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.
Cardiac minute work has two separate components: pressure and volume work.
Pressure work, is the amount of work the left ventricle must produce in order to push the
blood past any downstream resistance, which can be physiological, like aortic pressure,
or pathological, in conditions like aortic stenosis, when the aortic valve has a narrower
opening, so the left ventricle must exert a much greater force to push the blood into
the aorta.
Volume work is the amount of blood that the left ventricle moves per time and can be thought
to be equivalent to cardiac output which is defined as stroke volume times heart rate.
For example, in strenuous exercise, the heart rate increases and therefore increases the
cardiac output or volume work.
Additionally cardiac minute work is directly correlated with myocardial oxygen consumption.
Interestingly, pressure work takes up much more myocardial oxygen consumption than volume
work which is why overall myocardial oxygen consumption does not correlate well with cardiac
output.
Looking at the aortic stenosis example again, the increase in pressure work greatly increases
myocardial oxygen consumption, but less blood actually makes it past the narrowed aortic
valve opening, so volume work decreases.
It’s also interesting to look at the difference between the two ventricles with regard to
the pressure work they normally perform.
So, even though both ventricles pump the same amount of blood, the left ventricle has to
overcome the mean aortic pressure, of about 100 mmHg, while the right ventricle pressure
needs to surpass pulmonary artery pressure, which is only 15 mm Hg.
So although the volume work of both ventricles is the same, the pressure work of the left
ventricle is greatly increased.
So normally, the left ventricular wall is thicker than the right ventricular wall, because
it performs more pressure work and therefore requires more oxygen.
In pathological conditions like systemic hypertension, where the systemic vascular resistance is
increased, aortic pressure is higher than normal, so the left ventricle must perform
even more pressure work.
Over time, the left ventricular wall thickens even more in order to compensate - which is
called left ventricular hypertrophy.
Similarly, in conditions like pulmonary hypertension, pulmonary pressure is higher than normal,
and that causes right ventricular hypertrophy.
This increases myocardial oxygen consumption and keeps things balanced for a while.
Unfortunately, though, if left untreated, ventricular hypertrophy can eventually progress
to ventricular failure, which is when the ventricles can’t pump enough blood to meet
the body’s needs.
All right, as a quick recap, stroke work is defined as stroke volume times mean aortic
pressure and cardiac minute work is stroke work times heart rate.
Stroke work is best represented by a pressure-volume loop, and stroke work during one heartbeat
is proportional to the area inside the loop.
Cardiac minute work is directly correlated to myocardial oxygen consumption and can be
divided into pressure work and volume work.
Pressure work uses much more oxygen than volume work.
In some conditions that increase ventricular pressure work, like systemic or pulmonary
hypertension, the ventricles adapt by thickening their walls.