Arterial and venous contributions to near-infrared cerebral oximetry

Background: Cerebral oximetry Is a noninvasive bedside technology using near-infrared light to monitor cerebral oxygen saturation (Sco(2)) 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. Methods: Twenty anesthetized children aged <8 yr with congenital heart disease of varying arterial oxygen saturation (Sao(2)) were studied during cardiac catheterization. Sco(2), Sao(2), and jugular bulb oxygen saturation (Sjo(2)) were measured by frequency domain near-infrared spectroscopy and blood oximetry at normocapnia room air, normocapnia 100% inspired O-2, and hypocapnia room air. Results: Among subject conditions, Sao(2) ranged from 68% to 100%, Sjo(2) from 27% to 96%, and Sco(2) from 29% to 92%. Sco(2) was significantly related to Sao(2) (y = 0.85 x -17, r = 0.47), Sjo(2) (y = 0.77 x +13, r = 0.70), and the combination (Sco(2) = 0.46 Sao(2) + 0.56 Sjo(2) - 17, R = 0.71). The arterial and venous contribution to cerebral oximetry was 16 +/- 21% and 84 +/- 21%, respectively (where Sco(2) = alpha Sao(2) + beta Sjo(2) with alpha and beta being arterial and venous contributions). The contribution was similar among conditions but differed significantly among subjects (range, approximate to 40:60 to approximate to 0:100, arterial:venous). Conclusions: Cerebral 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.