An ARRHYTHMIA disorder in which an electrical conduction defect in the HEART results in delayed repolarization of myocardial cells. Repolarization is the process by which myocardial cells restore themselves to receive another electrical impulse. With LQTS, the myocardial cells hold a positive charge much longer than normal, preventing the heart from recharging for the next CARDIAC CYCLE. Most LQTS is hereditary, and researchers have isolated a number of GENE mutations that affect the heart’s ion channels (conductive pathways), usually potassium channels though sometimes sodium channels. The condition can also develop as a consequence of STROKE or as a medication SIDE EFFECT, notably with antiarrhythmia and ANTIDEPRESSANT MEDICATIONS though numerous drugs affect the QT interval.
The points Q and T on the ELECTROCARDIOGRAM (ECG) identify polarization or discharge of electrical activity (the Q wave) and repolarization (the T wave). Doctors call the amount of time it takes for this phase to complete the QT interval. The longer the QT interval, the greater the risk for a dangerous arrhythmia called TORSADE DE POINTES, a form of highly unstable ventricular tachycardia (rapid though regular contractions of the ventricles, typically exceeding 100 contractions a minute). Torsade de pointes can quickly lead to VENTRICULAR FIBRILLATION, in which the ventricular contractions are rapid, irregular, and nonfunctional. Ventricular fibrillation quickly becomes life-threatening and may require emergency DEFIBRILLATION. LQTS is a common cause of SUDDEN CARDIAC DEATH in young people who are apparently healthy.
Often, people who have LQTS do not have symptoms, and doctors detect the condition during ECG done for other reasons. The most common symptom that does occur is unexplained syncope (fainting), especially with intense exercise or emotional response (such as anger or fear). ECG generally provides the diagnosis, though the cardiologist may do an exercise STRESS TEST to evaluate the heart’s electrical response with increased physical activity.
The standard medical treatment for LQTS is a beta blocker medication, which helps slow and stabilize the HEART RATE. The beta blockers most commonly prescribed for LQTS are propanolol (Inderal), metoprolol (Lopressor or Toprol), nadolol (Corgard), and atenolol (Tenormin). Beta blockers control LQTS in about 70 percent of people who have the condition. When medications fail to prevent arrhythmias, the next level of treatment is an IMPLANTABLE CARDIOVERTER DEFIBRILLATOR (ICD), an electronic device similar to a PACEMAKER. The ICD monitors the heart’s rhythm and can deliver an electrical shock to return the heart to a normal rhythm should dangerous arrhythmias occur.
Because a prolonged QT interval is most likely to occur during intense physical exercise that puts high demand on the heart LQTS may require lifestyle modifications, especially for people who participate in competitive sports. Most people are able to enjoy recreational athletic and physical activities, however.
Gene mutations establish the foundation for LQTS, probably even in secondary LQTS (LQTS that results as a side effect of medication or other events). Family history of LQTS or sudden cardiac death is an important diagnostic clue. When LQTS is secondary, removing the cause often ends the conductive irregularities. There are no known preventive measures for primary LQTS. Treatment often controls symptoms and prevents life-threatening arrhythmia.
See also CARDIAC ARREST; MEDICATIONS TO TREAT CARDIOVASCULAR DISEASE; TAMPONADE, CARDIAC.
Resource: Facts On File Encyclopedia Of Health And Medicine
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