Blunted parasympathetic modulation in salt-sensitive patients with essential hypertension: evaluation by power-spectral analysis of heart-rate variability

Abstract

Objective: To evaluate autonomic nervous function by power-spectral analysis of heart-rate variability in salt-sensitive and non-salt-sensitive patients with essential hypertension under the conditions of low and high salt intakes.

Design and methods: The blood pressures, heart rates, and electrocardiogram R-R intervals of 20 hypertensive patients were measured at intervals of 30 min during a 24 h period using a portable recorder (TM-2425) on the last day of the high- (250 mmol NaCl/day) and low-salt (25 mmol NaCl/day) diet periods. The patients whose 24 h average mean blood pressures were increased by more than 10% by the high salt intake were defined as salt-sensitive (n = 10); the other patients were considered non-salt-sensitive (n = 10). Power-spectral analysis of R-R intervals was performed to obtain the low-frequency component (0.05-0.15 Hz) and the high-frequency component (0.15-0.40 Hz).

Results: The average 24 h blood pressure in the salt-sensitive patients was increased by the high salt intake [by 19.1 +/- 2.0/9.1 +/- 0.8 mmHg (mean +/- SEM)], whereas the heart rate did not change. In contrast, the increase in 24 h blood pressure in the non-salt-sensitive patients caused by the high salt intake was not significant and the heart rate was decreased significantly by the high salt intake (by 5.9 +/- 1.4 beats/min). The high-salt diet increased significantly the high-frequency component and decreased the low-frequency:high-frequency component ratio both during the daytime and during the night-time for the non-salt-sensitive patients. In contrast, the high-frequency component and the night-time low-frequency: high-frequency component ratio of the salt-sensitive patients did not respond to dietary salt manoeuvres.

Conclusions: Responses of the parasympathetic and sympathetic nervous systems to dietary salt manoeuvres were blunted in salt-sensitive patients. These altered modulations of the autonomic nervous system may contribute to the salt sensitivity of the blood pressure in patients with essential hypertension.