Cardiac Work & The Fick Principle

Here we will learn about cardiac work (aka, stroke work), myocardial oxygen consumption, and, the Fick Principle. Cardiac work, aka, stroke work: the work that the heart performs to eject blood; thus, it is a measurement of ventricular power. Stroke work is equivalent to the area under the curve in the pressure-volume loop. Be careful not to confuse stroke work (stroke power) with stroke volume (the amount of blood). Cardiac minute work: the amount of cardiac work done in a minute (work per unit time). Cardiac output: the volume of blood ejected per minute. We'll use the Fick Principle to calculate this. As we'll see, cardiac output represents the "volume work" in our equation for cardiac minute work. Let's begin our diagram with the stroke work of the left ventricle, which is responsible for pumping blood into the systemic circulation. In physics, "work" equals force multiplied by distance. Thus, left ventricular stroke work is equal to: The Aortic Pressure that the left ventricle must overcome to move blood into the systemic circulation, multiplied by The Stroke Volume, which is the volume of blood that the left ventricle ejects in a single contraction. Aortic pressure * Stroke Volume According to this equation: Aortic pressure is the force, Stroke volume is the distance Let's quickly illustrate this concept: Draw a heart and aorta in coronal section, and label the left atrium and ventricle. Show that left ventricular stroke work is the effort required to move a given amount of blood, the stroke volume, against the force of aortic pressure. We can see from this that, if aortic pressure rises, stroke work must increase to maintain stroke volume. Next, we want to calculate cardiac minute work, which is cardiac work per unit time. We use heart rate for the time variable. Cardiac minute work = Left ventricular Stroke Work * Heart Rate/ Then, remind ourselves that left ventricular stroke = Aortic Pressure * Stroke Volume. We can re-write the equation to show that: Cardiac minute work = Aortic Pressure Stroke Volume Heart Rate. Now, notice something familiar in this equation: Stroke volume * Heart rate = Cardiac output. Re-write the equation again to show that Cardiac Minute Work = Cardiac Output * Aortic Pressure. With this re-arrangement, show that cardiac minute work is the product of both volume work and pressure work. Now, we can predict that increased cardiac output and/or aortic pressure causes increased cardiac work. Later in this tutorial, we'll learn how to calculate cardiac output. Next, let's learn about myocardial oxygen consumption; it directly correlates with cardiac minute work. As we just showed, cardiac minute work comprises pressure work and volume work. Of these, pressure work is the primary driver, since it is more metabolically costly than volume work. Stated another way, cardiac minute work correlates strongly with aortic pressure, and less so with cardiac output. Other important variables include contractility and heart rate. Let's look at some clinical examples to see this principle in action. Aortic stenosis, in which the narrowed aortic valves increase the resistance for blood passing through the aorta – this increases pressure work. As a result of increased pressure work, myocardial oxygen consumption also increases. Furthermore, to overcome the increased aortic pressure, the myocardial tissue of the left ventricle hypertrophies. Although this compensatory growth is adaptive in the short term, it can lead to heart failure over time. Systemic hypertension, which also increases aortic pressure, has similar effects. On the right side of the heart, pulmonary hypertension increases the work of the right ventricle to pump blood to the lungs – this can lead to right ventricle hypertrophy. We can use the Fick principle to calculate cardiac output by measuring oxygen consumption. The Fick principle is based on the understanding that in a steady state, the rate of oxygen consumption must be equal to the difference in oxygen leaving and entering the lungs. We can arrange the equation to solve for cardiac output as follows: Total oxygen consumption divided by (O2 content of the pulmonary vein) - (O2 content of the pulmonary artery). Let's do an example problem to solve for cardiac output: A patient's total body oxygen consumption is 250 ml of oxygen per minute; The oxygen content of the pulmonary vein is 0.20 ml oxygen per ml blood; The oxygen content of the pulmonary artery is 0.15 oxygen per ml blood. This gives us a cardiac output of 5000 ml, or 5 liters, per minute, which is within the normal cardiac output range for adults.