Can the corrected QT interval be measured in a patient with right bundle branch block?

Can QTc Be Measured with Right Bundle Branch Block?

Yes, QTc can and should be measured in patients with right bundle branch block (RBBB), but the measured QT interval must be adjusted because RBBB artificially prolongs the QT through delayed depolarization rather than true repolarization abnormalities. 1

Why RBBB Affects QT Measurement

The fundamental issue is that bundle branch block increases the total QT interval mechanically through delayed depolarization, not through prolonged repolarization, and should not be considered indicative of acquired long QT syndrome or torsades de pointes risk. 2 The critical concept is that RBBB causes secondary repolarization abnormalities—ST-T changes that result from altered depolarization sequence rather than changes in cellular action potential duration. 2

Patients with right bundle-branch block usually can be monitored successfully because the ST-T wave is not so extremely deviated compared to left bundle branch block. 1 This makes RBBB more amenable to QTc assessment than LBBB.

How to Measure QTc in RBBB: Two Recommended Methods

Method 1: Subtract the Excess QRS Duration

When RBBB is present, the measured QT interval is artificially prolonged and requires adjustment. 1 The AHA/ACCF recommends subtracting the difference in QRS widths before and after the block from the measured QT interval. 1, 2

The practical formula is: Wide QRS complex-adjusted QTc = QTc - (QRS duration - 120 ms) 1

For example, if a patient's RBBB has yielded a QRS complex of 160 ms and a measured QTc of 500 ms, the wide QRS complex-adjusted QTc would be 500 ms - (160 - 120 ms) = 500 - 40 = 460 ms, which changes the clinical interpretation from high-risk to borderline. 1

Method 2: Measure the JT Interval

The alternative approach is measuring the JT interval from the end of the QRS complex to the end of the T wave, which eliminates the QRS contribution entirely. 1, 2 This method is particularly useful because the JT interval represents true ventricular repolarization independent of depolarization abnormalities. 3

Research confirms that right ventricular pacing increases the QT interval without increasing the JT interval, validating JTc as an index of repolarization independent of ventricular depolarization. 3

Which Correction Formula to Use

After adjusting for RBBB, apply standard heart rate correction formulas. The Fridericia formula is preferred over Bazett's formula, especially at heart rates >80 bpm, because Bazett systematically overcorrects at faster heart rates. 1, 4

Recent research comparing multiple formulas for RBBB found that the Yankelson formula showed the most consistent and accurate agreement when estimating baseline QTc in the presence of RBBB. 5 The Bogossian formula, originally developed for LBBB, can be applied to RBBB but shows slight overestimation. 6, 7

Clinical Thresholds After RBBB Adjustment

Once you've adjusted for RBBB, apply standard risk thresholds:

• Males: QTc >470 ms is abnormal 2
• Females: QTc >480 ms is abnormal 2
• Both sexes: QTc >500 ms is highly abnormal and significantly increases torsades de pointes risk 1, 2, 4
• An increase >60 ms from baseline is high-risk regardless of absolute value 1, 4

Critical Pitfalls to Avoid

Do not use standard QTc calculations without adjustment in RBBB, as this leads to falsely prolonged values and inappropriate medication discontinuation or unnecessary interventions. 2 The unadjusted QTc will systematically overestimate the true repolarization time.

BBB-related QT prolongation does not confer torsades de pointes risk because the prolongation reflects depolarization delay, not repolarization abnormality. 2 This is a crucial distinction when making clinical decisions about QT-prolonging medications.

Patients with frequent intermittent right bundle-branch block should not be monitored with automated ST-segment monitoring because of false ST alarms whenever the block appears or disappears. 1 However, this does not preclude manual QTc measurement during stable RBBB.

Practical Algorithm

1. Identify RBBB on ECG (QRS ≥120 ms with typical RSR' pattern in V1) 2
2. Measure the raw QT interval from QRS onset to T-wave end 1
3. Apply RBBB adjustment:
   • Either subtract (QRS duration - 120 ms) from the measured QT 1, 2
   • Or measure JT interval directly (QRS end to T-wave end) 1, 2, 3
4. Apply heart rate correction using Fridericia formula 1, 4
5. Interpret using standard thresholds (>470 ms males, >480 ms females, >500 ms high-risk) 2, 4
6. Document your method and use consistently for serial assessments 1