Defining thresholds for critical ischemia by using near-infrared spectroscopy in the adult brain

Object. Signal changes in adult extracranial tissues may have a profound effect on cerebral near-infrared spectroscopy (NIRS) measurements. During carotid surgery NIRS signals provide the opportunity to determine the relative contributions from the intra- and extracranial vascular territories, allowing for a more accurate quantification. In this study the authors applied multimodal monitoring methods to patients undergoing carotid endarterectomy and explored the hypothesis that NIRS can define thresholds for cerebral ischemia, provided extracranial NIRS signal changes are identified and removed. Relative criteria for intraoperative severe cerebral ischemia (SCI) were applied to 103 patients undergoing carotid endarterectomy. Methods. One hundred three patients underwent carotid endarterectomy. An intraoperative fall in transcranial Doppler-detected middle cerebral artery flow velocity (%Delta FV) of greater than 60% accompanied by a sustained fall in cortical electrical activity were adopted as criteria for SCI. Ipsilateral frontal NIRS recorded the total difference in concentrations of oxyhemoglobin and deoxyhemoglobin (Total Delta Hb(diff)). Interrupted time series analysis following clamping of the external carotid artery (ECA) and the internal carotid artery (ICA) allowed the different vascular components of Total Delta Hb(diff) (ECA Delta Hb(diff) and ICA Delta Hb(diff)) to be identified. Data obtained in 76 patients were deemed suitable. A good correlation between %Delta FV and ICA Delta Hb(diff) (r = 0.73, p < 0.0001) was evident. Sixteen patients (21%) fulfilled the criteria for SCI. All patients who demonstrated an ICA Delta Hb(diff) of greater than 6.8 mu mol/L showed SCI, and in two patients within this group nondisabling watershed infarction developed, as seen on postoperative computerized tomography scans. No patient with an ICA Delta Hb(diff) less than 5 mu mol/L exhibited SCI or suffered a stroke. Within the resolution of the criteria used an ICA Delta Hb(diff) threshold of 6.8 mu mol/L provided 100% specificity for SCI, whereas an ICA Delta Hb(diff) less than 5 mu mol/L was 100% sensitive for excluding SCI. When Total Delta Hb(diff) was used without removing the ECA component, no thresholds for SCI were apparent. Conclusions. Carotid endarterectomy provides a stable environment for exploring NIRS-quantified thresholds for SCI in the adult head.