CRT devices have at least two leads, one passing through the "vena cava" and the right atrium into the right ventricle to stimulate the septum, and another passing through the "vena cava" and the right atrium and inserted through the coronary sinus to pace the epicardial wall of the left ventricle. Often, for patients in normal sinus rhythm, there is also a lead in the right atrium to facilitate synchrony with the atrial contraction. Thus, timing between the atrial and ventricular contractions, as well as between the septal and lateral walls of the left ventricle can be adjusted to achieve optimal cardiac function.