Torsades de pointes

 Torsades de pointes:

This form of polymorphic VT (Ventricular Trachycardia) is a complication of prolonged ventricular repolarisation (prolonged QT interval). The ECG shows rapid irregular complexes that seem to twist around the baseline as the mean QRS axis changes. The arrhythmia is usually non-sustained and repetitive, but may degenerate into ventricular fibrillation. During periods of sinus rhythm, the ECG will usually show a prolonged QT interval (>0.44 sec in men, >0.46 sec in women when corrected to a heart rate of 60/min).

  The arrhythmia is more common in women and is often triggered by a combination of factors, such as administration of QT syndromes are a family of genetic disorders that are characterized by mutations in genes that code for cardiac sodium or potassium channels. Long QT syndrome subtypes have different triggers, which are important when counselling patients. Adrenergic stimulation through vigorous exercise is a common trigger in long QT type 1, and a sudden noise may trigger arrhythmias in long QT type 2. Arrhythmias are more common during sleep in Type 3.

Fig: Causes of long QT interval and torsades de pointes

Management

Intravenous magnesium (8 mmol over 15 mins, then 72 mmol over 24 hrs) should be given in all cases. If this is ineffective, atrial pacing should be tried, since it can suppress the arrhythmia through rate-dependent shortenig of the QT interval. Intravenous isoprenaline is a reasonable alternative to pacing but should be avoided in patients with the congenital long QT syndromes. Once initial control has been achieved, efforts should be made to identify and treat the underlying cause or stop medications that predispose to the arrhythmia. If the underlying cause cannot be corrected or the arrhythmia is the result of an inherited syndrome, then long-term pharmacological therapy may be necessary. Beta-blockers are effective at preventing syncope in patients with congenital long QT interval prolongation (>500 msecs) or certain high-risk genotypes, should be considered for an implantable defibrillator. Left stellate ganglion block may be of value in patients with resistant arrhythmias.

      Brugada syndrome is a related genetic disorder that may present with polymorphic VT or sudden death. It is characterized by a defect in sodium channel function and an abnormal ECG(right bundle branch block and ST elevation in V1 and V2 but not usually prolongation of the QT interval). The only known effective treatment is an implantable defibrillator.