What is Inappropriate Sinus Tachycardia?
Inappropriate sinus tachycardia (IST) is a condition in which an individual's resting heart rate is abnormally high (greater than 100 beats per minute), their heart rate increases rapidly with minimal exertion, and their rapid heart rate is accompanied by symptoms of palpitations, fatigue, and exercise intolerance. IST is not associated with an abnormal electrical pattern on the ECG. The heart rhythm in IST appears to arise from within the sinus node, the cardiac structure that generates the normal heart rhythm. (Click here for a quick and easy review of the normal heart rhythm.)What are the characteristics of IST?
While IST can be seen in anybody, it is most often a disorder of young women. 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 can also be associated with a host of other symptoms including a drop in blood pressure upon standing, blurred vision, dizziness, tingling, shortness of breath, and sweating.These patients most often have a resting heart rate of greater than 100 beats per minute, but it characteristically drops to 80 – 90 beats per minute while they are asleep. With even minimal exertion, the heart rate rapidly accelerates to 140 – 150 beats per minute. Palpitations (an unusual awareness of the heart beat) are a prominent symptom even though (as is often the case) there are no “abnormal” heart beats occurring. 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 10 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.
What causes IST?
Nobody knows.The main question seems to be whether IST represents a primary disorder of the sinus node, or whether it represents a more general derangement of the autonomic nervous system – a condition called dysautonomia. Click here for a general description of the dysautonomia disorders.
Patients with IST are hypersensitive to adrenaline, such that a little bit of adrenaline (like a little bit of exertion) causes a marked rise in heart rate. There is evidence that the sinus node itself has structureal changes. But much other evidence suggests that a more general dysautonomia is present in these patients. (This would explain why symptoms most often seem out of proportion to the increase in heart rate.)
It is the notion 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 autonomic dysfunction, and substance abuse. These conditions generally can be ruled out with blood and urine tests.In addition, other abnormal heart rhythm disorders – various types of SVT – can sometimes be confused with sinus tachycardia. Such SVTs need to be ruled out, because they can often be treated quite definitively.
