Effect of isoproterenol on QRS complex morphology during ventricular pacing: implications for pace mapping

Ventricular pace mapping may be used to identify the site of origin of idiopathic ventricular tachycardia. Isoproterenol is often required to induce this type of ventricular tachycardia, but its effect on QRS morphology during pace mapping is unknown. Therefore, this study was performed to evaluate the effect of isoproterenol on QRS morphology during ventricular pacing. The study population consisted of 20 patients (mean age 38 +/- 14 years) undergoing a clinically indicated electrophysiology procedure. Ventricular overdrive pacing was performed in trains of 12 stimuli at cycle lengths of 400, 350, 300, and 250 ms, first in the baseline state during an infusion of isoproterenol, and again after isoproterenol washout. Pacing was performed at the right ventricular apex in 10 patients, in the right ventricular outflow tract in 6 patients, and in the left ventricle in 4 patients. Visual evaluation revealed no apparent effects of isoproterenol on QRS morphology at any of the three pacing sites or at any of the pacing cycle lengths. It was concluded that QRS morphology during ventricular pacing is not affected by isoproterenol infusion. Therefore, in patients with idiopathic ventricular tachycardia, even if the induction of tachycardia requires infusion of isoproterenol, successful pace mapping may be performed in its absence.