EFFECT OF TEMPERATURE ON PHRENIC NERVE ACTIVITY INVESTIGATIONS WITH CONTROL LOOP OPENED .2. HYPERTHERMIA

The influence of an elevated body temperature on the electric activity of the phrenic nerve was investigated in 12 dogs which were artificially ventilated with oxygen. The integrated electric activity of the phrenic nerve was taken as equivalent to the tidal volume, its product with the burst frequency as equivalent to the total ventilation. One group of dogs was hypoventilated (high PaCO2) the other one hyperventilated (low PaCO2). The CO2 reactivity was tested by using short increases of PaCO2. During an increase in body temperature the following results were obtained: 1. With an increase in blood temperature the burst frequency increased similarily as in panting dogs, whereas the integrated electric activity decreased. 2. Above 42° C irreversible changes in the respiratory pattern occurred whereas the accompanying circulatory changes were reversible after cooling the dog. 3. Short increases in PaCO2 produced a sharp decrease of the burst frequency and an increase in the integrated electric activity. 4. In the hyperventilated group with a low PaCO2 panting frequencies were observed at a relatively low body temperature, whereas in the hypoventilated group with a high PaCO2 burst frequency never exceeded 70/min even at a very high blood temperature. 5. When the ventilatory data were plotted against PaCO2 and temperature, the ventilatory response to CO2 changed its direction if a critical temperature was reached. Above this temperature CO2 produced a decrease not only of the frequency, but also of the electric equivalent of the total ventilation. It is concluded that hyperthermia alters the set point as well as the gain of the controller of the CO2 homeostat. © 1969 Springer-Verlag.