ACUTE HYPERCAPNIA INCREASES THE OXYGEN-CARRYING CAPACITY OF THE BLOODIN VENTILATED DOGS

Objective: To test the hypothesis that Pace, levels generated during p ermissive hypercapnia may enhance arterial oxygenation, when ventilati on is maintained. Design: Prospective study. Setting: Research laborat ory in a hospital. Subjects: One group of eight mongrel dogs (four mal e; four female). Interventions: The dogs were anesthetized (30 mg/kg i v pentobarbital), intubated, and cannulated in one femoral artery and vein. While paralyzed with 0.1 mg/kg/hr iv vecouronium bromide, all su bjects were ventilated with room air. Anesthesia was maintained, using 2 to 3 mg/kg/hr iv pentobarbital. Arterial hypercapnia at the levels generated during permissive hypercapnia was produced by stepwise incre ases in the dry, inspired PCO2, (PiCO(2),) (0, 30, 45, 60 and 75 torr [0, 4, 6, 8, and 10 kPa]; 15 mins each). Measurements and Main Results : Blood gas profiles were determined at each level of hypercapnia. The minute volume was maintained at the baseline level during all exposur es. Arterial hypercapnia produced gradual and significant increases in the hemoglobin concentration. These increases were similar to 6%, 7%, 11%, and 14% at PiCO(2) of 30, 45, 60, and 75 torr (4, 6, 8, and 10 k Pa), respectively (p < .05; repeated analysis of variance followed by Dunnett multiple comparisons test). In parallel, the oxygen content in creased by similar to 6%, 7%, 11%, and 13%, respectively. During hyper capnic trials, the PaO2 remained at the normal range, whereas the dry, inspired PO2 (PiO(2)) was reduced from 150 to 138 torr (20 to 18.4 kP a). The average Pao, at the highest investigated level of arterial hyp ercapnia was at a normal range. The hemoglobin concentration and oxyge n content returned to baseline values 30 mins after hypercapnic trials . The PaCO2 and pH became normalized 15 mins after hypercapnic trials. Indirect evidence for a similar response to hypercapnia in humans is presented. Conclusions: Permissive hypercapnia due to inhaled CO2 incr eases oxygen-carrying capacity in dogs. The Pao, remains at normal ran ge even at a PiCO(2) of 75 torr (10 kPa). The benefits of these effect s during permissive hypercapnia, due to controlled hypoventilation, wa rrants investigation.