Attenuation of cardiac sympathetic drive in experimental myocardial ischemia in dogs.

Sympathetic discharges to the heart were recorded from the left inferior cardiac nerve of 16 dogs. Inferior cardiac nerve activity (ICNA) under normal conditions consisted of grouped discharges, synchronous with the cardiac cycle and modulated by respiration. After ligation of the circumflex branch of the left coronary artery, ICNA declined concomitant with a decline in heart rate and mean aortic pressure. After 30 minutes, when arterial pressure tended to recover toward control values (six dogs), ICNA remained low; in contrast, when arterial pressure dropped to shock levels (three dogs), ICNA increaed. When aortic pressure fell precipitously as a result of ventricular fibrillation, even during the first 30 minutes of ischemia (seven dogs), ICNA immediately increased greatly. The results of this study suggest that acute coronary occlusion produces a cardiocardiac depressor reflex with attenuation of sympathetic discharge to the heart. This reflex, under the experimental conditions studied, gives way to the baroreceptor reflex when aortic pressure drops to critically low levels.