Effect of Extracerebral Contamination on Near-infrared Spectroscopy as Revealed during Organ Donation: A Prospective Observational Study in Brain-dead Organ Donors

Background: Near-infrared spectroscopy (NIRS) has been utilized widely in anesthesia and intensive care to monitor regional cerebral oxygen saturation (rScO2). A normal oxygenation of extracerebral tissues may overlay and thereby mask cerebral desaturations, a phenomenon known as extracerebral contamination. The authors investigated the effect of a cessation of extracerebral tissue perfusion on rScO2 in patients with anoxic brains. Methods: In a single-center, prospective, observational study, brain-dead adults undergoing organ donation were investigated. rScO2 was measured bifrontally using the INVOS 5100C/7100 as well as the ForeSight Elite system. To achieve an efficient conservation of organs and to prevent a redistribution of the perfusion fluid to other tissues, the aorta was clamped before organ perfusion. rScO2 was monitored until at least 40 min after aortic clamping. The primary outcome was the amount of extracerebral contamination as quantified by the absolute decrease in rScO2 after aortic clamping. Secondary outcomes were the absolute rScO2 values obtained before and after clamping. Results: Twelve organ donors were included. Aortic clamping resulted in a significantly (P < 0.001) greater absolute decrease in rScO2 when comparing the INVOS (43.0 +/- 9.5%) to the ForeSight (27.8 +/- 7.1%) monitor. Before aortic clamping, near-normal rScO2 values were obtained by the INVOS (63.8 +/- 6.2%) and the ForeSight monitor (67.7 +/- 6.5%). The rScO2 significantly (P < 0.001) dropped to 20.8 +/- 7.8% (INVOS) and 39.9 +/- 8.1% (ForeSight) 30 min after clamping, i.e., a condition of a desaturation of both extracerebral and cerebral tissues. Conclusions: The abrupt end of extracerebral contamination, caused by aortic clamping, affected both NIRS monitors to a considerable extent. Both the INVOS and the ForeSight monitor were unable to detect severe cerebral hypoxia or anoxia under conditions of normal extracerebral oxygenation. While both NIRS monitors may guide measures to optimize arterial oxygen supply to the head, they should not be used with the intention to detect isolated cerebral desaturations.