Atrial pacing at progressively increasing frequencies was performed on unanaesthetized dogs through electrodes placed aseptically in the wall of the right atrium and exteriorized in the neck region. Heart rate and two atrioventricular conduction (AVC) parameters, namely the Wenckebach Point (one or two systole block at the end of expiration)‐and the maximum atrioventricular conduction frequency (the frequency of pacing for which ventricles do not follow any auricular stimulation) were measured by electrocardiography. Chloralose (0.08 g/kg i.v.) did not affect either heart rate or AVC but significantly reduced the effect of atropine (0.1 mg/kg i.v.) on all three parameters measured. The effect of isoprenaline (0.25 μg kg−1 min−1) remained unchanged. Pentobarbitone Na (25 mg/kg i.v.) increased heart rate and usually caused the Wenckebach Point to disappear. It reduced the effects of atropine but did not modify those of isoprenaline. In view of these results we suggest that pentobarbitone Na be avoided as an anaesthetic agent in AVC studies. It is possible to dispense with an anaesthetic agent if the technique described here is used.

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