CARDIAC ARRHYTHMIAS OCCURRING DURING HALOTHANE ANAESTHESIA IN CATS BY

The incidence of cardiac arrhythmias in cats during halothane anaesthesia is high, and the arrhythmias observed during thirty-seven periods of anaesthesia have been classified into four types. The effect of the concentration of halothane, the blood pressure, the arterial blood gases and sensory stimulation on the occurrence of the ventricular extra-systoles with contradirectional interference are described. It is suggested that, under certain conditions, halothane may produce cardiac arrythmias in cats through a direct action on the myocardium. Muir, Hall and Iittlewort (1959) reported cardiac arrhythmia in fourteen of sixteen cats anaes-thetized with 1.5-3.0 per cent halothane. Electro-cardiograms from the cats showed that the arrhythmia consisted of ventricular extrasystoles with a phasic variation in the amplitude of the R waves, indicating the presence of "interference dissociation". The arrhythmia disappeared rapidly on increasing the concentration of halothane in the inspired gases, but reappeared about 5 min-utes later. A decrease in the inspired concentra-tion of halothane had no immediate effect on the arrhythmia. Overventilation with an accompany-ing reduction in plasma bicarbonate also caused the arrhythmia to disappear. When 5 per cent carbon dioxide was added to the ventilating gases, however, overventilation had no effect. This report is an account of an investigation in which the incidence and nature of the arrhythmia, as well as some of the factors apparently respon-sible for its appearance, were studied. METHODS Anaesthesia was induced in the cats either with intravenous thiopentone (10 mg/kg) followed by a paralyzing dose of suxamethonium (1-2 mg) or with halothane-oxygen alone administered to the cat in a perspex tank. No premedication was used. A No. 2 or 3 Magill oral tube was inserted into the trachea under direct vision, and anaesthesia Present address: National Nutrition Research Insti