AGE-DEPENDENT INFLUENCE ON HEART-RATE-VARIABILITY IN SALT-SENSITIVE HYPERTENSIVE SUBJECTS

OBJECTIVE: The known association between systemic arterial hypertensio n in its initial stages and increased sympathetic nervous system drive prompted us to evaluate the influence of age on autonomic nervous sys tem function in subjects with salt-sensitive arterial hypertension. DE SIGN: In a randomized study, autonomic nervous system function was ass essed by power spectral analysis of heart-rate variability calculated with an autoregressive algorithm in salt-sensitive hypertensives and c ontrols 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 compa ring arterial pressures after a 7-day controlled dietary sodium intake of 20 mEq per day and after a 7-day period on 220 mEq sodium/day. SET TING: Geriatric Division at the I Medical Clinic of: the University of Rome ''La Sapienza''. PARTICIPANTS: Sixty-five patients with salt-sen sitive hypertension (age range 19 to 89 years) and 64 age-matched norm otensive controls, divided for data comparison into three age-groups: < 44 years; 44 to 64 years; and greater than or equal to 65 years. MEA SUREMENTS: With an autoregressive algorithm in a power spectral analys is of heart rate variability, we detected four spectral frequency-doma ins: 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 p ower (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 p ressures after a 7-day controlled dietary sodium intake of 20 mEq per day and after a 7-day period of 220 mEq sodium/day. RESULTS: Results w ere 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). A t baseline, the youngest hypertensives had lower natural logarithms an d 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 lo wer low-frequency natural logarithms than controls (P < .05) and norma lized units equal to those of controls. But the hypertensives of all a ges 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 o f heart rate variability, expressed in normalized units, at baseline ( P < .05) and after head-up tilt (P < .05). A significant inverse corre lation was found between age and the natural logarithm of low-frequenc y 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-frequen cy at baseline (r = - .595; P < .001) and after tilt (r = - .391; P < .001). The two regression line coefficients for age correlated signifi cantly (P < .001) with the natural logarithm of low-power frequency af ter tilt. CONCLUSION: Whereas sodium chloride-sensitive hypertension a ppears to be associated with sympathetic hyperactivity in young and mi ddle-aged subjects, in older people it is not. Sympathetic activity di minishes with age, declining faster in hypertensive subjects.