Tale of Two Cities: narrative review of oxygen

Abstract

The human brain contributes 2% of the body weight yet receives 15% of cardiac output and demands a constant supply of oxygen (O ₂) and nutrients to meet its metabolic needs. Cerebral autoregulation is responsible for maintaining a constant cerebral blood flow that provides the supply of oxygen and maintains the energy storage capacity. We selected oxygen administration-related studies published between 1975-2021 that included meta-analysis, original research, commentaries, editorial, and review articles. In the present narrative review, several important aspects of the oxygen effects on brain tissues and cerebral autoregulation are discussed, as well the role of exogenous O ₂ administration in patients with chronic ischemic cerebrovascular disease: We aimed to revisit the utility of O ₂ administration in pathophysiological situations whether or not being advantageous. Indeed, a compelling clinical and experimental body of evidence questions the utility of routine oxygen administration in acute and post-recovery brain ischemia, as evident by studies in neurophysiology imaging. While O ₂ is still part of common clinical practice, it remains unclear whether its routine use is safe.

Keywords: cerebral autoregulation; cerebral blood flow; cerebral ischemia; oxygen therapy; stroke.

Figure 1.. Hypothetical model representing changes in cerebral autoregulation during aging or ischemic events.

Panel A. A representation of cardiovascular coupling during normal or stressful events with variation in age, and compensation mechanisms. Panel B. Hypothetical model comparing changes in cerebral blood flow (CBF) in healthy volunteer compared to a patient with chronic ischemic stroke by magnetic resonance imaging. *Arbitrary unit=signal intensity; CBF of a young volunteer; CBF: cerebral blood flow; O ₂: oxygen; 1: pre O ₂; 2: during O ₂; 1: post O ₂.

Figure 2.. Model of cerebral autoregulation.

A. Cellular interaction (neurons, endothelial cells and glial cells); B. Nutrients/energy (glucose/oxygen); C. Activity/changes of demand, physiological; D. Pathological (seizures, stroke, trauma). Four mechanisms control mechanisms: myogenic, neurogenic, metabolic and endothelial. CBF: cerebral blood flow; MAP: mean arterial pressure; CMRO ₂: cerebral metabolic rate of oxygen; CMR: the cerebral metabolic rate; PaO ₂: O ₂ partial pressure in arterial blood; PaCO ₂: CO ₂ partial pressure in arterial blood; H+: hydrogen ion; pH: potential of hydrogen.