Long QT Syndrome overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [2]
Overview
The long QT syndrome (LQTS) is a heart condition associated with prolongation of repolarisation (recovery) following depolarisation (excitation) of the cardiac ventricles. It is associated with syncope (fainting) and sudden death due toventricular arrhythmias. Arrhythmias in individuals with LQTS are often associated with exercise or excitement. LQTS is associated with the rare, ventricular arrhythmia torsade de pointes, which can deteriorate into ventricular fibrillation and ultimately death.
Individuals with LQTS have a prolongation of the QT interval on the ECG. The Q wave on the ECG corresponds to ventricular depolarization while the T wave corresponds to ventricular repolarization. The QT interval is measured from the Q point to the end of the T wave. While many individuals with LQTS have persistent prolongation of the QT interval, some individuals do not always show the QT prolongation; in these individuals, the QT interval may prolong with the administration of certain medications.
Classification
There are multiple genetic mutations that account for LQTs, but LQT1, LQT2, and LQT3 account for 75% of cases of LQT.
Images
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The QT interval start at the onset of the Q wave and ends where the tangent line for the steepest part of the T wave intersects with the baseline of the ECG.
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A 12 lead ECG of a patient with acquired long QT syndrome. Notice the QT prolongation. The QTc is about 640ms.
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The three most common forms of LQTS can be recognized by the characteristic ECG abnormalities
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The ECG does not meet the baseline after the end of the T wave. Still, the crossing of the tangent and baseline should be used for measurements.
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A bifasic T wave. The tangent to the 'hump' with the largest amplitude is chosen. This can change from beat to beat, making it more important to average several measurements.
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The T wave is broad, but the tangent crosses the baseline before the T wave joins the baseline. The QT interval would be overestimated when this last definition of the end of the T wave would be used.
