The Heart & Cardiac Preload
First let's talk about the heart, one of the most important muscles in the body. It's made up of fibers that expand and contract in order to fill and empty the blood in the heart chambers. There are four chambers in the heart, two atria and two ventricles. The atria are pathways for the blood received by the body to go into the ventricles, and the ventricles contract, during which time they pump the blood out into the body.
Preload is one of the four heart functions that make up cardiac output, or the amount of blood ejected from the heart per minute.
The other functions constitute afterload, the resistance the ventricle needs to overcome to eject blood, contractility, the force of the contraction, and heart rate, also known as the pulse, or the number of beats per minute.
There are two types of cardiac preload: left ventricular preload and right ventricular preload. Measurement of left ventricular preload is recorded as pulmonary arterial wedge pressure (PAWP). Normal PAWP ranges between 6 and 12 millimeters of mercury (mmHg). Measurement of right ventricular preload is recorded as right arterial pressure (RAP). Normal levels of RAP range from 2 to 7 mmHg.
Alteration in Preload Volumes
At this point in the lesson, it is important to know that the heart pumps out all the blood it receives from the body. So how does cardiac preload fit into this discussion? Well, if the blood returned to the atria from the body has more volume than normal, the result is increased preload. This means that the heart pumps out to the body all the blood it receives, so the cardiac muscle is stretched to accommodate the blood. The stretched ventricular muscle contracts with greater force to pump out all the blood. One example of when someone may experience increased cardiac preload is during an illness involving pulmonary or lung congestion. Pulmonary congestion can be measured by PAWP. A PAWP value of greater than 20 mmHg shows moderate congestion in the lungs. At 30 mmHg or greater, one is diagnosed with acute pulmonary edema.
So, what happens when there is less volume of blood returned to the atria from the body? That is called decreased preload, and as expected, the ventricle has less blood to pump out to the body, so decreased contraction of the ventricle occurs. Conditions that cause reduced preload include those where there is less circulating blood volume. A few of those conditions include blood loss from any reason, like injury or surgery, decreased return of blood to the heart, pooling of the blood in the legs, or a blood clot.
Lesson Summary
Cardiac preload happens at the end of diastole before the ventricles contract. It represents the amount of pressure exerted on the ventricles by the volume of blood. Increased preload produces greater contraction of the ventricles to pump the increased volume of blood to the body. Decreased preload produces reduced contraction of the ventricles.
Normal cardiac preload in the left ventricle is measured by PAWP, or pulmonary arterial wedge pressure, ranging between 6 and 12 mmHg. A left ventricle preload is measured by a left arterial pressure, or PAWP, of 30 mmHg or greater. Right ventricle preload is measured by right arterial pressure, or RAP, and the normal levels range from 2 to 7 mmHg.
