Prognostic Significance of Heart Rate Variability in Post–Myocardial Infarction Patients in the Fibrinolytic Era

Abstract

Background Studies performed before the introduction of fibrinolysis showed that a low heart rate variability (HRV) is associated with higher mortality in post–myocardial infarction (MI) patients. We evaluated whether HRV adds information relevant to risk stratification in patients treated with fibrinolysis as well. Methods and Results From 24-hour ECG recordings obtained at discharge in patients treated with recombinant tissue-type plasminogen activator or streptokinase, we measured several time-domain indexes of HRV: standard deviation (SDNN), root-mean-square of successive differences (RMSSD), and number of RR interval increases >50 ms (“NN50+”). The prognostic value of HRV for total and cardiovascular mortality was assessed. Of 567 patients with valid recordings, 52 (9.1%) died during the 1000 days of follow-up, 44 (7.8%) of cardiovascular causes. All indexes of low HRV were able to identify patients (16% to 18% of total population) with a higher total mortality (20.8% to 24.2% versus 6.0% to 6.8%, depending on index used). The independent predictive value of low HRV was confirmed by the adjusted analysis with the following relative risks: NN50+, 3.5 (95% CI, 1.9 to 6.7); SDNN, 3.0 (95% CI, 1.55 to 5.9); and RMSSD, 2.8 (95% CI, 1.5 to 5.3). Advanced age, previous MI, Killip class at entry, and use of digitalis were also independent predictors. Similar data were obtained for cardiovascular mortality. Conclusions Time-domain indexes of HRV retain their independent prognostic significance even in post-MI patients of all ages treated with fibrinolysis.