Determination and Interpretation of the QT Interval

Abstract

BACKGROUND: Long QT syndrome (LQTS) is associated with potentially fatal arrhythmias. Treatment is very effective, but its diagnosis may be challenging. Importantly, different methods are used to assess the QT interval, which makes its recognition difficult. QT experts advocate manual measurements with the tangent or threshold method. However, differences between these methods and their performance in LQTS diagnosis have not been established. We aimed to assess similarities and differences between these 2 methods for QT interval analysis to aid in accurate QT assessment for LQTS. METHODS: Patients with a confirmed pathogenic variant in KCNQ1(LQT1), KCNH2(LQT2), or SCN5A(LQT3) genes and their family members were included. Genotype-positive patients were identified as LQTS cases and genotype-negative family members as controls. ECGs were analyzed with both methods, providing inter-and intrareader validity and diagnostic accuracy. Cutoff values based on control population's 95th and 99th percentiles, and LQTS-patients' 1st and 5th percentiles were established based on the method to correct for heart rate, age, and sex. RESULTS: We included 1484 individuals from 265 families, aged 33±21 years and 55% females. In the total cohort, QT Tangent was 10.4 ms shorter compared with QT Threshold (95% limits of agreement±20.5 ms, P<0.0001). For all genotypes, QT Tangent was shorter than QT Threshold (P<0.0001), but this was less pronounced in LQT2. Both methods yielded a high interand intrareader validity (intraclass correlation coefficient >0.96), and a high diagnostic accuracy (area under the curve >0.84). Using the current guideline cutoff (QTc interval 480 ms), both methods had similar specificity but yielded a different sensitivity. QTc interval cutoff values of QT Tangent were lower compared with QT Threshold and different depending on the correction for heart rate, age, and sex. CONCLUSION: The QT interval varies depending on the method used for its assessment, yet both methods have a high validity and can both be used in diagnosing LQTS. However, for diagnostic purposes current guideline cutoff values yield different results for these 2 methods and could result in inappropriate reassurance or treatment. Adjusted cutoff values are therefore specified for method, correction formula, age, and sex. In addition, a freely accessible online probability calculator for LQTS (www.QTcalculator.org) has been made available as an aid in the interpretation of the QT interval.

Prolongation of QT interval after pulmonary vein isolation for paroxysmal atrial fibrillation

“…The end of the T wave was defined as the intersection point of the tangent on the steepest point of the terminal limb of the T wave and the isoelectric baseline. 14 The Bazett (QTc = QT/(RR 1/2 )), Fridericia (QTc = QT/(RR 1/3 )), Framingham (QTc = QT + 0.154 (1 − RR)), and Hodges (QTc = QT + 1.75 (heart rate [HR] − 60)) formulas were obtained for each evaluation point. QT or QTc interval prolongation was identified when the QT or QTc interval was significantly increased compared with baseline.…”

Update on long QT syndrome

“…A U wave is always excluded from QT interval analysis. In a large cohort of genotype‐positive LQTS patients, Vink et al found that both methods have a high diagnostic accuracy, however, the cutoff for the tangent method should be shorter (for proposed cut‐off values by age and sex see Vink et al). An online probability calculator of LQTS is available as an aid in the interpretation of the QT interval (http://www.QTcalculator.org).…”

Handling of Ventricular Fibrillation in the Emergency Setting

“…Zhang et al showed that the relative risk of SCD comparing patients with the longest and shortest QT interval was 1.44 (Zhang et al, 2011). Vink et al proved that manual measurement of QT interval corrected according to Bazett formula has a specificity of 86% and a sensitivity of 85% simultaneously (Vink et al, 2018). QT dispersion is also a widely used ventricular arrhythmia risk marker; it is the difference between the longest and shortest QT intervals in the 12-lead electrocardiogram and can be used to estimate the spatial dispersion of ventricular repolarization.…”