The role of vagal tone and reflexes in the genesis of life-threatening arrhythmias was investigated in a clinically relevant animal model for sudden cardiac death. Forty-five dogs with a healed anterior myocardial infarction in which transient myocardial ischemia during exercise did not induce malignant arrhythmias were utilized for the study. They underwent a further exercise and ischemia test in which atropine (75 micrograms/kg) was injected before coronary artery occlusion. Novel occurrence of ventricular arrhythmia, or worsening of the type of arrhythmia present in the control test, occurred in 23 of 45 dogs (51%) and ventricular fibrillation occurred in 11 of 45 (24%, P = 0.001). Analysis of heart rate response to acute ischemia in the control test indicates that these 11 animals had powerful vagal reflexes during coronary artery occlusion, compared with the 34 survivors (-32 +/- 35 vs. +2 +/- 27 beats/min, P = 0.003). This study indicates that approximately 75% of animals resistant to ventricular fibrillation are characterized by weak sympathetic reflexes in response to acute myocardial ischemia. In the remaining 25% powerful vagal reflexes counteract concomitant reflex sympathetic hyperactivity, decrease heart rate, and are essential for survival.
Left stellectomy in the prevention of ventricular fibrillation caused by acute myocardial ischemia in conscious dogs with anterior myocardial infarction.