| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|---|
|
|
|
|||||||
|
|||||||||
a Department of Autonomic Neuroscience, Research Institute of Environmental Medicine, Nagoya University, Japan
b Department of Health and Psychosocial Medicine, Aichi Medical University, Japan
c Department of Hygiene and Public Health, Osaka Medical College, Takatsuki, Japan
Tadaaki Mano, Research Institute of Environmental Medicine, Nagoya University Furo-cho, Chikusa-ku, Nagoya 461-8601, Japan E-mail: mano{at}riem.nagoya-u.ac.jp.
Decision Editor: William B. Ershler, MD
 |
Abstract |
|---|
 Top Abstract MethodsResultsDiscussionReferences |
|---|
Methods. A chest electrocardiogram, blood pressure by Finapres, and stroke volume by impedance method were measured in healthy young men (23–31 years old) and healthy elderly men (74–80 years old) during supine rest, at 90° head-up tilt and thermoneutral head-out water immersion. Spectral analysis was applied to the time series data of the R-R intervals (heart rate variability [HRV]) and systolic blood pressure (blood pressure variability [BPV]). The arterial baroreflex gain for heart rate was estimated using frequency transfer function analysis.
Results. The young subjects had stable blood pressure, despite the larger amount of fluid shift induced by both tilt and immersion, and had marked changes in HRV and BPV. The elderly subjects failed to maintain stable blood pressure during these perturbations, despite less fluid shift and no significant changes in HRV and BPV. The arterial baroreflex gain for heart rate was not changed in the elderly subjects, whereas the gain decreased with upright in the young subjects and showed an increasing tendency during immersion compared with upright posture.
Conclusions. These findings suggest that the adaptivity of the autonomic nervous system to gravity-related fluid shift is reduced in elderly people, and this may cause blood pressure instability.
GRAVITY is a natural physical stimulus that humans live with. It affects cardiovascular function because we change posture in our active daily life (upright, sitting, squatting, and lying), which influences hydrostatic pressure gradient from the foot to the head. Changing one's posture resets the hydrostatic pressure gradient, and a gravity-related fluid shift is produced along the Gz axis. This shift induces neural responses of the autonomic nervous system that regulate cardiovascular functions to buffer blood pressure fluctuations induced by postural changes.
Head-up tilt induces a fluid shift from the upper to the lower part of the body, resulting in a decrease in stroke volume. To compensate for the hypotensive effect of reduced stroke volume, heart rate and vascular resistance are increased by enhancing both cardiac and vasomotor sympathetic nerve activities and suppressing cardiac vagal activity through the unloading of arterial and cardiopulmonary baroreceptors (1)(2). In contrast, head-out water immersion induces cephalad fluid shift, resulting in an increase in cardiac filling and loading mainly of the cardiopulmonary baroreceptor. Slowing the heart rate and reducing vascular resistance buffer the hypertensive effects of the increased stroke volume by enhancing cardiac vagal activity and suppressing sympathetic nerve activity (3). These buffering effects of autonomic nerve activity contribute to maintaining stable blood pressure.
Some investigators have tried to relate the autonomic nervous function with aging by using a spectral analysis method. Advancing age results in diminished heart rate variability (HRV), whereas the spectral powers for blood pressure variability (BPV) are greater in elderly persons than in those who are young and middle when supine or standing (4)(5)(6)(7). However, there is no report about the effects of aging on responses in heart rate and blood pressure during both head-up tilt and head-out water immersion. It is important to determine how the age-related changes in the autonomic nervous system correlate with gravity-related fluid shift in elderly persons.
To clarify the effects of aging on the autonomic responses to gravity-related fluid shift, the cardiac sympathovagal balance and vasomotor sympathetic nerve activity were estimated noninvasively by applying spectral analysis to HRV and BPV during head-up tilt and head-out water immersion. In addition, arterial baroreflex gain for heart rate was also evaluated by applying frequency transfer function analysis with coherence analysis (8).
|
Methods |
|---|
|
TopAbstractMethodsResultsDiscussionReferences |
|---|
Protocol
We used the water immersion facilities at the Space Medicine Research Center, affiliated with the Research Institute of Environmental Medicine, Nagoya University. The ambient temperature of the experimental room was maintained at 27 ± 1.0°C with an air conditioner. The subject lay on a tilt table with the right arm stretched on the table. We recorded a chest electrocardiogram (ECG, lead CM5), noninvasive continuous finger arterial pressure by Finapres (model 2300, Ohmeda, Louisville, CO), and intermittent blood pressure in the left upper arm with an automatic sphygmomanometer (model BP-203NP, Colin Electronics, Komaki, Japan). The ECG and arterial pressure wave form were stored in a multichannel frequency modulation magnetic tape recorder (model KS-616U, Sony-Magnescale, Tokyo, Japan). The stroke volume was estimated by an impedance cardiograph (model NCCOM3R7, BoMed Co., Irvine, CA), and the digitized data were transmitted to a personal computer (model PC9801NV, NEC, Tokyo, Japan) through a serial port RS-232C. The ECG, blood pressure, and impedance cardiogram were recorded in the horizontal supine position (supine) for 15 minutes. The tilt table was then inclined to 90° so that the subject stood in an upright position on the foot plate with his hip strapped by a wide belt (upright). His right shoulder was abducted at 100° to not wet the finger cuff and the sensor. The upright position was maintained for 5 minutes to record the variables. Thermoneutral water (34.5°C) was then added up to the acromion of the subject while in the upright position (immersion). It took about 5 minutes to raise the water level from the foot to the acromion. We began to record the variables immediately after the water level reached the acromion, and this condition was maintained for 5 minutes.
Data Analysis
The ECG and blood pressure wave form were played back from the frequency modulation tape and converted to digital data at the sampling frequency of 1,000 Hz through an analog-to-digital converter (model ADX-98E, Canopus, Kobe, Japan). Data from the last 2 minutes of the supine rest, head-up tilt, and water immersion were used for frequency domain analysis. Temporal positions of the R-wave peaks were detected on a personal computer (model PC-9801DA, NEC, Tokyo, Japan). The original ECG, with markers indicating the positions of all the detected R waves, was scanned to eliminate falsely detected error peaks. After confirmation of the positions of all the R-wave peaks, the consecutive R-R intervals (RRI) were calculated from the temporal positions of the waves. Beat-to-beat systolic blood pressure was obtained by detecting each systolic peak on the digitized blood pressure wave form.
Spectral decomposition of HRV and BPV was performed by the Maximum entropy method (MemCalc 1000, Suwa Trust, Sapporo, Japan) on the respective time series data for the beat-to-beat RRI and systolic blood pressure (9). This method is superior to the fast Fourier transform in that it has higher spectral resolution and shorter time series data with no window function. The optimum lag of the prediction-filter order for 2 minutes of data was determined as 35, on the basis of information criteria such as Akaike's information theoretical criterion and the characteristic correlation time. After calculating the power spectral density, the magnitude of the power for HRV and BPV was calculated by measuring the area under the spectral density curve with the trapezoidal formula (10). The values were divided into three major bands: very low frequency (VLF -0.04 Hz), low frequency (LF 0.04–0.15 Hz), and high frequency (HF 0.15–0.40 Hz) domains (11)(12).
Frequency transfer function analysis with coherence analysis was performed using signal processing software DADisp/Pro-32 with advanced DSP module (Version 4.1, Astrodesign, Kawasaki, Japan). Equidistant time series data with a sampling frequency of 2 Hz were made from the time series data of the beat-to-beat RRI and systolic blood pressure by applying cubic spline interpolation. After the DC elimination, frequency transfer function and coherence were calculated by Welchian method with a real data number of 128, an overlapped data number of 112, and a data number of 256 for spectral decomposition. Arterial baroreflex gain was estimated by calculating the mean value of the amplitude for the transfer function when the coherence value was higher than 0.5 in the high frequency domain (0.15–0.40 Hz) (8).
Statistical Analysis
Values are expressed as mean ± standard error (SE). Differences in both between conditions and aging were analyzed by two-way factorial analysis of variance. When the independent variables produced significant effects in the dependent variable, Fisher's protected least significant difference was calculated. Significance was set at p < .05.
|
Results |
|---|
|
TopAbstractMethodsResultsDiscussionReferences |
|---|
|
|
|
|
|
Changes in Arterial Baroreflex Gain for Heart Rate
Arterial baroreflex gains for heart rate in both groups under the three conditions are summarized in Fig. 5. When supine, the arterial baroreflex gain for heart rate was significantly higher in the young subjects than in the elderly subjects (17.6 ± 3.7 vs 6.1 ± 2.6 ms/mmHg; young vs elderly, p < .05). The arterial baroreflex gain for heart rate was decreased by the postural change from supine to upright in the young subjects and showed a significant increase during water immersion compared with head-up tilt (17.6 ± 3.7 vs 2.6 ± 0.4 ms/mmHg; supine vs upright, p < .05). On the other hand, the elderly subjects showed no significant changes in the gain among these three conditions.
|
|
Discussion |
|---|
|
TopAbstractMethodsResultsDiscussionReferences |
|---|
Effects of Aging on Autonomic Responses to Gravity-Related Fluid Shift
The gravity-related fluid shift caused by head-up tilt produces a decrease in central blood volume and unloads the baroreceptors. In the young subjects, unloading of the baroreceptors caused the cardiac sympathovagal balance to become sympathetic predominant and vasomotor sympathetic nerve activity to increase, resulting in stable blood pressure in the face of a large fluid shift. On the other hand, elderly people have been described to have consistently lower spectral powers for HRV during supine rest and head-up tilt (6)(7). In this study, the elderly subjects had lower power for HRV throughout the frequency range considered in this study. However, the changes in LH/HF ratio were not altered by the postural change from supine to upright. Moreover, the VLF power for BPV in the elderly subjects showed no difference from that in the young subjects. These observations are at odds with previous studies, which reported that LH/HF ratio for HRV increased similarly in both young and elderly people by head-up tilt and that the VLF power for BPV is larger in elderly people (6)(7)(11). Data length analyzed from the present study is shorter than theirs. We applied head-up tilt using a tilt table with the help of a belt, so this condition differed from active standing. In addition, the right shoulder was abducted at 100° so as not to wet the finger cuff and the sensor. These factors could have possibly affected the results.
Head-out water immersion induces a cephalad fluid shift (13)(14)(15) that loads baroreceptors, eliciting activation of cardiac vagal activity and relative suppression of cardiac and vasomotor sympathetic nerve activities to compensate an increased cardiac filling (3)(16). There is little in the literature that discusses the neural control of systemic circulation during head-out water immersion, especially in elderly people. Elevation of blood pressure in elderly people during immersion, despite less fluid shift, was reported by Stachenfeld and coworkers (5) and by Tajima and coworkers (17). In a previous study, we reported that vasomotor sympathetic nerve activity, recorded as muscle sympathetic nerve activity, was suppressed by head-out water immersion in the young subjects and that the suppression of muscle sympathetic nerve activity by immersion is reduced with advancing age (3). In the present study, blood pressure was significantly elevated in the elderly subjects with a lack of buffering autonomic responses, in spite of less fluid shift. Our present findings confirm the previous findings, that elderly subjects show a lesser buffering effect of the autonomic functions.
Arterial Baroreflex Function in the Elderly Subjects
We assessed the arterial baroreflex gain for heart rate by applying frequency transfer function analysis with coherence analysis to clarify whether the arterial baroreflex function is involved in the blood pressure instability and less autonomic response to gravity-related fluid shift. As reported previously, the sensibility of arterial baroreflex for heart rate decreased by postural change from supine to upright (8)(18). Moreover, the sensitivity was increased significantly during immersion compared with the upright position in the young subjects. Thus, the young subjects showed an adaptive response of baroreflex function to the evoked gravity-related fluid shift. The decreased sensitivity may maintain higher heart rate, compensating for the hypotensive effect of a reduced stroke volume during head-up tilt. Unlike the young subjects, the elderly subjects showed no changes in the arterial baroreflex gain for heart rate among the three conditions. Thus, in elderly subjects the impaired adaptive response of arterial baroreflex function for heart rate to the evoked gravity-related fluid shift may cause inappropriate autonomic responses, resulting in an instability of systemic blood pressure.
Limitations
We applied frequency domain analysis to BPV and impedance cardiography. These methods are indirect estimates of vasomotor sympathetic nerve activity and stroke volume respectively. Vasomotor sympathetic nerve activity can be directly recorded using a microneurographic technique as muscle sympathetic nerve activity. In a previous study, we recorded muscle sympathetic nerve activity during head-out water immersion from subjects aged 19 to 67 years (3). Stroke volume or central venous pressure can be also measured by echocardiography or intravenous catheterization into the thorax. It may be hard for elderly people to undergo these operations during head-up tilt and water immersion. We therefore calculated only the arterial baroreflex gain for heart rate. Another arterial baroreflex function to control vasomotor sympathetic nerve activity and the cardiopulmonary baroreflex functions were not analyzed.
Although physical activity is known to affect the cardiovascular function, physical activity and energy expenditure were not measured in this study. Taylor and coworkers reported that there was little difference in blood pressure and cardiovascular regulation between younger and older people who had the same V.O2max (19). Thus, physical activity should be taken into consideration for future studies.
|
Acknowledgments |
|---|
Received August 31, 1998
Accepted September 12, 1999
|
References |
|---|
|
TopAbstractMethodsResultsDiscussionReferences |
|---|
This article has been cited by other articles:
|
|
 |
|
  V. E. Claydon and A. V. Krassioukov Clinical correlates of frequency analyses of cardiovascular control after spinal cord injury Am J Physiol Heart Circ Physiol, February 1, 2008; 294(2): H668 - H678. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|---|
| All GSA journals | The Gerontologist |
| Journals of Gerontology Series B: Psychological Sciences and Social Sciences | |
p < .05 vs supine, *p < .05 vs upright, #p < .05 vs young subjects.
