Cerebral tolerance to asphyxial hypoxia in the harbor seal

Abstract Young unanesthetized restrained harbor seals, Phoca vitulina were subjected to simulated maximum dives by submerging their heads until the appearance of reversible characteristic hypoxic patterns in their electroencephalogram (EEG endpoint). The mean endpoint P O 2 , defined as the oxygen tension in the blood at the EEG endpoint, was 10 mm Hg in arterial and 2.5 mm Hg in cerebral venous blood. These values, lower than those reported for terrestrial laborabory mammals, held over a wide range of blood pH and P CO 2 values. During the last five minutes of the dive (averaging 18 min in duration) cerebral (a-v)C O 2 , diminished and cerebral (v-a) lactate content increased. Independent measurements of blood flow in the intravertebral vein showed that cerebral blood flow during diving was about two-thirds of the pre-dive value and that it remained substantially unchanged throughout the dive. It is concluded that during prolonged apneic asphyxia the seal's brain, unlike that of terrestrial mammals, tolerates a definite period of v O 2 reduction prior to the onset of gross functional impairment.