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Harvard Medical School

long qt syndrome

What Is It?

Long QT syndrome is an uncommon inherited condition — meaning it's caused by genes passed on to you from your parents. The electrical activity of heart cells is controlled by a set of channels that pump minerals, such as sodium and potassium, in and out of cells. If you inherit genes that cause alterations in these channels, it can affect the action of heart cells.

Normally, an electrical impulse starts in the sinus node, located in the upper chamber of the heart. The electrical impulse then travels down to the lower chambers of the heart, called the ventricles, causing contraction of the ventricles' muscle cells. This contraction causes the blood to flow out of your heart, like how squeezing a balloon filled with water, but not tied at the top, causes the water to squirt out the top.

The heart muscle cells then relax. During this relaxation phase, the electrical charges of the cells need to recover.

There are normal amounts of times for each of these phases of the heartbeat, called intervals. A test called an electrocardiogram (EKG) can show how long it takes for the impulse to pass through certain sections of the heart, and how long it takes for the heart muscle cells in the ventricles to recover. The recovery time is known as the QT interval.

The QT interval lasts just a fraction of a second; it normally lasts about one-third of an entire heartbeat. In people with long QT syndrome, the interval lasts longer than it should because heart muscle cells are having trouble getting ready for the next contraction.

A prolonged QT interval does not cause symptoms. However, the delay in recharging upsets the electrical timing of the heart. This makes the heart more likely to develop abnormal heart rhythms, called arrhythmias. Sometimes these arrhythmias can be dangerous, even deadly.

People with long QT syndrome may not always show an abnormally long QT interval on an EKG. Many different factors can influence the recovery time of even normal heart cells. Sometimes long QT intervals only occur during exercise, at times of intense emotion, or after being startled.

Inherited long QT syndrome is not nearly as common as a prolonged QT interval caused by a medication or an imbalance of certain minerals in the blood stream. More than 50 medications are known to lengthen the QT interval and upset the heart rhythm.

The two best understood inherited long QT syndromes are the Romano-Ward syndrome and the Jervell and Lange-Nielsen syndrome. Of the two, the Romano-Ward syndrome is more common. It is an autosomal dominant inherited condition — meaning you can have a long QT if you inherit one abnormal gene from either parent. The Jervell and Lange-Nielsen syndrome is rare. People with this syndrome have a very long QT interval and are also deaf. The Jervell and Lange-Nielsen syndrome is an autosomal recessive inherited condition — meaning that you need to inherit two abnormal genes, one from each parent, to get this syndrome.