Inappropriate Sinus Tachycardia

IST - A misunderstood cardiac arrhythmia

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What is Inappropriate Sinus Tachycardia?

Inappropriate sinus tachycardia (IST) is a condition in which a person's resting heart rate is abnormally high (often greater than 100 beats per minute), and it rapidly increases to very high rates with minimal exertion. These high resting and exertional 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), it is not associated with an abnormal electrical pattern on the ECG.


What Are the Characteristics of IST?

While IST can occur in anybody, it is much more common in younger adults and affects women more often than men.  The "average" IST sufferer is a woman in her late 20s or early 30s who has been having symptoms for months to years. In addition to the most prominent symptoms of palpitations, fatigue and exercise intolerance, IST is often also associated with a host of other symptoms including a drop in blood pressure upon standing, blurred vision, dizziness, tingling, shortness of breath, and sweating.

With IST, the resting heart rate is most often greater than 100 beats per minute, but during deep sleep, it may drop to 80 or 90 beats per minute, or even lower. With even minimal exertion the heart rate rapidly accelerates to as high as 140 or 150 beats per minute. Palpitations are a prominent symptom even though (as is often the case) there are no "abnormal" heart beats 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.

IST was recognized as a syndrome only as recently as 1979 and has been generally accepted as a true medical entity only for the past 20 years or so.

And even today, while IST is 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").

What Causes IST?

Nobody knows.

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.)

Patients with IST are hypersensitive to adrenaline; a little bit of adrenaline (like a little bit of exertion) causes a marked rise in heart rate. While there is indeed evidence that the sinus node itself has structural changes 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.) It is the idea that the sinus node itself is intrinsically abnormal that has led electrophysiologists to resort to ablation of the sinus node as a treatment for IST (more on this below).

What Else Needs To Be Considered in Diagnosing IST?

Several other specific and treatable medical disorders can be confused with IST, and in a patient 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, 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 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.

Drug Therapy For IST

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

Beta-blockers block the effect of adrenaline on the sinus node, and since patients with IST have an exaggerated response to adrenaline, using beta blockers is logical.

These drugs often help quite a bit in reducing the symptoms of IST. 

Calcium blockers can slow the action of the sinus node, but have been only marginally effective in for IST.

Recently, the new drug ivabradine has been used successfully patients with IST. Ivabradine directly affects the "firing rate" of the sinus node, and so reduces the heart rate. Ivabradine is marketed outside of the U.S. as a treatment for angina and for heart failure in patients who cannot tolerate beta blockers. It was recently approved by the U.S. Food and Drug Administration for the treatment of heart failure. However, reports suggest that this drug can have a very favorable effect in at least some patients with IST.

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, 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, and reduce symptoms.
  • Midodrine, a drug that causes an increase in vascular tone, helping to prevent low blood pressure.
  • Serotonin-reuptake inhibitors (the Prozac family of drugs) 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. Effective combinations can only be discovered on a trial-and-error basis, and so require a certain amount of patience, understanding, and trust between the doctor and patient. This is difficult to achieve if the doctor thinks the patient is just nuts. In order to be successfully treated, patients with IST (and the other dysautonomias) must often do a fair amount of doctor shopping.

Non-drug therapy For IST

Increase salt intake. This should be done with the approval of your doctor because it goes against society's and the medical profession's prejudice against sodium. But 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.

Sinus node ablation. Cardiologists, 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). Subscribing to this point of view, as we have noted, has limited their horizons in administering drug therapy. It has also created a certain amount of enthusiasm for using radiofrequency ablation (a technique in which part of the cardiac electrical system is cauterized through a catheter) to modify the function of, or even destroy the function of, the sinus node.

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

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 patients. "Doing nothing" may not be an option in patients who are severely symptomatic, but many patients with 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 - especially once they are enlightened as to the treatment options.


Krahn, AD, Yee, R, Klein, GJ, Morillo, C. Inappropriate sinus tachycardia: Evaluation and therapy. J Cardiovasc Electrophysiol 1995; 6:1124.

Schulze, V, Steiner, S, Hennersdorf, M, Strauer, BE. Ivabradine as an alternative therapeutic trial in the therapy of inappropriate sinus tachycardia: a case report. Cardiology 2008; 110:206.

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