Gas density dependence of regional V̇A/V and V̇A/Q̇ 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 (V̇A/Q̇) inequality appears. This V̇A/Q̇ 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 V̇A/Q̇ 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 V̇A/Q̇ 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 < 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 > 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 step change of inspired concentration was slower during CFV-He than during CFV-N₂. Moreover, CFV-He did not reduce interlobar differences in the rate of washout compared with CFV-N₂. We conclude that substitution of He for N₂ as the background gas during CFV increases arterial PCO₂ by decreasing alveolar ventilation and does not improve the V̇A/Q̇ inequality because it does not correct the maldistribution of ventilation, which apparently is the cause of the gas exchange defect.