Heart rate variability as an index of autonomic imbalance in patients with recent myocardial infarction.

Autonomic nervous activities are estimated in three groups of patients: Group A consists of patients who had experienced myocardial infarction (MI) within 2-6 weeks before the tests; Group B consists of patients who had MI more than one year previously; Group C consists of matched controls, free of cardiac disease. Autonomic activity is estimated using postural effects on heart rate variability (HRV): a sympathetic activity index is defined as HRV power within a low frequency band (0.04-0.12 Hz) in the tilt position and a parasympathetic activity index is defined as HRV power in a high frequency band (0.18-0.28 Hz) in the supine position. Results show that, relative to controls, Group A patients have reduced parasympathetic activity index (5 + 3 against 13 + 8, normalised units; p < 0.05) and an increased ratio of sympathetic to parasympathetic activity (17 + 17 against 4 + 2; p < 0.05). Group B is not significantly different from Group A or C. The period of 2-6 weeks post-MI thus appears to be characterised by depressed parasympathetic nervous activity which can be measured using HRV analysis.