REDUCTION OF CYTOCHROME AA(3) MEASURED BY NEAR-INFRARED SPECTROSCOPY PREDICTS CEREBRAL ENERGY-LOSS IN HYPOXIC PIGLETS

Near-infrared spectroscopy is a noninvasive monitoring technique that allows quantitative measurement of changes in cerebral oxygenated Hb ( HbO(2)), deoxygenated Hb (Hb), total Hb, and oxidized cytochrome aa(3) (CytO(2)). Changes in cerebral Hb oxygenation and CytO(2) have been m easured in human neonates and infants under a variety of conditions. H owever, the association of these measurements with cerebral high-energ y phosphate loss is not known. We studied simultaneous changes in cere bral HbO(2), Hb, total Hb, and CytO(2) by near-infrared spectroscopy a nd changes in nucleoside triphosphate (NTP, mostly ATP) and phosphocre atine (PC) concentrations and intracellular pH by in vivo P-31-labeled magnetic resonance spectroscopy. Four-wk-old piglets (n = 8) underwen t sequential hypoxic episodes of increasing severity (inspired O-2 con centration, 12, 8, 6, 4, and 0%). Animals were anesthetized and mechan ically ventilated. At all levels of hypoxia, cerebral HbO(2) decreased , and Hb increased. Loss of PC or NTP was not observed until inspired O-2 concentration was decreased to less than 12%. With such severe hyp oxia, hypotension, intracellular acidosis, and increasingly severe PC and NTP depletions occurred. Decreases in PC and NTP correlated closel y with decreased CytO(2) and arterial blood pressure (p < 0.0001) but not with changes in HbO(2) and Hb. In conclusion, cerebral hypoxemia i s readily detected by near-infrared spectroscopy as a decrease in HbO( 2) and an increase in Hb. However, relative changes in cerebral HbO(2) and Hb have low predictive value for cerebral energy failure. Reducti on of CytO(2) is highly correlated with decreased brain energy state a nd may indicate impending cellular injury.