1.14.2 Pressure Volume Loop Definition

Pressure Volume Loop Definition is a graphical representation of the relationship between ventricular pressure and ventricular volume as both change together throughout a single cardiac cycle, produced by continuously plotting instantaneous pressure on one axis against instantaneous volume on the other. The resulting closed, roughly quadrilateral trajectory captures the complete mechanical behavior of the ventricle, with each segment of the loop corresponding to one of the four major phases of the cardiac cycle.

Construction of the Loop

The pressure–volume loop is built from continuous, simultaneous measurement of ventricular pressure and volume.

Axes of the Loop

Ventricular volume is conventionally plotted along the horizontal axis, while ventricular pressure is plotted along the vertical axis, allowing the changing relationship between the two variables to be traced as a single continuous curve over time.

Tracing a Complete Cycle

As the ventricle proceeds through filling, isovolumetric contraction, ejection, and isovolumetric relaxation, the plotted point moves around a closed path, returning to its starting position at the completion of each cardiac cycle.

The Four Segments of the Loop

Each side of the pressure–volume loop corresponds to a distinct mechanical phase of the cardiac cycle.

Filling Segment

The filling segment traces the progressive rise in ventricular volume during diastole, with a relatively shallow rise in pressure reflecting normal ventricular compliance, extending from the end-systolic point to the end-diastolic point.

Isovolumetric Contraction Segment

The isovolumetric contraction segment appears as a nearly vertical line at the end-diastolic volume, reflecting the rapid rise in pressure that occurs while all valves remain closed and volume does not change.

Ejection Segment

The ejection segment traces the fall in ventricular volume as blood is expelled, extending from the end-diastolic point to the end-systolic point while pressure first rises and then falls.

Isovolumetric Relaxation Segment

The isovolumetric relaxation segment appears as a nearly vertical line at the end-systolic volume, reflecting the rapid fall in pressure that occurs while all valves remain closed and volume does not change.

Physiological Information Contained in the Loop

The pressure–volume loop encodes several important measures of cardiac function within its geometry.

Stroke Volume

The horizontal width of the loop, the difference between end-diastolic and end-systolic volume, directly represents stroke volume.

Stroke Work

The area enclosed by the loop represents the mechanical work performed by the ventricle during one cardiac cycle, since it integrates pressure across the changing volume during ejection.

Diagrammatic Summary

Clinical Relevance

The pressure–volume loop provides a unified analytical framework for evaluating ventricular performance under varying conditions of preload, afterload, and contractility, and shifts in the size, shape, or position of the loop are used to characterize the mechanical alterations underlying conditions such as systolic heart failure, diastolic dysfunction, and pressure- or volume-overload states.