The Effect of Inspired Hypoxia and Hyperoxia on Respiratory Sinus Arrhythmia Reactivity

Abstract

Respiratory sinus arrhythmia (RSA) is the normal fluctuation in heart rate (HR) in phase with the respiratory cycle, whereby HR increases during inspiration and decreases during expiration. The underlying mechanisms and potential utility remain elusive. RSA magnitude is thought to be affected by autonomic balance, whereby a dominant parasympathetic nervous system (NS) at rest facilitates RSA, and sympathetic NS activation attenuates its magnitude. Carotid body stimulation via acute hypoxia elicits sympathetic activation, whereas hyperoxia facilitates sympathetic withdrawal. In addition, RSA has been hypothesized to facilitate improvements in ventilation/perfusion (V/Q) matching, improving oxygenation when hypoxic. We hypothesized that (a) acute hypoxia and hyperoxia would increase and decrease the magnitude of RSA reactivity, respectively and (b) when hypoxic, those with larger RSA magnitude during deep breathing would have larger improvements in oxygen saturation (SpO2). Healthy participants (n=13) were instrumented with a pneumotachometer and coached to breathe at three percentages (30, 40 and 50%) of their forced vital capacity (FVC) using visual cues while inspiring room air (21% FIO2), hypoxia (13.5% FIO2), hyperoxia (100% FIO2). RSA was quantified via the peak‐valley approach, and RSA reactivity (RSAR) was quantified via linear regression to calculate the slope of changes in RSA magnitude across all three FVC levels. RSA magnitude increased linearly with increases in inspired tidal volume in all three gases (R2>0.98). No significant differences were found in RSAR slopes between room air (0.32±0.2 ΔBPM/%FVC), hypoxia (0.33±0.26 ΔBPM/%FVC), and hyperoxia (0.34±0.26 ΔBPM/%FVC; P=0.97). There was also no correlation between RSA magnitude during deep breathing (50%FVC) and the associated improvements in SpO2 in hypoxia (r=−0.11, P=0.72). Given that RSAR is not affected by oxygen availability, our data provides evidence that hypoxic stimulation of the carotid bodies elicits activation of both arms of the autonomic nervous system, maintaining autonomic balance and normal heart rate variability. In addition, RSA magnitude does not appear to bestow advantages on V/Q matching in the context of acute hypoxia.Support or Funding InformationMRU Faculty of Science and Technology