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Inappropriate sinus tachycardia

A misunderstood cardiac arrhythmia

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Updated March 03, 2014

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

What is Inappropriate Sinus Tachycardia?

Inappropriate sinus tachycardia (IST) is a condition in which a person's resting heart rate is abnormally high (greater than 100 beats per minute), and it increases to abnormally 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 generates 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 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 heart beat 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 hyperthyroidism, pheochromocytoma, diabetes-induced dysautonomia, and substance abuse. These conditions generally can be ruled out with 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 through medical history. Making this distinction is very important, because the treatment of SVT is quite often relatively straightforward.

Page 2 - Treating IST

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