Apparent cerebral cytochrome aa₃ reduction during cardiopulmonary bypass in hypoxaemic children with congenital heart disease. A critical analysis of in vivo near-infrared spectrophotometric data

The purpose of this analysis was critically to examine the changes of cerebral cytochrome oxidase as detected by near-infrared spectrophotometry (NIRS) during induction of cardiopulmonary bypass (CPB) in 14 children undergoing open heart surgery. Five children were hypoxaemic (arterial oxygen saturation 49%-84%) before the switch to CPB and 9 children were not. In the hypoxaemic children, the total cerebral haemoglobin concentration (tHb) decreased rapidly and markedly, reaching a plateau after 2 min. Surprisingly, cytochrome aa₃ concentration in its oxygenated form (CytO ₂) decreased in parallel while the cerebral haemoglobin oxygenation index (HbO₂)-(Hb) increased gradually. In the 8 normoxaemic children, changes in the NIRS signals were insignificant. When a standard NIRS algorithm was used, the magnitude of the change in (CytO₂) was closely associated with the magnitude of the change in (tHb) (p<0.0001), the time courses of the (CytO₂) and (tHb) signals were parallel in 11 of the 14 children and the error of fit in the NIRS multicomponent analysis increased 10- to 100-fold over system noise. A new NIRS algorithm, using wavelength specific optical pathlengths, did not improve the error of fit but produced smaller estimates of (CytO₂) changes, which were unrelated to the (tHb) changes. In the authors' opinion the interesting possibility of monitoring cytochrome oxidation by NIRS requires further validation.