What is inappropriate sinus tachycardia (IST)?

Inappropriate sinus tachycardia (IST) is a condition where the heart beats very rapidly for no known reason. This type of an abnormal heart rhythm is called an arrhythmia. Whereas, a fast heart rate is called tachycardia.

A resting heart rate of greater than 100 beats per minute in adults is considered tachycardia. Infants and children normally have faster heart rates than adults.

People with IST commonly have a resting heart rate greater than 100 beats per minute or at rest, their average heart rate is greater than 90 beats per minute. Other people with IST experience a normal heart rate when resting, but find it shoots up very quickly during light physical activity.

The heart's electrical signal typically starts in the sinoatrial (SA) node, which is found in the heart's right upper chamber. This area triggers an electrical signal that conducts throughout the heart and creates the heartbeat. The SA node receives signals from nerves – that can cause the heart to beat more rapidly or slowly, depending on the body's needs.

Several situations may normally cause the heart to beat faster - such as an elevated body temperature (fever), moderate or intense exercise, anxiety or increased stress. But in IST, the heart beats faster for unknown reasons. The heart rate may increase on its own without any stimulation. Other times, the heart rate rises because of a trigger. But it increases more than it should. If the heart beats too rapidly, it may not be able to fill properly between beats. Slightly less blood may then circulate out to the body.

Although a rare condition, IST is more common in young women than in men. IST is also a multifaceted health disorder. Patients should seek a cardiologist who is trained in the use of complex cardiac medications and electrophysiologic treatments.

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