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How Is Long QT Syndrome Diagnosed?

Cardiologists diagnose and treat long QT syndrome (LQTS). Cardiologists are doctors who specialize in diagnosing and treating heart diseases and conditions. To diagnose LQTS, your cardiologist will consider your:

  • EKG (electrocardiogram) results
  • Medical history and the results from a physical exam
  • Genetic test results

EKG (Electrocardiogram)

An EKG is a simple test that detects and records the heart's electrical activity. This test may show a long QT interval and other signs that suggest LQTS. Often, doctors first discover a long QT interval when an EKG is done for another suspected heart problem.

Not all people who have LQTS will always have a long QT interval on an EKG. The QT interval may change from time to time; it may be long sometimes and normal at other times. Thus, your doctor may want you to have several EKG tests over a period of days or weeks. Or, your doctor may have you wear a device called a Holter monitor.

A Holter monitor records the heart's electrical activity for a full 24- or 48-hour period. It can detect heart problems that occur for only a few minutes out of the day.

You wear small patches called electrodes on your chest. Wires connect the patches to a small, portable recorder. You can clip the recorder to a belt, keep it in a pocket, or hang it around your neck.

While you wear the monitor, you do your usual daily activities. You also keep a notebook, noting any symptoms you have and the time they occur. You then return both the recorder and the notebook to your doctor to read the results. Your doctor can see how your heart was beating at the time you had symptoms.

Some people have a long QT interval only while they exercise. For this reason, your doctor may recommend that you have a stress test.

During a stress test, you exercise to make your heart work hard and beat fast. An EKG is done while you exercise. If you can't exercise, you may be given medicine to increase your heart rate.

Medical History and Physical Exam

Your doctor will ask whether you've had any symptoms of an abnormal heart rhythm. Symptoms may include:

  • Unexplained fainting
  • A fluttering feeling in your chest, which is the result of your heart beating too fast
  • Loud gasping during sleep

Your doctor may ask what over-the-counter, prescription, or other drugs you take. He or she also may want to know whether anyone in your family has been diagnosed with or has had signs of LQTS. Signs of LQTS include unexplained fainting, drowning, sudden cardiac arrest, or sudden death.

Your doctor will check you for signs of conditions that may lower blood levels of potassium or sodium. These conditions include the eating disorders anorexia nervosa and bulimia, excessive vomiting or diarrhea, and certain thyroid disorders.

Genetic Tests

Genetic blood tests can detect some forms of inherited LQTS. If your doctor thinks that you have LQTS, he or she may suggest genetic testing. Genetic blood tests usually are suggested for family members of people who have LQTS as well.

However, genetic tests don't always detect LQTS. So, even if you have the disorder, the tests may not show it.

Also, some people who test positive for LQTS don't have any signs or symptoms of the disorder. These people may have silent LQTS. Less than 10 percent of these people will faint or suddenly die from an abnormal heart rhythm.

Even if you have silent LQTS, you may be at increased risk of having an abnormal heart rhythm while taking medicines that affect potassium ion channels or blood levels of potassium.

Types of Inherited Long QT Syndrome

If you have inherited LQTS, it may be helpful to know which type you have. This will help you and your doctor plan your treatment and decide which lifestyle changes you should make.

To find out what type of LQTS you have, your doctor will consider:

  • Genetic test results
  • The types of situations that trigger an abnormal heart rhythm
  • How well you respond to medicine
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Long QT Syndrome Clinical Trials

Clinical trials are research studies that explore whether a medical strategy, treatment, or device is safe and effective for humans. To find clinical trials that are currently underway for Long QT Syndrome, visit www.clinicaltrials.gov.

 
September 21, 2011 Last Updated Icon

The NHLBI updates Health Topics articles on a biennial cycle based on a thorough review of research findings and new literature. The articles also are updated as needed if important new research is published. The date on each Health Topics article reflects when the content was originally posted or last revised.