QT interval correction in LBBB: Improving the accuracy of an imperfect measurement

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Excerpt

QT interval prolongation is associated with a risk of polymorphic ventricular tachycardia and torsades de pointes. QT interval normally shortens with increasing heart rate and correction for this effect is necessary for a meaningful QT interval assessment. Correction of the QT interval in general is important. A knowledge of true QT interval is important in assessing the prognosis in many heart diseases and is of course fundamental in diagnosing Long QT syndrome (LQTS). But, it is in drug loading with class III antiarrhythmic drugs, including sotalol and dofetilide, that repeated, reproducible assessment of QT interval at differing heart rates impacts day‐to‐day clinical practice of most cardiac rhythm specialists.
There is a debate on the optimal QT correction method to determine the degree of the QT interval change in relation to heart rate. The Bazett formula (the quotient of the QT interval in milliseconds and the square root of the RR interval in seconds), which normalizes the QT interval to a heart rate of 60 bpm, is the most commonly used formula in clinical practice. The formula is not perfect outside physiologic heart rates and overestimates QT interval when the heart rate is high (>100 bpm).1
These issues are especially significant during drug loading in atrial fibrillation (AF). For example, Musat et al.2 compared the QTc corrected using Bazett's formula in AF and sinus rhythm (SR), with corrections by Fredericia (which uses the cube root of the RR interval rather than the square root) and by the Framingham method (which calculates QT from linear regression based on patients in the Framingham heart study according to: QTc = QT + 0.154(1–RR)), during dofetilide loading of AF patients to determine which correction in AF most accurately predicted the QT in SR. The QTc interval on the last electrocardiogram in AF was compared with the first electrocardiogram in SR. Bazett's correction overestimated QTc during AF compared with SR, whereas Framingham underestimated it. However, there was no significant difference between the QTc interval in AF and SR when assessed by Fredericia's formula.
Patients with left bundle brunch block (LBBB) present a particular set of problems in assessing QT interval. QT interval is usually prolonged in comparison with people with normal depolarization, but the prolongation of QT interval could result both from the delay in depolarization and a delay in repolarization—or both. The accurate measurement of the QT and QTc intervals in patients with LBBB is a serious and important issue that we face in our daily practice. So, how to resolve this? One approach is to bypass QT measurement completely and assess the JT interval. This approach is attractive in that it bypasses the depolarization phase of the ventricles and concentrates solely on repolarization. But, given the delay in depolarization in LBBB, repolarization might have begun in some cells before the J point so the JT interval likely underestimates the duration of repolarization. Another widely used approach is to apply Bazett's formula to the uncorrected QT interval and accept a longer QTc as the upper limit of acceptable (550 ms rather than 500 ms). This method, however, only addresses LBBB‐induced changes in the depolarization phase. The repolarization phase in the presence of LBBB may also follow different patterns than the repolarization period in the presence of normal conduction, and these concerns are not addressed.
In this issue of Journal of Cardiac Electrophysiology, Wang et al.3 describe a semiempiric formula that allows estimation of the true QT in LBBB on a per patient basis.
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