Short QT Syndrome


Diagnosis

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The hallmark of SQTS is a QT interval shorter than normal. Several studies have been done to define a normal QT interval, but the lower limit of normal for the QT interval in such studies has varied considerably. It is probably fair to state, however, that a QT interval at a heart rate of 60 beats/min, which is less than 350 msec, is a very rare finding in a normal person.


ECG from first patient diagnosed with Short QT Syndrome

Such a QT-interval can be seen in certain clinical scenarios with high serum calcium or potassium levels, hyperthermia or acidosis, and both acetylcholine and catecholamine can shorten the QT interval significantly. Also, a short QT-interval is a physiologic consequence of an increase in heart rate. Characteristic for the QT-interval in SQTS is the independence of heart rate. The QT interval in these patients will  often be normal during tachycardia, but show none or minimal prolongation with a decrease in heart rate. In order to make the diagnosis it is therefore mandatory that the QT-interval is measured at a heart rate close to 60 beats/min and not corrected for heart rate. This is particularly important to remember in children where the heart rate even under normal condition can be more than 100 beats/min at rest. In hospitalized patients telemetry will be helpful in getting an ECG during slow heart rates and in an out-patient setting 24 hour Holter-monitoring  will be able to show the short QT-interval during slow heart rates and the lack of a significant change in QT-interval with change in heart rate (Figure 1 and 1a).


Figure 1: Holter strip from Short QT patient with heart rate of 68 and 119
 

 


Figure 1a: Holter image enlarged and shifted.  QT interval measured 280ms at both heart rates.

 

There are no strict criteria for the duration of QT-interval in patients with SQTS, but in all published cases the QT-interval was less than 345 ms. It is likely, however, that some people with slightly longer QT-interval than this may turn out to have a similar clinical picture than the patients described so far with SQTS, and until more is known about the syndrome all patients with a short QT-interval should be further evaluated.

Another characteristic finding in the published ECGs from patients with SQTS is a short ST-segment followed by tall and narrow T waves seen especially in precordial leads (Figure 2). In some of the first patients with SQTS treated with ICD’s this led to multiple inappropriate shocks due to T-wave oversensing and double counting.


Figure 2.

The usefulness of programmed electrical stimulation for diagnosis and risk assessment of patients with suspected SQTS has not been well-established, but could become important in borderline cases. A short effective refractory period in both atria and ventricles with induction of either atrial or ventricular fibrillation would strongly favor a diagnosis of SQTS and possibly increased risk of sudden cardiac death.

When to suspect SQTS 

1.      Lone atrial fibrillation especially at a young age

2.      Family member with short QT interval

3.      Family member with SCD

4.      QT-interval at 60 beats/min < 350 ms

5.      ECG with tall and peaked T-waves

6.      Lack of QT-interval prolongation with slowing of heart rate

 

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