QTc with bundle branch block or a wide QRS

Why a wide QRS lengthens the QTc

A wide QRS lengthens the QTc because the QT interval measures two things at once: ventricular depolarization (the QRS complex) and ventricular repolarization (the JT interval, from the end of the QRS to the end of the T wave). The risk QT monitoring looks for, torsade de pointes, comes from long repolarization. In bundle branch block or right ventricular pacing, depolarization spreads slowly through the muscle instead of the conduction system, so the QRS widens to 120 ms or more and the QT grows with it, while the JT interval can stay the same. Bogossian and colleagues showed this directly: when they paced 60 patients with a narrow QRS from the right ventricle, creating a left bundle branch block pattern, the QT lengthened but the JT interval did not change significantly. A QTc calculated from that QT with any rate formula then reads long even though repolarization is normal, so a correction for the QRS is needed before the rate correction.

Option 1: use the JT interval

The JT interval is the QT interval minus the QRS duration, and it removes depolarization from the measurement entirely. The 2009 AHA/ACCF/HRS recommendations for the standardization and interpretation of the ECG suggest focusing on the JT interval when there is a ventricular conduction defect such as bundle branch block, and a 2020 review by Bogossian and colleagues summarises the same advice. The method is simple: measure the QT and the QRS in the same lead, subtract one from the other, and follow the JT over time. Its drawback is practical. Clinicians, monitoring protocols and drug safety thresholds are built around QT and QTc values, such as 450 ms in men, 460 ms in women and 500 ms in both, not around JT values, so a JT result is harder to compare with familiar limits. It works best for tracking change in one patient, such as a rise in JT after a new QT-prolonging medicine, rather than for a single reading.

Option 2: the Bogossian modified QT

The Bogossian formula subtracts about half of the QRS duration from the measured QT, giving a modified QT that can be rate-corrected and read against the usual QTc limits. Bogossian and colleagues derived it in Heart Rhythm in 2014 from 60 patients in sinus rhythm with a narrow QRS who were paced from the right ventricular apex and outflow tract during an electrophysiology study. The rise in QT during pacing was 48.3% and 48.8% of the paced QRS width at the two sites, a mean of 48.5%, and the JT interval was unchanged. So the extra QT in left bundle branch block is about 48.5% of the QRS, and the modified QT is the measured QT minus 48.5% of the QRS. Their 2020 review rounds this to 50% at the bedside, for left or right bundle branch block. The heart rate correction, such as Bazett or Hodges, is applied to the modified QT afterwards, exactly as for a normal QT.

Which rate formula to use after the QRS correction

The rate formula still matters after the QRS correction, and the studies so far favour Hodges over Bazett. Weipert and colleagues applied the Bogossian formula, then Bazett, in 163 pacemaker patients whose right ventricular pacing created a left bundle branch block pattern, and compared the result with the same patients' intrinsic QTc. With apical pacing the paced QTmc was 444 ms against an intrinsic 413 ms, and with non-apical pacing 430 ms against 416 ms; the authors advised allowing for an overestimate of about 30 ms. Erkapic and colleagues tested right bundle branch block with fascicular block in 71 patients: Bogossian plus Hodges came within a mean of −3 ± 24 ms and −6 ± 25 ms of the intrinsic QTc, while Bogossian with Bazett or Fridericia deviated significantly. For pacing, a 2026 study by Abdin and colleagues derived a different, rate-independent estimate of the unpaced QT, which beat the Bogossian method in 487 validation patients.

Worked example: left bundle branch block at 75 bpm

Take an ECG with left bundle branch block, a QT of 480 ms, a QRS of 160 ms and a rate of 75 bpm (RR 0.8 seconds). Corrected without regard to the QRS, the Bazett QTc is 480 / √0.8 = 537 ms, and the Hodges QTc is 480 + 1.75 × 15 = 506 ms, both over 500 ms and in the markedly prolonged range. The Bogossian modified QT is 480 − 0.485 × 160 = 402 ms. Rate-corrected, that gives a QTmc of 450 ms with Bazett, 433 ms with Fridericia or Framingham, and 429 ms with Hodges. The JT interval is 480 − 160 = 320 ms. So an ECG that seemed to show dangerous QT prolongation shows borderline or normal repolarization once the QRS is accounted for. Most of the apparent prolongation came from the wide QRS, and Bazett overstates it most, in line with the 30 ms overestimate found by Weipert.

To reproduce these numbers, subtract 48.5% of the QRS from the QT first, then enter the modified QT and the heart rate in the QTc calculator, which shows all four rate formulas.