Mechanical Properties of the Heart
Adapted from Dr. Marc Dickstein's Lecture Notes

What can we tell about ventricular performance from the P-V Loop?

Almost everything.

1. Contractility. Consider that for a given volume, the greater the elastance, the more pressure will be generated. In other words, a stronger ventricle will achive greater stiffness (i.e. greater maximal elastance) than a weak ventricle. The slope of a line through the end-systolic pressure-volume point (the left upper corner of the P-V loop) is termed End-Systolic Elastance (Ees) and is a measure of ventricular contractility.



2. Diastolic Compliance. Look at the filling portion of the loop. A healthy ventricle is very compliant during the filling phase, such that there is not a large increase in pressure as it is being filled. Scar tissue, impaired active relaxation, myocardial hypertrophy all cause an upward and leftward shift in this compliance curve. This means that for a given end-diastolic volume, venous pressures would be elevated. Or, for a given venous pressure, there would be less filling.
3. Preload. The stretch on the sacromeres just prior to contraction is proportional to end-diastolic volume. End-diastolic Pressure reflects the pressure that is required of the venous system to achieve that particular preload volume. The end-diastolic pressure-volume point is the bottom right corner of the P-V loop.
4. Afterload. This is the most confusing concept. At the least, know that afterload is represented on the P-V diagram by the slope of a line that starts on the volume axis at the end-diastolic volume, and goes through the end-systolic pressure-volume point. This line is called the Arterial Elastance line (Ea). As afterload is increased, this line becomes steeper, and the Ea line would intersect with the Ees line at a higher point (meaning that ESV would be higher and SV would be less). Details for the brave are available in the Appendix.
5. Stroke Volume. The difference between end-diastolic volume (EDV) and end-systolic volume (ESV). Obvious as the width of the P-V loop.
6. Stroke Work. The area of the P-V loop is stroke work. This is a useful measure of the amount of energy the heart imparts to the blood. Consider that when afterload is high, more of the energy goes into pressure development, and less to flow. Conversely, when afterload is low, more of the energy goes into flow generation, and less to pressure development. However, the product of pressure and flow, or the Stroke work, stays relatively constant.
7. Ejection Fraction. This is a common index of contractility used clinically. It is defined as SV/EDV. You can gestalt it from the width of the P-V loop relative to the width of the y-axis to isovolumic contraction line. In the diagram, it's more than 50%.
8. Arterial Systolic and Diastolic Blood Pressures. Arterial systolic blood pressure (SBP) is the peak pressure in the arteries. This is the same as peak pressure as in the ventricle and on the P-V loop. The arterial diastolic pressure (DBP) is the pressure at which the aortic valve opens during systole. This can be identified as the right upper corner of the P-V loop.