For accurate measurement of the QT interval, the relationship between QT and the R–R intervals should
be repeatable,. This issue is important particularly when the heart rate is < 50 bpm and >120 bpm.
Moreover, aAccurate measurement of the QT interval is also important in athletes and children who havea significant beat-to-beat variability of the R–R interval. In such cases, prolonged and numerous
recordings may be necessary. The lLongest QT interval is generally observed in the right precordial leads.

Long QT syndrome (LQTS) is a congenital disorder characterized by, which shows a protracted QTinterval on the electrocardiogramECG. LQTS This condition influences ventricular tachyarrhythmias todevelopment in peoplepatients, which may lead to syncope, cardiac arrest, or sudden cardiac death.
AdditionallyIn LQTS, QT prolongation can lead to polymorphic ventricular tachycardia, which is also
referred to as torsade de pointes. This condition itself may lead to cause ventricular fibrillation and sudden
cardiac death.

Torsade de pointes is widely thought to be triggered by calcium channel reactivation, a delayed sodium
current reactivation, or a diminished outward potassium current that results in early afterdepolarization(EAD). This leads to enhanced transmural dispersion of repolarization (TDR) and is usually associatedComment [A1]: An abbreviation is generally
defined at its first use in the text and the abbreviatedform is consistently used thereafter.

with a prolonged QT interval. TDR serves as a functional reentry background to maintain torsade depointes. ItTDR provides a reentry background for reentry and increases the likelihood of EAD, the trigger
for torsade de pointes, by the extension ofextending the time window for calcium channels to remain
open. Any additional condition accelerating the reactivation of calcium channels (e.g., increased
sympathetic tone), increases the risk of EAD.

Prolonged recovery from excitation increases the probability chance of dispersion of refractoriness, whensome parts of the myocardium are refractory to subsequent depolarization. From a physiological
viewpoint, dispersion occurs with repolarization of the three layers of the heart, and the repolarization
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Comment [A2]: Some singular nouns refer to one
specific thing (the only one of its kind), andtherefore, “the” is placed before the noun. Here, thehas been used to denote specificity.

phase tends to be prolonged in the myocardium. This is the reason whyTherefore, the T wave is usuallywide and the interval from the peak of the T -wave to its end (Tp-e) represents the transmural dispersionof repolarization (TDR). In long QT syndrome (LQTS), TDR increases and creates a functional
background for transmural reentry.

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