The fetal brain: metabolic and circulatory responses to asphyxia.

The fetal environment is well suited for normal brain growth and development with oxygen availability well in excess of oxidative needs. With impairments in blood gas exchange, cerebral oxidative metabolism is initially maintained by an increase in cerebral blood flow, thus protecting the 'oxygen margin of safety', and, when this response becomes limited, by an increase in the brain's fractional extraction of oxygen. Additional adaptive mechanisms involve substrate alterations and a decrease in energy consuming processes, including growth restriction and behavioural state alterations. Although protective insofar as essential metabolic functions are maintained, pathologic change may become evident as the 'oxygen margin of safety' becomes limited or energy conserving measures give rise to abnormal growth and development.