EFFECTS OF INSPIRED CARBON-DIOXIDE ON VENTILATION-PERFUSION MATCHING IN NORMOXIA, HYPOXIA, AND HYPEROXIA

We studied the effect of low concentrations (2 to 4%) of inspired CO2 on gas exchange and ventilation-perfusion (VA/Q) relationships in heal thy normocapnic anesthetized dogs during constant mechanical ventilati on by the multiple inert gas elimination technique (MIGET). One group was studied at normal tidal volumes (12 to 14 ml/kg) and rates (13 to 15/min) in normoxia, and the other in mild hyperoxia (Fl(O2) = 0.50) a nd hypoxia (Fl(O2) = 0.15). In normoxic dogs there were progressive in creases in arterial PO2 and reductions in the alveolar-to-arterial PO2 and arterial-to-mixed expired PCO2 differences in response to increas es in Fl(CO2). This increased gas exchange efficiency was characterize d by reductions in both dead space ventilation and VA/Q mismatch. Bett er VA/Q matching was characterized by reduction in the log standard de viation of ventilation (log SDV) without significant change in the log standard deviation of perfusion (log SDQ). Gas exchange parameters re turned to baseline when dogs were returned to CO2-free inspired gas. I n the second group, the effects of 3% inspired CO2 were of comparable magnitude in both mild hypoxia and hyperoxia. In this group (taking hy peroxic values as baseline), there were improved gas exchange and less VA/Q heterogeneity with inspired hypoxia, both with and without inspi red CO2. In contrast to the effects of added inspired CO2, improved VA /Q matching with hypoxia was characterized by reductions in both log S DV and log SDQ. These data show that inspired CO2 and hypoxia reduce V A/Q heterogeneity, but the dissimilar pattern of changes in MIGET-deri ved ventilation and perfusion distributions with CO2 and hypoxia sugge st differing airway and vascular effects that act to reduce VA/Q misma tch.