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 ine rt gas elimination technique. Dogs anesthetized with pentobarbital sod ium were hyperventilated with an inspired fraction of O2 of 0.21 by us ing 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 t ension (Pa(CO2)) was varied to three levels (20, 35, and 52 Torr) by a ltering the inspired PCO2. The orders of type of ventilation and Pa(CO 2) level were randomized. Compared with normocapnia, VA/Q heterogeneit y was increased during hypocapnia induced by increased respiratory rat e ventilation, which was indicated by an increase in dispersion indexe s and arterial-alveolar inert gas partial pressure difference areas (P < 0.01). In contrast, VA/Q heterogeneity was not affected by hypocapn ia when a large VT ventilation was used. Under the conditions of our s tudy, hypercapnia did not result in statistically significant changes in VA/Q heterogeneity with either type of ventilation. Increased VT ve ntilation reduced dead space at all Pa(CO2) levels (P < 0.01) and redu ced the log standard deviation of the ventilation distribution during normocapnia (P < 0.05) and hypocapnia (P < 0.01). We conclude that hyp ocapnia increased VA/Q heterogeneity when hyperventilation was achieve d with a rapid respiratory rate. Therefore, a lack of improvement in V A/Q matching with inhaled CO2 may be associated with the use of a larg e VT. These data suggest that hypocapnic bronchoconstriction may be im portant in mediating hypocapnia-induced VA/Q inequality in dogs.