Minute ventilation changes in rabbits during experimental hypothermia

The effect of experimental hypothermia on changes of the electrophysiological equivalent of minute ventilation (Veq) was studied in rabbits under urethane-chloralose general anaesthesia with muscle relaxation and artificial ventilation. The animals were subjected to bilateral vagotomy prior to the experiment. During normothermia (37.5 +/- 0.7 degree C) and hypothermia (29.9 +/- 1.7 degrees C) the animals were given for breathing a hypercapnic mixture of gases (CO2 5% with O2 95%) and asphyxia was produced by switching off the respirator. The arterial blood pressure, blood flow in the common carotid artery, end-expiratory CO2 concentration, "integrated" phernic nerve activity and brain-stem temperature were recorded. The partial pressure of carbon dioxide and oxygen, hydrogen ion concentration and arterial acid-base balance were determined with correction for temperature changes. The equivalent of minute ventilation (being the product of the frequency and amplitude of "integrated" phrenic nerve activity) decreased in hypothermia by 91%, with a simultaneous fall of PaCO2 from 33,48 +/- 3.84 mmHg to 23.40 +/- 3.59 mmHg (by 30%). The hypercapnic stimulus applied during hypothermia produced a fivefold lower Veq value than in normothermia and under control conditions (despite a similar value of PaCO2 of 28.89 +/- 3.12 mmHg). The Veq value approaching that found under normal conditions in normothermia was observed during hypothermia only when asphyxia was induced when the value of PaCO2 was 37.07 +/- 8.74 mm Hg and that of PaO2 was 37.41 +/- 29.11 mmHg. During hypothermia the blood flow in the common carotid artery decreased by 16% when the animals were breathing the hypercapnic mixture. The analysis of the obtained results showed a direct effect of temperature on respiratory activity generation and regulation of arterial blood flow to the brain. It may be supposed also that hypothermia raises the response threshold to CO2 level in the breathed air.