To diagnose an arrhythmia or find its cause, doctors use tests including:

• EKG. An electrocardiogram records the electrical activity of your heart. You wear small electrode patches on your chest, arms, and legs for the quick, painless test, which you take in your doctor's office.

• Holter monitor. This is a portable EKG (also called an "ambulatory electrocardiogram" or an ECG) about the size of a postcard or digital camera that you'll use for 1 to 2 days or up to 2 weeks. The test measures the movement of electrical signals or waves through your heart. These signals tell your heart to contract (squeeze) and pump blood. You'll have electrodes taped to your skin. It's painless, although some people have mild skin irritation from the tape used to attach the electrodes to the chest. You can do everything but shower or bathe while wearing the electrodes. After the test period, you'll go back to see your doctor. They'll download the information.

• Event monitor. If your symptoms don't happen often, your doctor may suggest that you wear one of these, usually for about a month. When you push a button, it records and stores your heart's electrical activity for a few minutes. Try to get a reading when you notice symptoms. Your doctor will interpret the results.

• Implantable loop recorder. Your doctor puts this under your skin, where it constantly records your heart’s electrical activity. It can send information to your doctor’s office.

• Stress test. There are different kinds of stress tests. The goal is to check how much stress your heart can manage before having a rhythm problem or not getting enough blood. For the most common type of stress test, you'll walk on a treadmill or pedal a stationary bike while you get an EKG and have your heart rate and blood pressure monitored. Technicians slowly raise the intensity level of your exercise.

• Echocardiogram. This test uses ultrasound to check your heart muscle and valves.

• Cardiac catheterization. Your doctor will insert a long, thin tube, called a catheter, into a blood vessel in your arm or leg. They’ll guide it to your heart with help from a special X-ray machine. Then, they’ll inject dye through the catheter to help make X-ray videos of your heart valves, coronary arteries, and chambers.

• Electrophysiology study. This test records your heart's electrical activities and pathways. It can help find out what's causing heart rhythm problems and find the best treatment for you. During the test, your doctor will safely trigger your unusual heart rhythm. Then, they may give you medications to see which one controls it best or to see what procedure or device you need to treat it.

• Head-up tilt table test. Doctors use this test to find out what's causing fainting spells. It measures the difference in heart rate and blood pressure when you're standing up and lying down. You'll get this test in a lab. You'll lie on a stretcher, tilted at different angles, while you get an EKG and specialists check your blood pressure and oxygen level. This shows if symptoms of passing out are due to your electrical system, nervous system, or vascular system.