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Long QT syndrome

Synonyms: LQTS

German: Long-QT-Syndrom

1 Definition

The long-QT-syndrome is a heart disease that belongs to the group of ion channel diseases. Its name is derived from the pathologically elongated QT interval present in the ECG.

ICD10-Code I49.8

2 Etiology

Etiologically two distinct types of the long-QT-syndrome are distinguished:

  • Congenital (primary) LQT-syndrome
  • Acquired (secondary) LQT-syndrome

3 Congenital LQTS

The congenital long-QT-syndrome originates from a mutation which gives rise to a structural modification of an ion channel or rather its anchor proteins (LQT5, see below) and thus leads to its disrupted functioning. Depending on the which mutation different types of the long-QT-syndrome (LQT1-10) are classified (see table).

Type Gene Gene product Function
I* KCNQ1 Alpha subunit of the voltage dependent potassium channel KvLQT1 Slow repolarizing current IKS
II HERG/KCNH2 Alpha subunit of the current dependet potassium channel HERG Rapid repolarizing current IKR
III** SCN5 Voltage dependent sodium channel Depolarisation
IV AncB ankyrin B Cytoskeletal protein ankering ion channels in the myocite mebrane
V KCNE1 ß subunit of the voltage dependent potassium channel KvLQT1 IKR
VI KCNE2 Regulatory subunit of the HERG channel IKS
VII KCNJ2 Potassium channel subunit IKS
and others

* Dependent on inheritance the LQT1 can be further divided into:

**The LQTIII has to be distinguished from the Brugada syndrome, which also originates from a SCN5-gene mutation.

3.1 Acquired LQTS

Acquired long-QT syndrome can occur because of various medications including class I- and III- antiarrhythmics, antibiotics and psychotropic drugs or be the result of hypokalemia.

4 Pathophysiology

The pathophysiologic mechanism is based on an ion channel defect resulting in:

These result in an elongation of the

Of the action potential in cardiomyocytes.

The delayed refractoriness can lead to afterdepolarizations within the vulnerable phase. This can lead to the non-physiologic excitement of neighboring cells (ectopic conduction system) with the effect of life threatening ventricular tachycardia.

5 Physiological Background

The action potential of the heart displays a steep upward slope with overshoot at +40mV (phase 1). After a short, K+ dependent repolarization a plateau phase follows (phase 2). The plateau phase happens because of the inward current of Ca2+ ions from the extracellular space and delayed outward current of K+ (IKS). Then the Ca2+ outward current and the opening of delayed activated K+ channels (IKS, IKR) lead to the repolarization of the cells (phase 3). After brief hyperpolarization the membrane rest potential (phase 4) is reached.

Similar to the neurogenic action potential the refraction phase (phase 3) can be divided into

  • relative refractory, during which postdepolarizations are possible and
  • absolute refractory.

6 Clinic

The clinical symptoms are often stress related, suddenly appearing Torsades de pointes tachycardias presenting with:

6.1 Complications

Ventricular tachycardias that are untreated can result in ventricular fibrillation and ultimately in cardiac arrest.

7 Diagnostics

Diagnosis is possible by resting ECG or stress ECG. The Criteria are an increase in:

Since the long-QT syndrome is a hereditary disease the family history can provide possible hints. In addition there is the molecular genetic possibility of finding mutations in leukocyte DNA via PCR amplification and sequencing of known risk genes. 2-5ml of EDTA-blood are sufficient for this examination.

8 Therapy

Long QT syndrome is treated by minimizing the risk of post depolarizations by reducing the heart rate via:

In case of refractory complaints the implantation of a cardiac pacemaker or AICD is indicated.

In case of a medication induced LQTS the discontinuation of the drug is obligatory. Hypokalemia caused LQTS should be treated by potassium substitution and continued control of the potassium serum level.

9 Weblinks

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