Determining operating lung volumes during a CO2 rebreathing test: does the inspiratory capacity maneuver influence the hypercapnic ventilatory response?

Previous studies using the modified Duffin CO2 rebreathing test have not accounted for changes in operating lung volumes, which may affect the hypercapnic ventilatory response (HCVR). Importantly, determining operating lung volumes requires the performance of an inspiratory capacity (IC) maneuver, which could influence the HCVR. Therefore, the purpose of this study was to determine if serial IC measurements during a modified Duffin CO2 rebreathing test influenced HCVR. Thirty young healthy participants (15 male:15 female) completed two modified Duffin CO2 rebreathing tests on separate days (order randomized). Serial IC maneuvers were completed at 2-min intervals throughout rebreathing (IC condition only), and expired-gas data were acquired to determine ventilation (V̇E) and the partial pressure of end-tidal CO2 ([Formula: see text]). Basal V̇E, the V̇E recruitment threshold for CO2 (VRTCO2), and the V̇E:CO2 slope (V̇ES) were determined, and expired gas and sensory data were compared between conditions at standardized timepoints. There were no between-condition differences (IC vs. no-IC trial) in basal V̇E, VRTCO2, and V̇ES (all P > 0.05). Repeated measure analysis revealed no between-condition differences in V̇E or [Formula: see text] as a function of rebreathing time (all P > 0.05). Intraclass correlation coefficient values for basal V̇E, VRTCO2, and V̇ES were 0.745, 0.692, and 0.828, respectively, indicating good agreement between tests. The HCVR during a standardized CO2 rebreathing protocol was unaffected when including serial measurements of IC. The findings from the current study support inclusion of IC measurements to determine operating lung volumes in healthy young adults, which in turn will improve the characterization of the HCVR.NEW & NOTEWORTHY Previous studies using the modified Duffin CO2 rebreathing test have not determined changes in operating lung volumes, assessed by an inspiratory capacity (IC) maneuver, which may affect the hypercapnic ventilatory response (HCVR). We determined if IC maneuvers during CO2 rebreathing test influenced HCVR. IC maneuvers had no effect on HCVR. These findings support the inclusion of IC measurements to improve the characterization of the HCVR in healthy young adults.