Pressure-volume loop
Also known as: pv loop, left ventricular pressure-volume loop
A pressure-volume loop is a graph of left ventricular pressure against left ventricular volume across one cardiac cycle. Its width represents stroke volume, and its shape shifts predictably with changes in preload, afterload, and contractility.
A pressure-volume (PV) loop plots left ventricular pressure (y-axis) against left ventricular volume (x-axis) through a single heartbeat. Traced counterclockwise, the loop captures the four phases of the cardiac cycle: diastolic filling along the bottom, isovolumetric contraction up the right side, ejection along the top, and isovolumetric relaxation down the left side.
Key landmarks read directly off the graph. The bottom-right corner is end-diastolic volume (EDV), reached when the mitral valve closes; the top-left region ends at end-systolic volume (ESV), when the aortic valve closes. The loop's width, EDV − ESV, is the stroke volume, and the area enclosed by the loop approximates stroke work — the work the ventricle performs per beat. The vertical segments are "isovolumetric" because both valves are closed and volume cannot change while pressure rises or falls.
PV loops are powerful because hemodynamic changes reshape them predictably. Increased preload widens the loop to the right (larger EDV, larger stroke volume). Increased afterload forces the ventricle to generate higher pressure before ejection, narrowing the loop and raising ESV. Increased contractility steepens the end-systolic pressure-volume relationship, ejecting more blood and shrinking ESV.
The USMLE Step 1 tests PV loops as a core cardiovascular physiology topic: identifying the phases and valve events at each corner, and predicting how preload, afterload, contractility, and valvular lesions distort the loop. Analogous pressure-volume reasoning also appears in respiratory physiology, where compliance curves describe the mechanics of breathing.
Key takeaways
- A PV loop graphs left ventricular pressure versus volume over one cardiac cycle, traced counterclockwise.
- Loop width equals stroke volume (EDV − ESV); loop area approximates stroke work.
- Valve events mark the corners: mitral closure at EDV, aortic opening, aortic closure at ESV, and mitral opening.
- Preload widens the loop, afterload raises its peak and ESV, and contractility shifts the end-systolic point leftward.
- USMLE Step 1 tests reading loop landmarks and predicting shape changes from hemodynamic alterations.