Pressure volume diagram

Thermodynamics

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Generalized PV diagram

A pressure volume diagram (or P-V diagram, or volume-pressure loop)) is used to describe a thermal cycle involving the following two variables:

• Volume (on the X axis)
• Pressure (on the Y axis)

This is in fact enough information to fully describe a simple system from a thermodynamic standpoint. The diagrams are useful when one wants to calculate the work done by the system, the integral of the pressure with respect to volume. One can often quickly calculate this using the PV diagram as it is simply the area enclosed by the cycle.

History

The pressure volume diagram, originally called an 'indicator diagram', was invented by John Southern in 1796.

In the heart

In cardiovascular physiology, the diagram is often applied to the left ventricle, and it can be mapped to specific events of the cardiac cycle.

PV loop studies are widely used in basic research and preclinical testing, to characterize the intact heart's performance under various situations (effect of drugs, disease, characterization of mouse strains)

Screenshot from software for analyzing PV loops

The sequence of events occurring in every heart cycle is as follows. The figure shows a PV loop from a real experiment; letters refer to points.

• A is the end-diastolic point; this is the point where contraction begins. Pressure starts to increase, becomes rapidly higher than the atrial pressure, and the mitral valve closes. Since pressure is also lower than the aortic pressure, the aortic valve is closed as well.
• Segment AB is the contraction phase. Since both the mitral and aortic valves are closed, volume is constant. For this reason, this phase is called isovolumic contraction.
• At point B, pressure becomes higher than the aortic pressure and the aortic valve opens, initiating ejection.
• BC is the ejection phase, volume decreases. At the end of this phase, pressure lowers again and falls below aortic pressure. The aortic valve closes.
• Point C is the end-systolic point.
• Segment CD is the isovolumic relaxation. During this phase, pressure continues to fall. The mitral valve and aortic valve are both closed again so volume is constant.
• At point D pressure falls below the atrial pressure and the mitral valve opens, initiating ventricular filling.
• DA is the diastolic filling period. Blood flows from the left atrium to the left ventricle. Atrial contraction completes ventricular filling.

As it can be seen, the PV loop forms a roughly rectangular shape and each loop is formed in an anti-clockwise direction.

Very useful information can be derived by examination and analysis of individual loops or series of loops, for example:

• the horizontal distance between the top-left corner and the bottom-right corner of each loop is the stroke volume
• the line joining the top-left corner of several loops is the contractile or inotropic state.

See external links for a much more precise representation.

See also

• Wiggers diagram
• stroke volume
• cyclic process
• PV loop experiments
• Pressure-Volume Loop Analysis in Cardiology

External links

• Diagram at hofstra.edu
• Diagram at cvphysiology.com
• Interactive demonstration at davidson.edu
• Lohff B (1999). "Otto Frank". Sudhoffs Arch. 83 (2): 131–51. PMID 10705804. 

it:Piano di Clapeyron