<P> Cardiac rate control via pharmacology is common today; for example, the therapeutic use of digoxin, beta adrenoceptor antagonists, or calcium channel blockers are important historical interventions in this condition . Notably, individuals prone to hypercoagulability (abnormality of blood coagulation) are at decided risk of blood clotting, a very serious pathology requiring therapy for life with an anticoagulant if it cannot be corrected . </P> <P> The atrial chambers each contains one valve: the tricuspid valve in the right atrium opens into the right ventricle, and the mitral (or bicuspid) valve in the left atrium opens into the left ventricle . Both valves are pressed open during the late stages of ventricular diastole; see Wiggers diagram at the P / QRS phase (at right margin). Then the contractions of atrial systole cause the right ventricle to fill with oxygen - depleted blood through the tricuspid valve . When the right atrium is emptied--or prematurely closed--right atrial systole ends, and this stage signals the end of ventricular diastole and the beginning of ventricular systole (see Wiggers diagram). The time variable for the right systolic cycle is measured from (tricuspid) valve - open to valve - closed . </P> <P> The contractions of atrial systole fill the left ventricle with oxygen - enriched blood through the mitral valve; when the left atrium is emptied or closed, left atrial systole is ended and ventricular systole is about to begin . The time variable for the left systolic cycle is measured from (mitral) valve - open to valve - closed . </P> <P> Atrial fibrillation represents a common electrical malady in the heart that appears during the time interval of atrial systole (see figure at right margin). Theory suggests that an ectopic focus, usually situated within the pulmonary trunks, competes with the sinoatrial node for electrical control of the atrial chambers and thereby diminishes the performance of the atrial myocardium, or atrial heart muscle . The ordered, sinoatrial control of atrial electrical activity is disrupted, causing the loss of coordinated generation of pressure in the two atrial chambers . Atrial fibrillation represents an electrically - disordered but well perfused atrial mass working (in an uncoordinated fashion) with a (comparatively) electrically - healthy ventricular systole . </P>

The atrioventricular (av) valves are closed when the ventricles are in systole