Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
An electrophysiologic study (EPS) is one of a number of tests of the electrical conduction system of the heart performed by a cardiac electrophysiologist, a specialist in the electrical conduction system of the heart. These tests are performed using catheters situated within the heart.
There are a number of different tests that fall under the heading of an electrophysiologic study (EP study). If the heart is electrically stimulated via the intracardiac catheter it is known as programmed electrical stimulation (PES). PES is used to help diagnose and risk stratify individuals with a number of rhythm disturbances of the heart. Other studies that do not require extrinsic stimulation of the heart include conduction studies to determine if the individual's heart block is at risk of progressing to complete heart block.
EP studies also allow the ablation of ectopic electrical foci causing supraventricular arrythmias
In order to perform an EP study, one or more catheters must be placed within the heart. The catheters are typically placed in the right atrium and/or right ventricle via one or more incisions made in the central venous system. The incisions are typically made in either the internal jugular vein, subclavian vein, or common femoral vein.
Catheters placed via the venous system enter the heart via the right atrium. The advantage of this is that the right atrium is where the SA node is located. The right atrium is also the location of most of the common re-entrant pathways associated with atrial flutter. If a catheter is placed with the distal tip in the right ventricle, it may be possible to measure conduction through the bundle of His, which is useful to determine the level of heart block. If the catheter is placed in the coronary sinus, it is possible to measure electrical activity in both the left atrium and the left ventricle without entering the high pressure arterial system associated with the left side of the heart.
Additional advantages of using these veins is that they are all part of the central venous system and do not have the valves that peripheral veins contain. The advantage of approaching the heart via the venous system as opposed to via the arterial system is that the risk of stroke is dramatically less if the arterial system is not used. Also, since the venous system is lower pressure, complications such as hematomas are typically easier to manage.