Background: The prognostic implications of alterations in heart rate variability have not been studied in a large community-based population.
Methods and results: The first 2 hours of ambulatory ECG recordings obtained on original subjects of the Framingham Heart Study attending the 18th biennial examination were reprocessed to assess heart rate variability. Subjects with transient or persistent nonsinus rhythm, premature beats > 10% of total beats, < 1 hour of recording time, processed time < 50% of recorded time, and those taking antiarrhythmic medications were excluded. The associations between heart rate variability measures and all-cause mortality during 4 years of follow-up were assessed. There were 736 eligible subjects with a mean age (+/- SD) of 72 +/- 6 years. The following five frequency domain measures and three time domain measures were obtained: very-low-frequency power (0.01 to 0.04 Hz), low-frequency power (0.04 to 0.15 Hz), high-frequency power (0.15 to 0.40 Hz), total power (0.01 to 0.40 Hz), the ratio of low-frequency to high-frequency power, the standard deviation of total normal RR intervals, the percentage of differences between adjacent normal RR intervals that are > 50 milliseconds, and the square root of the mean of the squared differences between adjacent normal RR intervals. During follow-up, 74 subjects died. In separate proportional hazards regression analyses that adjusted for relevant risk factors, very-low-frequency power (P < .0001), low-frequency power (P < .0001), high-frequency power (P = .0014), total power (P < .0001), and the standard deviation of total normal RR intervals (P = .0019) were significantly associated with all-cause mortality. When all eight heart rate variability measures were assessed in a stepwise analysis that included other risk factors, low-frequency power entered the model first (P < .0001); thereafter, none of the other measures of heart rate variability significantly contributed to the prediction of all-cause mortality. A 1 SD decrement in low-frequency power (natural log transformed) was associated with 1.70 times greater hazard for all-cause mortality (95% confidence interval of 1.37 to 2.09).
Conclusions: The estimation of heart rate variability by ambulatory monitoring offers prognostic information beyond that provided by the evaluation of traditional risk factors.