Inappropriate Sinus Tachycardia

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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 100 beats per minute that can raise to very high levels with even minimal exertion. These inappropriately elevated heart rates are usually accompanied by symptoms of palpitations, fatigue, and 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.

While IST can occur in anybody, it is much more common in younger adults and affects women more often than men. The average person with IST is a woman in her 20s or early 30s who has been having symptoms for months to years.

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, many practicing physicians either haven't heard of it or write it off as a psychological problem (namely, 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
  • Tingling
  • Sweating
  • Chest pain
  • Anxiety
  • Headaches
HEART RATES IN IST
  Resting Sleep Exertion
Beats per Minute 100 or more 80-90 140-150

Palpitations are a prominent symptom even though (as is often the case) there are no "abnormal" heartbeats occurring. (That is, each heartbeat arises 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 question seems to be whether IST represents a primary disorder of the sinus node, or whether, instead, it represents 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.)

People who have IST are hypersensitive to adrenaline; a little bit of adrenaline (as with a little bit of exertion) causes a marked rise in heart rate.

While there is indeed evidence of structural changes in the sinus node in IST, a lot of other evidence suggests that a more general disorder affecting the autonomic nervous system is present in many of these patients. (A more general dysautonomia would explain why symptoms with IST most often seem out of proportion to the increase in heart rate.)

The idea that the sinus node itself is intrinsically abnormal has led electrophysiologists to resort to ablation of the sinus node as a treatment for IST (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, hyperthyroidismpheochromocytoma, diabetes-induced dysautonomia, 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 include both drug therapy and non-drug therapy.

Drug Therapy

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

Beta-blockers are often the first drugs doctors 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 often help quite a bit in reducing 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.

Studies show the drug ivabradine can successfully treat 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 angina and for heart failure in patients who cannot tolerate beta blockers, but not for IST. However, it is at least as effective as other drugs, 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.

Many cardiologists tend not to subscribe to the "generalized autonomic dysfunction" theory of IST, and therefore have not tried prescribing drugs that have been helpful in patients with other forms of dysautonomia. However, since there is often a lot of overlap between IST and the 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:

  • Florinef (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.
  • ProAmitine, Orvaten (midodrine), a drug that causes an increase in vascular tone, helping to prevent low blood pressure.
  • Serotonin-reuptake inhibitors, including Prozac, are used primarily to treat depression and anxiety, but also have proven useful in treating several of the dysautonomia syndromes.

Often, the symptoms of IST can be controlled to a reasonable degree by using a combination of drugs. In general, beta-blockers are attempted first, and ivabradine is added (or substituted) if the beta blocker does not control symptoms sufficiently.

Effective drug therapy often requires persistence, working on a trial-and-error basis. A certain amount of patience, understanding, and trust between the doctor and patient is required.

Successful treatment is difficult to achieve if the doctor thinks the patient is just nuts. Many people with IST (and other dysautonomias) must often do a fair amount of doctor shopping before they find someone competent to treat them.

Non-Drug Therapy

Sinus node ablation: Many cardiologists, and especially electrophysiologists, have largely been swayed by the data suggesting that IST is primarily a disorder of the sinus node (as opposed to a more generalized disorder of the autonomic nervous system). This belief has created a certain amount of enthusiasm for using 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.

Increase salt intake. Salt increases the blood volume, and to the extent that a reduced blood volume contributes to symptoms, increasing the salt intake might help alleviate symptoms in IST. (This should be done with the approval of your doctor, because of our current prejudice in favor of low sodium diets.)

WaitingOne 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. Usually, a beta blocker will be attempted first, followed by a trial of ivabradine (either alone or in combination with a beta blocker). If these treatment attempts don't control symptoms, you can try several other drugs and combinations of drugs. Most experts now recommend ablation therapy only if at least two drug trials have failed.

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