Long QT syndrome
Some familial have a rare hereditary abnormality called congenital long QT syndrome (LQTS). Individual with LQTS are often discovered because the individual or a family member presents to a physician with episodes of syncope (fainting) or because an otherwise healthy person dies suddenly and an alert physician suggests that their close relatives get an ECG. The ECG of affected individuals reveals either a long, irregular T wave, a prolonged ST segment, or both. Their hearts have delayed repolarization which prolongs the ventricular action potential. In addition, when repolarization occur, the freshly
repolarized myocardial is subject to sudden, early depolarization, called afterdepolarization. These occur because the membrane potential in a small region of myocardium begins to depolarize before it has stabilized at the resting value. Afterdepolarization may disrupt the normal, synchronized pattern of depolarization, and the ventricles may begin to depolarize in a chaotic pattern called ventricular fibrillation. With ventricular fibrillation, there is no synchronized contraction of ventricular muscle and the heart cannot pump the blood. Arterial pressure drops, blood flow to the brain and other parts of the body ceases, and sudden death occur.
A single mutation of one of at least four genes, each of which codes for a particular cardiac muscle ion channel, cause LQTS. Mutations of three potassium channels have been discovered. The mutations decrease their function, decreasing potassium current and, thereby, increasing the tendency of the membrane to depolarize. A mutation of the sodium channel has been found in some patients with LQTS. This mutation increases the sodium channel function, increasing sodium current and the tendency of the membrane to depolarize.
Individual with congenital LQTS may be children or adults when the abnormality is identified. It is now apparent that at least cause of sudden infant death syndrome (SIDS) involves a form of LQTS.
Long QT is managed with:
- Life style changed; avoid, if possible, the specific triggers you have abnormal heart beat. Many people who have Long QT syndrome also benefit from adding more potassium to their diet.
- Medications: Beta blocker (Propranolol) prevents the hearting form fast beating. Some patient is benefit from sodium channel block (mexiletine) which make sodium channel less active.
- Surgery: Implantable Cardioverter Defibrillators (ICD). An ICD or ICD/pacemaker combination may be used. And some times Left-sided sympathetic denervation surgery is done.
References:
- Harrison’s Principles of Internal Medicine, 17th edition.
- Davidson’s Principles and Practice of Medicine, 20th Edition
- Medical physiology, Lippincott Williams & Wilkins 3rd edi.
- mayoclinic.org
- www.nhlbi.nih.gov
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