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Long QT syndrome
The long QT syndrome (LQTS) is a heart condition in which there is an
abnormally long delay between the electrical excitation (or
depolarization) and relaxation (repolarization) of the ventricle. The
point Q on the ECG corresponds to ventricular depolarization and the T
point corresponds to repolarization. Two types of Long QT are genetic
and drug-induced. A diagnosis of LQTS may indicate a greater propensity
for sudden cardiac death.
Genetic LQT can arise from mutation to several genes. These mutations
tend to prolong the duration of the ventricular action potential (APD),
thus lengthening the QT interval.
The following is a list of the three most common mutations:
LQT1 - mutations to the alpha subunit of the slow delayed rectifier
potassium channel (KCNQ1 or KvLQT1). The current through the heteromeric
channel (KCNQ1+KCNE1) is known as IKs. This mutation is thought to cause
LQT by reducing the amount of repolarizing action potential current that
prolongs action potential duration (APD). These mutations tend to be the
most common yet least severe.
LQT2 - mutations to the alpha subunit of the fast delayed rectifier
potassium channel (HERG). Current through this channel is known as IKr.
This phenotype is also probably caused by a reduction in repolarizing
LQT3 - mutations to the alpha subunit of the sodium channel (SCN5A).
Current through is channel is commonly referred to as INa. Depolarizing
current through the channel late in the action potential is thought to
prolong APD. The late current is due to failure of the channel to remain
inactivated and hence enter a bursting mode in which significant current
can enter when it should not. These mutations are more lethal but less
Other mutations affect the beta subunits ion channels or even ryanodine
receptors. For example LQT6 affects MinK (aka KCNE1) which is the beta
subunit that coassembles with KCNQ1 to form IKs channels.
Drug induced LQT is usually a result of treatment by anti-arrhythmic
drugs such as amiodarone. Genetic mutations may make one more prone to
drug induced LQT.
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