BackgroundCerebral oximetry is a noninvasive bedside technology using near-infrared light to monitor cerebral oxygen saturation (Sco2) in an uncertain mixture of arteries, capillaries, and veins. The present study used frequency domain near-infrared spectroscopy to determine the ratio of arterial and venous blood monitored by cerebral oximetry during normoxia, hypoxia, and hypocapnia.MethodsTwenty anesthetized children aged < 8 yr with congenital heart disease of varying arterial oxygen saturation (Sao2) were studied during cardiac catheterization. Sco2, Sao2, and jugular bulb oxygen saturation (Sjo2) were measured by frequency domain near-infrared spectroscopy and blood oximetry at normocapnia room air, normocapnia 100% inspired O2, and hypocapnia room air.ResultsAmong subject conditions, Sao2ranged from 68% to 100%, Sjo2from 27% to 96%, and Sco2from 29% to 92%. Sco2was significantly related to Sao2(y = 0.85 × −17, r = 0.47), Sjo2(y = 0.77 × +13, r = 0.70), and the combination (Sco2= 0.46 Sao2+ 0.56 Sjo2− 17, R = 0.71). The arterial and venous contribution to cerebral oximetry was 16 ± 21% and 84 ± 21%, respectively (where Sco2= &agr; Sao2+ &bgr; Sjo2with &agr; and &bgr; being arterial and venous contributions). The contribution was similar among conditions but differed significantly among subjects (range, ≈ 40:60 to ≈ 0:100, arterial:venous).ConclusionsCerebral oximetry monitors an arterial/venous ratio of 16:84, similar in normoxia, hypoxia, and hypocapnia. Because of biologic variation in cerebral arterial/venous ratios, use of a fixed ratio is not a good method to validate the technology.