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Cardiac Preload: Definition & Reduction

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  • 0:00 What is Cardiac Preload?
  • 0:25 The Heart & Cardiac Preload
  • 1:46 Alteration in Preload Volumes
  • 3:16 Lesson Summary
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Lesson Transcript
Instructor: Nadine James

Nadine has taught nursing for 12 years and has a PhD in Nursing research

In this lesson, you'll be introduced to cardiac preload. We'll examine what happens when cardiac preload is increased or decreased and how various cardiac preload conditions can affect our health.

What Is Cardiac Preload?

Cardiac preload is the pressure of the blood on the muscle fibers in the ventricles of the heart at the end of diastole. It is actually measured as an estimate, representing the amount of blood volume in the left ventricle of the heart at that particular point in the cardiac cycle, just before the heart contracts and when it is filled with the most blood, which is the end of diastole.

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.

Diagram of the Heart

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.

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