Stroke Work and Cardiac Work
In physics, work is the product of force times distance. Therefore, considering a solid object of a given mass, the work done to move the object is the force applied to the object times the distance that the objects moves. In the case of the work done to move a volume of fluid, work is defined as the product of the volume of fluid and the pressure required to move the fluid.
Stroke work (SW) refers to the work done by the ventricle to eject a volume of blood (i.e., stroke volume) into the aorta. The force that is applied to the volume of blood is the intraventricular pressure. Therefore, ventricular stroke work can be estimated as the product of stroke volume and mean aortic pressure during ejection. The use of aortic pressure instead of intraventricular pressure assumes that kinetic energy is negligible, which is generally true at resting cardiac outputs. Sometimes this is simplified to stroke volume (SV) times mean aortic pressure (MAP), although this will further underestimate the stroke work.
SW = SV × MAP
Stroke work is sometimes used to assess ventricular function. If stroke work is plotted against ventricular preload, the resulting ventricular function curve will appear qualitatively similar to a Frank-Starling curve. Like the Frank-Starling relationship, there will be a family of curves depending upon the inotropic state of the ventricle.
Cardiac work (also called cardiac minute work or ventricular minute work) is the product of stroke work and heart rate, which is the equivalent of the triple product of stroke volume, aortic pressure and heart rate. Because this is the rate of doing work (work per unit time), it represents the power of the ventricle.