Assessment of cerebrovascular reactivity in patients with carotid artery disease using Near-infrared spectroscopy

The aim of this study was to assess Near-infrared spectroscopy (NIRS) as a tool for testing CO2 reactivity in patients with carotid occlusive disease. One hundred sixty patients were examined (age range 44 to 85 years). Monitored parameters included transcranial Doppler flow velocity (FV), changes in concentration of oxy (HbO(2)) and deoxy (Hb) haemoglobin, cutaneous Laser Doppler blood flow (LDF), endtidal CO2, ABP, and SaO(2). Hypercapnia was induced using a 5% CO2 air mixture for inhalation. To estimate the skin flow contribution to NIRS during reactivity testing, the superficial temporal artery was compressed, and the NIRS changes in response to the fall in LDF recorded. FV and HbO(2) derived reactivity values were related to the severity of the stenosis (p =0.0001 and 0.021 respectively). The correlation between the two modalities was significant (r = 0.47, p < 0.000001). The average estimated skin contribution to NIRS changes was 16.5%. Reproducibility of HbO(2)-reactivity was similar but worse than FV reactivity (19.1% and 13.8% variation respectively). The clinical correlations improved when our method of correction for skin influence was used. NIRS shows potential as an alternative technique for testing CO2 reactivity in patients with carotid disease provided the conditions are carefully controlled and the contribution from extracranial tissue is taken into account.