Interactive effects of anoxia and general anesthesia during birth on the degree of CNS and systemic hypoxia produced in neonatal rats

A model of global hypoxia during Caesarean-section (C-section) birth has be en widely used to study long-term effects of birth hypoxia on central nervo us system (CNS) function. However, the actual degree of CNS and systemic hy poxia produced by the birth insult in this model has never been characteris ed. Additionally, the way in which the dam is anaesthetised during the C-se ction procedure may impinge on the degree of hypoxia experienced by the neo nate. This study examined how a period of global birth anoxia and isofluran e/N2O anaesthesia interact to affect measures of CNS and systemic hypoxia i n neonatal rats born by C-section compared with control, vaginally born ani mals. A 10-min period of global anoxia just before birth increased blood la ctate, a metabolic indicator of systemic hypoxia, increased brain lactate a nd decreased brain ATP to a similar extent in pups born by C-section from e ither decapitated, unanaesthetised darns or dams anaesthetised with 2.5% is oflurane. Thus, this model does produce systemic and CNS hypoxia in the neo nate. Pups born by C-section with a higher concentration of isoflurane (3.5 %), in the absence of added global anoxia, also showed reductions in brain ATP at birth. In addition, 10 min of global anoxia produced greater increas es in blood lactate in pups born from dams anaesthetised with the higher co ncentration of isoflurane. Thus, the concentration of an aesthetic used in this model may affect the degree of CNS or systemic hypoxia experienced by the neonate. Compared with vaginal birth, pups born by C-section with 2.5% or 3.5% isoflurane (and no added global anoxia) showed decreased pO(2) and pH, and increased pCO(2) in systemic blood taken <30 s after birth. Exposur e to global anoxia during C-section birth actually increased systemic pO(2) at <30 s after birth, presumably due to ventilatory responses to hypoxemia and hypercapnia; this effect of anoxia was reduced in anaesthetised compar ed with unanaesthetised pups. Thus, global anoxia acts as a stimulus for ra pid recovery of systemic pO(2) at birth, and this stimulus is dampened by i soflurane/N2O anaesthesia. These results should aid in understanding how CN S and systemic hypoxia at birth contribute to long-term changes in brain bi ochemistry and behaviour in this model.