Resolving extra- and intracranial signal changes during adult near infrared spectroscopy

Extracranial tissues have a significant effect on cerebral near infrared spectroscopy (NIRS) measurements in adults. Carotid surgery provides the opportunity to determine the relative contributions from the intra- and extracranial vascular territories. To assist, a specifically gated Laser Doppler flowmetry probe can be inserted between the NIRS optodes to co-monitor cutaneous blood flow associated with external carotid artery (ECA) clamping, whilst transcranial Doppler can be employed to monitor relative changes in the intracranial blood flow seen during internal carotid artery (ICA) clamping. Established criteria for severe cerebral ischemia (SCI) following carotid cross clamping was applied to 703 consecutive patients undergoing carotid surgery for high grade stenosis. Ipsilateral frontal NIRS recorded the difference (Total-deltaHbdiff) in concentrations of oxy- and deoxy-hemoglobin. Interrupted time series analysis following clamping of the ECA and ICA allowed the different vascular components of Total-deltaHbdiff (ECA-deltaHbdiff and ICA-deltaHbdiff) to be identified. Data from 76 patients was suitable. Sixteen (21%) patients fulfilled the criteria for SCI. Patients who demonstrated an ICA-deltaHbdiff of > 6.8 micromol l(-1) all showed SCI. No patient with an ICA-deltaHbdiff < 5.0 micromol l(-1) showed SCI. Thus an ICA-deltaHbdiff threshold of 6.8 micromol l(-1) provided a 100% specificity for SCI, whereas an ICA-deltaHbdiff < 5.0 micromol l(-1) was 100% sensitive for excluding SCI. When Total-deltaHbdiff was used without removing the ECA component, thresholds for SCI could not be resolved. NIRS can provide quantified thresholds for severe ischemia in the adult brain provided the extracranial component is removed.