Proper measurement of the QT interval on the 12-lead body-surface ECG is challenging
in daily practice. Even more difficult is its correct estimation in the presence of
repolarization abnormalities, arrhythmias or bundle-branch blocks (BBB). 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.
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 US recommendations for the standardization and interpretation of the ECG suggest
focusing on the JT interval in presence of BBB. However, in clinical practice physicians
have become more familiar with the interpretation of QT-interval measurements than
with the interpretation of the JT Interval. 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. 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)). Subsequently, rate-correction formula should be applied
as usual. 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.
(C) 2020 The Author(s). Published by Elsevier B.V. This is an open access article
under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
