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 anesthesia, in rats can reduce ventilatory demands without compromising pulmonary 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, tracheostomized 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 monitored. The respiratory variables and gas-exchange profiles were determined at 38 degrees C (normothermia), and during stepwise hypothermia at 37, 35, 33, 31 and 29 degrees C. The arterial pressure of carbon dioxide (PaCO2), pH and arterial pressure of oxygen (PaO2) during hypothermia were corrected at body temperature.

MEASUREMENTS AND RESULTS

Graded systemic hypothermia, with maintained anesthesia, produced a strong correlation between reduction in the respiratory frequency and rectal temperature (r2 = 0.55; p < 0.0001; n = 66). The minute volume was significantly reduced, starting at 35 degrees C, without significant changes in the tidal volume (repeated measures of analyses of variance followed by Dunnett multiple comparisons test). No significant changes occurred in the PaCO2, pH, PaO2, hemoglobin oxygen saturation, the calculated arterial oxygen content and estimated alveolar-arterial oxygen difference during mild hypothermia (37-33 degrees C). The PaO2, however, was significantly reduced below 31 degrees C. The MAP remained stable at different levels 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 observations in spontaneously breathing rats imply that a combination of mild hypothermia with anesthesia could be safely utilized to maintain adequate ventilation, using relatively low minute ventilation. We speculate that such a maneuver, if applied during mechanical ventilation, may prevent secondary pulmonary damage by allowing the use of lower ventilator volume-pressure settings.