LONG QT SYNDROME

The QT interval varies in comparison with other intervals according to the heart rate in all age groups. The corrected QT (QTc) is calculated by Bazzet’s formula which is as follows. All are calculated in seconds

Normal QT interval in a heart rate of 75 beats/min is 0.352 s in men and children and 0.362 s in women. The upper limits in men and children is 0.384 s and 0.394 s in women. The QTc is considered abnormal if it is greater than 0.42 s.

In long QT syndrome the QT interval is 0.5 s to 0.7 s.

There are two types:

  1. Congenital long QT syndrome.
  2. Acquired long QT syndrom.

CONGENTIAL LONG QT SYNDROME

It is characterized by considerable genetic and phenotypic heterogeneity. There are recurrent syncope, ventricular arrhythmia (Torsades de pointes [TdP]) or Ventricular fibrillation which may give rise to sudden death. It may be seen in presence or absence of congenital deafness. In presence of congenital deafness it is seen in Jervell’s syndrome and Lange-Nielsen syndrome, while in Rumano-Ward syndrome congenital deafness is absent.

ACCUIRED LONG QT SYNDROM

Cause

  1. Electrolytic imbalance (hypokalaemia, hypomagnesemia rarely hypocalcaemia).
  2. Drugs-Antiarrhythmic and antidepressant drugs.
  3. Cardiac diseases: IHD, myocarditis.
  4. Neurological diseases: intracranial trauma, subarachnoid haemorrhage, CVA.
  5. Toxic exposures to organophosphates and Cesium.
  6. Bradycardia.

TREATMENT

Congenital: B-blockers or unilateral sympathectomy may be done. B-blockers are often used with permanent pacing. Since low heart rate predisposes to ventricular arrhythmia, agents that prolong the QT interval such as class 1A, 1C and III are contraindicated. antiarrhythmic drugs

Acquired: Treatment is directed according to the underlying cause.

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