EXTRACEREBRAL ABSORPTION OF NEAR-INFRARED LIGHT INFLUENCES THE DETECTION OF INCREASED CEREBRAL OXYGENATION MONITORED BY NEAR-INFRARED SPECTROSCOPY

The detection of increased cerebral oxygenation secondary to cerebral hyperaemia, induced by hypercapnia has been studied in anaesthetised p atients using a near infrared, reflectance mode, cerebral oxygenation monitor (Invos 3100 Somanetics, Troy, Michigan, USA). Two studies were performed, with and without a pneumatic scalp tourniquet, to distingu ish between extracranial and intracranial changes in tissue oxygenatio n. In the control study a mean increase in end tidal CO2 of 23.1 mm Hg was accompanied by a mean increase in middle cerebral artery flow vel ocity of 116%. Regional cerebral oxygen saturation (rSo(2)) measured t ranscutaneously in the frontal distribution of the middle cerebral art ery increased significantly from 70.5% to 74.6% (p = 0.001). During th e second study with a scalp tourniquet inflated to maintain the extrac ranial tissues in a state of stable ischaemia a mean increase in end t idal CO2 of 22.3 mm Hg was accompanied by a mean increase in middle ce rebral artery flow velocity of 121%. The change in rSo(2) from 62.6% t o not significant (p = 0.085). no correlation between the change in mi ddle cerebral artery flow velocity and rSo(2) in the control or scalp ischaemia group. This study shows that the Invos 3100 monitor is sensi tive to tissue oxygenation but does not reliably detect changes in cer ebral oxygenation as a result of profound cerebral hyperaemia. The con tribution of extracerebral tissue to the attenuation of near infrared light and the lack of spatial resolution remain major problems to be o vercome before this or other near infrared spectroscopy instruments ca n be introduced into clinical practice.