From the Guidelines
Left bundle branch block (LBBB) itself does not directly cause a prolonged QTc interval, but it can affect the accuracy of QTc measurement due to the widened QRS complex. When evaluating patients with LBBB, it's crucial to recognize that the prolonged QRS contributes to QT interval prolongation, but this doesn't necessarily reflect true repolarization abnormalities, as noted in the guidelines for electrocardiographic monitoring in hospital settings 1. The standard formulas for QTc calculation may not be reliable in the presence of LBBB, which can lead to inaccurate assessments of repolarization. Some key points to consider in the context of LBBB and QTc interval measurement include:
- The QT interval is measured from the onset of the QRS complex to the end of the T wave, but in cases of LBBB, this measurement can be confounded by the widened QRS complex 1.
- An alternative approach is to use the JT interval (from the end of the QRS to the end of the T wave) to assess repolarization, as this can help eliminate the impact of the widened QRS complex on QT interval measurement 1.
- If a U wave is discrete, it should not be included in the QT measurement, and the end of the entire T wave complex should be considered the end of the QT interval 1. Given the potential for LBBB to affect QTc measurement and the importance of accurately assessing repolarization abnormalities, it is recommended to consult a cardiologist or electrophysiologist for proper interpretation of QTc intervals in patients with LBBB, especially when there are concerns about potential QTc prolongation, as true QTc prolongation represents a risk factor for dangerous arrhythmias like torsades de pointes, while QT prolongation simply due to LBBB does not carry the same risk.
From the Research
Left Bundle Branch Block and QTc Interval
- A left bundle branch block (LBBB) can cause a prolonged QTc interval, but this prolongation is mainly driven by the prolonged time of ventricular depolarization, rather than the time of ventricular repolarization 2, 3, 4.
- The corrected QT interval (QTc) is often overestimated in patients with LBBB, as conventional measurement methods do not account for the increased QRS duration 2, 3, 5, 6.
- Several alternative correction formulae exist to estimate the QT interval in patients with LBBB, including the QT-LBBB formula and the JTc interval 3, 5, 6.
- These formulae can help to accurately assess the QT interval in patients with LBBB and identify those at high risk of potentially fatal cardiac arrhythmias 3, 5, 6.
Assessment of QT Interval in LBBB
- The JTc interval, which is calculated by subtracting the QRS duration from the QTc, can be used as an index of repolarization and is independent of ventricular depolarization 3.
- The QT-LBBB formula, which takes into account the net increase in QRS duration, can be used to estimate the true QT interval in patients with LBBB 6.
- Other formulae, such as the QTcLBBBNEW formula, have also been developed to estimate the QT interval in patients with LBBB and have shown high correlation with intrinsic QTc 5.
Clinical Implications
- Accurate assessment of the QT interval is important in patients with LBBB, as it can help to identify those at high risk of cardiac arrhythmias and guide treatment decisions 2, 4, 5, 6.
- The use of alternative correction formulae, such as the QT-LBBB formula and the JTc interval, can help to improve the accuracy of QT interval assessment in patients with LBBB 3, 5, 6.