Background and purpose: Decreased heart rate variability (HRV) has been shown to be a predictor of both sudden cardiac death and cardiovascular death among patients surviving acute myocardial infarction (MI). However, the prognostic significance of time domain HRV in patients with MI is still controversial. In addition to the mean of all normal-to-normal (NN) intervals, we analyzed the prognostic significance of the 5 most widely used time domain parameters of HRV during long-term follow-up in patients surviving their first MI.
Methods: Five time domain measures of HRV from 24-hour ECG were obtained after discharge in 260 consecutive patients (207 males; mean age, 60 years) surviving their first MI. These parameters included: standard deviation of all NN intervals, mean of standard deviations of all NN intervals in all 5-minute segments (SDNNi), standard deviation of the average of all NN intervals in all 5-minute segments, the square root of the mean of the sum of the squares of differences between adjacent NN intervals, and percentage of differences between adjacent NN intervals > 50 ms.
Results: The study endpoint was cardiovascular death. After follow-up for 67 months (range, 1 to 114 months), there were 55 cardiovascular deaths (21%) including 39 sudden deaths (15%). Multivariate Cox regression analysis showed that SDNNi < 30 ms was a significant predictor of cardiovascular death (hazard ratio, 4.98; 95% confidence interval [CI], 2.03 to 12.21; p < 0.001) as was number of ventricular premature complexes >/= 10 beats/hour (hazard ratio, 5.34; 95% CI, 2.26 to 12.62; p < 0.001), age >/= 70 years (hazard ratio, 3.65; 95% CI, 1.44 to 9.23; p = 0.006), and left ventricular ejection fraction < 45% (hazard ratio, 3.29; 95% CI, 1.13 to 9.57; p = 0.03). SDNNi < 30 ms was also an important predictor of sudden cardiac death (hazard ratio, 5.02; 95% CI, 1.49 to 16.85; p = 0.009).
Conclusions: These data suggest that SDNNi is a significant time domain parameter of HRV for long-term prognosis in post-MI patients.