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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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