Torsades de Pointes (TdP) is a special type of polymorphic VT characterized by periodic changes in the amplitude and peaks of the QRS complexes, appearing to twist around the isoelectric line, with a frequency of 200 - 250 bpm. It can be triggered when a premature ventricular contraction occurs late in diastole, falling on the terminal part of the preceding T wave (R-on-T), and may progress to ventricular fibrillation and sudden death. Causes include electrolyte imbalances (such as hypokalemia and hypomagnesemia), antiarrhythmic drugs (such as class Ia and III), phenothiazines, tricyclic antidepressants, intracranial lesions, and bradycardia (especially third-degree AV block). Some cases are congenital. Efforts should be made to identify and eliminate acquired causes of QT interval prolongation and discontinue drugs known or suspected to induce TdP. Treatment primarily involves intravenous magnesium sulfate. Class Ia or III drugs should be avoided as they may further prolong the QT interval.
Figure 1 R-on-T phenomenon and Torsades de Pointes
In lead II, the arrow indicates an R wave overlapping the preceding T wave (R-on-T phenomenon). The QT interval is prolonged to 0.64 seconds, and during the tachycardia episode, the main direction of the QRS complexes appears to twist continuously around the isoelectric line.