Cerebrovascular reactivity assessment with O(2)-CO(2) exchange ratio under brief breath hold challenge

BACKGROUND: Hypercapnia during breath holding is believed to be the dominant driver behind the modulation of cerebral blood flow (CBF). However, increasing evidence show that mild hypoxia and mild hypercapnia in breath hold (BH) could work synergistically to enhance CBF response. We hypothesized that breath-by-breath O(2)-CO(2) exchange ratio (bER), defined as the ratio of the change in partial pressure of oxygen (ΔPO(2)) to that of carbon dioxide (ΔPCO(2)) between end inspiration and end expiration, would be able to better correlate with the global and regional cerebral hemodynamic responses (CHR) to BH challenge. We aimed to investigate whether bER is a more useful index than end-tidal PCO(2) to characterize cerebrovascular reactivity (CVR) under BH challenge. METHODS: We used transcranial Doppler ultrasound (TCD) to evaluate CHR under BH challenge by measuring cerebral blood flow velocity (CBFv) in the middle cerebral arteries. Regional changes in CHR to BH and exogenous CO(2) challenges were mapped with blood oxygenation level dependent (BOLD) signal changes using functional magnetic resonance imaging (fMRI). We correlated respiratory gas exchange (RGE) metrics (bER, ΔPO(2), ΔPCO(2), end-tidal PCO(2) and PO(2), and time of breaths) with CHR (CBFv and BOLD) to BH challenge. Temporal features and frequency characteristics of RGE metrics and their coherence with CHR were examined. RESULTS: CHR to brief BH epochs and free breathing were coupled with both ΔPO(2) and ΔPCO(2). We found that bER was superior to either ΔPO(2) or ΔPCO(2) alone in coupling with the changes of CBFv and BOLD signals under breath hold challenge. The regional CVR results derived by regressing BOLD signal changes on bER under BH challenge resembled those derived by regressing BOLD signal changes on end-tidal PCO(2) under exogenous CO(2) challenge. CONCLUSION: Our findings provide a novel insight on the potential of using bER to better quantify CVR changes under BH challenge.