A recent Quick Quiz on our Facebook page
resulted in a mix of responses. Do you know what word is used to describe the amount of stretch on the myocardium at the end of diastole? The responses were split between preload and afterload.
Let’s take a closer look at what these terms mean.
Preload, also known as the left ventricular end-diastolic pressure (LVEDP), is the amount of ventricular stretch at the end of diastole. Think of it as the heart loading up for the next big squeeze of the ventricles during systole. Some people remember this by using an analogy of a balloon – blow air into the balloon and it stretches; the more air you blow in, the greater the stretch.
Afterload, also known as the systemic vascular resistance (SVR), is the amount of resistance the heart must overcome to open the aortic valve and push the blood volume out into the systemic circulation. If you think about the balloon analogy, afterload is represented by the knot at the end of the balloon. To get the air out, the balloon must work against that knot.
Cardiac Output & Cardiac Index
Cardiac output is the volume of blood the heart pumps per minute. Cardiac output is calculated by multiplying the stroke volume by the heart rate; normal cardiac output is about 4 to 8 L/min, but varies depending on the body’s metabolic needs. Cardiac index is a calculation of the cardiac output divided by the person’s body surface area (BSA).
So, if you answered ‘D’ to the quiz above, you’re right!