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.