A procedure which studies the heart's electrical system. During this procedure one or more catheters are placed into the large blood vessels in either the arm or the leg or sometimes even both. The tips of the catheters are then moved into the heart. Once they're in the heart, they record electrical signals from the normal electrical system which gives a much more precise result than an ECG. Sometimes in this procedure the heart may be stimulated to beat irregularly. By triggering this, the catheter can then record the heart's response and the way the electricity is moved around in the heart during a tachycardia. This helps the cardiologist in understanding the nature of the arrhythmmia. This procedure is usually undergone for people who have irregular heartbeat and tachycardia (ie. supraventricular tachycardia).