Cat brain cytochrome-c oxidase redox changes induced by hypoxia after blood-fluorocarbon exchange transfusion

We used rapid-scanning near-infrared (NIR) spectroscopy (730-960 nm) to study the effects of graded or acute hypoxia on cerebral cytochrome-c oxidase (cyt aa₃) redox state in blood-perfluorocarbon-exchanged cats with somatosensory evoked potential (SEP) monitoring. In graded hypoxia [10 min each at fractional inspiratory O₂ concentration (FIO₂) 0.9, 0.8, 0.7, 0.6, and 0.5], cyt aa₃ reduction occurred at FIO₂ 0.6 when cerebral O₂ delivery was < 3.5 ml · 100 g^-1 · min^-1. In acute hypoxia (FIO₂ 0.6 for 10 min), significant cyt aa₃ reduction occurred from 5 to 10 rain (cerebral O₂ delivery 3.1 ± 0.3 ml · 100 g^-1 · min^-1) and recovered with reoxygenation (FIO₂ 1.0). Cyt aa₃ redox changes preceded or coincided with SEP alterations in both hypoxia protocols. These results demonstrate that cerebral cyt aa₃ reduction occurs with severe reduction of cerebral O₂ delivery, but no significant change in cerebral cyt aa₃ redox state occurs with small reductions of cerebral O₂ delivery. We conclude that substantial changes in cerebral cyt aa₃ do riot occur at physiological levels of O₂ delivery and that current NIR clinical instruments would detect oxygen- dependent cerebral cyt aa₃ redox changes only when O₂ delivery is extremely compromised.