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

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 degrees 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 degrees 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.