Age-dependent influence on heart rate variability in salt-sensitive hypertensive subjects

OBJECTIVE: The known association between systemic arterial hypertension in its initial stages and increased sympathetic nervous system drive prompted us to evaluate the influence of age on autonomic nervous system function in subjects with salt-sensitive arterial hypertension. DESIGN: In a randomized study, autonomic nervous system function was assessed by power spectral analysis of heart-rate variability calculated with an autoregressive algorithm in salt-sensitive hypertensives and controls at baseline and under sympathetic stress (passive head-up tilt). For 1 week before the study, all subjects kept to a diet supplying 120 mEq sodium. Sodium sensitivity was assessed by measuring and comparing arterial pressures after a 7-day controlled dietary sodium intake of 20 mEq per day after a 7-day period on 220 mEq sodium/day. SETTING: Geriatric division at the I Medical Clinic of the University of Rome "La Sapienza". PARTICIPANTS: Sixty-five patients with salt-sensitive hypertension (age range 19 to 89 years) and 64 age-matched normotensive controls, divided for data comparison into three age-groups: < 44 years; 44 to 64 years; and > or = 65 years. MEASUREMENTS: With an autoregressive algorithm in a power spectral analysis of heart rate variability, we detected four spectral frequency-domains: total power (0.0033 to 0.40 Hz), high-frequency power (0.16 to 0.40 Hz), low-frequency power (0.04 to 0.15 HZ) and very-low-frequency power (0.0033 to 0.04 Hz). To determine sodium sensitivity, for 1 week before the study all subjects kept to a diet supplying 120 mEq sodium. Sodium sensitivity was assessed by measuring and comparing arterial pressures after a 7-day controlled dietary intake of 20 mEq per day and after a 7-day period of 220 mEq sodium/day. RESULTS: Results were expressed as natural logarithms of power and normalized units. The hypertensive patients of all ages had significantly lower total power of heart rate variability than the normotensive controls (P < .05). At baseline, the youngest hypertensives had lower natural logarithms and low-frequency normalized units than controls (P < .001). After tilt, only their low-frequency normalized units exceeded those of controls (P < .001). The middle-aged hypertensive group had higher low-frequency normalized units than controls at baseline (P < .05) and after tilt (P < .001). At baseline and after tilt, the oldest hypertensives had lower low-frequency natural logarithms than controls (P < .05) and normalized units equal to those of controls. But the hypertensives of all ages were less able than controls (P < .001) to increase low-frequency power after head-up tilt. In the less than 44-year-old hypertensives, diastolic pressure correlated significantly with low-frequency power of heart rate variability, expressed in normalized units, at baseline (P < .05) and after head-tilt (P < .05). A significant inverse correlation was found between age and the natural logarithm of low-frequency power at baseline (r = -.682, P < .001) and after tilt (r = -.800; P < .001). Also, a significant inverse correlation was found to exist in normotensive subjects between the natural logarithm of low-frequency at baseline (r = -.595; P < .001) and after tilt (r = -.391; P < .001). The two regression line coefficients for age correlated significantly (P < .001) with the natural logarithm of low-power frequency after tilt. CONCLUSION: Whereas sodium chloride-sensitive hypertension appears to be associated with sympathetic hyperactivity in young and middle-aged subjects, in older people it is not. Sympathetic activity diminishes with age, declining faster in hypertensive subjects.