Breath-to-breath variations of alveolar Po2 and Pco2 at barometric pressures of 490, 745 and 1500 Toor in resting awake dogs.

Two awake, resting dogs born and raised at low altitude were studied, breathing air (1) at 745 torr, (2) during a 12 days sojourn at 490 torr in an altitude chamber, and (3) during 5 days sojourn at 1500 torr in a hyperbaric chamber. The respired gas was continuously sampled an end-tidal PCO2 and PO2 of sequences of thirty breaths were measured by fast analyzers. The mean value of alveolar PCO2 was 29 torr at high altitude; 35 torr at 745 torr; and 40 torr in hyperbary. The changes of PCO2 indicate different alveolar ventilations which result mainly from the changes of the chemoreceptor drive which is enhanced at high altitude and decreased in hyperbary. The scattering of PCO2 is about the same at the three pressures. The scattering of PO2 is less at high altitude than at sea level, and less at sea level than in hyperbary. On a PCO2 vs PO2 diagram end-tidal PCO2 and PO2 points form elliptical clouds whose mean slopes decrease with the increase of total pressure. The characteristics of the dispersion of the alveolar pressures and of the slopes of the alveolar clouds depend on several factors among which the relevant steepness (i.e. capacitance) of the O2 and CO2 blood abosrption curves at the three pressures presumably plays the major role.