Circulatory responses to asphyxia in pregnant and fetal animals: a comparative study of Weddell seals and sheep.

Diving mammals share with fetal and newborn animals a high resistance to asphyxia when compared, respectively, with terrestrial and adult animals. Adaptations for conservation or sparing of oxygen and for adjustment to the effects of accumulated metabolic products, such as carbon dioxide and hydrogen ions, have been discovered in both diving marine mammals and asphyxiated terrestrial species. The major requirement in all instances of prolonged oxygen deprivation is for protection of the least resistant vital organs, the brain and the heart. Dawes has recently reviewed the evidence supporting the view that integrity of the circulation contributes importantly to survival time of the asphyxiated fetus or newborn.' The mechanism is dependent on the concentration of glycogen in cardiac muscle before the asphyxial episode,' and requires an adequate circulatory supply of glucose to the brain for glycolytic energy production. Survival is shortened by blocking glycolysis with iodoacetate and prolonged by the administration of additional glucose when accompanied by base to maintain near normal pH.' The brain of the newborn dog also has a specific resistance to total ischemia, which enables it to survive longer than the similarly treated adult brain.' Scholander suggested that fetal animals may react to asphyxia in a manner like that of adult diving seals' which experience a profound reduction and redistribution of cardiac output such that oxygen stored in blood is preferentially directed to the cerebral and coronary circulations. Selective vasoconstriction produces marked decreases in muscle, skin, mesenteric and renal blood flows, which are accompanied by bradycardia and little change in central arterial blood pressure. These events and similar ones in diving or asphyxiated land mammals, including man, are well documented and have recently been reviewed.67 Similar, though less profound, changes in distribution of cardiac output have recently been demonstrated in partially asphyxiated, anesthetized lambs delivered by Caesarean section.8