Seizure-associated pulmonary edema and cerebral oxygenation in the rat

Cerebral partial pressure of O₂ (PO₂), relative changes in the ratio of reduced/oxidized cytochrome aa₃, blood flow, and the arteriovenous difference in O₂ content were measured during seizures with and without pulmonary edema. Seizures were induced with bicuculline (0.2-1.2 mg/kg iv) in rats anesthetized with 70% N₂O and paralyzed with curare. Briefer seizures were accompanied by increased cerebral PO₂ and increased oxidation of cytochrome aa₃. Lung water content and arterial O₂ partial pressure (Pa(O₂)) remained normal. Longer duration seizures were also accompanied initially by increases in cerebral oxygenation. Within minutes, however, Pa(O₂) fell from a mean of 118 to 51 mmHg, and lung water content increased from 76.2 to 83.6%. Cerebral PO₂ fell but most often rose back to or above control levels, while cytochrome aa₃ became markedly reduced. Simultaneously, cerebral blood flow increased more than 300% above preseizure values and O₂ delivery increased more than O₂ consumption. The reductive shift of cytochrome aa₃ was greater than that produced by lowering Pa(O₂) to equivalent values in seizures-free rats. The reductive shift of cytochrome aa₃, despite increased O₂ delivery, may be indicative of derangements in cerebral O₂ diffusion or energy metabolism.