Excerpt from Torsade de Pointes

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Background

Torsade de pointes (TDP), often referred to as torsade, is an uncommon variant of ventricular tachycardia (VT). The underlying etiology and management of torsade are, in general, quite different from those of garden-variety VT. The management of torsade with group IA antidysrhythmic drugs can have disastrous consequences. Differentiating between these entities, therefore, is critically important.

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Pathophysiology

Torsade is defined as a polymorphous VT in which the morphology of the QRS complexes varies from beat to beat. The ventricular rate can range from 150 beats per minute (bpm) to 250 bpm. The original report described regular variation of the morphology of the QRS vector from positive to net negative and back again. This was symbolically termed torsade de pointes, or "twisting of the point" about the isoelectric axis, because it reminded the original authors of the torsade de pointes movement in ballet. Most cases exhibit polymorphism, but the axis changes may not have regularity.

The definition also requires that the QT interval be increased markedly (usually to 600 msec or greater). Cases of polymorphous VT, which are not associated with a prolonged QT interval, are treated as generic VT. Torsade usually occurs in bursts that are not sustained; thus, the rhythm strip usually shows the patient's baseline QT prolongation.

The underlying basis for rhythm disturbance is delay in phase III of the action potential. The delay is mediated by the hERG potassium channel. This prolonged period of repolarization and the inhomogeneity of repolarization times among myocardial fibers allow the dysrhythmia to emerge. The initiating electrophysiologic mechanism may be triggered activity or reentry.

Six genetic variants currently are recognized. Genotypes LQT1 and LQT2 have slow potassium channels, while LQT3 shows defects in the sodium channels. Treatment modalities soon may be based on the genotype of the individual.

Frequency

United States

Prevalence of torsade is still unknown.

Mortality/Morbidity

In the United States, 300,000 sudden cardiac deaths occur per year. TDP probably accounts for fewer than 5%.

Race

Brugada syndrome is more frequent in Southeast Asians.¹

Sex

Women are 2-3 times more likely to develop TDP than men.

• Women have more QT prolongation secondary to drug therapy.
• Congenital long QT syndrome is autosomal dominant but shows greater frequency of expression and a greater lengthening of the QT interval in women than in men.

Age

The highest frequency is in patients aged 35-50 years.

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