Estimation of Cerebral Vasomotor Reactivity with Near Infrared Spectroscopy in Young Adults

Cerebral vasomotor reactivity (CVMR) reflects the ability of cerebral blood vessels to regulate cerebral blood flow (CBF) in response to changes in carbon dioxide (CO2). Transcranial Doppler (TCD) measurement of CBF velocity (CBFV) in the large cerebral arteries has been widely used to evaluate CVMR. However, assessment of CVMR in the cerebral microcirculation using near infrared spectroscopy (NIRS) has not been fully explored. The aims of this study are to characterize CVMR during hypo‐ and hypercapnia using NIRS, and to investigate the relationship between CVMRs assessed by NIRS and TCD. Thirty‐six healthy adults aged 21 to 45 (women: n=23) underwent CBFV and brain tissue oxygenation measurements using both NIRS and TCD during hyperventilation (hypocapnia) and modified CO2 rebreathing (hypercapnia). NIRS was placed on the forehead to measure brain tissue oxygenation index (TOI), TCD was used to measure CBFV in the middle cerebral artery, and capnography was used to measure end‐tidal CO2 (EtCO2). The percentage changes in TOI and CBFV relative to the baseline level in response to changes in EtCO2 were calculated to assess CVMR. Both TOI (−6±2 %) and CBFV (−41±10 %) were reduced during hypocapnia, and elevated during hypercapnia (TOI: 6±2 %, CBFV: 62±20 %) (Fig. ). CVMR assessed by NIRS was significantly lower than that measured with TCD in both hypocapnia (0.33±0.16 vs. 2.21±0.43 %/mmHg, p< 0.001) and hypercapnia (0.52±0.19 vs. 3.88±1.22 %/mmHg, p< 0.001, Fig. ). CVMRs assessed by NIRS and TCD were correlated in hypocapnia (r=0.40, p=0.02), but not in hypercapnia (r=0.10, p=0.55). These findings suggest that NIRS measurement of TOI in responses to changes in CO2 likely reflects CVMR of the cerebral microcirculation, which is in difference from that measured in the large cerebral arteries using TCD Support or Funding Information The study was supported by National Institutes of Health (R01AG033106 and R01HL102457). JingShang was supported by the China Scholarship Council. The typical recordings of hyperventilation (hypocapnia) and the modified CO2 rebreathing (hypercapnia). Figure 1 Average values of the relative change in CBFV and TOI in 4‐mmHg bins of ΔEtCO2 during hypercapnia induced by CO2 rebreathing. Error bars indicate standard deviation. ΔEtCO2 refers to change in EtCO2 from baseline. Figure 2