Cardiac work

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.

During the ejection phase, blood from the left ventricle goes into the aorta, decreasing left ventricular volume. The left ventricle continues to contract, so ventricular pressure rises further, but then falls slightly.

Finally, when aortic pressure exceeds left ventricular pressure, the aortic valve closes, marking the end of systole, or the end-systolic point.

At this point, left ventricular pressure is called end-systolic pressure, and left ventricular volume is called end-systolic volume. And the difference between end-diastolic volume and end-systolic volume is the stroke volume.