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.

After that, ventricular diastole begins, so the left ventricular muscle relaxes, making pressure fall. However, both the mitral and aortic valves are closed, so the volume remains constant.

This phase is isovolumetric relaxation, and the atria fill with blood during this time. Eventually, the pressure in the left atrium exceeds that of the left ventricle, so the mitral valve opens, and blood flows into the left ventricle.

As the left ventricle fills with blood, left ventricular volume rises back to its end-diastolic volume, and the pressure increases only slightly. This relaxation phase continues until the mitral valve closes, letting the loop start all over again.

All this happens during one heartbeat. And stroke work during one heartbeat is proportional to the area inside the loop. In other words, the bigger the loop and the more the area inside of it, the more stroke work our heart does.

In a related concept, cardiac minute work is defined as work per time, or how much work the heart muscle does over one minute.

In terms of cardiac function, cardiac minute work equals mean aortic pressure times cardiac output where cardiac output is defined as heart rate or beats per minute times the stroke volume.

And since mean aortic pressure can be calculated as stroke work divided by stroke volume, we can replace that in the equation, and see that cardiac minute work is equal to stroke work times heart rate.