Operation Everest II: An indication of deterministic chaos in human heart rate variability at simulated extreme altitude

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Abstract
It has been shown that fluctuation of human heartbeat intervals (heart rate variability, HRV) reflects variations in autonomic nervous system activity. We studied HRV at simulated altitudes of over 6000 m from Holter electrocardiograms recorded during the Operation Everest II study (Houston et al. 1987). Stationary, ∼30-min segments of HRV data from six subjects at sea level and over 6000m were supplied to (1) spectral analysis to evaluate sympathetic and parasympathetic nervous system (SNS, PNS) activity, (2) the analysis of Poincaré section of the phase space trajectory reconstructed on a delayed coordinate system to evaluate whether there was fluctuation with deterministic dynamics, (3) the estimation of the correlation dimension to evaluate a static property of putative attractors, and (4) the analysis of nonlinear predictability of HRV time series which could reflect a dynamic property of the attractor. Unlike HRV at sea level, the recordings at over 6000 m showed a strong periodicity (period of about 20 s) with small cycle-to-cycle perturbation. When this perturbation was expressed on a Poincaré section, it seemed to be likely that the perturbation itself obeyed a deterministic law. The correlation dimensions of these recordings showed low dimensional values (3.5 ± 0.4, mean±SD), whereas those of the isospectral surrogates showed significantly (P < 0.05) higher values (5.3 ±0.5) with embedding dimensions of 5.6 ± 0.9. At over 6000 m, the correlation coefficients between the observed and the predicted time series with the prediction time of < 4 beats were significantly (P < 0.01) higher than those for the surrogate data, whereas there was no significant difference in the nonlinear predictability between the observed and the surrogate data at sea level. The results of the spectral analyses showed that, at over 6000 m, there was hardly any power > 0.15 Hz in the HRV spectra possibly due to PNS withdrawal. Hence, these deterministic and/or chaotic dynamics might be mediated by variations in SNS activity at over 6000 m.

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