Heart rate variability in hypertensive subjects

Hypertension is often associated with findings of sympathetic hyperactivity. Evidence shows that adrenergic receptor stimulation can induce left ventricular hypertrophy. Using an autoregressive algorithm in a power spectrum analysis of heart-rate variability in 14 subjects with mild hypertension (mean age 41 +/- 9.0 years) and 9 age-matched normotensives we compared autonomic nervous system function at baseline (rest) and during sympathetic stress (passive head-up tilt). The spectrum comprised four spectral frequency-domains: total power (0.0033-0.40 Hz), high-frequency power (0.16-0.40 Hz), low-frequency power (0.04-0.15 Hz) and very-low-frequency power (0.0033-0.04). The high-frequency spectral component predominantly reflects vagal activity, the low-frequency component sympathetic nervous system activity. The ratio between low- and high-frequency power expresses the sympathovagal balance. Results were expressed as natural logarithms of power and normalized units. In addition, we compared spectral densities obtained, with the left ventricular mass index evaluated by M-mode echocardiography. Hypertensive subjects had greater low-frequency and low-high frequency ratio values (P < 0.001) than normotensive controls. They also had a low capacity for increase after tilt. Multiple regression analysis showed that the left-ventricular mass index was independently associated with the body mass index(P < 0.0027), very-low frequency (P < 0.043), and low frequency(P < 0.0138) expressed as the natural logarithm, low-high frequency ratio (P < 0.0172) and systolic blood pressure (P < 0.0353). Our findings confirm enhanced sympathetic activity in hypertensive subjects. They also indicate a close association between the left-ventricular mass index and spectral indices of sympathetic activation.