EFFECTS OF SPONTANEOUS AND HYPERCAPNIC HYPERVENTILATION ON INSPIRATORY EFFORT SENSATION IN NORMAL SUBJECTS

In order to elucidate if the inspiratory effort sensation (IES) associ ated with carbon dioxide (CO2) is independent of the concomitant incre ase in the ventilation, we studied 23 normal resting volunteers (mean age 34 +/- 11 yr) during CO2 rebreathing. Our main goal was to compare the IES at the same ventilation level under hypercapnic and isocapnic conditions. The protocol included: (1) basal measurements (BASAL); (2 ) hypercapnic ventilation (HV); (3) screen copy of ventilatory pattern during hypercapnia (COPY); (4) screen copy at basal end-tidal (partia l) carbon dioxide pressure (PETCO2) (ISO); and (5) recovery (REC). Dur ing HV, PETCO2 increased to 54.8 +/- 0.78 mm Hg (p < 0.001) and ventil ation (VE) from 12.0 +/- 0.50 to 28.1 +/- 1.19 L/min (p < 0.001). Borg value increased from 0.11 +/- 0.06 to 3.4 +/- 0.23 (p < 0.001). These values were not different during HV and COPY. During ISO, PETCO2 was 40.2 +/- 0.59 mm Hg (not significant [NS] from BASAL), while VE remain ed unchanged: 29.9 +/- 1.29 L/min (NS from HV and COPY). Interestingly , the Borg value during the ISO decreased to 1.86 + 0.28 (p < 0.001 co mpared with HV and COPY). The increased IES induced by hypercapnic ven tilation was reduced at the same ventilation level during isocapnic co nditions. We suggest that CO2 generates an IES independent of the conc omitant increase in ventilation.