Near-infrared spectroscopy in adults: effects of extracranial ischaemia and intracranial hypoxia on estimation of cerebral oxygenation.

We have studied the effects of extracranial ischaemia and intracranial hypoxia on measurement of cerebral oxygenation using near-infrared, reflectance-mode, cerebral oximetry (Invos 3100 cerebral oximeter) in healthy adult subjects. Under stable systemic conditions, scalp ischaemia induced by a pneumatic tourniquet caused an apparent reduction in mean regional cerebral oxygenation (rSO2) from mean 72 (SD 6)% to 59 (7)% (n = 8, P < 0.001). rSO2 returned to control values within 1 min of release of the tourniquet. Local scalp ischaemia induced by rapid frontalis muscle exercise caused a significant reduction (4.5 (2)% in rSO2 (n = 12, P < 0.001). The effect of systemic hypoxia on rSO2 was examined during controlled scalp ischaemia. A decrease in mean SpO2 from 98 (2)% to 66 (6)% was associated with a decrease in mean rSO2 from 57 (4)% to 41 (6)%. There was a significant correlation between the percentage reduction in rSO2 and SpO2 during hypoxia (r = 0.81, P < 0.001). We conclude that the Invos cerebral oximeter was capable of detecting tissue hypoxia deep to the scalp under carefully controlled conditions but that it also was affected significantly by changes in extracranial blood flow and oxygenation which may affect its reliability in clinical practice. Further work is necessary to define those situations in which cerebral oximetric monitoring is useful and valid.