Torsades de Pointes
Torsades de pointes is a specific form of polymorphic ventricular tachycardia that occurs in the presence of prolongation of the QT interval. It has a very characteristic appearance in which the QRS complex appears to twist around the isoelectric baseline. A prolonged QT interval reflects prolonged myocayte repolarisation due to ion channel malfunction and also gives rise to early after-depolarisations (EADs). EADs can manifest as tall U waves, which can cause premature ventricular contractions (PVCs). Torsade de pointes is initiated when a PVC occurs during the preceding T wave (‘R on T’ phenomenon). It can be caused by any cause of prolongation of the QT interval: Myocardial infarction Electrolyte disturbance e.g. hypokalaemia, hypomagnesaemia and hypocalcaemia Congenital e.g. Romano-ward syndrome and Lange-Nielson syndrome Drugs e.g. disopyramide, amiodarone, sotalol, terfenadine The drug treatment of choice for torsades de pointes is IV magnesium sulphate. Magnesium sulphate acts by decreasing the influx of calcium and lowering the amplitude of EADs. DC cardioversion is usually kept as a last resort in a haemodynamically stable patient because of the paroxysmal and recurrent nature of torsade de pointes. Torsade de pointes can degenerate into ventricular fibrillation and result in sudden cardiac death.
