Autonomic nervous system activity in essential hypertension: a comparison between dippers and non-dippers

Abstract

The present study was undertaken to investigate the changes in autonomic nervous system activity in essential hypertension. Fourteen normotensive controls and 33 age-matched untreated hypertensive subjects, diagnosed by ambulatory blood pressure (ABP) measurement (24-h systolic ABP value over 140 mm Hg or 24-h diastolic ABP over 90 mm Hg, or both) were recruited. ABP and 24-h electrocardiogram were monitored simultaneously. Power spectral analysis of the R-R interval was performed by a fast Fourier transformation method and the powers of low frequency (LF; 0.04 to 0.15 Hz) and high frequency (HF; 0.15 to 0.4 Hz) components were obtained. Hypertensive subjects were divided into ‘dippers’, whose night-time systolic ABP fell by more than 10% of their daytime ABP, and ‘non-dippers’ in whom this phenomenon was absent. In hypertensive subjects, electrocardiogram monitoring and power spectral analysis were also performed for 5 min before and during 90° tilt. There were no significant differences in the 24-h mean LF/HF power ratio, LF power or HF power between normotensive and hypertensive subjects. A significant negative correlation between the night-time systolic ABP level and the 24-h LF/HF power ratio was found (r = −0.36, P < 0.05) in the hypertensive subjects. a significant positive correlation was found between the 24-h lf/hf power ratio and the percentage nocturnal reduction of the daytime systolic abp in hypertensive subjects (r = +0.40, P < 0.01). the 24-h lf/hf power ratio was significantly lower in non-dippers than in dippers (2.09 ± 1.06 vs 3.24 ± 0.97, P < 0.01). the mean daytime lf/hf power ratio was significantly lower in non-dippers than in dippers (2.50 ± 1.43 vs 4.08 ± 1.27, P < 0.01). the night-time lf/hf power ratio was not significantly different between the two groups. the lf/hf power ratio increased significantly in dippers (from 1.32 ± 1.95 to 4.65 ± 1.54, P < 0.001) during 90° tilt, but there was no significant change in the lf/hf power ratio in non-dippers during tilt (from 1.13 ± 0.28 to 1.36 ± 0.78, ns). the 24-h lf/hf power ratio decreased according as the night-time systolic bp elevated in hypertensive subjects. during ambulatory monitoring, the non-dippers showed a significantly lower lf/hf power ratio than the dippers. the lf/hf power ratio increased significantly in dippers, but not in non-dippers during tilting. these results suggest that impaired cardiovascular reflexes might contribute to the decreased sympathovagal balance in non-dipper type hypertension.