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-stream bronchi. Because an asymmetric distribution of ventilation might introduce ventilation-perfusion (V̇A/Q̇) 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·kg^-1·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 Q̇) increased, compared with the findings during IPPV. Reducing CFV flow rate to 1 l·kg^-1·min^-1 at constant lung volume improved R-E* and log SD Q̇, but significant V̇A/Q̇ 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 V̇A/Q̇ inequality which may be due to nonuniform ventilation distribution and a redistribution of pulmonary blood flow.