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
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.
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.
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.