Prevention by dexamethasone of the marked antiarrhythmic effects of preconditioning induced 20 h after rapid cardiac pacing.

Dogs were paced, via a pacing electrode in the right ventricle, for four 5 min periods at a rate of 220 beats min-1. On the following day they were reanaesthetized, thoracotomized and the left anterior descending coronary artery occluded for 25 min. Pacing markedly reduced the severity of ischaemia-induced arrhythmias (e.g. reduction in VF from 45% in unpaced dogs to 10% in paced dogs; P < 0.05), an effect reversed by dexamethasone (4 mg kg-1 i.v., 45 min prior to pacing). This protection may be due to the induction of nitric oxide synthase or cyclo-oxygenase.