ReviewClinical and therapeutic aspects of congenital and acquired long QT syndrome
Section snippets
Cause
The QT interval can be prolonged by slowing outward repolarizing potassium currents, enhancing inward calcium currents, or slowing the inactivation of inward depolarizing sodium currents. The long QT syndrome is caused by similar types of changes in transmembrane ionic currents 5, 6, 7, 8.
Electrocardiographic manifestations
By definition, the long QT syndrome is characterized by a prolonged QTc interval on the surface electrocardiogram. Other electrocardiographic features, including increased QT dispersion and T- and U-wave abnormalities, are common 36, 37, 38, 39, 40, 41 (Figure 1). The QTc interval is not only prolonged but also more variable; thus, QT dispersion, a measure of the QT interval variability, is increased (36). T waves may be larger, prolonged, or bizarre looking, with a notched, bifid, or biphasic
Diagnosis
The diagnosis of the long QT syndrome is made by clinical characteristics, electrocardiographic findings, and family history. Unexplained syncope or sudden cardiac death in a child or young adult should suggest the diagnosis. The criteria in Table 349, 56 provide a quantitative approach to the diagnosis and classify patients as having a low, intermediate, or high probability of the syndrome. In borderline cases, exercise testing may be performed because of possible abnormal lengthening of the
Short-term treatment
Many episodes of torsade de pointes are short lived and terminate spontaneously. However, a prolonged episode may result in marked hemodynamic compromise, and immediate cardioversion should be implemented. Short-term treatment of the syndrome is aimed at preventing the recurrence of torsade de pointes, and includes administration of intravenous magnesium sulfate and temporary transvenous cardiac pacing, but rarely administration of intravenous isoproterenol. Intravenous magnesium and temporary
Long-term treatment of the congenital long QT syndrome
Long-term treatment has as its purpose shortening the QTc interval and preventing recurrences of torsade de pointes, treatment that reduces 10-year mortality to 3% to 4%. Treatment options include beta-blockers, permanent pacemaker placement, and implantation of a cardioverter-defibrillator. Recent advances in understanding the genetics of the congenital long QT syndrome have led to interest in the development of mutation-specific therapies.
Long-term treatment of the acquired long QT syndrome
Patients with torsade de pointes resulting from the acquired causes of a prolonged QT interval often require only acute treatment along with withdrawal of any offending agents and correction of any electrolyte imbalance. Long-term treatment is generally not required in these patients, because the QTc interval often becomes normal once the underlying cause is treated. Use of agents that prolong the QTc interval directly, or indirectly by causing electrolyte imbalance, should be avoided in
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