EFFECT OF HYPOBARIA ON VENTILATORY AND CO2 RESPONSES TO SHORT-TERM HYPOXIC EXPOSURE IN CATS

The effect of hypobaria on the ventilatory response to short-term hypo xia was studied by comparing the respiratory mechanical and inspired C O2 ventilatory responses to hypobaric hypoxia (438 mmHg) with normobar ic hypoxia (11.8% FI(O)2). Fifteen spontaneously breathing, anesthetiz ed cats were divided into three groups of five: time control, normobar ic hypoxia and hypobaric hypoxia. Measurements of ventilation, gas exc hange, and responses to intermittent CO, rebreathing were collected ov er a 4 h period. Pa-O2 fell to 44.5 +/- 2.7 mmHg, Pa-CO2 fell to 24.8 +/- 0.9, and pH rose to 7.49 +/- 0.01 in both hypoxic groups. Tidal vo lume did not change with respect to time or condition, but frequency a nd ventilation were significantly increased in the hypobaric hypoxic g roup. The slope of the CO, response was unchanged over time or by cond ition. These results suggest that hypobaric hypoxia may alter the patt ern of breathing responses to hypoxia but not the CO2-response. If met abolic rate remained constant, these results could be explained by a d ifference in dead space between hypoxic conditions. (C) 1998 Elsevier Science B.V.