isovolumetric contraction

Isovolumetric (isovolumic) contraction begins immediately after closure of the mitral valve (marking S1) when left ventricular pressure exceeds left atrial pressure. At this point, the aortic valve is still closed because ventricular pressure has not yet exceeded aortic pressure. Since both the mitral and aortic valves are closed, no blood enters or leaves the ventricle, so ventricular volume stays constant (isovolumetric) while pressure rises rapidly due to isovolumetric myocardial contraction. This phase ends when LV pressure exceeds aortic pressure, causing the aortic valve to open and ejection to begin. On the Wiggers diagram, this corresponds to the steep rise in ventricular pressure between the QRS complex and the upstroke of the aortic pressure tracing. On the pressure-volume loop, it is represented by the vertical line connecting the point of mitral valve closure to aortic valve opening (no change in volume, rapid rise in pressure). Clinically, this phase is important in understanding parameters like dP/dt (rate of pressure development), which is used as an index of contractility. Conditions affecting contractility (e.g., positive inotropes like catecholamines increase dP/dt) or valvular disease (e.g., aortic stenosis increases the pressure needed to open the aortic valve, prolonging this phase) can alter isovolumetric contraction. The equivalent phase in the right heart is analogous but at lower pressures. Isovolumetric relaxation is the corresponding phase in diastole after aortic valve closure and before mitral valve opening.