Poincare Analysis of Spontaneous Respiratory Sinus Arrhythmia during High Altitude Ascent: Effects on oxygenation

Abstract

Respiratory sinus arrhythmia (RSA) is the transient fluctuation in heart rate (HR) in phase with the respiratory cycle. Poincare spectral analysis is used to (a) quantify heart rate variability (HRV) through short term (SD1; i.e., RSA) and long term (SD2) fluctuations in heart rate and (b) determine components of autonomic nervous system activity (low and high frequency; LF and HF; sympathetic and parasympathetic, respectively). Although RSA likely represents the dominance of the parasympathetic nervous system at rest, there is controversy regarding the mechanisms underlying RSA, as well as the possible physiological utility. RSA may improve ventilation and perfusion (V/Q) matching in the lungs, and may protect against oxygen desaturation in the context of hypoxic exposure. We aimed to investigate the relationship between RSA magnitude and peripheral oxygen saturation (SpO2) during incremental ascent to high altitude (4370m). We hypothesized that (a) RSA magnitude (SD1/SD2) ratio would decrease during ascent to high altitude as a result of sympathetic nervous system activation and (b) individuals with a larger SD1/SD2 ratio during ascent would have increased SpO2 through improved V/Q matching. Measurements were conducted over eleven days at 1045m (baseline, day 1), 3440m (day 7), 3860m (day 9), and 4370m (day 11) during a trek to altitude in the Nepal Himalaya. Participants (26±2.1 years; BMI 23.8±1.1 kg/m2; 9 females, n=12) were instrumented with an ECG and finger pulse oximeter (SpO2 %) and tested on rest days after one night at each altitude. Spontaneous RSA was measured from the ECG periods during the last 7‐min of a 10‐min baseline and evaluated using Poincare plots. There was no significant difference between the SD1/SD2 ratio during incremental ascent to 4370m (P=0.65). There was also no significant difference observed in the LF/HF ratios during ascent to 4370m (P=0.66), suggesting that the sympathetic/parasympathetic ratio remained unchanged with ascent. Additionally, we correlated SD1/SD2 ratio and SpO2 at 4370m when individuals were most hypoxic at rest (SpO2 87.2±0.8%) and found no significant correlation (r=0.32, P=0.31). Our data suggests that during slow incremental ascent to high altitude, there is no difference in spontaneous RSA or the balance between sympathetic and parasympathetic nervous system activity. Individuals with larger SD1/SD2 ratios do not have improved SpO2, suggesting that RSA magnitude does not improve V/Q matching during high altitude ascent.Support or Funding InformationNatural Sciences and Engineering Research Council of Canada, Student Temporary Employment Program of Alberta, Alberta Innovates Health Solutions Summer Studentship,