Experimental critical care in ventilated rats: Effect of hypercapnia on arterial oxygen-carrying capacity

Purpose: We have previously demonstrated an increased arterial O-2-carrying capacity in normal ventilated dogs subjected to both acute and prolonged e xogenous hypercapnia. In the present study, we tested if arterial hypercapn ia, during controlled ventilation, can increase O-2-carrying capacity also in rats. Materials and Methods: Twenty you ng male Sprague Dawley rats were anesthet ized (60 mg/kg pentobarbital), tracheostomized, intubated, and one femoral vein and artery were cannulated. Anesthesia and paralysis were maintained u sing 15 mg/kg/h pentobarbital intravenously, and 2 mg/kg/h vecuronium bromi de. The fluid balance (5 mL/kg/h saline), normothermia, and minute volume w ere maintained. The mean arterial brood pressure and heart rate were contin uously monitored. Experiments included the following: (1) a control group, ventilated with normoxic air for 150 minutes (n = 5); (2) mild hypercapnia, a group of eight rats ventilated with normoxic air for 30 minutes and then ventilated with a mixture of normoxic air at 60 mm Hg CO2 (8 kPa) for 1 ho ur; and (3) severe hypercapnia, a group of seven rats were treated exactly as in group II, except a 90 mm Hg (12 kPa) CO2 during hypercapnia. Gas-exch ange profile, arterial hemoglobin (Hb) concentration, arterial Hb-oxygen sa turation (Hb-O-2), and arterial O-2 content were periodically determined du ring normocapnia and 1 hour of hypercapnia. Results: Exposures to mild and severe hypercapnia, in rats with maintained ventilation, significantly reduced the arterial O-2 content by 20% and 33%, respectively. without significant changes in the arterial Hb concentration (-2%). Severe hypercapnia generated a significant reduction of -14% in the Pao(2), but not in P(a)o(2)/FiO2 ratio. Conclusion: Rats subjected to controlled ventilation and permissive hyperca pnia, unlike dogs and perhaps humans, show no augmentation of Hb concentrat ion. Hypercapnia in rats also provokes much stronger Bohr effect than in do gs. Hypercapnia-induced Bohr effect in rats is accompanied with extreme des aturations of Hb-O-2, and substantial reduction in the O-2-carrying capacit y. We speculate that the strong hypercapnia-induced Bohr effect in rats may prevent hypoxia at the tissue level. However, to maintain a stable oxygen- carrying capacity in rats used for pulmonary critical care studies with hyp ercapnia, we suggest to use hyperoxia, with or without a mild hypothermia. Copyright (C) 1999 by W.B. Saunders Company.