Torsade de Pointes

Introduction
Background

Torsade de pointes, literally meaning twisting of points, is a distinctive form of polymorphic ventricular tachycardia (VT) characterized by a gradual change in the amplitude and twisting of the QRS complexes around the isoelectric line. Torsade de pointes (torsade) is associated with a prolonged QT interval, which may be congenital or acquired. Torsade (not long QT) usually terminates spontaneously but frequently recurs and may degenerate into ventricular fibrillation.


Torsade de pointes. Asymptomatic patient on erythromycin had marked QT prolongation on ECG findings. Patient was profoundly hypomagnesemic and hypokalemic. This shows an example of recurrent nonsustained torsade de pointes that occurred several hours after the ECG was performed. With discontinuation of the erythromycin and aggressive repletion of the magnesium and potassium, no further torsade de pointes occurred and the patient's QT interval returned to normal.

Pathophysiology

The association between torsade and a prolonged QT interval has long been known, but the mechanisms involved at the cellular and ionic levels have been made clearer in approximately the last decade.1 The abnormality underlying both acquired and congenital long QT syndromes is in the ionic current flow during repolarization, which affects the QT interval. A variety of changes in ionic current can result in the common effect of decreased repolarizing current, reflected in a long QT, and these changes can secondarily lead to subsequent depolarizing currents and sometimes action potentials, termed afterdepolarizations. This leads to a further delay in repolarization and causes early afterdepolarization (EAD), the triggering event for torsade. The following phases are described:

Phase 1: During initial upstroke of action potential in a normal cardiac cell, a rapid net influx of positive ions (Na+ and Ca++) occurs, which results in the depolarization of the cell membrane. This is followed by a rapid transient outward potassium current (Ito), while the influx rate of positive ions (Na+, Ca++) declines. This represents the initial part of the repolarization, or phase 1.
Phase 2 is characterized by the plateau, the distinctive feature of which is the cardiac repolarization. The positive currents flowing inward and outward become almost equal during this stage.
Phase 3 of the repolarization is mediated by activation of the delayed rectifier potassium current (IK) moving outward while the inward positive current decays. If a slow inactivation of the Ca++ and Na+ currents occurs, this inward "window" current can cause single or repetitive depolarization during phases 2 and 3 (ie, EADs). These EADs appear as pathologic U waves on a surface ECG, and, when they reach a threshold, they may trigger ventricular tachyarrhythmias.

These changes in repolarization do not occur in all myocardial cells. The deep endocardial region and midmyocardial layer (composed of M cells) of the ventricle are more prone to prolongation of repolarization and EADs because they have a less rapid delayed rectifier potassium current (IKr), while other regions might have short or normal cycles. This heterogeneity of repolarization in the myocardial cells promotes the spread of triggered activity, which is initiated by EADs by a reentrant mechanism and currently is thought to be responsible for the maintenance of torsade.2

Mortality/Morbidity

Torsade is a life-threatening arrhythmia and may present as sudden cardiac death in patients with structurally normal hearts.3
Race

For both sexes, the corrected QT interval is longer in white persons than in black persons, possibly explaining the lower susceptibility to acquired torsade in black persons.
Sex

Females are more prone to the development of torsade than males, potentially because they have longer QT intervals.4
Age

Torsade occurs in patients of a wide age range, from newborn to 86 years. If it occurs at an early age, the cause usually is due to congenital long QT syndrome. In later years, the cause usually is due to acquired long QT syndrome.
Clinical
History

Patients with torsade usually present with recurrent episodes of palpitations, dizziness, and syncope; however, sudden cardiac death can occur with the first episode. Nausea, cold sweats, shortness of breath, and chest pain also may occur but are nonspecific and can be produced by any form of tachyarrhythmia.

In a young patient with torsade, a diagnosis of congenital long QT syndrome should be considered, especially if a family history of sudden cardiac death or sudden infant death syndrome is present. In these patients, episodes of torsade are triggered by adrenergic stimulation such as stress, fear, or physical exertion, but other predisposing factors also should be considered. See eMedicine article Long QT Syndrome.
Patients with Jervell and Lange-Nielsen syndrome commonly have congenital sensorineural deafness representing an autosomal dominant pattern of inheritance for cardiac abnormalities, whereas deafness usually is autosomal recessive.
Another form of familial or congenital long QT syndrome is Romano-Ward syndrome, in which hearing is normal and an autosomal dominant pattern of inheritance is observed.
Patients with acquired long QT syndrome usually develop torsade during periods of bradycardia.
The most common causes of acquired long QT syndrome are medications and electrolyte disorders (eg, hypokalemia, hypomagnesemia). Drug-associated torsade de pointes is relatively rare, but is becoming increasingly common with its incidence as high as 2-3% with certain drugs.
Risk factors for torsade include the following:
Congenital long QT syndrome
Female gender
Acquired long QT syndrome (causes of which include medications and electrolyte disorders such as hypokalemia and hypomagnesemia)
Bradycardia
Baseline electrocardiographic abnormalities
Renal or liver failure
Physical

The physical findings in torsade depend on the rate and duration of tachycardia and the degree of cerebral hypoperfusion.
Findings include rapid pulse, low or normal blood pressure, or transient or prolonged loss of consciousness. This could be preceded by bradycardia or premature ventricular contractions (leading palpitations).
Pallor and diaphoresis may be noted, especially with a sustained episode.
Other physical signs depend on the etiology of torsade.
Causes

Congenital long QT syndromes (adrenergic-dependent)

Jervell and Lange-Nielsen syndrome
Romano-Ward syndrome

Acquired long QT syndromes
Drugs
Antiarrhythmic drugs
Class 1A - Quinidine, disopyramide, procainamide
Class III - Sotalol, amiodarone (rare), ibutilide, dofetilide, almokalant
Antibiotics - Erythromycin, clarithromycin, azithromycin, levofloxacin, moxifloxacin, gatifloxacin, trimethoprim-sulfamethoxazole, clindamycin, pentamidine, amantadine, chloroquine,
Antifungals - Ketoconazole, itraconazole
Antipsychotics - Haloperidol, phenothiazines, thioridazine, trifluoperazine, sertindole, zimeldine, fluoxetine (possible)
Tricyclic and tetracyclic antidepressants
Antihistamines (histamine1-receptor antagonists) - Terfenadine, astemizole, diphenhydramine, hydroxyzine
Cholinergic antagonists - Cisapride, organophosphates (pesticides)
Diuretics - Indapamide, hydrochlorothiazide, furosemide
Antihypertensives - Bepridil, lidoflazine, prenylamine, ketanserin
Lithium
Other drugs - Anticonvulsants (phenytoin), oral hypoglycemics, citrate (massive blood transfusions), vasopressin (possible), carbamazepine (possible), cocaine, organophosphorus poisoning
Some drugs (eg, amiodarone) routinely prolong QT but are less commonly associated with clinical consequences of long QT.5
For additional information, see QTsyndrome.ch.
Electrolyte abnormalities - Hypokalemia, hypomagnesemia, hypocalcemia
Endocrine disorders - Hypothyroidism, hyperparathyroidism, pheochromocytoma, hyperaldosteronism
Cardiac conditions - Myocardial ischemia, myocardial infarction, myocarditis, bradyarrhythmia, complete atrioventricular (AV) block
Intracranial disorders - Subarachnoid hemorrhage, thalamic hematoma, cerebrovascular accident, encephalitis, head injury
Nutritional disorders - Anorexia nervosa, starvation, liquid protein diets, gastroplasty and ileojejunal bypass, celiac disease

http://emedicine.medscape.com/article/158243-overview