Heart rate variability as an index of sympathovagal interaction after acute myocardial infarction

By analysis of spectral components of heart rate variability, sympathovagal interaction was assessed in patients after acute myocardial infarction (AMI). At 2 weeks after AMI (n = 70), the low-frequency component was significantly greater (69 ± 2 vs 53 ± 3 normalized units [NU], p < 0.05) and the high-frequency component was significantly smaller (17 ± 1 vs 35 ± 3 NU) than in 26 age-matched control subjects. This difference was likely to reflect an alteration of sympathovagal regulatory outflows with a predominance of sympathetic activity. At 6 (n = 33) and 12 (n = 29) months after AMI, a progressive decrease in the low- (62 ± 2 and 54 ± 3 NU) and an increase in the high-frequency (23 ± 2 and 30 ± 2 NU) spectral components was observed, which suggested a normalization of sympathovagal interaction. An increase in sympathetic efferent activity induced by tilt did not further modify the low-frequency spectral component (78 ± 3 vs 74 ± 3 NU) in a subgroup of 24 patients at 2 weeks after AMI. Instead, 1 year after AMI, this maneuver was accompanied by an increase in the low-frequency component (77 ± 3 vs 53 ± 3 NU, p < 0.05) of a magnitude similar to the one observed in control subjects (78 ± 3 vs 53 ± 3 NU). These data indicate that the sympathetic predominance that is detectable 2 weeks after AMI is followed by recovery of vagal tone and a normalization of sympathovagal interaction, not only during resting conditions, but also in response to a sympathetic stimulus.