Heart rate variability: an important new risk factor in patients following myocardial infarction.

After acute myocardial infarction, cardiac autonomic, and particularly parasympathetic, activity decreases, followed by a gradual return toward normal over the next few weeks and months. The easiest measurable index of autonomic activity is heart rate variability, which can be assessed in a number of different ways. Where heart rate variability is low after myocardial infarction, long-term survival is considerably reduced, independent of other known risk factors. This may be caused by patchy autonomic denervation, rendering the heart more susceptible to potentially fatal arrhythmias. Prophylactic drug therapy might reduce mortality in patients with low heart rate variability.