Effect of hypothermia on ventilation in anesthetized, spontaneously breathing rats: theoretical implications for mechanical ventilation

Objective: To test if hypothermia, induced by a sustained pentobarbital ane sthesia, in rats can reduce ventilatory demands without compromising pulmon ary gas-exchange efficiency. Design: Prospective study. Setting: Research laboratory in a hospital. Subjects: One group of 11 female Sprague Dawley rats. Interventions: The rats were anesthetized with 45 mg/kg pentobarbital, trac heostomized and intubated; their femoral veins and arteries were cannulated . After surgery, anesthesia and fluid balance were maintained (10 mg/kg per h pentobarbital, and 5 ml/kg per h saline, i.v.). Rectal temperature, mean arterial blood pressure (MAP), and heart rate (HR) were continuously monit ored. The respiratory variables and gas-exchange profiles were determined a t 38 degrees C (normothermia), and during stepwise hypothermia at 37, 35, 3 3, 31 and 29 degrees C. The arterial pressure of carbon dioxide (PaCO2), pH and arterial pressure of oxygen (PaO2) during hypothermia were corrected a t body temperature. Measurements and results: Graded systemic hypothermia, with maintained anes thesia, produced a strong correlation between reduction in the respiratory frequency and rectal temperature (r(2) = 0.55; p < 0.0001; n = 66). The min ute volume was significantly reduced, starting at 35 degrees C, without sig nificant changes in the tidal volume (repeated measures of analyses of vari ance followed by Dunnett multiple comparisons test). No significant changes occurred in the PaCO2, pH, PaO2, hemoglobin oxygen saturation, the calcula ted arterial oxygen content and estimated alveolar-arterial oxygen differen ce during mild hypothermia (37-33 degrees C). The PaO2, however, was signif icantly reduced below 31 degrees C. The MAP remained stable at different le vels of hypothermia, whereas HR was significantly reduced below 33 degrees C. Conclusions: Mild hypothermia in rats, induced by a sustained pentobarbital anesthesia, reduces ventilation without compromising arterial oxygenation or acid-base balance, as measured at body temperature. Theoretically, our o bservations in spontaneously breathing rats imply that a combination of mil d hypothermia with anesthesia could be safely utilized to maintain adequate ventilation, using relatively low minute ventilation. We speculate that su ch a maneuver, if applied during mechanical ventilation, may prevent second ary pulmonary damage by allowing the use of lower ventilator volume-pressur e settings.