Heart Rate Variability. : Standards of Measurement, Physiological Interpretation, and Clinical Use: Task Force of The European Society of Cardiology and the North American Society for Pacing and Electrophysiology

The powers of the low-frequency (LF) and high-frequency (HF) oscillations characterizing heart rate variability (HRV) appear to reflect, in their reciprocal relationship, changes in the state of the sympathovagal balance occurring during numerous physiological and pathophysiological conditions. However, no adequate information is available on the quantitative resolution of this methodology. We studied 22 healthy volunteers (median age, 46.5 years) who were subjected after a rest period to a series of passive head-up tilt steps randomly chosen from the following angles: 15 degrees, 30 degrees, 45 degrees, 60 degrees, and 90 degrees. From the continuous ECG, after appropriate analog-to-digital conversion, a personal computer was used to compute, with an autoregressive methodology, time and frequency domain indexes of RR interval variability. Spectral and cross-spectral analysis with the simultaneously recorded respiratory signal excluded its contribution to LF. Age was significantly correlated to variance and to the absolute values in milliseconds squared of very-low-frequency (VLF), LF, and HF components. The tilt angle was correlated to both LF and HF (expressed in normalized units [nu]) and to the LF-to-HF ratio (r = .78, -.72, and .68; respectively). Lower levels of correlation were found with HF (in ms2) and RR interval. No correlation was present between tilt angle and variance, VLF, or LF (in ms2). Individual analysis confirmed that the use of nu provided the greatest consistency of results. Spectral analysis of HRV, using nu or LF-to-HF ratio, appears to be capable of providing a noninvasive quantitative evaluation of graded changes in the state of the sympathovagal balance.

Of 22 randomized trials of rehabilitation with exercise after myocardial infarction (MI), one trial had results that achieved conventional statistical significance. To determine whether or not these studies, in the aggregate, show a significant benefit of rehabilitation after myocardial infarction, we performed an overview of all randomized trials, involving 4,554 patients; we evaluated total and cardiovascular mortality, sudden death, and fatal and nonfatal reinfarction. For each endpoint, we calculated an odds ratio (OR) and 95% confidence interval (95% CI) for the trials combined. After an average of 3 years of follow-up, the ORs were significantly lower in the rehabilitation than in the comparison group: specifically, total mortality (OR = 0.80 [0.66, 0.96]), cardiovascular mortality (OR = 0.78 [0.63, 0.96]), and fatal reinfarction (OR = 0.75 [0.59, 0.95]). The OR for sudden death was significantly lower in the rehabilitation than in the comparison group at 1 year (OR = 0.63 [0.41, 0.97]). The data were compatible with a benefit at 2 (OR = 0.76 [0.54, 1.06]) and 3 years (OR = 0.92 [0.69, 1.23]), but these findings were not statistically significant. For nonfatal reinfarction, there were no significant differences between the two groups after 1 (OR = 1.09 [0.76, 1.57]), 2 (OR = 1.10 [0.82, 1.47]), or 3 years (OR = 1.09 [0.88, 1.34]) of follow-up. The observed 20% reduction in overall mortality reflects a decreased risk of cardiovascular mortality and fatal reinfarction throughout at least 3 years and a reduction in sudden death during the 1st year after infarction and possibly for 2-3 years. With respect to the independent effects of the physical exercise component of cardiac rehabilitation, the relatively small number of "exercise only" trials, combined with the possibility that they may have had a formal or informal nonexercise component precludes the possibility of reaching any definitive conclusion. To do so would require a randomized trial of sufficient size to distinguish between no effect and the most plausible effect based on the results of this overview.