Sensitivity to CO2: Insights from a new standardized methodology

Objective Compare respiratory and cerebrovascular sensitivity to CO 2 measured using Steady state (SS) and Rebreathing methods. Method Three steady‐state, isocapnic, hypoxic responses and 2 dynamic, isoxic rebreathing responses were recorded and analyzed to obtain hypoxic and hyperoxic responses to CO 2 (Figure 1). Gases were controlled using a specialized device (RespirAct TM , Thornhill Research Inc.). Ventilation (VE) was monitored using a flowmeter and middle cerebral artery velocity (MCAv) using ultrasound. Results 18 subjects completed the study. The results from SS and Rebreathing are comparable for VE sensitivity to CO 2 when the response threshold is taken into account. The MCAv response slopes are often similar between SS and rebreathing tests but differ in some subjects, possibly due to variability in the SS responses. Figure 2 shows examples. Conclusion While both SS and rebreathing methods can be used for measuring respiratory sensitivity to CO 2 , the same does not apply to measuring MCAv sensitivity to CO 2 . We suggest that rebreathing is the preferred method. This study is funded by Thornhill Research Inc.