A test performed to record and measure the electrical impulses running through the heart during a heartbeat. Catheters containing electrodes are introduced into the heart through veins or arteries to “map” the heart signals. The test can also help a physician evaluate abnormal heart rhythms, determine their origin, and assess how well medicines are working to control them. The test also may be used to check how an implantable cardioverter defibrillator (ICD) functions during an abnormal heart rhythm.