Role of cycle length, cycle length alternans, and electrocardiographic lead in electrical alternans with rapid atrial pacing.

The role of cycle length and cycle length alternans in the induction of tachycardia-related QRS electrical alternans was investigated using an atrial pacing protocol in 16 patients. Pacing was performed at a cycle length less than 400 ms in 5 patients, greater than 400 ms in 5 and at both in 6 with 0, 6, 10, 20, 40 and 60 ms of atrial cycle length alternans. A 12-lead electrocardiogram and high right atrial, His bundle and right ventricular apical electrograms were simultaneously recorded after 30 to 60 seconds of pacing. Alternans was produced in 88% of patients. Alternans was 3 times more frequent at short (less than 400 ms) than long paced cycle lengths (greater than 400 ms) (66% vs 22%, p less than 0.0001). Alternans increased with increasing cycle length alternans and occurred with very little (less than or equal to 10 ms) atrioventricular nodal, His-Purkinje and ventricular cycle length alternans when paced cycle length was short. Alternans was more frequent in the precordial than the limb leads (45% vs 17%, p less than 0.001) and was most frequent in V3 and V2 (sensitivity 69% and 65%) and least frequent in leads I and aVL (sensitivity 4% and 10%). More leads per electrocardiogram showed alternans at short compared with long paced cycle lengths and the number of leads per electrocardiogram increased with increasing cycle length alternans. Occurrence of alternans was highly related to QRS amplitude by Spearman rank correlation (p less than 0.0005).(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Entrainment of ventricular tachycardia in postoperative tetralogy of Fallot.

The mechanism of ventricular tachycardia (VT) in postoperative tetralogy of Fallot has been ascribed to both reentry and triggered automaticity. We performed electrophysiologic studies on a patient with this condition and induced sustained uniform ventricular tachycardia by programmed extrastimulation. Pacing during the tachycardia at multiple cycle lengths from the right ventricular apex (RVA) and outflow tract (RVOT) produced constant but progressive fusion between the paced and tachycardia QRS. With termination of pacing, the last captured complex was unfused but coupled at the paced cycle length and then the tachycardia resumed at its intrinsic rate. Therefore, the VT was entrained. In addition, an area of slow conduction between the RVOT and RVA was demonstrated. These findings support a reentrant mechanism of this arrhythmia.