Blood pH and PaCO2 as chemical factors in myocardial blood flow control

The effect of metabolic and hypercapnic acidosis on myocardial blood flow was studied during intravenous infusions of hydrochloric acid solutions (n = 12) and during passive ventilation with 5% CO2 (n = 5) in anaesthetized, closed chest dogs. Below a pH of 7.2 metabolic acidosis at normal arterial CO2-tensions caused an increase of coronary blood flow and a decrease of coronary vascular resistance associated with a narrowed myocardial arteriovenous O2-difference, indicating vasodilation at unchanged myocardial oxygen consumption. In propranolol-pretreated dogs myocardial blood flow and coronary oxygen AV difference remained unaffected, suggesting that the coronary dilatory effect of metabolic acidemia involves beta adrenergic stimulation. Coronary vasodilation induced by increasing arterial pCO2 was found to the significantly greater as compared with the dilatory effect of metabolic acidosis at the same blood pH level. Blocking of beta receptors did not reduce the coronary response to increased arterial CO2-tensions. It is concluded that the coronary vasodilation observed during hypercapnic acidosis is neither mediated by a beta adrenergic stimulation nor dependent of the concomitant change in blood pH. The possible sites of the coronary dilatory actions of increased arterial CO2-tensions are discussed.