beta-Adrenergic activity and cardiovascular response to severe respiratory acidosis

The mechanism responsible for the depressive myocardial effects of severe respiratory acidosis is unclear; however, sympathetic stimulation and catecholamines are known to be involved. The influence of beta-adrenergic receptor activity on the myocardial response to severe respiratory acidosis was studied in 18 anesthetized, mechanically ventilated dogs. Arterial CO2 tension (PaCO2) was raised by increasing the inspired CO2 fraction in O2. In control animals, as PaCO2 increased, heart rate (HR) decreased (PaCO2 approximately 110 mmHg), then returned to control (PaCO2 approximately 220 mmHg), whereas arterial blood pressure (Pa) and cardiac output (Q) remained unchanged from prehypercapnia levels. At PaCO2 greater than 350 mmHg, Pa, HR, and Q decreased and left ventricular function (LVF) curves were depressed. Death occurred at a PaCO2 of 404 +/- 25 mmHg (pH 6.48 +/- 0.02). In a second group of animals, administration of isoproterenol during the increase in PaCO2 did not result in depression of myocardial function, and death did not occur even at a significantly higher PaCO2 (PaCO2 496 +/- 12 mmHg; pH 6.39 +/- 0.02) than in the control group. Administration of propranolol to a third group of animals as PaCO2 increased did not change Pa, HR, and Q; however, LVF curves indicated a more rapid and severe depression of myocardial performance than in control, and death occurred at a significantly lower PaCO2 (PaCO2 220 +/- 25 mmHg; pH 6.65 +/- 0.02). We conclude that beta-adrenergic receptor stimulation can prevent hypercapnic heart failure and that beta-adrenergic receptor activity is involved in the mechanism responsible for this failure.