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 (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. Methods: Twenty 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. Results: Among subject conditions, Sao₂ ranged from 68% to 100%, Sjo₂ from 27% to 96%, and Sco₂ from 29% to 92%. Sco₂ was significantly related to Sao₂ (y = 0.85 x -17, r = 0.47), Sjo₂ (y = 0.77 x +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 = α Sao₂ + β Sjo₂ with α and β being arterial and venous contributions). The contribution was similar among conditions but differed significantly among subjects (range, ≃ 40:60 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.