Birth asphyxia: pathophysiologic events and fetal adaptive changes.

We have made significant advances toward understanding birth asphyxia and its effects upon neurologic development in the newborn and infant. The fetus is well adapted to compensate for moderate alterations in oxygen delivery. However, near lethal hypoxemia, prolonged exposure, and survival result in cell death and permanent neurologic sequelae. Neuroelectrical measurements such as the EEG and visual evoked potential provide insight into the acute alterations in nerve transmission during asphyxia, and in the recovery phase may ultimately provide information for long-term prognosis. These measurements are limited, however, by their inability once lost to distinguish cell inactivity from cell death. Permanent neurologic damage from asphyxia appears now to be a complex process in which severe hypoxemia precipitates a cascade of events leading to glycolysis, glycogenolysis, hypotension, and ultimately the accumulation of high concentrations of lactate at the cell level. As a consequence, cellular and extracellular fluid shifts produce cerebral edema, further impairment of cerebral circulation, and ultimately cell death. Clinical studies have helped to identify the newborn at high risk for neurologic impairment, but a cause-effect relationship remains unclear. That birth asphyxia can produce severe neurologic damage and death is generally accepted. Moreover, improper resuscitation of a severely depressed newborn increases the chance of permanent sequelae. The important clinical question is: Can one alter the natural course of asphyxia as has been alluded to through pharmacologic and ventilator manipulation? Answers to this question will depend upon continued study of the mechanisms of asphyctic damage in the central nervous system.