Effect of inspired CO2 on ventilation and perfusion heterogeneity in hyperventilated dogs

We studied the effect of inspired CO2 on ventilation-perfusion (VA/Q) heterogeneity in dogs hyperventilated under two different tidal volume (VT) and respiratory rate conditions with the use of the multiple inert gas elimination technique. Dogs anesthetized with pentobarbital sodium were hyperventilated with an inspired fraction of O2 of 0.21 by using an increased VT (VT = 30 ml/kg at 18 breaths/min) or an increased respiratory rate (VT = 18 ml/kg at 35 breaths/min). The arterial CO2 tension (PaCO2) was varied to three levels (20, 35, and 52 Torr) by altering the inspired PCO2. The orders of type of ventilation and PaCO2 level were randomized. Compared with normocapnia, VA/Q heterogeneity was increased during hypocapnia induced by increased respiratory rate ventilation, which was indicated by an increase in dispersion indexes and arterial-alveolar inert gas partial pressure difference areas (P < 0.01). In contrast, VA/Q heterogeneity was not affected by hypocapnia when a large VT ventilation was used. Under the conditions of our study, hypercapnia did not result in statistically significant changes in VA/Q heterogeneity with either type of ventilation. Increased VT ventilation reduced dead space at all PaCO2 levels (P < 0.01) and reduced the log standard deviation of the ventilation distribution during normocapnia (P < 0.05) and hypocapnia (P < 0.01). We conclude that hypocapnia increased VA/Q heterogeneity when hyperventilation was achieved with a rapid respiratory rate. Therefore, a lack of improvement in VA/Q matching with inhaled CO2 may be associated with the use of a large VT. These data suggest that hypocapnic bronchoconstriction may be important in mediating hypocapnia-induced VA/Q inequality in dogs.