pressure–volume loop
A pressure–volume loop is a single picture that captures one heartbeat as a closed loop on a graph, with the ventricle's pressure on the vertical axis and its blood volume on the horizontal. As the cycle runs, the point travels around the loop and returns to where it started.
Reading it counterclockwise, the loop has four sides: filling along the bottom, the steep rise of isovolumetric contraction on the right, ejection across the top as volume drops, and the fall of isovolumetric relaxation on the left. The width of the loop equals the stroke volume.
The loop is powerful because it shows the effects of preload, afterload, and contractility in one view. Adding preload widens the loop to the right; raising afterload makes it taller and narrower; boosting contractility shifts the top-left corner leftward, ejecting more blood.
It is mostly a teaching and research tool rather than a routine bedside test, because measuring ventricular pressure requires a catheter inside the heart. Still, its logic underlies how clinicians reason about what is loading or weakening a given heart.
To illustrate aortic stenosis, a textbook draws a PV loop that is much taller than normal, showing the very high pressures the left ventricle must reach to eject past the narrowed valve.
A tall, narrow loop signals a ventricle ejecting against high afterload.
The area enclosed by the loop represents the stroke work done by the ventricle that beat. The loop's four corners correspond to the opening and closing of the inlet and outlet valves.