ASGSB 2006 Annual Meeting Abstracts
[93]
BEHAVIOR OF
VERY LOW- & LOW- FREQUENCY COMPONENTS IN HEART RATE VARIABILITY
POWER SPECTRA DURING 6 HOURS OF EXPOSURE TO MICROGRAVITY SIMULATED AS
THERMO-NEURAL, DRY, SUPINE IMMERSION
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K. K.
Tripathi, Mona
Dahiya Dangi, Ranjit Kumar, Dept of High
Altitude Physiology & Hyperbaric Medicine, Institute of Aerospace
Medicine,
Vimanapura PO, Bangalore- 560 017 (INDIA)
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Interpretation of
very low frequency (VLF) and low frequency (LF) components in the power
spectrum
of heart rate variability (HRV) is controversial. Recent reports
suggest
susceptibility of both LF and VLF power to parasympathetic influences.
We
examined behaviour of VLF and LF components of HRV power spectra in 7
healthy
male volunteers during 6 hours of dry, thermo-neutral (33-34°C),
supine
immersion, a manipulation known to
increase vagal outflow and attenuate renin-angiotensin mechanisms.
Indices of
HRV were estimated from ECG records of 8.5 minutes during immersion (at
20
minutes, 2 hours, 4 hours & 6 hours) and compared with ground-based
supine
values. Thoracic impedance, BP & Systolic Time Intervals (STI’s)
were also
measured. Statistical tools included Friedman’s non parametric ANOVA
for HRV
indices (due to departures from the assumptions of normality and
homogeneity of
variance) and a repeated measure parametric ANOVA with Greenhouse
Geisser
correction for other data. During immersion, all the time &
frequency
domain indices of HRV exhibited a significant increase [p=0.002, 0.021,
0.002
& 0.022, for SDNN, CV, pNN50 & RMSSD, respectively; p=0.007,
0.000
& 0.010 for VLF, LF & HF power, respectively]. The centre
frequencies
of the spectra did not change [p=0.771, 0.153 & 0.125 for VLF, LF
& HF,
respectively]. Increase in HRV was much more pronounced than that in
R-R
intervals (at 6 hours, R-R interval increased by ~15% but the total
power
in HRV spectrum increased ~four-fold)
and the increase in spectral indices was
appreciable even after normalisation with the square of mean R-R
interval. However,
variation in VLF, LF and HF was not significant when normalised to
total power.
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