Rate Reduction by Overdrive Stimulation at the Origin of an Ectopic Ventricular Tachycardia in the Intact Pig Heart‐Evidence of Vagal Activation

Overdrive stimulation is used in the treatment of tachycardias. In this study, the effect of overdrive stimulation on a focal ventricular tachycardia was investigated. In 12 landrace pigs thoracotomy was performed. Stable ectopic ventricular tachycardias with a rate of 120–170/min were induced by subepicardial infusion of a solution of norepinephrine (10−4 M). Unipolar cathodal stimulation was carried out by an epicardial screw-in electrode. During tachycardia, overdrive stimulation was undertaken in different distances to the ectopic focus, with different stimulus amplitude and duration. A significant rate reduction of the ventricular tachycardia was observed after stimulation. When stimulated directly at the focus the extent of rate reduction was larger than after stimulation in a distance of 10–40 mm. At the infusion site the rate of the tachycardia was lowered in direct relation to the stimulation amplitude and stimulus width. The marked reduction in rate after overdrive stimulation (18%) was nearly completely antagonized by atropine. This indicates that rate reduction caused by local overdrive stimulation is due to local acetylcholine liberation caused by vagal stimulation in the ventricular working myocardium in this experimental model.