Historical Appreciation of Brain Vulnerability from Pure Hypoxemia

Excerpt

Even more now than in the recent past, neurointensivists are asked to evaluate the effects of hypoxemia on the brain. The SARS-CoV-2 pandemic, characterized by widespread transmission and admitting patients by the thousands, requires aggressive management of profound hypoxemia. Many patients are initially in what has been called “silent hypoxemia.” A thoughtful explanation of why ‘silence’ is expected rather than unusual has been provided by Tobin et al., who emphasizes to recognize well-established physiology principles [1]. Hypoxia increases depth and rate of breathing but the carotid artery baroreceptors are set at quite low levels. Patients seldom experience dyspnea with moderate hypoxemia. This is further illustrated by the experience of climbers, in whom oxygen saturations of < 65% for prolonged periods may “dull the mind” but do not always increase the sensation of dyspnea [2]. Clinically, only a severely hypoxemic (< 50%) patient is tachypneic (assuming no further blunting effect from hypercarbia or enhancing effect from fever) and restless. The gradual hypotension and rising PaCO₂ are eventually more crucial factors in declining consciousness than hypoxemia alone. …