The premature fetus: Not as defenseless as we thought, but still paradoxically vulnerable?

Traditionally, it has been believed that the cardiovascular and hormonal re sponses to asphyxia in preterm fetuses are immature, and this immaturity co ntributes to their apparent vulnerability to neural injury. However, these data were derived from studies using relatively mild insults, which did not allow for the greater cardiac glycogen reserves and anaerobic capacity of the brain near midgestation. Here, we review the maturation of the cardiova scular and cerebrovascular and cerebral responses to asphyxia in experiment al animals and how these relate to the apparent vulnerability of the human premature brain. Most such investigations have been performed in the chroni cally instrumental fetal sheep. Recent studies have demonstrated that the p remature fetus has highly adaptive and relatively mature responses to asphy xia, and that in absolute terms the preterm brain is very resistant to asph yxial injury. These data suggest that the premature fetus is able to surviv e much more prolonged periods of asphyxia than the near-term fetus, but tha t, paradoxically, such survival is associated with exposure to prolonged pe riods of hypotension and hypoperfusion and consequently greater risk of sev ere neural damage. Copyright (C) 2001 S. Karger AG, Basel.