JOINT NASA-ESA-DARA STUDY - PART 2 - HYPERCAPNIC VENTILATORY RESPONSEIN HUMANS BEFORE, DURING, AND AFTER 23 DAYS OF LOW-LEVEL CO2 EXPOSURE

Alterations in ventilation and the chemoreceptor response to CO2 durin g 23 d of 1.2% inspired CO2 were studied in four male subjects. Restin g ventilation ((V) over dot E), tidal volume (VT), respiratory frequen cy (fR), inspired and end tidal O-2 and CO2 and the hypercapnic ventil atory response (HCVR) measured by CO2 rebreathing were measured once b efore entering the chamber, on days 2, 5, 11, and 22 of CO2 exposure, and one day after. Resting (V) over dot E slightly increased (5%) on d ay 2 of exposure and significantly increased (22%) by day 5 followed b y a progressive decrease to pre-chamber levels by day 22 and on the fi rst day of recovery. Tidal volume and fR were not statistically differ ent. During the exposure PetCO(2) was significantly elevated with day 2 having the largest increase (19.60%). PetCO(2) returned to normal wi thin 24 h post exposure. The HCVR was characterized by the slope (SHCV R), intercept at zero ventilation (B), and the ventilation at a PCO2 = 60 mmHg ((V) over dot E60). The SHCVR decreased (14%) on day 2, but w as not significant; the SHCVR On the other exposure days were also not different. The SHCVR On the first recovery day significantly increase d (37%). The HCVR B was shifted to the right on day 2 by 5.2 mmHg, the n progressively returned to the preexposure position. On recovery the B significantly shifted 6.9 mmHg to the right of pre-exposure B. The ( V) over dot E60 decreased by similar to 32% and 16% on day 2 and 5, re spectively, then returned within pre-exposure range for the remainder of the exposure and during recovery. During the early phase and one da y after the exposure the HCVR was right shifted. One day after exposur e chemoreceptor sensitivity to elevated CO2 was increased but, the B w as right shifted resulting in a reduced HCVR below PCO2 Of 60 mmHg and a greater HCVR above 60 mmHg.