An exact diagnosis is important because the treatment you receive depends on the type of tachycardia you have. Supraventricular tachycardia can sometimes be diagnosed simply on the basis of a medical history and physical examination and a few simple tests. The physical exam may include a carotid sinus massage. Tests that may be done to monitor your heart and diagnose the type of fast heart rate that you have include:

• Electrocardiogram (EKG, ECG), which measures the electrical impulses in the heart. If an electrocardiogram is performed while the fast heart rate is occurring, it often provides the most useful information.
• Ambulatory electrocardiogram. A portable EKG can record your heart rhythm on a continuous basis, usually over a 24-hour period. If your symptoms are infrequent, your health professional may use another type of ambulatory electrocardiogram called a cardiac event monitor. When you have symptoms, you activate the monitor, which records your heart rhythm.
• Electrophysiology study. In this test, flexible wires are inserted into a vein, usually in the groin, and threaded into the heart. Electrodes at the end of the wires transmit information about the heart's electrical activity. This information is used to determine whether there is an extra electrical pathway inside the heart and, if so, where it is located. Catheter ablation can be done during this test to treat abnormal pathways and correct the supraventricular tachycardia.
• Medicine trial. Giving certain medicines while you are experiencing a fast heart rate, and monitoring what happens, may sometimes help your doctor determine what type of fast heart rate problem you have.

After finding tachycardia, your doctor may need to search for its cause. The specific tests needed depend on the particular tachycardia. These tests may include:

• Blood tests to check thyroid function and electrolyte levels.
• Chest X-ray and/or pulmonary function tests to check for lung disease.