Changes of minute ventilation in rabbits in experimental hyperthermia

The relative effect of the temperature on respiratory rhythm generation was studied in muscle-relaxed, artificially ventilated and bilaterally vagotomized rabbits under general anaesthesia (urethane and chloralose). Hypercapnia was produced during normothermia (38.8 +/- 0.6 degrees C) and hyperthermia (40.5 +/- 0.3 degrees C). The basic physiological parameters, efferent phrenic nerve activity and gasometric determinations in arterial blood were recorded. In the animals ventilated with a classic respirator hyperthermia produced a 118% increase of Veq value with a simultaneous 28% rise of the partial pressure of CO2. An increase of the stroke volume of the respirator during hyperthermia (in a degree sufficient for achieving PaCO2 value equal to the control value during normothermia) produced a 2% fall of Veq value due to an 8% fall in amplitude of the respiratory movements without changes of respiratory rate. Breathing in of a hypercapnic mixture caused a 131% rise of Veq above the control value in normothermia. This rise was due both to the increased respiratory rate and respiratory amplitude. During ventilation by means of a respirator controlled by phrenic nerve activity hyperthermia increased the electrophysiological equivalent of minute ventilation by 34%, with a 109% rise in the respiratory rate and with no change in PaCO2. Breathing of a hypercapnic mixture increased Veq without inducing any statistically significant changes in the respiratory rate and amplitude. The analysis of the results suggests that the effect of raised temperature on respiratory rhythm generation is manifested mainly as an impairment of the respiratory amplitude. Maintaining of minute ventilation proportional to the magnitude of respiratory drive is decisive in this phenomenon.