Autonomic nervous system and sudden cardiac death. Experimental basis and clinical observations for post-myocardial infarction risk stratification

Abstract

The analysis of the autonomic control of the heart, by means of indirect markers, may represent a new approach for identifying patients at higher risk for sudden cardiac death after a myocardial infarction. This possibility is based on the evidence that autonomic responses during acute myocardial ischemia are a major determinant of the outcome (i.e., occurrence of ventricular fibrillation or survival). Specifically, sympathetic activation can trigger malignant arrhythmias, whereas vagal activity may exert a protective effect. Several experimental observations have provided new insights on the relation between sympatho-vagal interactions and the likelihood for the occurrence of ventricular fibrillation. In an established experimental model for sudden death involving conscious dogs with a healed myocardial infarction, either depressed reflex chronotropic responses during a blood pressure rise or reduced variability of heart rate (respectively, markers of reflex and tonic cardiac vagal activity) identify dogs at greater risk to develop malignant arrhythmias during a new ischemic episode. In anesthetized cats, direct neural recording of vagal activity to the heart confirmed that vigorous reflex vagal activation during acute myocardial ischemia is associated with protection from ventricular fibrillation. Furthermore, in these experiments the reflex neural response to acute myocardial ischemia was predicted by the analysis of baroreflex sensitivity. The antifibrillatory effect of vagal activation is confirmed by the prevention of ventricular fibrillation during acute ischemia in dogs susceptible to sudden cardiac death by direct electrical stimulation of the right vagus. The clinical counterpart of these experimental data lies in three separate prospective studies showing a higher cardiac mortality in patients who after a myocardial infarction have a depressed baroreflex sensitivity or a decreased heart rate variability. A definitive answer on the role that the analysis of markers of cardiac vagal activity may play in risk stratification of patients with coronary artery disease should be provided by Autonomic Tone and Reflexes After Myocardial Infarction (ATRAMI), an ongoing prospective study. In ATRAMI, baroreflex sensitivity and heart rate variability will be assessed within 20 days after a myocardial infarction in 1,200 patients enrolled in Europe, U.S.A., and Japan with a minimum follow up of one year.