Long QT Syndrome electrocardiogram
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [2]
Overview
The diagnosis of LQTS is difficult in so far as 2.5% of the healthy population have a prolonged QT interval, and 10% of LQTS patients have a normal QT interval. The presence of LQTs in the absence of QT prolongation underscores the importance of genetic testing in the diagnosis of LQTs. It should be noted that the QT interval is often overestimated in the presence of a u wave.
How to Measure the QT
The QT interval is often measured incorrectly. It is measured incorrectly by 33% of EP physicians and 75% of general cardiologists. It is measured incorrectly by <5% of Long QT syndrome experts who deal with this on a frequent basis. [1] The presence of a U wave can often lead to a false diagnosis of QT prolongation. In order to avoid this, the "teach the tangent" or "avoid the tail" rule is applied. In this method, a line is drawn on top of the downslope of the T wave as shown below with the dotted green line. The QT interval is measured where this green dotted line intersects with the isoelectric line as shown by the large green arrow. The red arrow is an incorrect assessment of the QT interval at the end of the U wave. Using the red arrow would lead to a misdiagnosis of QT prolongation.
The LQTS Diagnostic Score
A commonly used criterion to diagnose LQTS is the LQTS "diagnostic score". Its based on several criteria giving points to each. With 4 or more points the probability is high for LQTS, and with 1 or less point the probability is low. Two or 3 points indicates intermediate probability.
- QTc (Defined as QT interval / square root of RR interval)
- >= 480 msec - 3 points
- 460-470 msec - 2 points
- 450 msec and male gender - 1 point
- Torsades de Pointes ventricular tachycardia - 2 points
- T wave alternans - 1 point
- Notched T wave in at least 3 leads - 1 point
- Low heart rate for age (children) - 0.5 points
- Syncope (one cannot receive points both for syncope and Torsades de pointes)
- With stress - 2 points
- Without stress - 1 point
- Congenital deafness - 0.5 points
- Family history (the same family member cannot be counted for LQTS and sudden death)
- Other family members with definite LQTS - 1 point
- Sudden death in immediate family (members before the age 30) - 0.5 points
Electrocardiographic Examples of the Long QT Variants
Shown below are examples of ECGs demonstrating QT prolongation in Long QT syndrome.
- LQT 1 shows 'early onset' broad based T waves
- LQT 2 shows small late T waves
- LQT 3 shows a flat JT segment with a 'late onset' T wave that is normal in configuration.
References
- ↑ Heart Rhythm 2005;2:569-574