Vestibular Function: A Hidden Harbinger of Human Hypoxia

Abstract

The aim of this study was to evaluate the impact of hypoxia on the human vestibular system — the organs in our inner ear that sense head motion and gravity. The vestibular system contributes to self-motion perception, balance control, and several reflexive responses. Maintaining ideal vestibular sensation imposes a high metabolic load; consider that the human vestibular periphery has over 100,000 hairs cells requiring energy to maintain depolarization and roughly 40,000 peripheral neurons requiring energy to support an average firing rate of nearly 100 action potentials per second for each neuron. Consistent with this, a recent study concluded that the vestibular organs of old animals appear to be bordering on an energy crisis; such an energy crisis could result from inadequate O2 or inadequate fuel. To quantify vestibular impacts of reduced O2, vestibular function was quantified using self-motion perceptual thresholds. Specifically, we used standard forced-choice psychophysical methods to determine the smallest earth-vertical (i.e., upward/downward) translation that a seated human could reliably sense. To quantify blood oxygenation, we used a standard fingertip pulse oximeter. To manipulate blood oxygenation, we controlled the oxygen (O2) air content using a Reduced Oxygen Breathing Device (Environics). Our hypotheses were that: (1) human vestibular thresholds will increase with reduced blood oxygenation, and (2) vestibular degradation will monotonically increase as blood oxygenation decreases. All data were collected at Wright Patterson Air Force Base (elevation 823’). On different days, participants breathed air having O2 content of 20.9%, 15.4%, 14.3%, 12.9%, 11.8%, and 10.7% — chosen to simulate O2 content found at altitudes of 0’ (“baseline”), 8,000’, 10,000’, 12,500’, 15,000’, and 17,500’. Fifteen participants completed test sessions at 20.9% and 15.4% O2. Earth-vertical translation thresholds were quantified to be 23.5% greater when the oxygen content was 15.4% than at baseline (p=0.005, Wilcoxson Sign Rank Test, Z = -2.84). While all subjects were unable to complete all conditions, a second set of participants was invited to complete test sessions at 20.9%, 14.3%, 12.9%, 11.8%, and 10.7% O2. Thresholds increased as O2 content decreased. Subjects reported no spatial disorientation. We did not vary barometric pressure, but these results show that vestibular thresholds are both significantly and substantively impacted at simulated altitudes of 8,000’. This is noteworthy because it matches the cabin pressurization required in the US by the FAA for commercial flights. These results suggest that many of us who have flown in a commercial jet may have experienced degraded vestibular function. Behavioral impacts remain to be understood, but vestibular function does appear to be a harbinger of human hypoxia. Support provided by Offce of Navy Research via a Multidisciplinary University Research Initiative (MURI N00014-20-1-2163). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.