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

PURPOSE: We have previously demonstrated an increased arterial O2-carrying capacity in normal ventilated dogs subjected to both acute and prolonged exogenous hypercapnia. In the present study, we tested if arterial hypercapnia, during controlled ventilation, can increase O2-carrying capacity also in rats.
MATERIALS AND METHODS: Twenty young male Sprague Dawley rats were anesthetized (60 mg/kg pentobarbital), tracheostomized, intubated, and one femoral vein and artery were cannulated. Anesthesia and paralysis were maintained using 15 mg/kg/h pentobarbital intravenously, and 2 mg/kg/h vecuronium bromide. The fluid balance (5 mL/kg/h saline), normothermia, and minute volume were maintained. The mean arterial blood pressure and heart rate were continuously 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 hour; 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-exchange profile, arterial hemoglobin (Hb) concentration, arterial Hb-oxygen saturation (Hb-O2), and arterial O2 content were periodically determined during normocapnia and 1 hour of hypercapnia.
RESULTS: Exposures to mild and severe hypercapnia, in rats with maintained ventilation, significantly reduced the arterial O2 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 PaO2, but not in PaO2/ FiO2 ratio.
CONCLUSION: Rats subjected to controlled ventilation and permissive hypercapnia, unlike dogs and perhaps humans, show no augmentation of Hb concentration. Hypercapnia in rats also provokes much stronger Bohr effect than in dogs. Hypercapnia-induced Bohr effect in rats is accompanied with extreme desaturations of Hb-O2, and substantial reduction in the O2-carrying capacity. 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 hypercapnia, we suggest to use hyperoxia, with or without a mild hypothermia.