The effect of the lateral decubitus position on vagal tone

Transcription

1 The effect of the lateral decubitus position on vagal tone G.-Y. Chen 1 and C.-D. Kuo 2 1 Department of Medicine, Provincial Tao-Yuan General Hospital, Tao-Yuan, and Institute of Clinical Medicine, National Yang-Ming University, Taipei, Taiwan, Republic of China 2 Cardiopulmonary Laboratory, Respiratory Therapy Department, Veterans General Hospital-Taipei, and College of Medicine, National Yang-Ming University, Taipei, Taiwan, Republic of China Summary The average person spends about one-third of their time in a recumbent position. However, little is known about the effect of recumbent posture on autonomic nervous activity. Manoeuvres which can increase vagal tone have been sought both in the normal subject and in patients with heart disease. We have studied the autonomic effect of various recumbent positions, namely the supine, left lateral decubitus and right lateral decubitus positions, in healthy subjects by using spectral heart rate variability analysis. Both time- and frequency-domain measures were calculated and compared between the three recumbent positions. The normalised high-frequency power was used as the index of cardiac vagal activity, the normalised low-frequency power as the index of cardiac sympathetic activity and the low-frequency power/high-frequency power ratio as the index of sympathovagal balance. The normalised high-frequency power is highest in the right lateral decubitus position, followed in decreasing order by left lateral decubitus and supine positions. The low-frequency power/high-frequency power ratio has the reversed trend as compared with that of the normalised high-frequency power. These results suggest that cardiac vagal activity is greatest when the right lateral decubitus position is adopted. Keywords Parasympathetic nervous system; vagus. Position; effects, cardiovascular.... Correspondence to: Dr Cheng-Deng Kuo Accepted: 22 December 1996 Spectral heart rate variability analysis is a noninvasive, useful method for the assessment of autonomic nervous control of heart rate. The high-frequency spectral component in the power spectrum of RR intervals can reflect cardiac vagal activity [1, 2]. A decreased high-frequency component has been found in various physiological and pathological conditions, such as the upright posture (relative to supine) [3], ageing [4, 5], acute myocardial infarction [6], diabetes mellitus [7], chronic renal failure [8] and congestive heart failure [9]. The recovery of the reduced high-frequency component and the normalisation of sympathovagal balance are signs of resolution of acute myocardial infarction [6]. Pharmacological measures have been tried to increase vagal activity in normal subjects [1] and in patients with acute myocardial infarction [11] and congestive heart failure [12]. Any physiological means that can increase the vagal activity of the subjects may be better than pharmacological measures because of its safety, feasibility and lack of complications. Autonomic nervous activity is sensitive to physical and mental stress. The effect of posture on autonomic nervous activity has been studied in the supine position, the standing position and at a tilt; vagal tone was found to be increased in the supine position as compared with that of the standing position [5, 13]. There are benefits to be gained from the position which can produce the greatest vagal activity and/ or the least sympathetic activity. Such posture can be used as an effective physiological vagal enhancer in normal subjects and in patients with depressed vagal activity. However, the effect of various recumbent positions on the autonomic nervous activity has not been investigated in normal subjects or in patients with depressed vagal activity. The aim of this study was to examine the effect of various recumbent positions on the autonomic nervous activity by using spectral heart rate variability analysis. Methods Volunteers were recruited from a community college. Study subjects had no major diseases known to affect 653

2 G.-Y. Chen and C.-D. Kuo Lateral decubitus position and vagal tone Supine high-frequency power/total power: 19.7% low-frequency power/total power: 3.9% low-frequency power/high-frequency power: 1.57 Power spectral density (ms 2.Hz 1 ) Left lateral decubitus high-frequency power/total power: 43.3% low-frequency power/total power: 29.% low-frequency power/high-frequency power:.67. Right lateral decubitus high-frequency power/total power: 49.9% low-frequency power/total power: 19.5% low-frequency power/high-frequency power: Frequency (Hz).4 Figure 1 The RR interval power spectra in the supine, left lateral decubitus and right lateral decubitus positions in a representative study subject. The highfrequency power/total power ratio is lowest when the supine posture was assumed, followed in increasing order by left lateral decubitus and right lateral decubitus positions, whereas the lowfrequency power/total power and the lowfrequency power/high-frequency power ratio have the reverse trends. heart rate variability, nor did they take any drugs or smoke. Informed consent was obtained from all participants. All volunteers were instructed not to drink caffeinated or alcoholic beverages for at least 24 h prior to the study session. Three recumbent positions, namely the supine, left lateral decubitus and right lateral decubitus, were assumed in random order by the study subject. After a 5-min rest, a rhythm electrocardiograph (ECG) tracing was obtained (Model 9621A Spacelabs Inc., Redmond, WA, USA). The ECG signal data were recorded on a personal computer for 15 min for each recumbent position. The sampling frequency for the ECG recording was 5 Hz. During the rest and recording periods, the subject was asked to relax as much as possible with closed eyes. The recorded ECG signals were analysed later to measure the consecutive RR intervals. The last 512 stationary RR intervals were obtained in each recumbent position for later spectral heart rate variability analysis. The mean, standard deviation and coefficient of variation of 512 stationary RR intervals were calculated for each subject using standard formulae. The power spectra of these RR intervals were obtained by means of fast Fourier transformation (Mathcad, Mathsoft Inc., Cambridge, MA, USA). The direct current component was excluded in the calculation of the power spectrum. The high-frequency spectral peak in the range of.15.4 Hz was identified and the area-under-the-curve of the spectral peaks was defined as the high-frequency power. Similarly, the area of the spectral peaks within 654

3 G.-Y. Chen and C.-D. Kuo Lateral decubitus position and vagal tone Table 1 Postural effects on heart rate variability. Results are expressed as mean (SD). Left lateral Right lateral Supine decubitus decubitus Time domain Mean RR interval; ms (87.4) (92.1) (9.3) Standard deviation of RR interval; ms 48.2 (16.4) 46.7 (15.4) 44.9 (15.1) Coefficient of variation of RR interval; % 5.6 (1.7) 5.4 (1.6) 5.3 (1.7) Frequency domain Normalised high-frequency power; % 21.5 (14.5)* 27.8 (15.)* 36.3 (16.7)* Normalised low-frequency power; % 32.6 (11.3) 29.4 (11.1) 27.4 (9.9) Low/high-frequency power ratio 2.8 (1.92)* 1.9 (2.8)* 1.12 (1.1)* Frequency of high-frequency peak; Hz.28 (.4).28 (.4).28 (.4) * Significant difference (p <.5). the range of.4.15 Hz was defined as the low-frequency power and the area of the peaks within the whole range (.1.4 Hz) as the total power [14]. The normalised high-frequency power (= high-frequency power/ total power) was used as the index of vagal activity, the normalised low-frequency power (= low-frequency power/total power) as the index of sympathetic activity and the low-frequency power/high-frequency power ratio as the index of sympathovagal balance of the study subject [2, 3, 14]. Statistical analysis The Friedman repeated measures analysis of variance on ranks (SigmaStat statistical software, Jandel Scientific, San Rafael, CA, USA) was employed to compare the mean, standard deviation and coefficient of variation of the RR intervals and normalised high-frequency power, normalised low-frequency power, low-frequency power/highfrequency power ratio and the frequency of the highfrequency peak in the frequency domain among supine, left lateral decubitus and right lateral decubitus positions. Significant difference was further analysed by pairwise comparisons using the Student Newmann Keuls test. A p value <.5 was considered statistically significant. Results Twenty-eight young healthy adults (11 male, 17 female, mean age 24.7 (SD 3.2) years) volunteered to participate in this study. Figure 1 shows the RR interval power spectra in the supine, left lateral decubitus and right lateral decubitus positions in a representative study subject. The normalised high-frequency power was highest (49.9%) when the right lateral decubitus position was assumed, followed in decreasing order by the left lateral decubitus (43.3%) and supine positions (19.7%). Table 1 shows the postural effects on the measures of heart rate variability for the group of subjects. In the time domain, there were no significant differences in the mean, standard deviation and coefficient of variation of RR interval between the recumbent positions. In the frequency domain, the normalised high frequency was highest in the right lateral decubitus posture (36.3 (16.7)%), followed in decreasing order by left lateral decubitus (27.8 (15.)%) and supine (21.5 (14.5)%) positions. The low-frequency power/high-frequency power ratio had the reverse trend as compared with the normalised highfrequency power. Both normalised high-frequency power and low-frequency power/high-frequency power ratio were significantly different for all pairwise comparisons in the three recumbent positions. There were no significant differences in the normalised low-frequency power and in the frequency of the high-frequency peak between three recumbent positions. Discussion Our results demonstrate that the normalised highfrequency power, an index of cardiac vagal activity [2, 3, 14], is highest in the right lateral decubitus position when studied in healthy adults. This result suggests that a higher vagal activity can be induced when this position is assumed. Age and many diseased states are known to be associated with depressed vagal activity. To evaluate the clinical potential of vagomimetic intervention, studies have been conducted to increase the vagal activity in normal subjects [1] and in patients with acute myocardial infarction [11] and congestive heart failure [12]. A low dose of transdermal hyoscine was found to be effective in the augmentation of vagal activity and may be helpful in the management of patients with acute myocardial infarction [11] and congestive heart failure [12]. Our finding that the right lateral decubitus position can result in the highest vagal activity during recumbency might have important implications in normal subjects and even in patients with 655

4 G.-Y. Chen and C.-D. Kuo Lateral decubitus position and vagal tone depressed vagal activity. Since most people spend up to 8 h in bed, the cumulative effect of right lateral decubitus posture on vagal activity should not be overlooked. With its safety and feasibility, this physiological manoeuvre might be advisable in normal subjects and might also be beneficial to patients with various kinds of heart diseases with depressed vagal activity. It has always been considered that there is reciprocal regulation in the autonomic nervous activity, whereby conditions which activate sympathetic activity, will depress vagal activity at the same time [15]. With the advent of electrophysiological studies, recordings of the sympathetic and vagal discharges have indicated that not only reciprocal [16] but also nonreciprocal [17] organisation is involved in the autonomic regulation of heart rate. Therefore, the low-frequency power/high-frequency power ratio was suggested as a marker of changes in the sympathovagal balance in the heart rate variability analysis to exclude possible nonreciprocal regulation [3]. In this study, the low-frequency power/high-frequency power ratio has a reversed trend as compared with that of the normalised high-frequency power. This suggests that the right lateral decubitus position can simultaneously activate vagal activity and suppress sympathetic activity. The reason why the normalised low-frequency power did not show a significant difference between the three recumbent positions may be that the low-frequency component is mediated by both sympathetic and vagal activity [1 3], rendering it less sensitive to the subtle change in cardiac vagal activity. The postural effect on gas exchange during recumbency has been studied in patients with unilateral lung disorders [18 2] and pleural effusions [21, 22]. It was found that both arterial oxygen tension and arterial oxygen saturation change according to the recumbent positions assumed. Thus, taking the position which has the best arterial oxygen tension has been suggested to be the rule in the management of patients with lung disorders [22 26]. Unfortunately, the postural effect on gas exchange during recumbency has not been shown in normal subjects [18]. In contrast to gas exchange, the indices of vagal activity or sympathovagal balance are different between the three recumbent positions in normal subjects in this study. Our results suggest that autonomic nervous activity might be more sensitive to postural change than gas exchange during recumbency. The high-frequency component has been shown to be synchronous with respiration; it can be considered to be a quantitative measure of respiratory sinus arrhythmia [2], which has been shown to correlate negatively with respiratory frequency in adults [23]. Since the normalised high-frequency power is different between the three recumbent positions, whereas the frequency of the highfrequency peak is not, the increase in the normalised high-frequency power in the right lateral decubitus position cannot be explained by respiration itself and other mechanisms must be sought. Several mechanisms might be involved in the enhancement of vagal activity when the right lateral decubitus position is assumed. Firstly, the smaller left lung volume and compressive effect of the heart and mediastinum on contiguous lung might compromise lung function, resulting in the suppression of parasympathetic activity when the left lateral decubitus or supine positions were assumed. Secondly, the position of the heart is lower in the left lateral decubitus and supine positions than in the right lateral decubitus position. Gravity might exert an increased workload on cardiac function when the left lateral decubitus or supine posture is assumed. A larger workload required in the left lateral decubitus or supine position, as compared with the right lateral decubitus position, will produce more sympathetic and less vagal activity. The reduction in this workload when the study subject assumes the right lateral decubitus position will lead to an enhancement of vagal activity. Thirdly, the human sino-atrial node receives its vagal innervation mainly from the right vagus nerve. The right vagus nerve in the neck might be stimulated by periodic massage from the pulsation of the carotid artery in the right lateral decubitus position. Fourthly, because of the right-sided anatomical position of the right atrium, the venous return from the venous system via inferior and superior vena cavae to the right atrium is more favourable when assuming the right lateral decubitus position. Fifthly, the right lateral decubitus position might facilitate gastric emptying, resulting in an increase in the vagal activity. Finally, asymmetrical pressure on the shoulder, thorax and buttocks might induce ipsilateral nasal congestion and contralateral decongestion [24]. Since forced unilateral nostril breathing can cause selective contralateral hemisphere stimulation [25] and the vagal activity can be increased by right hemisphere stimulation [26], asymmetrical pressure on the shoulder, thorax and buttocks might be related to the elevated vagal activity in the right lateral decubitus position [25]. All these factors might have a role in the activation of vagal activity in the right lateral decubitus position; the detailed mechanism warrants further investigations. Acknowledgments This study was supported by the VGH-NTHU Joint Research Program and the Clinical Research Centre, Institute of Biomedical Sciences, Taipei, Taiwan, Republic of China. References 1 Akselrod S, Gordon D, Ubel FA, Shannon DC, Barger AC, 656

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