Slow Conducting Electroanatomic Isthmuses: An Important Link Between QRS Duration and Ventricular Tachycardia in Tetralogy of Fallot.

OBJECTIVES: This study sought to evaluate the influence of slow conducting anatomic isthmuses (SCAI) as dominant ventricular tachycardia (VT) substrate on QRS duration.

BACKGROUND: QRS prolongation has been associated with VT in repaired tetralogy of Fallot.

METHODS: Seventy-eight repaired tetralogy of Fallot patients (age 37 ± 15 years, 52 male, QRS duration 153 ± 29 ms, 67 right bundle branch blocks [RBBB]) underwent programmed stimulation and electroanatomic activation mapping during sinus rhythm. Right ventricular (RV) surface, RV activation pattern, RV activation time, conduction velocity at AI, and remote RV sites were determined.

RESULTS: Twenty-four patients were inducible for VT (VT+); SCAI was present in 22 of 24 VT+ but only in 2 of 54 patients without inducible VT (VT-). Conduction velocity through AI was slower in VT+ patients (median of 0.3 [0.3 to 0.4] vs. 0.7 [0.6 to 0.9] m/s; p < 0.01) but conduction velocity in the remote RV did not differ between groups. In non-RBBB, QRS duration was similar in VT+ patients (n = 6) and VT- patients (n = 5), but RV activation within SCAI exceeded QRS offset in VT+ patients (37 ± 20 ms vs. -5 ± 9 ms, p < 0.01). In RBBB, both QRS duration and RV activation time were longer in VT+ patients (n = 18, 17 of 18 QRS > 150 ms) compared with VT- patients (n = 49, 27 of 49 QRS > 150 ms) (173 ± 22 ms vs. 156 ± 20 ms; p < 0.01; 141 ± 22 ms vs. 129 ± 21 ms; p = 0.04). In VT+ patients, QRS prolongation >150 ms (n = 17) was due to SCAI or blocked isthmus in 15 patients (88%) and 1 (6%). In contrast, in VT- patients, QRS prolongation >150 ms (n = 27) was due to enlarged RV or blocked isthmus in 10 patients (37%) and 8 (30%), but due to SCAI in only 1 (4%). After exclusion of a severely enlarged RV, a QRS duration >150 ms was highly predictive for SCAI/blocked AI (OR: 17; 95% CI: 3.3 to 84; p < 0.01).

CONCLUSIONS: A narrow QRS interval does not exclude VT-related SCAI. In the presence of RBBB, SCAI further prolongs QRS duration. QRS duration >150 ms is highly suspicious for SCAI or isthmus block distinguishable by electroanatomic mapping.