THE EFFECTS OF INDUCED APNEIC EPISODES ON CEREBRAL CORTICAL OXYGENATION IN NEWBORN PIGLETS

The effect of different inspiratory oxygen levels (FiO(2)) on cortical oxygenation (pO(2)) during and after recovery from apnea, was investi gated in 18 anesthetized, paralyzed, and mechanically ventilated newbo rn piglets. Heart rate (HR) and mean arterial blood pressure (MABP) we re continuously monitored as the piglets were subjected to repeated ep isodes of apnea initiated by disconnecting the ventilator and terminat ed when HR decreased to less than 80 beats/min by reconnecting the ven tilator. A closed cranial window was placed over the parietal cortex o f the animals and cortical pO(2) was measured optically by phosphoresc ence quenching. Apneic episodes induced in animals ventilated with 15% , 22% and 40% oxygen had mean duration's of apnea (time before HR decr eased to less than 80 beats/min) of 80, 128 and 134 s, respectively. B y the end of the apnea the MABP decreased to 82%, 64%, and 54% of cont rol, respectively. The cortical pO(2) decreased from control values of 24.1, 32.3 and 38.3 Torr at 15%, 22% and 40% oxygen, respectively, to 1.7 to 3.2 Torr at the end of the apneic episode. The duration of apn ea necessary for the cortical pO(2) to drop below 20.3 Torr was 18, 44 and 81 s at 15%, 22% and 40% oxygen, respectively. There was an inver se correlation between the rate of decline of cortical pO(2) and basel ine FiO levels. With reventilation, the cortical pO(2) reached maximal values of 42.8, 51.9 and 57.2 Torr at 15%, 22%, and 40% oxygen, respe ctively, before returning to the pre-apnea values. The present results show that apnea of less than 30 s duration at an FiO(2) of 22% do not result in significant cortical hypoxia in hemodynamically stable pigl ets. Increasing the FiO(2) to above 22% may possibly increase the rate of recovery of tissue oxygenation but it also may facilitate post-hyp oxic cortical hyperoxia, a factor that may predispose the immature bra in to free radical injury.