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

Abstract

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 maneuver (IC), 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-minute intervals throughout rebreathing (IC condition only) and expired-gas data were acquired to determine ventilation (⩒_E) and the partial pressure of end-tidal CO₂ (P_ETCO₂). Basal ⩒_E, the ⩒_E recruitment for threshold for CO₂ (VRTCO₂), and the ⩒_E:CO₂ slope (⩒_ES) were determined and expired-gas and sensory data were compared between-conditions at standardized time points. There were no between-condition (IC vs no-IC trial) differences in basal ⩒_E, VRTCO₂, and ⩒_ES (all p>0.05). Repeated measure analysis revealed no betweencondition differences in ⩒_E or P_ETCO₂ as a function of rebreathing time (all p>0.05). Intraclass correlation coefficient values for basal ⩒_E, VRTCO₂, and ⩒_ES were 0.745, 0.692, and 0.828, indicating good between test agreement. 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 characterization of the HCVR.