Determination of the QT Interval in Left Bundle Branch Block: Development of a Novel Formula
Jacky K K Tang1, Simon W Rabkin1
1Division of Cardiology, University of British Columbia, Vancouver, British Columbia, Canada.
Insights
A new formula accurately calculates the corrected QT (QTc) interval in patients with left bundle branch block (LBBB), improving risk stratification for potentially fatal cardiac arrhythmias.
Area of Science:
- Cardiology
- Medical Diagnostics
- Electrocardiography
Background:
- Determining the prolonged QT interval in left bundle branch block (LBBB) is crucial for identifying patients at high risk of fatal cardiac arrhythmias.
- Accurate QT interval assessment in LBBB patients has been a significant clinical challenge.
Purpose of the Study:
- To develop and validate a new formula for calculating the corrected QT (QTc) interval specifically in the context of LBBB.
- To compare the performance of the new formula against existing methods for QTc calculation in LBBB.
Main Methods:
- Developed a novel QTc formula using electrocardiograms (ECGs) from 17 patients with intermittent LBBB.
- Validated the new formula and compared it with five other formulas in a larger cohort of 2610 LBBB patients.
- Assessed heart rate (HR) dependency using Bazett (QTcBZT) and spline (QTcRBK) correction methods.
Main Results:
- The new formula (QTcLBBBNEW) demonstrated the highest correlation with intrinsic QTc (without LBBB) and minimal HR dependency.
- Existing formulas, except one, showed significant HR dependency; QTcBZT markedly increased the proportion of individuals with prolonged QTc, especially at higher HRs.
- The 99th and 97.5th percentiles of QTcLBBBNEW effectively identified abnormal QT prolongation in both men and women with LBBB.
Conclusions:
- A novel formula effectively predicts the QT interval in the absence of LBBB by modifying QT and JT intervals.
- Utilizing the 99th and 97.5th percentiles of the new formula can identify LBBB patients with QT interval prolongation.
- This new method enhances the ability to identify high-risk individuals for cardiac arrhythmias in the LBBB population.
Background:
Determination of the prolonged QT interval in left bundle branch block (LBBB), which should be of special concern to identify individuals at high risk of potentially fatal cardiac arrhythmia risk, has been problematic.
Methods:
Electrocardiograms (ECGs) (n = 17) in intermittent LBBB were used to develop a new formula for the calculation of QTc in LBBB. This formula and 5 others were compared in a population with LBBB (n = 2610). The QT was corrected for heart rate (HR) using the Bazett formula (QTcBZT) and the spline QT formula (QTcRBK), which is relatively independent of HR. The JT interval was significantly related to HR.
Results:
The new approach (QTcLBBBNEW = 0.945*QTcRBKLBBB - 26) in LBBB showed the highest correlation with intrinsic QTc (without LBBB) and had minimal HR dependency. Previous formulae to determine the QT interval in LBBB showed significant HR dependency except for one proposed by Rautaharju et al. Inclusion of an HR correction factor in existing formulae blunted HR dependency but not if the QT interval was adjusted by the QTcBZT. In men and women, use of the QTcBZT markedly increases the proportion of individuals with prolonged QTc, which was more evident with increasing HR. The 99th and 97.5th percentiles for QTcLBBBNEW for men and women identified abnormal QT prolongation in LBBB.
Conclusions:
A new formula that modifies the QT and JT intervals in LBBB predicts the QT interval in the absence of LBBB. Abnormal QT intervals in the 99th and 97.5th percentiles can identify patients with LBBB who have QT interval prolongation.
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