Inappropriate sinus tachycardia is a condition in which the patient has a normal heart but an unusually fast heart rate (tachycardia), occurring both at rest and in response to physical activity. The condition is generally considered a harmless type of abnormal heart rhythm (arrhythmia) that can continue for months or years. A normal heart rate is generally considered to be between 60 and 100 beats per minute, with infants and children having faster normal heart rates than older adults.

The cause of inappropriate sinus rhythm remains unclear. Some physicians recommend performing a test called a tilt table test to help distinguish inappropriate sinus tachycardia from a rare syndrome known as postural orthostatic tachycardia syndrome (POTS).

In some cases, the heart rate can become persistently and severely elevated, potentially damaging the pumping ability of the heart. This is quite rare, however.