. Cardiac outputCardiac output depends on the interrelated factors of heart rate (HR) and stroke volume (SV)--that is, CO=HR*SV. Stroke volume, is the volume of blood ejected in a single ventricular contraction. This, in turn, depends on three things--contractility, preload, and afterload.Contractility is the amount of mechanical work done for a given preload and afterload. It is increased by sympathetic stimulation and inotropes. It is reduced by parasympathetic stimulation, negative inotropeshttp://www.studentbmj.com/issues/04/05/education/190.php.