventilation per unit volume (sV) of selected areas was computed from local 13NN concentration vs. time curves during washout. To separate the regional contributions of CFV and cardiogenic oscillation to enhanced molecular diffusion, additional supine dogs were also studied during unilateral CFV. In this protocol the CFV jet flow was delivered to a single lung while the contralateral lung was left apneic. For each lung, washout data were obtained under CFV and apnea both living and postmortem animals. The local contributions of diffusion, CFV jet effects, and cardiac activity to gas transport were evaluated and tested for additive and multiplicative synergistic interactions. The regional distribution of gas transport during CFV was found to be highly nonuniform and characterized by higher ventilation to regions located close to the main bronchi and those located in the direction in which the CFV jet pointed. No major differences were observed between supine and prone positions. This regional pattern of ventilation distribution was found to be the result of complementary and nearly multiplicative interaction between the regional effects of the CFV jet, concentrated in the central airways, and the preferential cardiogenic gas transport enhancement in ventral regions close to the heart. The data were also analyzed with a model that divides the regional diffusive gas transport resistance into a central component, affected by the CFV jet, and a peripheral component, affected only by cardiac activity. This analysis showed substantial regional heterogeneities in the effects of the different gas transport mechanisms, which are consistent with the geometry of the bronchial tree and the location of the heart in the dog. The results indicate that regional nonuniformities must be considered when modeling gas transport in CFV.     

Alveolar pressure inhomogeneity and gas exchange during constant-flow ventilation in dogs

Analysis of momentum transfer between inflow jets and resident gas during constant-flow ventilation (CFV) predicts inhomogeneity of alveolar pressures (PA) and volume, which might account for specific ventilation-variance in the lung. Using alveolar needles to measure pressures (PA) during CFV in eight anesthetized dogs with wide thoracotomy, we observed random dispersion of PA among lobes of up to 12.5 cmH2O. Within each lobe, the PA dispersion was up to 10 cmH2O at CFV of 90 l/min; when flow decreased, PA at all sites decreased, as did the intralobar dispersion. These pressure differences were not observed during conventional mechanical ventilation (CMV). During CFV with room air, dogs were hypoxemic [arterial PO2 (Pao2) 54 +/- 15 Torr] and the venous admixture (Qva/QT) was 50 +/- 15%. When inspiratory O2 fraction was increased to 0.4, Pao2 increased to 172 +/- 35 Torr and Qva/QT dropped to 13.5 +/- 8.4%, confirming considerable ventilation-perfusion (VA/Q) variance not observed during CMV. We conclude that momentum transfer between the inflow stream and resident gas caused inhomogeneities of alveolar pressures, volumes, and ventilation responsible for VA/Q variance and hypoxemia during CFV. Conceivably, the abnormal ventilation distribution is minimized by collateral ventilation and forces of interdependence between regions of high and low alveolar pressures. Momentum transfer also predicted the mucosal damage observed on histological evaluation of the bronchial walls near the site of inflow jet impact.     

Constant-flow ventilation in pigs

Constant-flow ventilation (CFV) is a ventilatory technique in which physiological blood gases can be maintained in dogs by a constant flow of fresh gas introduced via two catheters placed in the main-stem bronchi (J. Appl. Physiol. 53: 483-489, 1982). High-velocity gas exiting from the catheters can create uneven pressure differences in adjacent lung segments, and these pressure differences could lead to gas flow through collateral channels. To examine this hypothesis, we studied CFV in pigs, animals known to have a high resistance to collateral ventilation. In three pigs we examined steady-state gas exchange, and in six others we studied unsteady gas exchange at three flow rates (20, 35, and 50 l/min) and three catheter positions (0.5, 1.5, and 2.5 cm distal to the tracheal carina). During steady-state runs we were unable to attain normocapnia; the arterial CO2 partial pressure (PaCO2) was approximately 300 Torr at all flow rates and all catheter positions, compared with 20-50 Torr at similar flows and positions in dogs studied previously. The initial unsteady gas-exchange experiments indicated no consistent effect of catheter position or flow rate on the rate of rise of PaCO2. In three other pigs, the rates of rise of PaCO2 were compared with the rates observed with apneic oxygenation (AO). At the maximum flow and deepest position, the rate of rise of PaCO2 was lower during CFV than during AO. These data suggest that flow through collateral channels might be important in producing adequate gas transport during CFV; however, other factors such as airway morphometry and the effects of cardiogenic oscillations may explain the differences between the results in pigs and dogs.     

Ventilation-perfusion inequality during constant-flow ventilation

Previous work by Lehnert et al. (J. Appl. Physiol. 53:483-489, 1982) has demonstrated that adequate alveolar ventilation can be maintained during apnea in anesthetized dogs by delivering a continuous stream of inspired ventilation through cannulas aimed down the main-stem bronchi. Because an asymmetric distribution of ventilation might introduce ventilation-perfusion (VA/Q) inequality, we compared gas exchange efficiency in nine anesthetized and paralyzed dogs during constant-flow ventilation (CFV) and conventional ventilation (intermittent positive-pressure ventilation, IPPV). Gas exchange was assessed using the multiple inert gas elimination technique. During CFV at 3 l X kg-1 X min-1, lung volume, retention-excretion differences (R-E*) for low- and medium-solubility gases, and the log standard deviation of blood flow (log SD Q) increased, compared with the findings during IPPV. Reducing CFV flow rate to 1 l X kg-1 X min-1 at constant lung volume improved R-E* and log SD Q, but significant VA/Q inequality compared with that at IPPV remained and arterial PCO2 rose. Comparison of IPPV and CFV at the same mean lung volume showed a similar reversible deterioration in gas exchange efficiency during CFV. We conclude that CFV causes significant VA/Q inequality which may be due to nonuniform ventilation distribution and a redistribution of pulmonary blood flow.     

Gas density dependence of regional VA/V and VA/Q inequality during constant-flow ventilation

Constant-flow ventilation (CFV) is achieved by delivering a constant stream of inspiratory gas through cannulas aimed down the main stem bronchi at flow rates totaling 1-3 l.kg-1.min-1 in the absence of tidal lung motion. Previous studies have shown that CFV can maintain a normal arterial PCO2, although significant ventilation-perfusion (VA/Q) inequality appears. This VA/Q mismatch could be due to regional differences in lung inflation that occur during CFV secondary to momentum transfer from the inflowing stream to resident gas in the lung. We tested the hypothesis that substitution of a gas with lower density might attenuate regional differences in alveolar pressure and reduce the VA/Q inequality during CFV. Gas exchange was studied in seven anesthetized dogs by the multiple inert gas elimination technique during ventilation with intermittent positive-pressure ventilation, CFV with O2-enriched nitrogen (CFV-N2), or CFV with O2-enriched helium (CFV-He). As an index of VA/Q inequality independent of shunt, the log SD blood flow increased from 0.757 +/- 0.272 during intermittent positive-pressure ventilation to 1.54 +/- 0.36 (P less than 0.001) during CFV-N2. Switching from CFV-N2 to CFV-He at the same flow rate did not improve log SD blood flow (1.45 +/- 0.21) (P greater than 0.05) but tended to increase arterial PCO2. In excised lungs with alveolar capsules attached to the pleural surface, CFV-He significantly reduced alveolar pressure differences among lobes compared with CFV-N2 as predicted. Regional alveolar washout of Ar after a stap change of inspired concentration was slower during CFV--He than during CFV-N2.(ABSTRACT TRUNCATED AT 250 WORDS)     

Lobar contribution to VA/Q inequality during constant-flow ventilation

Previous studies have shown that normal arterial PCO2 can be maintained during apnea in anesthetized dogs by delivering a continuous stream of inspired ventilation through cannulas aimed down the main stem bronchi, although this constant-flow ventilation (CFV) was also associated with a significant increase in ventilation-perfusion (VA/Q) inequality, compared with conventional mechanical ventilation (IPPV). Conceivably, this VA/Q inequality might result from differences in VA/Q ratios among lobes caused by nonuniform distribution of ventilation, even though individual lobes are relatively homogeneous. Alternatively, the VA/Q inequality may occur at a lobar level if those factors causing the VA/Q mismatch also existed within lobes. We compared the efficiency of gas exchange simultaneously in whole lung and left lower lobe by use of the multiple inert gas elimination technique in nine anesthetized open-chest dogs. Measurements of whole lung and left lower lobe gas exchange allowed comparison of the degree of VA/Q inequality within vs. among lobes. During IPPV with positive end-expiratory pressure, arterial PO2 and PCO2 (183 +/- 41 and 34.3 +/- 3.1 Torr, respectively) were similar to lobar venous PO2 and PCO2 (172 +/- 64 and 35.7 +/- 4.1 Torr, respectively; inspired O2 fraction = 0.44 +/- 0.02). Switching to CFV (3 l.kg-1.min-1) decreased arterial PO2 (112 +/- 26 Torr, P less than 0.001) and lobar venous PO2 (120 +/- 27 Torr, P less than 0.01) but did not change the shunt measured with inert gases (P greater than 0.5).(ABSTRACT TRUNCATED AT 250 WORDS)     

Catheter position and blood gases during constant-flow ventilation

We studied the effect of catheter position and flow rate on gas exchange during constant-flow ventilation (CFV) in eight anesthetized, paralyzed dogs. The distal tips of the insufflation catheters were positioned 0.5, 2.0, 3.5, and 5.0 cm from the tracheal carina. Flow rates were varied between 10 and 55 l/min and steady-state arterial blood gases were measured. At a given flow rate, arterial CO2 pressure (PaCO2) decreased as CFV was administered further into the lung up to a distance of 3.5 cm from the carina; there were no significant differences in PaCO2 at 3.5 and 5.0 cm. For a given catheter position, PaCO2 decreased with increasing flow rate up to a flow rate of 40 l/min. Further increases in flow rate had no significant effect on PaCO2. Arterial O2 pressure (PaO2) was relatively constant at all flow rates and catheter positions. We conclude that, up to a point, CO2 elimination can be improved by positioning the catheters further into the lung; advancing the catheters further than 3.5 cm from the carina may cause over-ventilation of specific lung regions resulting in a relative plateau in CO2 elimination and relatively constant PaO2's. Positioning the catheters further into the lung permits the use of lower flow rates, thus potentially minimizing the risk of barotrauma.     

Pressure, flow, and density relationships in airway models during constant-flow ventilation

Adequate CO2 elimination and normal arterial PCO2 levels can be maintained in dogs during apnea by delivering a continuous flow of inspired gas at high flow rate (1-3 l.min-1.kg-1) through tubes placed in the main-stem bronchi. However, during constant-flow ventilation (CFV) the mean alveolar pressure is increased, causing increased lung volume despite low pressures in the trachea. We hypothesized that the increased dynamic alveolar pressures during CFV were due to momentum transfer from the high-velocity jet stream to resident gas in the lung. To test this, we simulated CFV in straight tubes and in a branched airway model to determine whether changes in gas flow rate (V), gas density (rho), and tube diameter (D) altered the pressure difference (delta P) between alveoli and airway opening in a manner consistent with that predicted by conservation of momentum. Momentum analysis predicts that delta P should vary with V2, whereas measurements yielded a dependence of V1.69 in branched tubes and V1.9 in straight tubes. Substitution of heliox (80% He-20% O2) for air significantly reduced lung hyperinflation during CFV. As predicted by momentum transfer, delta P varied with rho 1.0. Momentum analysis also predicts that delta P should vary with D-2.0, whereas measurements indicated a dependence on D-2.02. The influence of V and rho on depth of penetration of the jet down the airway was explored in a straight tube model by varying the flow rate and gas used. The influence of geometry on penetration was measured by changing the ratio of jet-to-airway tube diameters.(ABSTRACT TRUNCATED AT 250 WORDS)     

Gas transport during high-frequency ventilation

During high-frequency small-volume ventilation (HFV), the transport rate of gas from the mouth to a lung region is a function of two conductances (conductance is the transfer rate of a gas divided by its partial pressure difference): regional longitudinal gas conductance along the airways (Grlongi) and gas conductance between lung regions (Ginter). Grlongi per unit regional lung (gas) volume [Grlongi/(Vr beta g)] was determined during HFV in 11 anesthetized paralyzed dogs lying supine. The distribution of Grlongi/(Vr beta g) was nearly uniform during HFV when stroke volumes were less than approximately two-thirds of the Fowler dead-space volume. By contrast, the distribution of Grlongi/(Vr beta g) was nonuniform when the stroke volume exceeded approximately two-thirds of the Fowler dead-space volume and the oscillation frequency was 5 Hz. Gas conductance along the airways per unit lung gas volume [average Glongi/(V beta g)], for the entire lung, increased with stroke volume at all frequencies, but for a given product of oscillation frequency and stroke volume, the average Glongi/(V beta g) was greater when stroke volume was large and oscillation frequency was low. The average Glongi/(V beta g) increased with frequency up to a maximal value; the frequency at which the maximum occurred depended on the kinematic viscosity of the inspired gas mixture.     

Combination of constant-flow and continuous positive-pressure ventilation in canine pulmonary edema

Constant-flow ventilation (CFV) maintains alveolar ventilation without tidal excursion in dogs with normal lungs, but this ventilatory mode requires high CFV and bronchoscopic guidance for effective subcarinal placement of two inflow catheters. We designed a circuit that combines CFV with continuous positive-pressure ventilation (CPPV; CFV-CPPV), which negates the need for bronchoscopic positioning of CFV cannula, and tested this system in seven dogs having oleic acid-induced pulmonary edema. Addition of positive end-expiratory pressure (PEEP, 10 cmH2O) reduced venous admixture from 44 +/- 17 to 10.4 +/- 5.4% and kept arterial CO2 tension (PaCO2) normal. With the innovative CFV-CPPV circuit at the same PEEP and respiratory rate (RR), we were able to reduce tidal volume (VT) from 437 +/- 28 to 184 +/- 18 ml (P less than 0.001) and elastic end-inspiratory pressures (PEI) from 25.6 +/- 4.6 to 17.7 +/- 2.8 cmH2O (P less than 0.001) without adverse effects on cardiac output or pulmonary exchange of O2 or CO2; indeed, PaCO2 remained at 35 +/- 4 Torr even though CFV was delivered above the carina and at lower (1.6 l.kg-1.min-1) flows than usually required to maintain eucapnia during CFV alone. At the same PEEP and RR, reduction of VT in the CPPV mode without CFV resulted in CO2 retention (PaCO2 59 +/- 8 Torr). We conclude that CFV-CPPV allows CFV to effectively mix alveolar and dead spaces by a small bulk flow bypassing the zone of increased resistance to gas mixing, thereby allowing reduction of the CFV rate, VT, and PEI for adequate gas exchange.     

Similar ventilation distribution in normal subjects prone and supine during tidal breathing.

Multiple-breath washout (MBW) tests, with end-expiratory lung volume at functional residual capacity (FRC) and 90% O(2), 5% He, and 5% SF(6) as an inspired gas mixture, were performed in healthy volunteers in supine and prone postures. The semilog plot of MBW N(2) concentrations was evaluated in terms of its curvilinearity. The MBW N(2) normalized slope analysis yielded indexes of acinar and conductive ventilation heterogeneity (Verbanck S, Schuermans D, Van Muylem A, Paiva M, Noppen M, and Vincken W. J App Physiol 83: 1907-1916, 1997). Also, the difference between SF(6) and He normalized phase III slopes was computed in the first MBW expiration. Only MBW tests with similar FRC in the prone and supine postures (P > 0.1; n = 8) were considered. Prone and supine postures did not reveal any significant differences in curvilinearity, N(2) normalized slope-derived indexes of conductive or acinar ventilation heterogeneity, nor SF(6)-He normalized phase III slope difference in the first MBW expiration (P > 0.1 for all). The absence of significant changes in any of the MBW indexes suggests that ventilation heterogeneity is similar in the supine and prone postures of normal subjects breathing near FRC.     

Continuous tracheal gas insufflation during partial liquid ventilation in juvenile rabbits with acute lung injury.

To examine the hypothesis that combined treatment with tracheal gas insufflation (TGI) and partial liquid ventilation (PLV) may improve pulmonary outcome relative to either treatment alone in acute lung injury (ALI), saline lavage lung injury was induced in 24 anesthetized, ventilated juvenile rabbits that were then randomly assigned to receive (n = 6/group) 1) conventional mechanical ventilation (CMV) alone, 2) continuous TGI at 0.5 l/min, 3) PLV with perfluorochemical liquid, and 4) combined TGI and PLV (TGI + PLV), and subsequently ventilated with minimized pressures and tidal volume (Vt) to keep arterial Po(2) (Pa(O(2))) >100 Torr and arterial Pco(2) (Pa(CO(2))) at 45-60 Torr for 4 h. Gas exchange, lung mechanics, myeloperoxidase, IL-8, and histomorphometry [including expansion index (EI)] were assessed. The CMV group showed no improvement in lung mechanics and gas exchange; all treated groups had significant increases in compliance, Pa(O(2)), ventilation efficacy index (VEI), and EI, and decreases in PaCO(2), oxygenation index, physiological dead space-to-Vt ratio (Vd/Vt), myeloperoxidase, and IL-8, relative to the CMV group. TGI resulted in lower peak inspiratory pressure, Vt, Vd/Vt, and greater VEI vs. PLV group; PLV resulted in greater compliance, Pa(O(2)), and EI vs. TGI. TGI + PLV resulted in decreased peak inspiratory pressure, Vt, Vd/Vt, and increased VEI compared with TGI, improved compliance and EI compared with PLV, and a further increase in Pa(O(2)) and oxygenation index and a decrease in PaCO(2) vs. either treatment alone. These results indicate that combined treatment of TGI and PLV results in improved pulmonary outcome than either treatment alone in this animal model of ALI.     

Distribution of pulmonary ventilation using Xe-enhanced computed tomography in prone and supine dogs.

Xe-enhanced computed tomography (CT; Xe-CT) is a method for the noninvasive measurement of regional pulmonary ventilation in intact subjects, determined from the washin and washout rates of the radiodense, nonradioactive gas Xe, as measured in serial CT scans. We used the Xe-CT ventilation method, along with other quantitative CT measurements, to investigate the distribution of regional lung ventilation and air content in healthy, anesthetized, mechanically ventilated dogs in the prone and supine postures. Vertical gradients in regional ventilation and air content were measured in five mongrel dogs in both prone and supine postures at four axial lung locations. In the supine position, ventilation increased with dependent location, with a mean slope of 7.3%/cm lung height, whereas no ventilation gradients were found at any location in the prone position. These results agree quantitatively with other published studies. In addition, six different animals were studied (3 supine, 3 prone) to examine the longitudinal distribution of ventilation and air content. The prone lungs were more uniformly inflated compared with the supine, which were less well expanded at the base than apex. Ventilation index, a measure of regional ventilation relative to whole lung ventilation, increased steeply from apex to base in the supine animals, whereas it was again more uniform in the prone condition. We conclude that the Xe-CT method provides a reasonable, quantitative measurement of regional ventilation and promises to be a valuable tool for the noninvasive determination of regional lung function.     

Effect of high-frequency ventilation on gas exchange and pulmonary vascular resistance in lambs

We studied the effects of conventional mechanical ventilation (CMV) (15 ml/kg tidal volume delivered at 18-25 breaths/min) and high-frequency oscillatory ventilation (HFOV) (less than or equal to 2 ml/kg delivered at 10 Hz) on pulmonary hemodynamics and gas exchange during ambient air breathing and hypoxic gas breathing in 10 4-day-old lambs. After instrumentation and randomization to either HFOV or CMV the animals breathed first ambient air and then hypoxic gas (inspired O2 fraction = 0.13) for 20 min. The mode of ventilation was then changed, and the normoxic and hypoxic gas challenges were repeated. The multiple inert gas elimination technique was utilized to assess gas exchange. There was a significant increase with HFOV in mean pulmonary arterial pressure (Ppa) (20.1 +/- 4.2 vs. 22 +/- 3.8 Torr, CMV vs. HFOV, P less than 0.05) during ambient air breathing. During hypoxic gas breathing Ppa was also greater with HFOV than with CMV (29.5 +/- 5.7 vs. 34 +/- 3.1 Torr, CMV vs. HFOV, P less than 0.05). HFOV reduced pulmonary blood flow (Qp) during ambient air breathing (0.33 +/- 0.11 vs. 0.28 +/- 0.09 l . kg-1 . min-1, CMV vs. HFOV, P less than 0.05) and during hypoxic gas breathing (0.38 +/- 0.11 vs. 0.29 +/- 0.09 l . kg-1 . min-1, P less than 0.05). There was no significant difference in calculated venous admixture for sulfur hexafluoride or in the index of low ventilation-perfusion lung regions with HFOV compared with CMV.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of hypercarbia on arterial and alveolar oxygen tensions in a model of gram-negative pneumonia

Inspired CO2 causing changes from hypo- to normocapnia has previously been shown to improve arterial O2 tension (PaO2) and to reduce alveolar-arterial O2 difference. The effect of further increases in inspired CO2 to hypercarbic levels has not been studied in inflammatory lung disease. Three days after induction of sublobar Pseudomonas pneumonia, Suffolk sheep were anesthetized and ventilated with a fixed-volume ventilator. After 2.5 h, CO2 was added to the inspired gas to raise arterial CO2 tension (PaCO2) to 60-65 Torr. Four hours later the CO2 was withdrawn and ventilation continued for an additional 2 h. Constant minute ventilation and inspired O2 fraction were maintained. Regional lung perfusion was measured by injection of radioactive microspheres. With the administration of CO2, PaO2 increased significantly from 65.5 to 77.5 Torr as did alveolar O2 tension (from 109.7 to 120.0 Torr) with no significant change in alveolar-arterial O2 difference. There were no significant changes in cardiac output, shunt fraction, O2 uptake, O2 delivery, respiratory quotient, or distribution of regional lung perfusion. We conclude that the increases in alveolar O2 tension and PaO2 with the added CO2 resulted from improved alveolar ventilation.     

Reduction of 15-hydroxyeicosatetraenoic acid (15-HETE) in tracheal fluid by high frequency oscillatory ventilation

The effects of high frequency oscillatory ventilation (HFOV) and conventional mechanical ventilation (CMV) on tracheal secretion were compared in 6 anesthetized dogs. Using a double-balloon endotracheal catheter, 5 ml of saline was instilled into an isolated tracheal segment during HFOV and CMV for 10 min respectively. Two eicosanoids, 15-hydroxyeicosatetraenoic acid (15-HETE) and 11-dehydrothromboxane B2 (11-dehydro-TXB2) were measured by radioimmunoassay in each sample. HFOV (stroke volume: 6 ml/kg, f: 10 Hz, bias flow: 5 l/min) and CMV (stroke volume: 12 ml/kg, f: 15/min) were performed in random sequence and achieved comparable gas exchange. The concentration of 15-HETE in tracheal fluid during HFOV (87 +/- 67 pg/ml) was decreased to less than half of that during CMV (286 +/- 184 pg/ml, P less than 0.05), while there was no significant change of 11-dehydro-TXB2 either in tracheal fluid or in plasma. This reduction of 15-HETE was tended to be enhanced by vagotomy (HFOV: 42 +/- 14, CMV: 120 +/- 103 pg/ml) with the concentration ratio of CMV/HFOV remaining unchanged. HFOV may provide hitherto unrecognized advantage over CMV by reducing airway secretion of 15-HETE, a potent inflammatory mediator.     

Local gas transport in eucapnic ventilation: effects of gravity and breathing frequency

The effects of body position and respiratory frequency (f) on regional gas transport during eucapnic conventional ventilation (CV) and high-frequency ventilation (HFV) were assessed from the washout of nitrogen 13 (13NN) using positron-emission tomography. In one protocol, six dogs were ventilated with CV or HFV at f = 6 Hz and tidal volume (VT) selected supine for eucapnia. A coronal cross section of the lung base was studied in the supine, prone, and right and left lateral decubitus positions. In a second protocol, six dogs were studied prone: apical and basal cross sections were studied in CV and in HFV with f = 3 and 9 Hz at eucapnic VT. Regional alveolar ventilation per unit of lung volume (spVr) was calculated for selected regions and analyzed for gravity-dependent cephalocaudal and right-to-left gradients. In both CV and HFV, nonuniformity in spVr was highest supine and lowest prone. In CV there were vertical gradients of spVr in all body positions: nondependent less ventilated than dependent regions, particularly in the supine position. In HFV there was a moderate vertical gradient in spVr in addition to a preferentially ventilated central region in all body positions. Overall lung spV was unaffected by body position in CV but in HFV was highest supine and lowest prone. Nonuniformity in eucapnic prone HFV was unaffected by f and always higher than in CV.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of lung inflation on regional lung expansion in supine and prone rabbits

At functional residual capacity, lung expansion is more uniform in the prone position than in the supine position. We examined the effect of positive airway pressure (Paw) on this position-dependent difference in lung expansion. In supine and prone rabbits postmortem, we measured alveolar size through dependent and nondependent pleural windows via videomicroscopy at Paw of 0 (functional residual capacity), 7, and 15 cmH2O. After the chest was opened, alveolar size was measured in the isolated lung at several transpulmonary pressures (Ptp) on lung deflation. Alveolar mean linear intercept (Lm) was measured from the video images taken in situ. This was compared with those measured in the isolated lung to determine Ptp in situ. In the supine position, the vertical Ptp gradient increased from 0.52 cmH2O/cm at 0 cmH2O Paw to 0.90 cmH2O/cm at 15 cmH2O Paw, while the vertical gradient in Lm decreased from 2.17 to 0.80 microns/cm. In the prone position, the vertical Ptp gradient increased from 0.06 cmH2O/cm at 0 cmH2O Paw to 0.35 cmH2O/cm at 15 cmH2O Paw, but there was no change in the vertical Lm gradient. In anesthetized paralyzed rabbits in supine and prone positions, we measured pleural liquid pressure directly at 0, 7, and 15 cmH2O Paw with dependent and nondependent rib capsules. Vertical Ptp gradients measured with rib capsules were similar to those estimated from the alveolar size measurements. Lung inflation during mechanical ventilation may reduce the vertical nonuniformities in lung expansion observed in the supine position, thereby improving gas exchange and the distribution of ventilation.     

Gas exchange in dogs in the prone and supine positions.

To determine the cause of the difference in gas exchange between the prone and supine postures in dogs, gas exchange was assessed by the multiple inert gas elimination technique (MIGET) and distribution of pulmonary blood flow was determined using radioactively labeled microspheres in seven anesthetized paralyzed dogs. Each animal was studied in the prone and supine positions in random order while tidal volume and respiratory frequency were kept constant with mechanical ventilation. Mean arterial PO2 was significantly lower (P less than 0.01) in the supine [96 +/- 10 (SD) Torr] than in the prone (107 +/- 6 Torr) position, whereas arterial PCO2 was constant (38 Torr). The distribution of blood flow (Q) vs. ventilation-to-perfusion ratio obtained from MIGET was significantly wider (P less than 0.01) in the supine [ln SD(Q) = 0.75 +/- 0.26] than in the prone position [ln SD (Q) = 0.34 +/- 0.05]. Right-to-left pulmonary shunting was not significantly altered. The distribution of microspheres was more heterogeneous in the supine than in the prone position. The larger heterogeneity was due in part to dorsal-to-ventral gradients in Q in the supine position that were not present in the prone position (P less than 0.01). The decreased efficiency of oxygenation in the supine posture is caused by an increased ventilation-to-perfusion mismatch that accompanies an increase in the heterogeneity of Q distribution.     

Effect of tracheal bias flow on gas exchange during high-frequency chest percussion

High-frequency chest percussion (HFP) with constant fresh gas flow (VBF) at the tracheal carina is a variant of high-frequency ventilation (HFV) previously shown to be effective with extremely low tracheal oscillatory volumes (approximately 0.1 ml/kg). We studied the effects of VBF on gas exchange during HFP. In eight anesthetized and paralyzed dogs we measured arterial and alveolar partial pressures of CO2 (PaCO2) and O2 (PaO2) during total body vibration at a frequency of 30 Hz, amplitude of 0.17 +/- 0.019 cm, and tidal volume of 1.56 +/- 0.58 ml. VBF was incrementally varied from 0.1 to 1.2 l.kg-1.min-1. At low flows (0.1-0.4 l.kg-1.min-1), gas exchange was strongly dependent on flow rate but became essentially flow independent with higher VBF (i.e., hyperbolic pattern). At VBF greater than 0.4 l.kg-1.min-1, hyperventilatory blood gas levels were consistently sustained (i.e., PaCO2 less than 20 Torr, PaO2 greater than 90 Torr). The resistance to CO2 transport of the airways was 1.785 +/- 0.657 l-1.kg.min and was independent of VBF. The alveolar-arterial difference of O2 was also independent of the flow. In four of five additional dogs studied as a control group, where constant flow of O2 was used without oscillations, the pattern of PaCO2 vs. VBF was also hyperbolic but at substantially higher levels of PaCO2. It is concluded that, in the range of VBF used, intraairway gas exchange was limited by the 30-Hz vibration. The fresh gas flow was important only to maintain near atmospheric conditions at the tracheal carina.