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 (V̇_E) and the partial pressure of end-tidal CO₂ ([Formula: see text]). Basal V̇_E, the V̇_E recruitment threshold for CO₂ (VRTCO₂), and the V̇_E:CO₂ slope (V̇_ES) 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 V̇_E, VRTCO₂, and V̇_ES (all <i>P</i> > 0.05). Repeated measure analysis revealed no between-condition differences in V̇_E or [Formula: see text] as a function of rebreathing time (all <i>P</i> > 0.05). Intraclass correlation coefficient values for basal V̇_E, VRTCO₂, and V̇_ES 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.<b>NEW & NOTEWORTHY</b> 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.