Pressure volume diagram

  A pressure volume diagram (or P-V diagram, or volume-pressure loop)^[1]) is used to describe a thermal cycle involving the following two variables:

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

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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.

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. The flow of events is as follows, and in the opposite direction to the diagram above (counterclockwise). See external links for a much more precise representation:

The two vertical lines are usually much more straight than the two horizontal lines. The distance between the two vertical lines is the stroke volume.^[2]

A line tangent to the end of the ejection phase (the upper left hand corner) can be used to estimate inotropic state.^[3]. (It is easier to identify this line when multiple consecutive cycles are plotted on top of each other.)

See also

• Wiggers diagram
• stroke volume
• cyclic process

References

1. ^ Physiology at MCG 3/3ch5/s3ch5_16
2. ^ Diagram at uc.edu
3. ^ http://www.cvphysiology.com/Heart%20Failure/HF005.htm