EFFECTS OF HALOTHANE ON BLOOD-GASES AND A CID-BASE-BALANCE IN INTACT VS CHEMODENERVATED RATS

Halothane decreases the ventilatory response to hypoxia and the activi ty of peripheral arterial chemoreceptors, resulting in ''chemical chem odenervation. '' In order to evaluate the role of this halothane-induc ed ''chemical denervation'' in acid-base and arterial blood gas change s, these values were measured in intact and chemodenervated rats, awak e and under anaesthesia Since the depth of anaesthesia could be modifi ed by the anatomical chemodenervation, the ED50 of inspired halothane was determined in six rats Wore and after anatomical chemodenervation. To prevent haemodynamic changes due to halothane and/or anatomical ch emodenervation from interfering with the results, systemic arterial bl ood pressure and heart rate were measured in six intact rats, awake an d then anaesthetized, and in the same rats after chemodenervation, awa ke and then anaesthetized. In nine intact rats and in 19 chemodenervat ed rats, arterial pH, arterial bicarbonate concentration, and arterial blood gases (PaO2 and PaCO2) were measured before and after administr ation of halothane. Anatomical chemodenervation modified neither the i nspired ED50 (1.1%), nor the mean arterial blood pressure or heart rat e. The haemodynamic effects of halothane were comparable in intact and in chemodenervated rats. Changes in arterial blood gases and acid-bas e balance due to halothane in intact rats and due to chemodenervation in awake rats were not different, but there was a decrease in PaO2 and pHa, and an increase in PaCO2. In chemodenervated rats, halothane cau sed a further decrease in PaO2 and a further increase in PaCO2. The fa ct that halothane and anatomical chemodenervation have similar effects on arterial blood gases and acid-base balance favours a ''chemical ch emodenervating '' action of halothane. However, the additional effects of halothane in the anatomically chemodenervated animal show that the action of halothane on blood gases and acid-base balance is the resul t of multiple sites of impact on the respiratory system.