The purpose of this investigation was to develop an algorithm on the basis of the QRS morphology observed on the 12-lead ECG that would rapidly locate the site of origin of the monomorphic ventricular tachycardia arising from the septal portion of the RVOT. Radiofrequency catheter ablation guided by pace-mapping techniques has proven effective in eliminating the ventricular tachycardia originating from the RVOT in the absence of structural heart disease. A method that would rapidly identify the portion of the RVOT septum toward which more detailed pace-mapping should be directed before catheter ablation would be useful in decreasing procedure time and radiation exposure and potentially facilitating a successful ablation procedure. The RVOT septum was divided into nine sites. In 11 patients, bipolar pacing was performed at each of the nine designated sites to mimic ventricular tachycardia. A standard 12-lead surface ECG was recorded during pacing. The QRS morphology in the limb leads was characterized and the site of the R-wave transition was determined in the precordial leads. A QS in lead a VR and a monophasic R wave in leads II, III, aVF, and V6 were noted in each patient at all paced sites. In lead I, pacing at the three posterior septal sites always resulted in an R wave. Pacing at the three anterior sites produced a dominant Q wave (either QS or Qr) at 17 (52%) of 33 sites or a qR complex at 16 (48%) of 33 sites.(ABSTRACT TRUNCATED AT 250 WORDS)