CLINICAL-EVALUATION OF NEAR-INFRARED SPECTROSCOPY FOR TESTING CEREBROVASCULAR REACTIVITY IN PATIENTS WITH CAROTID-ARTERY DISEASE

Background and Purpose Near-infrared spectroscopy (MRS) derives inform ation about the concentrations of oxyhemoglobin (HbO(2)) and deoxyhemo globin (Hb) from measurements of light attenuation caused by these chr omophores. The aim of this study was to assess NIRS as a tool for test ing CO2 reactivity in patients with carotid artery disease. Methods On e hundred patients with symptomatic carotid occlusive disease were exa mined (age range, 44 to 83 years). The severity of stenosis ranged fro m 30% to 100% (median, 80%) on the ipsilateral side and 0% to 100% (me dian, 30%) on the contralateral side. Monitored parameters included tr anscranial Doppler flow velocity, changes in concentration of HbO(2) a nd Hb, cutaneous laser-Doppler blood flow, endtidal CO2, arterial bloo d pressure, and arterial oxygen saturation. Hypercapnia was induced wi th the use of a 5% CO2/air mixture for inhalation. To estimate the con tribution of skin flow to NIRS during reactivity testing, the superfic ial temporal artery was compressed, and the NIRS changes in response t o the fall in laser-Doppler blood flow were recorded. Finally, reprodu cibility of reactivity testing was assessed in 10 patients who were su bjected to repeated examinations over 3 days. Results Flow velocity- a nd HbO(2)-derived reactivity values were related to the severity of th e stenosis (P=.0001 and P=.017, respectively). The correlation between the two reactivity modalities was significant (r=.49, P<.000001). The median estimated contribution of skin flow to NIRS changes was 15.8%. Another variable affecting HbO(2) signal changes during the CO2 chall enge was arterial blood pressure (P=.025). Reproducibility of HbO(2) r eactivity was similar to flow velocity reactivity (14.3% and 18.6% var iation, respectively). Conclusions NIRS shows potential as an alternat ive technique for testing CO2 reactivity in patients with carotid dise ase provided that conditions are carefully controlled. Marked changes in arterial blood pressure may render the NIRS reactivity indices unre liable, and the contribution from extracranial tissue must be taken in to account when significant.