Cardiac work

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Cardiac work


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USMLE® Step 1 questions

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Cardiac work

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USMLE® Step 1 style questions USMLE

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A researcher is conducting a study on cardiac physiology. The test subject has a heart rate of H and an ejection fraction of F. The subject’s pressure-volume loop is shown below. Which of the following best approximates the cardiac work (also termed stroke work) performed by this patient’s heart?

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Cardiac work, also known as stroke work, is similar to the concept of work in physics. In physics, work is defined as force times distance.

Stroke work can be thought of as the work performed by the left ventricle to eject a volume of blood, defined as stroke volume multiplied by mean aortic pressure.

And here, stroke volume corresponds to distance, whereas mean aortic pressure corresponds to force. Stroke work is best represented by a pressure-volume loop.

Pressure- volume loops are graphs, where the pressure inside the left ventricle is on the y axis and the volume of the left ventricle is on the x axis.

Each loop represents changes in ventricular pressure and volume over the course of one cardiac cycle, or one heartbeat, which includes both ventricular systole, or contraction, and diastole, or relaxation.

The lower right hand corner is the end-diastolic point, and it’s the point in the cardiac cycle when diastole is over. Αt this point, the mitral valve between the left atrium and the left ventricle, closes, leaving the left ventricle filled with the maximum volume of blood, called the end-diastolic volume.

And then, systole begins, which is when the left ventricle contracts to push that blood into the aorta. Ventricular contraction makes the pressure shoot up, but for a brief period of time, both the mitral and aortic valves are closed, so left ventricular volume doesn’t change.

This phase is isovolumetric contraction, but it doesn’t last long, because eventually the pressure inside the left ventricle exceeds aortic pressure, making the aortic valve pop open, and that starts the ejection phase.


The cardiac cycle, also called the stroke work, is the work performed by the heart's left ventricle during an ejection of a blood volume. It is the performance of the heart from the beginning of one heartbeat to the beginning of the next, and equals to the product of the mean aortic pressure and stroke volume, which is the amount of blood pumped by the left ventricle in one beat. Cardiac work done in a full minute will be referred to as cardiac minute work. It is equal to the product of mean aortic pressure and cardiac output, since the cardiac output equals heartbeats in one minute times the stroke volume.


  1. "Medical Physiology" Elsevier (2016)
  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)

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