The QT interval refers to the time between two events on the electrocardiogram (ECG)—from the beginning of the Q wave to the end of the T wave.
(See also Overview of Abnormal Heart Rhythms Overview of Abnormal Heart Rhythms Abnormal heart rhythms (arrhythmias) are sequences of heartbeats that are irregular, too fast, too slow, or conducted via an abnormal electrical pathway through the heart. Heart disorders are... read more and Ventricular Tachycardia Ventricular Tachycardia Ventricular tachycardia is a heart rhythm that originates in the ventricles (lower chambers of the heart) and produces a heart rate of at least 120 beats per minute (the normal heart rate is... read more .)
ECG: Reading the Waves
An electrocardiogram (ECG) represents the electrical current moving through the heart during a heartbeat. The current's movement is divided into parts, and each part is given an alphabetic designation in the ECG.
Each heartbeat begins with an impulse from the heart's pacemaker (sinus or sinoatrial node). This impulse activates the upper chambers of the heart (atria). The P wave represents activation of the atria.
Next, the electrical current flows down to the lower chambers of the heart (ventricles). The QRS complex represents activation of the ventricles.
The ventricles must then undergo an electrical change to get ready for the next heart beat. This electrical activity is called the recovery wave, which is represented by the T wave.
Many kinds of abnormalities can often be seen on an ECG. They include a previous heart attack (myocardial infarction), an abnormal heart rhythm (arrhythmia), an inadequate supply of blood and oxygen to the heart (ischemia), and excessive thickening (hypertrophy) of the heart's muscular walls.
Certain abnormalities seen on an ECG can also suggest bulges (aneurysms) that develop in weaker areas of the heart's walls. Aneurysms may result from a heart attack. If the rhythm is abnormal (too fast, too slow, or irregular), the ECG may also indicate where in the heart the abnormal rhythm starts. Such information helps doctors begin to determine the cause.
Any abnormality that prolongs the QT interval increases the risk of a dangerous heart rhythm called torsades de pointes ventricular tachycardia. Some people are born with a genetic abnormality that causes a long QT interval (called long QT syndrome Long QT Interval Syndromes The long QT interval syndromes are disorders of the heart's electrical activity that predispose people to dangerous heart rhythms and sudden death. People may be born with an abnormality that... read more ). In other people, the long QT interval results from low serum levels of potassium (hypokalemia Hypokalemia (Low Level of Potassium in the Blood) In hypokalemia, the level of potassium in blood is too low. A low potassium level has many causes but usually results from vomiting, diarrhea, adrenal gland disorders, or use of diuretics. A... read more ), a very slow heart rhythm, or a drug. Often, drugs used to treat abnormal heart rhythms cause a long QT interval, but certain antidepressants and certain antiviral and antifungal drugs can also cause it.
Ventricular tachycardia Ventricular Tachycardia Ventricular tachycardia is a heart rhythm that originates in the ventricles (lower chambers of the heart) and produces a heart rate of at least 120 beats per minute (the normal heart rate is... read more is a heart rhythm that originates in the ventricles (lower chambers of the heart) and produces a rapid heart rate. People with a long QT interval may develop a particular form of ventricular tachycardia called torsades de pointes ventricular tachycardia (torsades). Torsades has a characteristic appearance on the electrocardiogram (ECG) and often turns into ventricular fibrillation Ventricular Fibrillation Ventricular fibrillation is a potentially fatal, uncoordinated series of very rapid, ineffective contractions of the ventricles (lower chambers of the heart) caused by many chaotic electrical... read more , in which the heart stops beating, which is rapidly fatal.
Sometimes, exercise brings on the ventricular tachycardia (see Sudden Cardiac Death in Athletes Sudden Cardiac Death in Athletes An estimated 1 to 3 per 100,000 apparently healthy young athletes develop an abrupt-onset heart rhythm abnormality and die suddenly during exercise. Males are affected up to 10 times more often... read more ). Other factors that increase the risk of torsades include female sex, older age, an underactive thyroid gland (hypothyroidism Hypothyroidism Hypothyroidism is underactivity of the thyroid gland that leads to inadequate production of thyroid hormones and a slowing of vital body functions. Facial expressions become dull, the voice... read more ), brain disorders such as a stroke, and certain types of heart disease such as a heart attack or inflamed heart (myocarditis).
Symptoms of Torsades de Pointes Ventricular Tachycardia
People who develop torsades de pointes ventricular tachycardia may have palpitations (awareness of heartbeats) and feel light-headed or faint. Torsades de pointes runs of ventricular tachycardia usually stop on their own but frequently recur. Ventricular fibrillation causes cardiac arrest Cardiac Arrest and CPR Cardiac arrest is when the heart stops pumping blood and oxygen to the brain and other organs and tissues. Sometimes a person can be revived after cardiac arrest, particularly if treatment is... read more and sudden collapse.
Diagnosis of Torsades de Pointes Ventricular Tachycardia
Electrocardiography Electrocardiography Electrocardiography (ECG) is a quick, simple, painless procedure in which the heart’s electrical impulses are amplified and recorded. This record, the electrocardiogram (also known as an ECG)... read more (ECG) is used to detect torsades.
If doctors diagnose torsades in a person, they ask whether there is a family history of the disorder or if relatives have died unexpectedly because of a heart problem. Such people may then have genetic testing for congenital long QT syndrome, and their close family members should be evaluated.
Treatment of Torsades de Pointes Ventricular Tachycardia
Converting heartbeat to normal rhythm by applying an electric shock (defibrillation)
Preventing further episodes
Defibrillation Cardioversion-Defibrillation There are many causes of abnormal heart rhythms (arrhythmias). Some arrhythmias are harmless and do not need treatment. Sometimes arrhythmias stop on their own or with changes in lifestyle,... read more is needed if ventricular fibrillation develops. Sometimes doctors also give magnesium sulfate.
If a drug is the cause, it is stopped.
People may need to limit their physical activity to prevent a recurrence. They may also need to take beta-blockers (see table ) or have an artificial pacemaker Artificial Pacemakers There are many causes of abnormal heart rhythms (arrhythmias). Some arrhythmias are harmless and do not need treatment. Sometimes arrhythmias stop on their own or with changes in lifestyle,... read more or cardioverter-defibrillator Implantable Cardioverter-Defibrillator (ICD) There are many causes of abnormal heart rhythms (arrhythmias). Some arrhythmias are harmless and do not need treatment. Sometimes arrhythmias stop on their own or with changes in lifestyle,... read more implanted.
The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
American Heart Association: Arrhythmia: Information to help people understand their risks of arrhythmias as well as information on diagnosis and treatment