Hypoxia, hyperoxia, ischemia, and brain necrosis

Abstract

Background: Human brains show widespread necrosis when death occurs after coma due to cardiac arrest, but not after hypoxic coma. It is unclear whether hypoxia alone can cause brain damage without ischemia. The relationship of blood oxygenation and vascular occlusion to brain necrosis is also incompletely defined. Methods: We used physiologically monitored Wistar rats to explore the relationship among arterial blood oxygen levels, ischemia, and brain necrosis. Hypoxia alone (PaO₂ = 25 mm Hg), even at a blood pressure (BP) of 30 mm Hg for 15 minutes, yielded no necrotic neurons. Ischemia alone (unilateral carotid ligation) caused necrosis in 4 of 12 rats, despite a PaO₂ > 100 mm Hg. To reveal interactive effects of hypoxia and ischemia, groups were studied with finely graded levels of hypoxia at a fixed BP, and with controlled variation in BP at fixed PaO₂. In separate series, focal ischemic stroke was mimicked with transient middle cerebral artery (MCA) occlusion, and the effect of low, normal, and high PaO₂ was studied. Results: Quantitated neuropathology worsened with every 10 mm Hg decrement in BP, but the effect of altering PaO₂ by 10 mm Hg was not as great, nor as consistent. Autoradiographic study of cerebral blood flow with ¹⁴C-iodoantipyrine revealed no hypoxic vasodilatation during ischemia. In the MCA occlusion model, milder hypoxia than in the first series (PaO₂ = 46.5 ± 1.4 mm Hg) exacerbated necrosis to 24.3 ± 4.7% of the hemisphere from 16.6 ± 7.0% with normoxia (PaO₂ = 120.5 ± 4.1 mm Hg), whereas hyperoxia (PaO₂ = 213.9 ± 5.8 mm Hg) mitigated hemispheric damage to 7.50 ± 1.86%. Cortical damage was strikingly sensitive to arterial PaO₂, being 12.8 ± 3.1% of the hemisphere with hypoxia, 7.97 ± 4.63% with normoxia, and only 0.3 ± 0.2% of the hemisphere with hyperoxia (p < 0.01), and necrosis being eliminated completely in 8 of 10 animals. Conclusions: Hypoxia without ischemia does not cause brain necrosis but hypoxia exacerbates ischemic necrosis. Hyperoxia potently mitigates brain damage in this MCA occlusion model, especially in neocortex. Key words: Hypoxia—Ischemia—Brain—Necrosis—Coma—Blood pressure—Cerebral blood flow