Ventilatory responses to dead space and CO2 breathing under inspiratory resistive load

To investigate how breathing is controlled during CO2 stimulation, steady-state ventilatory responses to rebreathing through a tube (DS) and inspiring a fixed PCO2 (INH) were compared in healthy human subjects. Tests were performed in hyperoxia with (IRL) and without (NL) an inspiratory resistive load (15 cmH2O.l-1.s at 1 l/s). The mean slope of the minute ventilation (VE)-end-tidal PCO2 relationship was significantly higher in DS-IRL than in INH-IRL 1.86 +/- 0.67 (SD) vs. 1.40 +/- 0.32 l.min-1.Torr-1, P < 0.01, and it was significantly different between INH-NL and INH-IRL (1.64 +/- 0.41 vs. 1.40 +/- 0.32 l.min-1.Torr-1, P < 0.05) but not between DS-NL and DS-IRL (1.85 +/- 0.72 vs. 1.86 +/- 0.67 l.min-1.Torr-1). The slope of the VE-tidal volume relationship was significantly lower in DS-NL than in INH-NL (19.6 +/- 3.8 vs. 21.2 +/- 5.1 min-1, P < 0.05), but other comparisons in breathing pattern between NL and IRL and between DS and INH failed to reach significance. We concluded that 1) alterations in alveolar PCO2 temporal profile by DS could induce changes in VE-end-tidal PCO2 sensitivity and ventilatory pattern, 2) these changes may be modified by increased mechanical impairment resulting from IRL, and 3) carotid chemoreceptor mediation is not necessary for the observed effects of DS.