Cardiac pacemaker discharge at about 40-60 beats per minute, and are called the "secondary pacemaker". Further down the electrical conducting system of the heart is the Bundle of His. The left and right branches of this bundle, and the Purkinje fibres, will also produce a spontaneous action potential at a rate of 30-40 beats per minute, so if the SA and AV node both fail to function, these cells can become pacemakers. It is important to realize that these cells will be initiating action potentials and contraction at a much lower rate than the primary or secondary pacemaker cells. The SA node
Isovolumetric contraction Isovolumetric contraction In cardiac physiology, isovolumetric contraction is an event occurring in early systole during which the ventricles contract with no corresponding volume change (isovolumetrically). This short-lasting portion of the cardiac cycle takes place while all heart valves are closed. In a healthy young adult, blood enters the atria and flows to the ventricles via the opened atrioventricular valves (tricuspid and mitral valves). Atrial contraction rapidly follows, actively pumping about 20% of the returning blood. As diastole ends, the ventricles begin depolarizing and, while ventricular pressure starts to rise owing to contraction, the atrioventricular valves close in order to prevent