From the Guidelines
Long QT syndrome is not a normal finding with Right Bundle Branch Block (RBBB). While RBBB itself can cause some changes in the QT interval measurement, it should not significantly prolong the QT interval. RBBB typically affects the terminal portion of the QRS complex, widening it to greater than 120 milliseconds, but this does not inherently lead to QT prolongation 1. If a patient has both RBBB and a prolonged QT interval, these should be considered as two separate findings that require individual evaluation.
Key Considerations
- Long QT syndrome is a potentially serious cardiac electrical disorder that can predispose patients to dangerous arrhythmias like Torsades de Pointes and sudden cardiac death.
- Causes of long QT include congenital syndromes, electrolyte abnormalities (particularly low potassium, magnesium, or calcium), and medications 1.
- Any patient with RBBB and prolonged QT interval should undergo thorough cardiac evaluation, including review of medications, electrolyte testing, and possibly genetic testing if a congenital long QT syndrome is suspected.
Evaluation and Management
- Treatment depends on the underlying cause but may include medication adjustments, electrolyte correction, or in some cases, beta-blockers or implantable cardioverter-defibrillators for high-risk patients 1.
- It's essential to consider the guidelines for the management of patients with ventricular arrhythmias and the prevention of sudden cardiac death, as outlined in the 2017 AHA/ACC/HRS guideline 1.
- The presence of RBBB does not inherently lead to QT prolongation, and thus, the focus should be on identifying and managing the underlying cause of the long QT interval, rather than attributing it to the RBBB alone 1.
From the Research
QT Interval in Right Bundle Branch Block (RBBB)
- The QT interval is often overestimated in patients with RBBB due to the conduction defect reflected by the QRS duration 2, 3.
- Several alternative correction formulae exist to estimate the QT interval in the presence of RBBB, including the Bogossian formula, Rautaharju formula, and Yankelson formula 4, 5.
- A study found that the Yankelson formula showed the most consistent and accurate agreement in estimating the QTc interval in patients with RBBB 5.
- Another study proposed a new approach to recalculate the QT intervals in RBBB using the formula QTcRBBB = 0.945*QTcRBBB - 26, which accurately predicts the QT interval in the absence of RBBB 6.
Clinical Implications
- Failure to adjust the QT interval in RBBB can produce an overestimate of the QT interval, which can lead to incorrect diagnosis and treatment 6.
- The use of correction formulae can help to accurately estimate the QT interval in patients with RBBB, reducing the risk of misdiagnosis and improper treatment 4, 5.
- Further research is needed to determine the clinical significance of QT prolongation in patients with RBBB and to establish clear guidelines for QT interval measurement in this population 2, 3.
Measurement and Calculation
- The QT interval can be measured using standard electrocardiographic readings, and correction formulae can be applied to estimate the QTc interval in patients with RBBB 4, 5.
- Different heart rate correction formulae can be used to calculate the QTc interval, including the Bazett formula, Hodges formula, and Fridericia formula 4, 6.
- The choice of correction formula and heart rate correction formula can affect the accuracy of the estimated QTc interval in patients with RBBB 4, 5.