DISCUSSION

RR interval fluctuations occurring at respiratory frequencies are vagally mediated and have been shown to be nearly abolished by large-dose atropine (5). Baseline HF power expressed in absolute units of power quantifies vagally mediated fluctuations in instantaneous heart rates due to respiration. LF power expressed in absolute units of power quantifies RR interval fluctuations due to baroreflex-mediated changes in vagal nerve traffic to the heart (5). Total power quantifies heart rate variability due to LF and HF components. The ratio of low frequency to high frequency spectral powers has been used as an index of sympathovagal balance (5). However, in healthy subjects in the supine position, power in both the LF and HF ranges is nearly abolished by large dose atropine (5). Therefore, the assumption that LF/HF ratio signifies sympathovagal balance, especially in the supine position, is problematic (5). This is why we have only analyzed changes in absolute powers of the LF and HF components of HRV. We have not attempted to analyze overall HRV or quantitative sympathovagal balance.

In this study, we have noted large variations in spectral measures of HRV in both the groups, at baseline as well as immediately after the study period. This could possibly be due to inter-individual variations in cardiac responsiveness to changes in vagal nerve traffic to the heart. Singh et al have demonstrated that a substantial proportion of the variance in HRV noted in a population is due to genetic factors (10). Gold Berger at al have postulated that HRV initially increases with increasing vagal nerve traffic to the heart and then decreases with further increase in vagal tone (11). The absence of a significant change in LF and HF powers in our subjects may have been due to the fact that their baseline HRV was saturating high. Secondly, the intensity of training may not have been adequate to produce a quantifiable change in HRV. 

In conclusion, the present study shows that shavasan training for 15 minutes a day, 4 days a week, for six weeks does not significantly affect heart rate variability in young healthy subjects. Further studies may be undertaken to determine the effect of longer duration of shavasan training on heart rate variability. This study demonstrates a useful method of examining the effects of yogic techniques on autonomic modulation of cardiovascular function. Since the BP and HR lowering effects of shavasan have been previously demonstrated in subjects with hypertension, it follows that the effects of shavasan and other relaxation techniques may be more apparent in subjects with reduced baseline HRV. Controlled studies done on subjects with reduced baseline HRV may provide us with more useful information of clinical significance.

ACKNOWLEDGEMENTS 

This study was supported by a grant from the Central Council for Research in Yoga and Naturopathy (CCRYN), New Delhi. J. Amudhan, currently third year medical student in JIPMER, was on an Indian Council of Medical Research (ICMR) studentship during this study. We are grateful to yoga therapist and teacher Mr. G. Kumaran for training the students. We thank Miss R. Lalithambiga, B.Sc., for providing secretarial and technical assistance. We thank Dr. Kaviraja Udupa, Senior Resident, Department of Physiology, JIPMER, for his critical comments on the manuscript.