Guidelines for QTc Measurement in Right Bundle Branch Block (RBBB)

Rationale for Adjusted QTc Assessment

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, as stated by the American Heart Association 5.
The critical concept is that BBB causes secondary repolarization abnormalities—ST-T changes that result from altered depolarization sequence rather than changes in cellular action potential duration, according to the American College of Cardiology 6.

In BBB, the QRS duration is ≥120 ms with characteristic morphologic patterns, including Right BBB (RBBB) with QRS width >120 ms and RSR' pattern in V1, and Left BBB (LBBB) with QRS width ≥120 ms and broad, deep S waves, as described by the American Heart Association 7, 8, 9.
ST-T wave vectors are directed opposite to the mean QRS vector in LBBB, and opposite to the terminal QRS forces in RBBB, as noted by the American College of Cardiology 6.

When BBB is present, the measured QT interval is artificially prolonged and requires adjustment, according to the American Heart Association and American College of Cardiology 5, 10.
The AHA/ACCF recommends two approaches: subtracting the difference in QRS widths before and after the block from the measured QT interval, or measuring the JT interval from the end of the QRS complex to the end of the T wave, which eliminates the QRS contribution entirely, as stated by the American Heart Association 5, 10.

The American Heart Association recommends assessing QRS duration and morphology, and if QRS ≥120 ms with typical BBB morphology, proceeding to check for hyperkalemia features, including obtaining immediate serum potassium level and looking for peaked T waves, PR prolongation, or P wave loss, as described by the European Heart Journal 11.
True QT prolongation thresholds after BBB adjustment are: males QTc >470 ms, females QTc >480 ms, and both sexes QTc >500 ms is highly abnormal, as stated by the American Heart Association 5, 10.

The American Heart Association advises against using standard QTc calculations without adjustment in BBB, as this leads to falsely prolonged values and inappropriate medication discontinuation or unnecessary interventions 5, 10.
The American College of Cardiology notes that BBB-related QT prolongation does not confer torsades de pointes risk, and severe hyperkalemia can mimic BBB and is immediately life-threatening 5, 6.