Cardiac Pressure-Volume Relationship
- The Cardiac Pressure-Volume Relationship is a graphical comparison of ventricular pressure and ventricular volume that occurs within a single heart beat. It should be noted that Ventricular Pressure and Volume are also presented in the cardiac cycle where they are plotted with respect to time. In the Cardiac Pressure-Volume Relationship, ventricular pressure and volume are plotted against one another, generating a loop-shaped curve. Below we discuss the major segments of this loop-shaped curve and discuss the processes which occur during each stage.
- Ventricular Filling
- This phase represents passive filling of the ventricle with blood and is characterized by an increase in the ventricular volume from the Ventricular End Systolic Volume (VESV) to the Ventricular End Diastolic Volume (VEDV). Although a large amount of atrial blood volume is injected into the ventricles during this phase through the atrioventricular valves, the intraventricular pressure rises only slightly, representing the relatively large compliance of the relaxed ventricle. As drawn here, this phase also includes the "Atrial Ejection" phase of the cardiac cycle.
- Isovolumetric Ventricular Contraction
- During this phase the ventricle's volume remains the same as no blood enters the chamber from the atria and no blood is ejected into the aorta. However, as active tension is generated within the ventricle, the intraventricular pressure sharply rises. This segment ends when the intraventricular pressures equals that of the aortic pressure and the semilunar valves open.
- Ventricular Ejection
- Ventricular Ejection occurs when the intraventricular pressure rises above that of the aorta, causing opening of the aortic valve and ejection of blood. The intraventricular pressure continues to rise, reaches a peak, and then falls, correlating with the amount of active tension within the myocardium. During this period, the ventricular volume reduces from the VEDV back to that of the VESV. The difference between these two volumes is also known as the stroke volume which represents the amount of blood volume the ventricles eject in one beat.
- Isovolumetric Ventricular Relaxation
- During this period of ventricular relaxation, all valves remain closed as the intraventricular pressure remains above that of the atria. Consequently, the pressure within the ventricle falls while the ventricular volume remains constant. The end of this phase returns the heart to the beginning of the ventricular filling phase which is initiated by the opening of the AV valves.