USA Surgery Today

Oxygen-based metabolism is necessary to maintain cell life. The cellular milieu typically requires an oxygen tension of about 1 to 4 mm Hg to sustain baseline VO₂ levels (1). Intravascular venous oxygen tensions of at least 20 mm Hg are required to maintain an appropriate oxygen gradient to achieve these minimal levels of intracellular oxygen tension (2). Oxygen is necessary to provide reduction of cytochromes A and A3 to allow oxidative phosphorylation to occur (Fig. 10-1) (3).

Hypoxemia results in a decrease in the availability of oxygen to mitochondria. The consequence is inhibition of Krebs cycle activity with reduction in adenosine triphosphate (ATP) production. With a decrease in perfusion, metabolism of other substrates such as glucose by the glycolytic pathway is necessary to maintain cellular metabolic processes. As ATP stores diminish, cellular synthetic and transport functions become impaired and eventually stop. With continued hypoxia, mitochondrial and endoplasmic reticulum swelling is observed, and lysosomal rupture and intracellular proteolysis follow.