Hypocapnia and hypercapnia in the dog: effects on myocardial blood-flow and haemodynamics during beta- and combined alpha- and beta-adrenoceptor blockade.

We have recently demonstrated an increase in myocardial blood-flow (MBF) during systemic hypercapnia independent of myocardial oxygen consumption. During hypocapnia no significant decrease of MBF was observed; however, a significant increase of myocardial oxygen extraction resulted. The present study was undertaken to examine whether these effects were caused by changes in the sympathoadrenal activity. During beta-adrenoceptor blockade and combined alpha- and beta-blockade the effects of hypocapnia and hypercapnia on MBF and haemodynamics were examined. Closed-chest dogs were anaesthetized with pentobarbital and hypocapnia was induced by hyperventilation. Carbon dioxide was added to the inspiratory gas to create normocapnia and hypercapnia. Hypocapnia did not result in any changes of MBF, coronary vascular resistance or myocardial oxygen extraction, neither during beta-adrenoceptor blockade nor during combined alpha- and beta-blockade. Hypercapnia did not increase MBF, neither during beta-blockade nor during combined alpha- and beta-blockade. However, both myocardial oxygen consumption and mechanical performance of the heart were reduced during hypercapnia. Thus, a relative myocardial overperfusion, indicated by decreased coronary vascular resistance and myocardial oxygen extraction, was observed during hypercapnia. In conclusion, the unaltered MBF during hypocapnia and the coronary overperfusion during hypercapnia were not related to changes in the sympathoadrenal activity.