Key words :inverted posture, skin conductance, level heart rate, variability, finger blood flow

INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

INTRODUCTION

The head stand (sirsasana) has been described as the king of all yoga postures. It is believed that mastering it gives one physical and mental balance and poise (1).

Reports on the physiological effects of this practice have been contradictory, with some results suggesting physiological activation, while others suggested the reverse effect. An early study described an increase in brachial blood pressure in eleven subjects during five minutes of the head stand (2). A 68.6 percent increase in oxygen consumption during the head stand was also reported in six subjects (3). In contrast to these reports suggesting physiological activation, in another study, the heart rate decreased during five minutes of this practice, compared to standing erect before, with a further reduction while supine after it (4). While a decrease in heart rate has been associated with reduced mental arousal, the reduction in heart rate during this inverted yoga posture, compared to standing erect before it, may have been reflexly brought about by baroreceptor stimulation. The change in brachial blood pressure during the head stand may be related to better left ventricular filling and cardiac output because of the inverted posture with better blood flow to the 'dependent' upper parts of the body. These changes during the head stand were comparable, to the effects of 70 degrees head down tilt, which resulted in an increase in BP despite peripheral vasodilation, along with increased cardiac output and left ventricular filling (5). Hundred degrees head down tilt brought about a decrease in forearm vascular resistance, and in the absolute and relative low frequency component of the heart rate variability spectrum (6).

While studies on the effects of the head stand reported changes whose mechanisms (in terms of sympathetic activation or withdrawal) were inferred, the present study aimed at understanding sympathetic and vagal activity, following the head stand, using the heart rate variability spectrum. Also, since the head stand is practiced both with the support of a wall and without support (7), the present study assessed subjects practicing both methods to see whether the effects were different.
top

METHODS

Subjects

Forty adult male volunteers who practiced the head stand in two ways (i.e., with the support of a wall and without), were studied, with 20 subjects practicing each method. For the 'With support group’, the group average age ± SD was 26.0 ± 4.8 years. For the 'Without support group' the group average age ± SD was 24.0 ± 3.2 years.

Design of the study

The subjects of both groups were assessed in a single session of 12 minutes. Recordings were made for 5 minutes each, both before and after 2 minutes of the head stand, but not during the practice, as artifacts related to muscular contraction due to the inverted posture interfered with the recording. Recordings were taken both before and after the practice while subjects were seated, to provide a contrast to the inverted posture.

Measurements

A 4-channel polygraph (Medicaid Systems, Chandigarh, India) was used to record the electrocardiogram (EKG), respiration, finger plethysmogram amplitude and skin conductance level. EKG was recorded using standard limb lead I configuration. The EKG was digitized using a 12 bit analog-to-digital converter (ADC) at a sampling rate of 500 Hz. The data recorded were visually inspected off-line and only noise free data were included for analysis. The HRV analysis software was developed at the Dept. of Electrical Engineering, Indian Institute of Science, Bangalore (by A.G. Ramakrisnan, Ph.D.). The R waves were detected to obtain a point event series of successive R-R intervals, from which the beat to beat heart rate series was computed (8). Simultaneously, polygraph recordings of the variables mentioned below were also carried out. Skin conductance was recorded using Ag/AgCl disc electrodes with electrode gel (Medicon, Madras, India), placed in contact with the volar surfaces of the distal phalanges of the index and middle fingers of the left hand. A low-level DC preamplifier was used and a constant voltage of 0.5V was passed between the electrodes. Respiration was recorded using a nasal thermistor clipped to the more patent nostril. Finger plethysmogram was recorded placing the photoplethysmograph on the volar surface of the distal phalanx of the index finger of the right hand. The amplitude was sampled from the ascending portion of the wave (9). The blood pressure was recorded with a sphygmomanometer by auscultation over the right brachial artery.

Data extraction

The following data were extracted from the polygraph records : The breath rate (in cycles per minute) was calculated by counting the breath cycles in 60 second epochs, continuously. The skin conductance level (SCL in micro siemens) and finger plethysmogram amplitude (in mm) were sampled at 20 second intervals. For each subject, the average of the values obtained during the 5-minute session was used for analysis. Frequency domain analysis of heart rate variability (HRV) data was carried out for the 5-minute recordings before and after the head stand. The mean heart rate was obtained from this record. The HRV power spectrum was obtained using Fast Fourier Transform (FFT). The energy in HRV series in the following specific frequency bands was studied, viz. the very low frequency band (0.0-0.05 Hz), low frequency band (0.05-0.15 Hz), and high frequency band (0.15-0.50 Hz). According to guidelines of the Task force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, the low frequency and high frequency values were expressed as normalized units (10).

Statistics

A two factor analysis of variance (ANOVA) was used to check for significant differences between the two categories of subjects, i.e., factor A, and for differences between recordings before and after, i.e., factor B. The Tukey test for the least significant difference between means was used for multiple comparisons. The 't' test for paired data was used to assess the significance between after and before values of each group, separately, to detect changes which were not significant with the Tukey test.

The head stand (sirsasana)

At the start of the practice, subjects kept the crown of their head on a firm surface supported with interlocked fingers behind and forearms on either side. With the knees flexed, the legs were gradually raised, till the body was stretched upwards, resting on the head (7). During this practice one group of subjects were supported by a wall, while the other group was not.
top

RESULTS

The data before and after the practice were found to be (a) normally distributed and (b) not of unequal variance. Hence parametric statistics were used for analysis.

Two factor analysis of variance (ANOVA)

There was a significant difference between the recordings before and after the head stand (i.e., factor B) in the power of the low frequency component of HRV [F = 6.79, since F .05 (2) 1, 76 = 5.23, hence P<.05], in the high frequency power value [F = (6.74, since F .05 (2) 1, 76 = 5.23, hence P<.05], and in low frequency/high frequency ratio [F = 9.84, since F .005 (2) 1, 76 = 9.79, hence P<.005]. The heart rate, skin conductance level, breath rate, finger plethysmogram amplitude and both systolic and diastolic blood pressure were not significantly different between categories or recordings, with interaction between the factors being not significant, i.e., A X B (P>.50). Here, the F values for the degrees of freedom (1, 76) have been derived by linear interpolation from the values for denominator equal to 70, 80 in the standard table, according, to the method described (11).

Tukey test for multiple comparisons between mean values

None of the values were significantly different after the practice compared to before, for both categories of subjects. Also, the baseline values of the two categories of subjects did not differ (P>.10, for all comparisons).

The 't' test for paired data

Following the head stand compared to before, there were the following changes: (i) In both groups, there was a significant increase in the low frequency and reduction in high frequency power values of the, heart rate variability (HRV) spectrum expressed in normalized units (in subjects who practiced without wall support, for both LF and HF, P<.02) and in subjects who practiced with wall support, for both LF and HF, P<.05). This contributed to a significant increase in low frequency/high frequency power ratio (P<.02 for 'Without support' subjects; P<.05 for 'With support' subjects). The heart rate was significantly reduced following the head stand (P<.02 for 'Without support' subjects; P<.05 for 'With support' subjects). (ii) The 'Without support' subjects showed a significant increase in skin conductance level (P<.05) and (iii) the 'With support' subjects showed a reduction in the amplitude of the finger plethysmogram (P<.05). No significant changes were observed in respiratory rate or systolic and diastolic blood pressure values in either group following the head stand.

The group average values ± S.E.M are given in Table I. Sample tracings of the HRV spectra and polygraph recordings in two subjects each belonging to one of the two groups, are given in Fig. 1, 2 and 3, 4 respectively.

Table I

click for full view

Table I: Components of heart rate variability spectrum and other autonomic variables before and after the head stand in two categories of subjects (n=20) values are group mean ± SEM.

Fig. 1

click for full view

Fig. 1: Sample records of heart rate variability spectrum made before (upper record) and after (lower record) two minutes of the head stand practiced with wall support in a single subject (RAV/24/M). The vertical axis gives the power values in BPM²/Hz. The two vertical dotted lines separate the three frequency components, viz. very low frequency (VLF), low frequency (LF) and high frequency (HF). The present record shown an increase in the low frequency power (stippled portion) and a decrease in high frequency power (hatched portion) following two minutes of the head stand.

Fig. 2

click for full view

Fig. 2: Sample records of heart rate variability spectrum made before (upper record) and after (lower record) two minutes of the head stand practiced without wall support in a single subject (SRI/27/M). The rest of the details and the findings are as for Figure 1. However note the magnitude of changes is less in the subject who practiced the head stand with wall support (RAV; Fig.1) compared to this subject (SRI) who practiced it without wall support.

Fig 3.

click for full view

Fig. 3: Polygraphic tracing of subject (RV/27/M) as a representative of the ‘With Support’ group. Four variables are shown here: (a) volar skin conductance, (b) EKG (lead I), (c) respiration with nasal thermistor, and (d) finger plethysmogram. For each variable the upper trace was taken before and the lower trace was after the practice of sirsasana. Note the decrease in finger plethysmograpm amplitude after sirsasana (tracing 4b) compared to before (4a).

Fig 4.

click for full view

Fig. 4: Polygraph tracing of subject (SRI/27/M) as a representative of the ‘Without Support’ group. The rest of the details are the same as for Figure 3. Note the increase in skin conductance after sirsasana (tracing 1b) compared to before (1a).

top

DISCUSSION

Following two minutes of the head-stand compared to the preceding period while sitting erect, there was a significant increase in the power of the low frequency component of the heart variability spectrum, a decrease in the high frequency power value and decrease in heart rate in both categories of subjects (i.e., those who practiced without support and those who used support). Also, the 'Without support' group showed a significant increase in skin conductance level. The 'With support' group showed a significant reduction in finger plethysmogram amplitude.

The low frequency component (0.05- 0.15 Hz) of the heart rate variability spectrum when expressed in normalized units, is a quantitative marker of sympathetic modulations; the high frequency component (0.15-0.50 Hz) has been correlated with parasympathetic activity and the LF/HF ratio reflects the sympathovagal balance (10). From the present study it appears that in both categories of subjects the cardiac sympathetic activity has increased, vagal activity has reduced and sympathovagal balance has changed.

The electrodermal activity recorded from the distal phalanges is a sensitive index of changes in sympathetic tone (12). The skin resistance is the reciprocal of skin conductance (13) and is a measure of arousal. In a subject at rest, an increase in skin resistance is an index of relaxation (14), while reduced skin resistance has the reverse interpretation. The 'Without support' subjects in the present study showed increased skin conductance level, while the finger plethysmogram amplitude was significantly lower in the 'With support' group subjects. The finger photoplethysmograph measures the peripheral cutaneous blood flow and in turn the sympathetic vasomotor changes. Decreased amplitude of the plethysmogram record as observed in the present study indicates peripheral vasoconstriction due to sympathetic arousal (15).

A previous study reported a decrease in heart rate during the head stand, which was more in magnitude after the practice, while lying supine (4). In the present study also, the heart rate significantly decreased in both groups after the practice. While a decrease in heart rate during the head stand may be a reflex response due to baroreceptor activation, a decrease in heart rate while seated erect after the practice, is less easy to explain and may be due to a continuation of changes which occurred during the head stand.

The inverted posture increases left ventricular filling and cardiac output (5). This can be expected to activate the baroreceptors. When resuming the erect posture, the arterial pressure in the head and upper part of the body tends to fall and the falling pressure at the baroreceptors elicits an immediate reflex, resulting in sympathetic discharge throughout the body (16).

In the present study, this mechanism may explain the following changes : increased power of the low frequency component of the HRV spectrum, increased skin conductance level, and decreased finger plethysmogram amplitude.

The two categories of subjects ('With support', 'Without support') showed activation of different subdivisions of the sympathetic nervous system selectively, viz., sudomotor and vasomotor respectively. In attempting to explain this, a previous study reported that during hypothermia, static hand grip caused sustained increase in skin sympathetic nerve activity (SNA) and in electrodermal activity along with a transient increase in skin vascular resistance (17). There were directionally opposite changes in estimated skin vascular resistance : with exercise-induced vasodilatation during hyperthermia, and exercise induced vasoconstriction during hypothermia. It was concluded that static exercise markedly increases sympathetic out-flow to skin; the increases in skin SNA appear to be caused mainly by central command rather than by muscle afferent reflexes and this cutaneous sympathetic activation appears to be targeted both to sweat glands and to vascular smooth muscle, with relative targeting being temperature dependent.

Hence we concluded that sirsasana was a form of 'static exercise' which produced both of the reported effects (increased electrodermal (sudomotor) activity and increased cutaneous vasomotor activity). The presence of differential effects in the two groups was not easy to explain, however an early report showed that different stimuli generate activity in different divisions of the sympathetic nervous system (18). This shows that under different types of exercise the different subdivisions get selectively and differentially activated. It is possible that the two methods of practicing the postures (i.e., with support and without) may activate different mechanisms, and even induce different, minor temperature changes, according to the method of practice. Further studies are needed for better understanding.

The findings of the present study have to be interpreted with discretion, as recordings were not made during the practice of the headstand. However, the present results suggest that the practice causes sympathetic activation, which required further investigation to understand applications and precautions related to this posture.
top

REFERENCES

1. Swami Mukthibodhananda Saraswathi. Hatha Yoga Pradipika - The Light on Hatha Yoga, edited by Swami Karmananda Saraswathi. Bihar School of Yoga, Bihar, India 1985.

2. Kuvalayananda Swami. Blood pressure experiments on shirsasana. Yoga Mimamsa 1926; 2(l): 92-99.

3. Rao S. Metabolic cost of headstand posture. J App Physiol 1962; 17(l): 117-118.

4. Rao S. Cardiovascular responses to head stand posture. J Appl Physiol 1963; 8(5): 987-990.

5. Dekulander G, Lecroart JL, Chammas C, Goullard L, Houdas Y. Intracardiac haemodynamics in man during short periods of head down and head up tilt. Aviat Space Eniviron Med 1993; 64(l): 43-49.

6. Weise F, London GM, Guerin AP, Pannier BM, Elghozi EL. Effect of head down tilt on cardiovascular control in healthy subjects : a spectral analytic approach. Clin Sci 1995; 88(l): 87-93.

7. Iyengar BKS. Light on yoga - Yoga Dipika. Harper Collins Publishers India (Pvt.) Ltd. New Delhi 1992.

8. Raghuraj P, Ramakrishnan AG, Nagendra HR, Telles S. Effect of two selected yogic breathing techniques on heart rate variability. Ind J Physiol Pharmacol 1998; 42(4): 467-472.

9. Roy M, Steptoe A. The inhibition of cardiovascular responses to mental stress following aerobic exercise. Psychophysiol 1991; 28: 689-699.

10. Task force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Heart rate variability : Standards of measurement, Physiological interpretation, and Clinical Use. Circulation 1996; 93(5):1043-1065.

11. Zar JH. Biostatistical analysis. Second edition. Prentice-Hall, London 1984.

12. Scerba AS, Freedman LW, Raine A, Dawson ME, Venables P. A major effect of recording sites on measurement of electrodermal activity. Psychophysiol 1992; 29: 241-246.

13. Dawson EM, Schell AM, Filion DL. The electrodermal system. In : Principles of Psychophysiology, edited by JT Cacioppo and LG Tassinary. Cambridge University Press, Cambridge 1990;p.295-324.

14. Collet C, Roure R, Rada H, Dittmar A, Maury V. Relationships between performance and skin resistance evolution involving various motor skills. Physiol Behauior 1996; 59(4/5): 953-963.

15. James FP, David S. The cardiovascular system. In Principles of Psychophysiology, edited by JT Cacioppo, LG Tassinary. Cambridge University Press, Cambridge 1990; p. 456-512.

16. Guyton AC, Hall JE. Text book of Medical Physiology. Ninth Edition, Prism books (Pvt.) Ltd. Bangalore 1996.

17. Vissing SF, Scherrer U, Victor RG. Stimulation of sympathetic nerve discharge by central command. Differential control of sympathetic outflow to skin and skeletal muscle during static exercise. Circ Res 1991; 69(l): 228-238.

18. Engel BT. Stimulus-response and individual response specificity. Arch Gen Psychiatr 1960; 2: 305-313.