Effect of preexisting bundle branch block on the electrocardiographic diagnosis of ventricular tachycardia.

The electrocardiograms (ECGs) of 80 ventricular tachycardias (VTs) occurring in 52 patients with a normal baseline QRS duration (group 1) were compared with 26 VTs in 18 patients with preexisting bundle branch block (group 2). The effects of bundle branch block on the sensitivity of previously defined electrocardiographic criteria for differentiating VT from supraventricular tachycardia with aberration were under investigation. Specificity was examined by comparing VT to the baseline ECG in group 2 patients. The VTs in groups 1 and 2 were comparable with respect to rate, bundle branch pattern, R-wave pattern in V1 with right bundle, frequency of an R/S ratio less than 1 in V6 with right bundle, Q-wave frequency in V6 with left bundle and quadrant of the frontal plane axis. Precordial QRS concordance was more frequent (35 vs 15%, p = 0.045) and a greater than 30 ms R wave in V1 or V2 with left bundle pattern was less frequent (18 vs 63%, p = 0.015) in group 2 vs group 1 arrhythmias. Right bundle pattern VT usually had a monophasic R wave in V1 (69%), whereas preexisting right bundle usually had a biphasic R wave in V1 (82%, p = 0.001). The quadrant of the frontal plane axis was significantly different between the VT ECGs and the ECGs with preexisting bundle branch block (p = 0.029) with a right superior quadrant axis only seen in VT (19%). A greater than 30 ms R wave in V1 or V2 with left bundle was also only seen in VT (52 vs 0%, p = 0.052).(ABSTRACT TRUNCATED AT 250 WORDS)