Initiating sequences in exercise induced idiopathic ventricular tachycardia of left bundle branch-like morphology.

Initiating sequences for VT may infer the underlying arrhythmogenic mechanisms. This study examines the initiating sequences of exercise induced idiopathic VT of left bundle branch block-like (LBBB-like) morphology and makes an attempt to relate these to clinical aspects and the mechanisms of arrhythmia. Thirty-two patients (mean age 33.4 +/- 13.2 years; 18 men) with exercise induced VT in the absence of structural cardiac abnormality on history, clinical examination, and noninvasive and invasive investigations were divided into two groups on the basis of the initiating sequence of VT on exercise. Group I consisted of patients with long-short sequence of RR intervals prior to the onset of VT (initiating/preinitiating cycle length ratio < or = 0.78). Group II consisted of patients without changes in cycle length prior to VT. Group I mechanism would suggest delayed afterdepolarizations (DADs) or reentry whereas group II mechanism triggered activity due to early afterdepolarizations. Fourteen patients (group I) had long-short sequence and 18 patients (group II) were without cycle length changes prior to VT initiated during exercise. VT axis was inferior in all 18 patients in group II but only in 9 patients in group I (P = 0.02). In these predefined patient groups, sustained monomorphic VT could not be initiated by programmed stimulation in any patient in group I, whereas four patients in group II had inducible VT. Patients in group II also had higher incidence of sustained VT on ambulatory monitoring (P < 0.05). The two groups did not differ in other respects. This study demonstrates the existence of at least two possible mechanisms of initiation of exercise induced idiopathic VT of LBBB-like morphology. VT initiated without cycle length changes is more common, more likely to have an inferior axis suggesting an outflow tract origin, and is probably related to triggered activity secondary to DADs. VT initiated with a long-short sequence is more often nonsustained and may have a superior axis suggesting an origin from the body or septal region of the ventricle. The two groups, therefore, exhibit differences in electrophysiological characteristics that may aid classification and therapy of this arrhythmia.