The effect of extracerebral contamination on near infrared spectroscopy as revealed during organ donation: A prospective observational study in brain dead organ donors

Near-infrared spectroscopy (NIRS) has been utilized widely in anesthesia and intensive care to monitor regional cerebral oxygen saturation (rS_cO₂). A normal oxygenation of extracerebral tissues may overlay and thereby mask cerebral desaturations, a phenomenon known as extracerebral contamination. We investigated the effect of a cessation of extracerebral tissue perfusion on rS_cO₂ in patients with anoxic brains.

In a single-center, prospective, observational study, brain dead adults undergoing organ donation were investigated. rS_cO₂ 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 prior to organ perfusion. rS_cO₂ was monitored until at least forty minutes after aortic clamping. The primary outcome was the amount of extracerebral contamination as quantified by the absolute decrease in rS_cO₂ after aortic clamping. Secondary outcomes were the absolute rS_cO₂-values obtained before and after clamping.

Twelve organ donors were included. Aortic clamping resulted in a significantly (p<0.001) greater absolute decrease in rS_cO₂ when comparing the INVOS (43.0 ± 9.5%) to the ForeSight (27.8 ± 7.1%) monitor. Before aortic clamping, near normal rS_cO₂-values were obtained by the INVOS (63.8 ± 6.2%) and the ForeSight-monitor (67.7 ± 6 .5%). The rS_cO₂ significantly (p<0.001) dropped to 20.8 ± 7.8% (INVOS) and 39.9 ± 8.1% (ForeSight) thirty minutes after clamping, i.e. a condition of a desaturation of both extracerebral and cerebral tissues.

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/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.