Very low frequency oscillations in the power spectra of heart rate variability during dry supine immersion and exposure to non-hypoxic hypobaria

Abstract

The origin of very low frequency (VLF) oscillations in the power spectra of heart rate variability (HRV) is controversial with possible mechanisms involving thermoregulation and/or renin-angiotensin-aldosterone system. Recently, a major contribution from vagal influences has been suggested. The present study investigated the behaviour of VLF (0.004-0.040 Hz) components of HRV power spectra in a group of healthy male volunteers during their exposure to (1) dry, supine, immersion in thermo-neutral water for 6 h (n = 7) and (2) non-hypoxic hypobaria (breathing 40-60% oxygen at 15,000' simulated in a decompression chamber) for 5 h (n = 15). The two manoeuvres are established to increase vagal outflow. During both the manoeuvres, all the frequency domain indices of HRV exhibited a significant increase. Increase in HRV was much more than that in the R-R interval. At 6 h of immersion, the R-R interval increased by ∼ 15% but the total power increased ∼ fourfold. Similarly, at 5 h of exposure to hypobaria, total power increased ∼ twofold with a very modest increase in an R-R of ∼ 9%. Increase in spectral power was appreciable even after normalization with mean R-R(2). Increase in VLF during immersion was more than reported during enalaprilat blockade of angiotensin convertase enzyme. Plasma renin activity did not vary during hypobaria. There was a significant increase in pNN50, an established marker of cardiac vagal activity. Centre frequencies of the spectra and slope (β) of the relation between log(PSD) and log(frequency) did not change. Results were supportive of the notion that the parasympathetic system is pre-potent to influence slower (than respiratory) frequency components in HRV spectrum. Additionally, such an effect was without a change in the time constant of effector responses or pacemaker frequencies of VLF and LF periodicities and HRV was not a simple linear surrogate for cardiac vagal effects. An invariance of spectral exponent (β) ruled out contamination of VLF and LF spectra from fractal power as an alternate explanation.

Residual vasomotor activity assessed by heart rate variability in a brain-dead case

“…Regarding the VLF band, there are still discussions about its origin 23 24. The VLF power probably represents several factors influencing the heart, such as the renin–angiotensin system, thermoregulation and sympathetic vasomotor activity.…”

Myogenic Oscillations in Rabbit Ocular Vasculature Are Very Low Frequency

“…A major contribution from vagal influences has been suggested, but recent studies do not support this thesis [34]. Recently, Aletti et al [35] showed that cardiac output buffers slow oscillations in the LF band and arterial BP variability of vasomotor origin.…”

Watching neutral and threatening movies: Subjective experience and autonomic responses in subjects with different hypnotizability levels

“…This may indicate a different efficacy of the renin-angiotensin system/ thermoregulatory activity as well as a different expression of the parasympathetic activity (Busek et al, 2005;Tripathi 2011) in males and females. A limitation of the study is that the interactions of hypnotizability and gender observed in the eT/eR and iT/iR subsamples could not be analyzed owing to the small number of subjects included in each subsample as a consequence of the unpredictable necessity to split the participants by their subjective reaction to the threatening movie.…”

“…The VLF functional role is not well-assessed. In fact, it has been associated with the activity of the renin-angiotensin system and with thermoregulation, but even more with the overall parasympathetic tone (Busek et al, 2005;Tripathi 2011); some authors have suggested also a sympathetic contamination of this frequency band (Yamamoto and Hughson, 1991). Moreover, the clinical findings regarding the VLF content of heart rate variability as a predictor of cardiac health are not univocal.…”