What Is Inappropriate Sinus Tachycardia?

Inappropriate sinus tachycardia (IST) is a condition in which a person's heart rate, at rest and during exertion, is abnormally elevated for no apparent reason.

IST is defined as a resting heart rate greater than 90 beats per minute on average that can rise to very high levels with even minimal exertion. These inappropriately elevated heart rates are accompanied by symptoms of palpitations, fatigue, lightheadedness, or exercise intolerance.

Because the heart rhythm in IST is generated by the sinus node (the cardiac structure that controls the normal heart rhythm), IST is not associated with an abnormal electrical pattern on the ECG.

IST was recognized as a syndrome only as recently as 1979 and has been generally accepted as a true medical entity only since the late 1980s. While IST is now fully recognized as a genuine medical condition by every university medical center, some practicing physicians may not have heard of it or may misdiagnose the rapid heart rate as anxiety.

Symptoms

Some people with IST don't have any symptoms. In those who do, the most prominent symptoms associated with IST include:

• Palpitations
• Fatigue
• Exercise intolerance
• Dyspnea (shortness of breath)

However, IST is often also associated with a host of other symptoms, including: 

• Orthostatic hypotension (a drop in blood pressure upon standing)
• Blurred vision
• Dizziness, fainting, or near-fainting
• Sweating
• Chest pain
• Anxiety
• Headaches

Palpitations are a prominent symptom even though there may not be any "abnormal" heartbeats occurring. (That is, each heartbeat seems to arise from the sinus node, just as with the normal heart rhythm.) The symptoms experienced by sufferers of IST can be quite disabling and anxiety-producing.

Causes

The main cause of IST is not yet known. It is likely related to a problem with how the neuroendocrine system regulates the heart rate, a problem within the sinus node of the heart itself, or both.

IST may be part of a more general derangement of the autonomic nervous system—a condition called dysautonomia. (The autonomic nervous system manages the "unconscious" bodily functions, such as digestion, breathing, and heart rate.)

A more general dysautonomia would explain why IST symptoms often seem out of proportion to the increase in heart rate.

Experts theorize that people who have IST may be hypersensitive to adrenaline; a little bit of adrenaline (as with a little bit of exertion) causes a marked rise in heart rate.

There is some evidence to suggest structural changes to the sinus node in IST, which is why some specialized cardiologists, electrophysiologists, sometimes treat IST with a sinus node ablation. (more on this below)

Diagnosis

Several other specific and treatable medical disorders can be confused with IST, and in a person presenting with an abnormal sinus tachycardia, these other causes need to be ruled out. These disorders include anemia, fever, infections, hyperthyroidism, pheochromocytoma, diabetes-induced dysautonomia, medication side effects, and substance abuse. These conditions generally can be ruled out with a general medical evaluation, and blood and urine tests.

In addition, other cardiac arrhythmias—most often, certain types of supraventricular tachycardia (SVT)—can sometimes be confused with IST. It is usually not difficult for a doctor to tell the difference between SVT and IST by carefully examining an ECG and taking a thorough medical history. Making this distinction is very important because the treatment of SVT is quite often relatively straightforward.

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Treatment

Treatments for IST may include drug therapy, non-drug therapy, or both. Sometimes, not having treatment is also an option.

Drug Therapy

In some patients with IST, drug therapy can be helpful. However, achieving optimal results often requires trial-and-error attempts with several medications, singly or in combination.

Studies show the drug ivabradine can successfully treat some people with IST. Ivabradine directly affects the "firing rate" of the sinus node, and thus reduces the heart rate.

Ivabradine is approved in the U.S. as a treatment for heart failure in certain patients, but not IST. However, it has shown a modest benefit for IST patients in trials, and many experts recommend ivabradine as a useful treatment for this condition. Furthermore, several professional organizations now also support its use for IST.

A drawback of this drug, though, is that it may not be safe during pregnancy. Because so many people treated for IST are women in their childbearing years, some researchers urge caution and careful study before recommending ivabradine.

Beta-blockers are another category of drugs doctors can prescribe for IST. They block the effect of adrenaline on the sinus node, and since people with IST have an exaggerated response to adrenaline, these drugs may help reduce the symptoms of IST. Even so, they're not effective in everyone and can have unpleasant side effects.

Calcium blockers can directly slow the action of the sinus node but have been only marginally effective in treating IST.Calcium blockers have also been associated with adverse side effects, such as hypotension.

IST is a difficult condition to treat, partly because the causes are incompletely understood, and partly because even a controlled heart rate does not necessarily improve symptoms.

Because there is some overlap between IST and other dysautonomia syndromes (especially POTS and vasovagal syncope), drugs that are effective in treating these conditions can occasionally be helpful in treating patients with IST. These drugs may include:

• Fludrocortisone which is a drug that causes sodium retention. Some dysautonomic syndromes, especially POTS and vasovagal syncope, have been shown to be related to decreases in blood volume, and a sodium-retaining drug can increase the blood volume toward normal, thereby reducing symptoms.
• Orvaten (midodrine), a drug that causes an increase in vascular tone, helping to prevent low blood pressure.
• Serotonin-reuptake inhibitors are used primarily to treat depression and anxiety, but also have proven useful in treating several of the dysautonomia syndromes.

Sometimes, the symptoms of IST can be improved by using one or more medications.

Successful treatment of IST (and other dysautonomias) is more likely if the patient and doctor share the same treatment goals and can communicate effectively. Sometimes this means the patient has to change doctors to find a good fit.

Non-Drug Therapy

Exercise Training. Once dangerous arrhythmias are ruled out and exercise is safe, a regular exercise program can help lower the heart rate naturally and improve symptoms over time.

Sinus node ablation: Because the sinus node is sometimes abnormal in IST, some patients may consider ablation therapy (a technique in which part of the cardiac electrical system is cauterized through a catheter) to modify the function of, or even destroy, the sinus node.

Sinus node ablation has so far achieved only limited success. While this procedure can eliminate IST in up to 80% of people immediately after the procedure, the IST recurs within a few months in the large majority of these individuals.

Cognitive behavioral therapy (CBT): CBT is an effective form of psychological treatment that may reduce the risk of adverse cardiovascular events in people who experience anxiety as a symptom (or trigger) of their heart condition.CBT will not cure IST nor does it aim to. Rather, the treatment focuses on correcting a person's negative thoughts and helping them face reality, thus reducing the likelihood that anxiety will trigger or aggravate their symptoms.

Waiting: One reasonable non-pharmacologic approach to managing IST is to do nothing. While the natural history of this disorder has not been formally documented, it seems likely that IST tends to improve over time in most people. "Doing nothing" may not be an option in people who are severely symptomatic, but many individuals with only mild IST can tolerate their symptoms once they are assured that they do not have a life-threatening cardiac disorder and that the problem is likely to improve on its own eventually.

A Word From Verywell

Once IST is diagnosed and it is determined that simply “waiting” is not going to be an adequate approach, most experts today recommend beginning with drug therapy and exercise. If initial treatment attempts don't control symptoms, you can try several other drugs and combinations of drugs. Most experts now recommend ablation therapy only in severe cases.