Various cardiovascular and non-cardiovascular drugs can also cause relevant QT prolongation, also.
This repolarization prolongation can result in triggered activity by promoting the genesis of early afterdepolarizations and is often spatially heterogeneous, providing a substrate for conduction block and reentrant arrhythmias. In these disorders, loss-of-function mutations in repolarizing K + channels, e.g., long-QT syndrome type 2 (LQT2) due to the rapid delayed rectifier (IKr) channel malfunction, or gain-of-function mutations in depolarizing sodium (Na +) or calcium (Ca 2+) channels lead to excessive repolarization prolongation. In the setting of prolonged repolarization inheritable arrhythmogenic disorders due to gene mutations were identified as an important cause of arrhythmogenesis. In this review, we discuss the determination of the QT-interval in the presence of BBB and summarize the origin and application of the modified QT-interval formula.Ģ. Arrhythmogenic risk in the setting of QT-interval prolongationĭuring the last two decades different mechanisms of arrhythmogenesis have been investigated. Subsequently, rate-correction formula should be applied as usual. In this formula, the modified QT interval is calculated by subtracting 50% of the length of the BBB-QRS from the measured QT interval (QT m = QT BBB − 50% QRS BBB). In the last decade, a simple formula for the estimation of the “modified QT interval” in the presence of left or right BBB has been developed and evaluated. However, in clinical practice physicians have become more familiar with the interpretation of QT-interval measurements than with the interpretation of the JT Interval. The US recommendations for the standardization and interpretation of the ECG suggest focusing on the JT interval in presence of BBB. Prolongation of the QRS width – like in the presence of BBB – entails prolongation of the QT interval, making the estimation of the true repolarisation time challenging. The QT interval results from two parts of the ECG: (1) the QRS complex, describing the excitation of the ventricles and (2) the JT interval, describing the repolarisation of the ventricles. Even more difficult is its correct estimation in the presence of repolarization abnormalities, arrhythmias or bundle-branch blocks (BBB). Proper measurement of the QT interval on the 12-lead body-surface ECG is challenging in daily practice.
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