POLYMORPHIC VENTRICULAR TACHYCARDIA, LONG Q-T SYNDROME, AND TORSADES DE POINTES - 02/09/11
Résumé |
Polymorphic ventricular tachycardia (PMVT) is defined as a ventricular rhythm faster than 100 beats/min characterized by clearly defined QRS complexes with frequent and rapid changes in morphology, axis, or both. The clinical spectrum of presentation of PMVT can range from a brief, asymptomatic, self-terminating episode to one of recurrent syncope or sudden cardiac death. The challenge in caring for patients with PMVT lies in elucidating the cause and preventing recurrences, although primary prevention in the at-risk population is the ultimate goal.
The term PMVT encompasses a variety of clinical syndromes with classification broadly based on the Q-T interval (Table 1). The Q-T interval is an electrocardiogram (ECG) measure of the time between the initiation of the QRS complex to the termination of the T wave and represents the algebraic sum of the duration between the onset of depolarization to completion of repolarization of all action potentials (Figure 1). It is generally accepted that the Q-T interval represents the integrated sum of repolarization from at least a substantial mass of myocardium, but the degree to which the Q-T interval reflects local repolarization is controversial. Prolonged repolarization of ventricular myocardium is manifested by a prolongation in the Q-T interval. The Q-T interval is heart rate, age, and gender dependent and exhibits diurnal variation, probably by fluctuations in adrenergic tone.7, 27, 36, 46 The corrected Q-T interval (Q-Tc) may be calculated from the observed QT interval (Q-To) using the formula from Bazett,6 where the Q-Tc is equal to the Q-To divided by the square root of the RR interval in seconds (Q-Tc = Q-To/√RR), or by a series of other formulas.
The first category consists of PMVT in the setting of prolonged repolarization (long Q-T interval), commonly known as torsades de pointes. Originally described by Dessertenne,10 the term torsades de pointes, or twisting of the points, was used to describe cycles of ventricular tachycardia with alternating electric polarity and amplitude, such that the peaks of the QRS complexes appeared to be twisting around the isoelectric line of the recording (Figure 2). The prolongation in repolarization necessary for this form of PMVT can be congenital or acquired, the latter typically as a result of a variety of drugs.
The second category of PMVT occurs independently of prolonged repolarization, and the Q-T interval is within normal limits. This form of PMVT may be induced by a variety of clinical scenarios, including acute ischemia, reperfusion, and organic heart disease. Less frequently, PMVT with a normal Q-T interval can occur in patients without apparent structural heart disease resulting from several underlying electrophysiologic abnormalities. Catecholaminergic PMVT is one such disorder. A rare disease of childhood, catecholaminergic PMVT typically presents with stress-induced or emotion-induced syncope that can be reproduced with exercise or infusion of isoproterenol. Believed to be a form of congenital long Q-T syndrome (LQTS) without the ECG manifestations of the disease, 30% of children with catecholaminergic PMVT have a family history of syncope or sudden cardiac death.29 Infants and adults rarely may be affected.43 PMVT can strike adults who are otherwise healthy and without structural heart disease in so-called idiopathic PMVT. This type of PMVT typically occurs with exercise or in the setting of coronary vasospasm, although PMVT occurring apart from these two conditions may occur and carries a more ominous prognosis.14 The short-coupled variant of torsades de pointes is another form of PMVT seen in patients without structural heart disease and with normal Q-T intervals. In contrast to typical torsades de pointes seen with LQTS, the short-coupled variant is initiated with ventricular extrasystoles that have a significantly shorter coupling interval to the preceding QRS complex. This disease is associated with a high incidence of sudden cardiac death, with 6 of 14 patients dying after a mean follow-up of 63 months in the largest series to date.28 A new disorder that may cause PMVT has been discovered. First reported in a series of 8 otherwise healthy patients with aborted sudden cardiac death, structurally normal hearts, and a distinct ECG pattern of right bundle-branch block with ST segment elevation in the right precordial leads and normal Q-T interval, the Brugada syndrome now has been recognized with increasing frequency (Figure 3).1 Although usually presenting with ventricular fibrillation, these patients also may exhibit PMVT.
Although there may be subtle differences in the appearance between PMVT with and without Q-T prolongation, morphologic criteria alone cannot differentiate reliably the two types of PMVT.23, 37, 45, 57 Additionally, use of the Q-T interval as the sole distinguishing variable is problematic because Q-T interval may vary with gender, age, autonomic tone, and time of day. For example, although women and men have a similar Q-To interval, the faster resting heart rate in women makes the Q-Tc interval longer in this group. As a result, women younger than age 60 have a longer Q-Tc interval than men, although this difference disappears with aging as the Q-T interval in men lengthens. The congenital LQTS registry has used a Q-Tc interval of less than 430 ms as the upper limits of normal in men and less than 450 ms in women. In children ages 1 to 15 years, less than 440 ms represents the upper limit of normal.42
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| Address reprint requests to Rod Passman, MD, Northwestern Memorial Hospital, 250 East Superior Street, Suite 520, Chicago, IL 60611. e-mail: r-passman@nwu.edu |
Vol 85 - N° 2
P. 321-341 - mars 2001 Retour au numéro
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- Nonsustained Ventricular Tachycardia : A Guide to the Clinical Significance and Management
- Marilyn J. Weigner, Alfred E. Buxton
- Implantable Cardiac Defibrillators
- Robert W. Peters, Michael R. Gold
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