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Short QT syndrome
Short QT syndrome is a genetic disease of the electrical system of the
heart. It is made up of a constellation of signs and symptoms, made up of
a short QT interval on EKG (≤ 300 ms) that doesn't significantly change
with heart rate, tall and peaked T waves, and a structurally normal
heart. Short QT syndrome appears to be inherited in an autosomal
dominant pattern, and a few affected families have been identified.
Causes
The etiology of short QT syndrome is unclear at this time. A current
hypothesis is that short QT syndrome is due to increased activity of
outward potassium currents in phase 2 and 3 of the cardiac action
potential. This would cause a shortening of the plateau phase of the
action potential (phase 2), causing a shortening of the overall action
potential, leading to an overall shortening of refractory periods and
the QT interval.
In the families afflicted by short QT syndrome, two different missense
mutations have been described in the human ether-a-go-go gene (HERG).
These mutations result in expression of the same amino acid change in
the cardiac IKr ion channel. This mutated IKr has increased activity
compared to the normal ion channel, and would theoretically explain the
above hypothesis.
Symptoms and signs
Individuals with short QT syndrome frequently complain of palpitations
and may have syncope (loss of conciousness) that is unexplained. Due to
the autosomal dominant inheritance pattern, most individuals will have
family members with a history of unexplained or sudden death at a young
age (even in infancy), palpitations, or atrial fibrillation.
Short QT syndrome is associated with an increased risk of sudden cardiac
death, most likely due to ventricular fibrillation.
Diagnosis
The diagnosis of short QT syndrome is made up of characteristic history
and findings on EKG and electrophysiologic testing. There are currently
no set guidelines for the diagnosis of short QT syndrome.
Electrocardiogam
The characteristic findings of short QT syndrome on EKG are a short QT
interval, typically ≤ 300 ms, that doesn't significantly change with the
heart rate. Tall, peaked T waves may also be noted. Individuals may also
have an underlying atrial rhythm of atrial fibrillation.
Electrophysiologic Studies
In the electrophysiology lab, individuals with short QT syndrome are
noted to have short refractory periods, both in the atria as well as in
the ventricles. Also, ventricular fibrillation is frequently induced on
programmed stimulation.
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Treatment
Currently, the only effective treatment option for individuals with
short QT syndrome is implantation of an implantable cardioverter-defibrillator
(ICD).
A recent study has suggested that the use of certain antiarrhythmic
agents, particularly quinidine, may be of benefit in individuals with
short QT syndrome due to their effects on prolonging the action
potential and by their action on the IK channels1. While the use of
these agents alone is not indicated at present, there may be benefit of
adding these agents to individuals who have already had ICD implantation
to reduce the number of arrhythmic events.
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