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IST (inappropriate sinus tachycardia)

For a quick view to familiarise yourself with the normal conduction system of the heart and heart rhythms please check out these links:

how your heart works by Dr. Carl Bianco, M.D.

the cardiac electrical system by Dr Rich Fogoros, M.D.

Inappropriate sinus tachycardia (IST) is a type of cardiac arrhythmia (an abnormal heart rhythm) in which a person's resting heart rate is usually greater than 90 - 100 beats per minute (or rapidly accelerates to greater than 90 - 100bpm very frequently), the heart rate increases dramatically with small amounts of exercise, emotional or physical stress.

People with IST often find that their fast heart rate causes them to experience symptoms including palpitations, shortness of breath and other symptoms and an inability to engage in exercise and physically demanding activities. The symptoms can be very frightening, especially when an individual first becomes aware of them.

People with IST often find their lives made a misery, not only by the symptoms and effects of the condition - but by the apparent lack of knowledge amongst healthcare professionals.

Many doctors/physicians and even some cardiologists do not appear to know a great deal about IST. To see an account of my own and others experiences visit the stories page. Cardiologists have many different aspects of cardiac disease to focus on, and whilst this may not be a reasonable excuse for them to not know about IST - it is realistic. IST is rare. My greatest wishes are for cardiologist and medics to become educated about IST, so that more people do not feel like they are being brushed off as being 'anxious' or 'crazy' and they can get appropriate care from professionals. A doctor who will have the greatest knowledge about IST is a Cardiac Electrophysiologist (or 'EP' for short).

What is IST?

IST is a type of tachycardia (a fast heart beat).

1st to explain the 'Inappropriate' in Inappropriate Sinus Tachycardia…….

Sinus tachycardia ('Appropriate' Sinus tachycardia) is a normal heart rhythm. Sinus Rhythm is the name given to the normal heart rhythm, with the heart rate in the range of 50 - 90 bpm (beats per minute). Older textbooks will say 60 - 100 bpm. But no-one would consider a resting heart rate of 100bpm to be normal or desirable. Sinus Tachycardia is described as a sinus rhythm over 100 bpm.

An 'Appropriate' Sinus Tachycardia can develop as a desirable response to pregnancy, physical exertion and emotion. Sinus tachycardia can also develop as a 'compensatory response' by the circulatory system in response to various conditions including low blood pressure, low blood volume (including via blood loss), anaemia, structural heart disease, pain, cancer, endocrine disorders, also in response to some prescription and non-prescription drugs and can result from drug toxicity.

When there is no known cause and all possible known causes of Sinus Tachycardia have been excluded, the heart rhythm is called 'Inappropriate' Sinus Tachycardia.

There are 2 main types of abnormal rapid heart rate that also need to be excluded before IST is considered - SVT (Supraventricular Tachycardia), and VT / V-Tach (Ventricular Tachycardia). These fast rhythms are generated in parts of the heart that do not normally initiate the heart beat. IST is generated in the Sinus Node (where normal sinus rhythm is generated).

So, on an ECG - IST looks exactly the same as a 'normal' sinus tachycardia.

Some info on IST :

Most people who have IST are women (not all, but most). This is reported in data collected for study. It has also been reported that many of these patients are young, usually under the age of 30. There are also said to be cases of patients who are elderly women who have IST. It is not unusual for patients to first present to cardiologists and other doctors with arrhythmias when they are in their teens and twenties. IST appears to be similar in this respect.

Many patients with IST find that they have to be very persistent when seeking diagnosis and treatment.

IST can also be confused with other conditions, notably one called POTS (Postural Orthostatic Tachycardia Syndrome) - notably a raise in heart rate from lying to standing of greater than 30 bpm or over 120pbm within 10 mins of standing without a drop in blood pressure (BP) and sometimes accompanyied by blurred vision, dizziness and pre-syncope (a feeling that you are about to faint) and syncope (fainting) amongst other symptoms.

For information on POTS please check out this link:

POTS UK

IST Symptoms

Most patients with IST have a resting heart rate of greater than 90 - 100bpm. During sleep the heart rate usually falls to 70 - 90 bpm. With tiny amounts of exertion, the heart rate can quickly rise to over 150 bpm. The main symptom experienced by IST sufferers is usually palpitations (being aware of your heart beat) and can be very distressing and exhausting. Please visit the symptoms page.

What causes IST?

The cause of IST isn't known at present. There are several possibilities (and certainly it could result from one or a combination of these or from another mechanism not yet discovered).

Some scientists and/or cardiologists especially electrophysiologists (cardiologists specialising in the diagnosis and treatment of heart rhythm disorders) think that IST is caused by the sinus node itself having an abnormal structure or function, some believe it is part of a problem called dysautonomia
(a disturbance with the autonomic nervous system). For information on dysautonomia please check out these links :

Dysautonomia by Dr Rich Fogoros M.D.

National Dysautonomia Research Foundation

What needs to be excluded before IST is diagnosed?

Other abnormal heart rhythm disorders - various types of SVT - can sometimes be confused with sinus tachycardia. Abnormal SVT needs to be excluded, they can often be diagnosed and cured. There are lots of treatable medical disorders that can be confused with IST (but often aren't as IST is very much a 'last resort' consideration). If someone has a persistent, frequent, sinus tachycardia other causes need to be considered and investigated. These can include hyperthyroidism, a type of benign tumor (called a phaeochromocytoma) that produces substances that behave like adrenaline in the body, diabetic autonomic dysfunction, and prescibed and non-prescription drugs.

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