Long QT Syndrome: Diagnosis and Management

Ijaz A. Khan, MD, FACP, FACC

Disclosures

Am Heart J. 2002;143(1) 

In This Article

Abstract and Introduction

Abstract

Background: Long QT syndrome (LQT) is characterized by prolongation of the QT interval, causing torsade de pointes and sudden cardiac death. The LQT is a disorder of cardiac repolarization caused by alterations in the transmembrane potassium and sodium currents. Congenital LQT is a disease of transmembrane ion-channel proteins. Six genetic loci of the disease have been identified. Sporadic cases of the disease occur as a result of spontaneous mutations. The acquired causes of LQT include drugs, electrolyte imbalance, marked bradycardia, cocaine, organophosphorus compounds, subarachnoid hemorrhage, myocardial ischemia, protein sparing fasting, autonomic neuropathy, and human immunodeficiency virus disease.
Methods: Data on the diagnosis and management of LQT were thoroughly reviewed.
Results and Conclusions: The diagnosis of LQT primarily rests on clinical and electrocardiographic features and family history. The clinical presentations range from dizziness to syncope and sudden death. Genetic screening is available primarily as a research tool. Short-term treatment of LQT is aimed at preventing the recurrences of torsades and includes intravenous magnesium and potassium administration, temporary cardiac pacing, withdrawal of the offending agent, correction of electrolyte imbalance, and, rarely, intravenous isoproterenol administration. The long-term treatment is aimed at reducing the QT-interval duration and preventing the torsades and sudden death and includes use of oral ß-adrenergic blockers, implantation of permanent pacemaker/cardioverter-defibrillator, and left thoracic sympathectomy. Sodium channel blockers are promising agents under investigation. Electrocardiograms are recorded for screening of family members. The data favor treating asymptomatic patients, if <40 years old at the time of diagnosis, with ß-adrenergic blockers.

Introduction

Long QT syndrome (LQT) is a disorder caused by lengthening of the repolarization phase of the ventricular action potential, although the QT interval on the surface electrocardiogram (ECG) represents the total duration of both the depolarization and the repolarization phases. Therefore it is the lengthening of the JT interval that determines the vulnerability for development of torsade de pointes (torsades), and the lengthening of QT interval, because of the mere lengthening of the duration of QRS complex, will not constitute LQT and will not precipitate torsades. The LQT is caused by congenital and acquired factors. Congenital LQT is a heritable ion channel disease caused by a number of mutations in the genes encoding for the transmembrane sodium or potassium ion channel proteins.[1] Six subsets of congenital LQT (LQT1 to LQT6) have been identified. Electrophysiologically, these mutations slow the inactivation of inward depolarizing sodium currents or cause retardation of outward repolarizing potassium currents, resulting in an increase in after depolarizations and dispersion of repolarization, both of which contribute to the final phenotype of the syndrome.[2,3] The acquired factors causing LQT include use of drugs, hypokalemia, hypomagnesemia, hypocalcemia, marked bradycardia, cocaine abuse, organophosphorus compound poisoning, subarachnoid hemorrhage, stroke, myocardial ischemia, protein-sparing fasting by using liquid protein diets, autonomic neuropathy, and human immunodeficiency virus disease.[4,5,6,7,8,9,10] Multiple risk factors may interplay in an individual patient to prolong QT interval and precipitate torsades.[11]

Comments

3090D553-9492-4563-8681-AD288FA52ACE
Comments on Medscape are moderated and should be professional in tone and on topic. You must declare any conflicts of interest related to your comments and responses. Please see our Commenting Guide for further information. We reserve the right to remove posts at our sole discretion.
Post as:

processing....