Response of mean and phasic coronary arterial blood flow to graded hypercapnia in dogs.

An electromagnetic flow meter was used to assess the changes in coronary blood flow in response to four levels of increased PaCO2 in six dogs. Mean, diastolic and systolic flow all increased during hypercapnia, the increase being maximal at mean PaCO2 11.3 KPa. Mean and diastolic coronary vascular resistances decreased progressively as PaCO2 increased, but systolic resistance, although decreasing with the lower levels of hypercapnia, returned to control at the greatest PaCO2. Although the oxygen available to the myocardium was increased markedly during hypercapnia, coronary sinus PO2 increased also, reflecting a reduction in myocardial oxygen extraction; thus myocardial oxygen consumption was unchanged. Cardiac output was increased significantly only at the greatest PaCO2. Total body oxygen handling was not altered significantly.