AUTONOMIC NERVOUS ACTIVITY DURING GRADED HEAD-UP TILT

M. Yamamoto¹, A. Kijima¹, M. Sato¹, M. Suzuki¹ and H. Sato²
¹ School of Nursing, Miyagi University,
1-1 Gakuen, Taiwa-cho, Kurokawa-gun, Miyagi, JAPAN
²The Cardiovascular Institute
7-3-10 Roppongi, Minato-ku, Tokyo, JAPAN

Abstract: The regulatory mechanisms of the cardiovascular system during orthostatic stress is an important focus of research in physiology, particularly in humans. We investigated the response of autonomic nervous activity during graded head-up tilt (HUT), using heart rate variability (HRV) and noninvasive baroreflex sensitivity (BRS). Nine healthy female (mean age 21.7years) performed graded HUT, after a 10-min rest in supine position.　Before and during HUT, blood pressure (BP) and ECG were monitored. BP was measured by the tonometry method. Data were collected in the 0 degree supine position and in the 25, 45, 75 degrees tilt positions for 8 min at each position. The heart rate, DBP and LF/HF increased during HUT-45°& HUT-75°, and BRS showed 52% & 69% reduction from 26.9 ±4.6 msec/mmHg at rest, to 12.8±3.3 msec/mmHg & 8.4±3.5 msec/mmHg during HUT-45°& HUT-75°, and HF also showed 67% & 83% reduction from 426±188 msec² at rest, to 139±61 msec² & 72±36 msec² during HUT-45°& HUT-75°. While SBP was unchanged. This study clearly demonstrated that HR, DBP, HF, LF/HF and BRS were unchanged during low-level tilt (< 25 degrees), while these were significantly and progressively changed during high-level tilt (> 45 degrees). HRV and BRS is useful for investigating the orthostatic regulatory mechanisms of the cardiovascular system.

INTRODUCTION

The regulatory mechanisms of the cardiovascular system during orthostatic stress is an important focus of research in physiology, particularly in humans. Spectral analysis of beat-to-beat variability of cardiovascular parameters has been used to investigate cardiovascular control system. Earlier studies found that a head-up tilt(HUT) reduced the amplitude of the high frequency(HF) component of heart rate variability(HRV), an index of cardiac vagal efferent activity, and increased ratio of low frequency(LF) power to HF power(LF/HF ratio), a marker of sympathovagal balance. However, the responses of these indexes to different levels of orthostatic stress remain unclear. We investigated the response of autonomic nervous activity during graded HUT, using HRV and noninvasive baroreflex sensitivity(BRS).

METHODS

Nine healthy volunteers (mean age 21.7years, female) were enrolled in this study. After a 10-min rest period in supine position, graded HUT were performed. Before and during HUT, blood pressure (BP) and ECG were monitored. BP was measured by the tonometry method as a noninvasive continuous BP monitor. Data were collected in the 0 degree supine position and in the 25, 45, 75 degrees tilt positions for 8 min at each position. For analysis of the 5-min records in resting position and during each tilt positions, the computer selected all sequences of three or more successive heart beats were analyzed in which there were concordant increases or decreases in systolic BP(SBP) and RR interval. If the correlation coefficient between SBP value and RR intervals had a value less than 0.9, the data was discarded from the analysis. A linear regression was applied to each of the sequences, and an average regression slope was calculated. This slope represents BRS (msec/mmHg). HRV was also analyzed in the 5-min records of resting position and during each tilt positions by CGSA method.

RESULTS

As shown in the table, the heart rate, DBP and LF/HF increased during HUT-45°& HUT-75°, and BRS showed 52% & 69% reduction from 26.9±4.6 msec/mmHg at rest, to 12.8±3.3 msec/mmHg & 8.4±3.5 msec/mmHg during HUT-45°& HUT-75°, and HF also showed 67% & 83% reduction from 426±188 msec² at rest, to 139±61 msec² & 72±36 msec² during HUT-45°& HUT-75°. While SBP was unchanged.

	HR(bpm)	SBP(mmHg)	DBP(mmHg)
REST	63±5	95±6	55±7
HUT-25°	66±4	96±5	60±7
HUT-45°	73±7 *	98±9	66±7 *
HUT-75°	88±15 *	99±6	71±6 *
	HF(msec²)	LF/HF	BRS(msec/mmHg)
REST	426±188	0.8±0.4	26.9±4.6
HUT-25°	289±131	1.5±0.9	23.8±6 *
HUT-45°	139±61 *	2.4±1.5 *	12.8±3.3 *
HUT-75°	72±36 *	3.9±2.8 *	8.4±3.5 *

REST vs HUT * P<0.001

DISCUSSION

This study clearly demonstrated that HR, DBP, HF, LF/HF and BRS were unchanged during low-level tilt (< 25 degrees), while these were significantly and progressively changed during high-level tilt (> 45 degrees). HRV and BRS is useful for investigating the orthostatic regulatory mechanisms of the cardiovascular system. And also these methods are noninvasive techniques and provide a useful assessment of a poor prognosis in patients with cardiac diseases in whom other methods can not be applicable.

REFERENCES

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