IST - Inappropriate Sinus Tachycardia

What is inappropriate sinus tachycardia (IST)?

Inappropriate sinus tachycardia (IST) occurs when the heart beats very quickly without a good reason. It is a type of heart rhythm abnormality called an arrhythmia.

Tachycardia is the medical term for a fast heart rate. In adults, a heart rate greater than 100 beats per minute when a person is at rest is considered tachycardia. Children and infants normally have faster heart rates than adults. Most people with IST have a resting heart rate greater than 100 beats per minute or an average resting heart rate greater than 90 beats per minute. In others, the heart rate is normal when resting, but it shoots up very quickly during light physical effort  

The heart’s electrical signal usually starts in the sinoatrial (SA) node, which is found in the heart’s right upper chamber. This area triggers the electrical signal that spreads throughout the heart and coordinates the heartbeat. The SA node receives signals from nerves. These nerves can cause the heart to beat more quickly or more slowly, depending on the body’s needs.  

There are many normal reasons why the heart might beat faster, such as with a fever, during exercise, or with anxiety or stress. But in IST, the heart beats faster for unknown reasons. The heart rate may increase on its own without any stimulation. Other times, the heart rate rises because of a trigger. But it increases more than it should. If the heart beats very quickly, it may not be able to fill fully between beats. Slightly less blood may then get out to circulate through the body.

IST is rare. It's more common in young women than in men. IST is also a complex disorder. It is generally best to see a cardiologist trained in the use of various cardiac medicines and electrophysiology treatments.

What causes inappropriate sinus tachycardia?

IST has many causes. Experts don't understand all of them well. Sometimes, IST results from inappropriate signaling of the nerves that increase the heart’s rate. The nerves that lower the heart’s rate may also not work as they should.

In some people, IST starts after an infection from a virus. In this situation, people may have their symptoms go away suddenly after several months to a few years.  

Many factors can trigger tachycardia itself. Some triggers may include: 

• Caffeine.
• Alcohol.
• Nicotine.
• Illegal drugs such as cocaine.
• Overactive thyroid (hyperthyroidism).
• Fever.
• Anxiety.
• Anemia.
• Very low blood pressure.
• Heart attack.
• Heart failure.
• Lung disease.
• Pain.

With these triggers, the tachycardia would be considered an appropriate response. But usually with IST, the heart rate is high even when these triggers are not present. In some cases of IST, the heart rate may increase even more than it should in response to these triggers.  

What are the symptoms of inappropriate sinus tachycardia?

Some people with IST don't have any symptoms. But others do. Possible symptoms include:

• Rapid heartbeat (palpitations), which may be uncomfortable.
• Shortness of breath.
• Dizziness.
• Fainting.
• Chest pain.
• Anxiety.
• Headaches.
• Decreased ability to exercise.

The symptoms of IST may look like other health problems. Always see your healthcare provider for a diagnosis.

You might be more likely to have symptoms from IST if you have another heart problem. These symptoms might come on in response to a trigger such as caffeine. These symptoms may happen from time to time over months or years. In some people, these symptoms go away after a few months.

How is inappropriate sinus tachycardia diagnosed?

Your healthcare provider will review your health history and do a physical exam. They can easily notice a fast heartbeat by taking your pulse. But it's important to rule out other causes for the fast heartbeat. It's also important to learn what type of tachycardia is present. Other types of tachycardia may need different treatment.  

Your healthcare provider might use tests to help make this diagnosis. These include:  

• Electrocardiogram. This is done to analyze the heart’s electrical rhythm and the type of tachycardia.
• Continuous monitoring of the heart rhythm (Holter monitor). This checks the rhythm for a longer period of time.
• Blood tests. These are done to look for other causes of the fast heartbeat.
• Echocardiogram. This checks the heart's size, structure, and pumping ability.
• Chest X-ray. This is done to view the heart and lungs.

IST is sometimes diagnosed by mistake as an anxiety disorder.  

How is inappropriate sinus tachycardia treated?

A variety of treatments are available to treat IST. Your healthcare provider might suggest that you:  

• Eliminate potential triggers or stimulants in your diet such as caffeine, nicotine, and alcohol.
• Take medicine to slow the heart rate such as ivabradine, beta-blockers, or calcium channel blockers.
• Exercise to improve quality of life and to maintain a healthy heart.

IST is often hard to treat. If you have severe symptoms that don’t respond to these treatments, you may be a candidate for catheter ablation. This procedure uses energy to destroy a very small part of the heart that is triggering the tachycardia. But it is not always successful. Also, there is some risk that T destroying too much heart tissue might make a permanent pacemaker necessary.  

What can I do to prevent inappropriate sinus tachycardia?

It may not be possible to prevent IST itself. If you have IST, staying away from triggers may help you prevent episodes of increased heart rate.  

Heart disease can make symptoms of IST worse. Talk with your healthcare provider about ways you can prevent heart disease. These include:  

• Eating a heart-healthy diet.
• Getting enough exercise and maintaining a healthy weight.
• Treating conditions such as unhealthy cholesterol levels, high blood pressure, and diabetes.

When should I call my healthcare provider?

Call your healthcare provider if your symptoms are becoming more frequent or severe. Call if you have symptoms such as severe shortness of breath or chest pain.  

Key points about inappropriate sinus tachycardia:

• In IST, the heart rate is typically greater than 100 beats per minute at rest. Or it's greater than 90 beats per minute when averaged throughout the day.
• Most often, this increase in heart rate occurs on its own in the absence of triggers. Other times, the heart rate increases because of a trigger. But the increase is much more than what would be expected.
• Some people don’t have any symptoms from IST. But others do.
• Possible treatments vary depending on the severity of your symptoms.
• It may help to stay away from possible triggers like caffeine, nicotine, and any other triggers that cause your IST.