No, not exactly. The 12 lead EKG preforms a tracing of the heart's electrical activity, from different "angles". The heart has two electrical nodes, known as the SA (pacemaker node) and AV node. These are groups of specialized cells that produce the characteristic EKG pattern. The SA node sits up in the atria, and fires at a rate of 60-100 impulses per minute. These impulses cause a contraction of the atria, and signal the AV node to fire, with a small delay to allow the blood from the atria to fill the ventricles. The AV node conducts a impulse down into the ventricles through certain fibers of conducting tissue.
The coronary arteries are not directly related to this process, and slight incomplete blockages will not show up. However, if a blockage becomes severe enough to cause ischemia, the cells beyond the blockage that are starving for oxygen will have a higher than normal resting voltage, creating a pattern known as ST elevation that will show up on the EKG. Old heart attacks may also be evidenced on the EKG due to permeant damage to the myocardium.
