The study looks at the health of the sinus node, the "natural pacemaker" in the heart that keeps the pulse even. It also measures the time it takes for a normal electrical impulse to travel from the sinus node to the atrio-ventricular node (the node that links the upper chambers to the lower chambers), and then thru the His-Purkinje system (a very fast electrical network that synchronizes the contraction of the lower chambers). It also looks to see if you have potential short circuits in the electrical system that can be the cause of fast, uncomfortable, sometimes unstable heart rhythms.
The EP study is extremely low risk, but still is an invasive procedure performed in the electrophysiology lab in the hospital. This test is not recommended for everyone who has an abnormal heart rhythm. There are other non-invasive tests that can be performed before going on to an EP study, and a consultation with a cardiologist or a cardiac electrophysiologist is important to figure out how best to evaluate an abnormal heart rhythm.