CEREBRAL AND VENTILATORY DEPRESSION DURING HYPOXIA IN ANESTHETIZED NEWBORN GUINEA-PIGS

The effects of hypoxia on ventilation and cerebral activity were studi ed in urethane-anaesthetized newborn guinea-pigs. Ventilation was meas ured by a pneumotachograph, and cerebral activity by a cerebral functi on monitor (CFM). All animals were subjected to either 9% O-2 or 6% O- 2 in N-2 for 10 minutes or until apnoea occurred. Hypoxia produced a b iphasic response in ventilation, that is, an increase followed by a de crease. The initial increase was attributed to the elevation of the re spiratory rate, whereas the tidal volume showed a pure decline. The re spiratory rate reached its peak at 3 minutes of hypoxia (170 +/- 12% d uring 9% O-2 and 169 +/- 12% during 6% O-2). Cerebral activity during both 9 and 6% O-2 breathing showed a small increase followed by a decr ease. In the group subjected to 9% O-2 the maximum CFM activity increa sed to 114 +/- 8% of the control level and the minimum activity increa sed to 113 +/- 7%, while in the group subjected to 6% O-2 the maximum CFM activity increased to 104 +/- 5% and the minimum CFM activity to 1 01 +/- 3%. The depression of CFM activity was more pronounced with 6% O-2 than with 9% O-2 Regression analysis showed a linear correlation b etween ventilation and cerebral activity during both 9 and 6% O-2 brea thing. The results suggest that hypoxic ventilatory depression may be the consequence of cerebral depression produced by acute severe hypoxi a.