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Oxygen-carrying capacity during 10 hours of hypercapnia in ventilated dogs
J Ramirez1, B R Totapally, E Hon
1Division of Critical Care Medicine, Miami Children's Hospital, FL, USA.
Objective:
To test if a relatively long-term exogenous hypercapnia, equivalent to those maintained during permissive hypercapnia, can persistently increase oxygen-carrying capacity in ventilated dogs.
Design:
Prospective study.
Setting:
Research laboratory in a hospital.
Subjects:
Six mongrel dogs (3 males; 3 females).
Interventions:
The dogs were anesthetized (30 mg/kg pentobarbital, i.v.), intubated, and cannulated in one femoral artery, one femoral vein, and the right jugular vein. The mean arterial blood pressure, heart rate, and mean pulmonary artery pressure were continuously recorded. Anesthesia, fluid balance, and normothermia were maintained. Arterial hypercapnia was generated by the addition of 60 torr dry CO2 (8 kPa) to the inspired air for 10 hrs, continuously. All subjects were paralyzed (vecuronium bromide) and ventilated with room air, while the ventilator settings were kept constant.
Measurements And Main Results:
Arterial and venous gas exchange profiles, hemoglobin concentration, oxygen saturation, oxygen content, cardiac output, and oxygen consumption were determined, before, during, and after 10 hrs of hypercapnia, periodically. Both hemoglobin concentration and oxygen content were gradually increased during hypercapnia and reached significant levels at 8 and 10 hrs of hypercapnia, respectively. These increases continued up to 2 hrs after termination of hypercapnia. The PaO2/FIO2, as an index of arterial oxygenation, was significantly increased during the first 3 hrs of hypercapnia and then remained at the normoxic level up to 10 hrs of hypercapnia. No significant changes occurred in the mean arterial blood pressure and oxygen consumption. The heart rate and cardiac output were significantly reduced at 4 and 8 hrs of hypercapnia, respectively. The mean pulmonary artery pressure was increased throughout the hypercapnic trial.
Conclusions:
A relatively long-term exogenous hypercapnia can significantly increase oxygen-carrying capacity in normal ventilated dogs. Whether this effect can occur during permissive hypercapnia because of controlled ventilation in patients warrants investigation.