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Ep7: Basic Hemodynamics | Stroke Volume, Cardiac Output, Preload, Afterload, Contractility

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This is out first video podcast.  Hemodynamics is hard to describe without images so I thought it would be best to cover this with a video.  This podcast provides a basic introduction to hemodynamics for nurses and nursing students to help clarify topics like stroke volume, preload, afterload, contractility, and cardiac output.  I cover how these components all play together to create the same goal . . . perfuse the body!

This podcast does include a free handout on hemodynamics which you can get at: NRSNG.com/Hemodynamics

Hemodynamics is such a complex issue but as you begin to understand the basic concepts you will see that your overall ability to grasp difficult physiological issues will increase.   Really, by grasping how the various components play together you will begin to realize that learning this subject becomes much easier.

PODCAST TRANSCRIPT:

When we talk about hemodynamics and specifically when we talk about the parameters that are important to know for nurses, it can get very complex and very complicated.

 

So what we’re going to do today is we’re just going to cover some of the very basic introductory terms for hemodynamics and some of the parameters that you need to know and how they all play in together.

 

Now we are going to have a chart available that will help you be able to learn hemodynamics and how they all play in together. That chart is going to be available at Nrsng.com/hemodynamics and that chart – we’re going to kind of go over pieces of that chart today. That’s going to be available for free. You can go to Nrsng.com/hemodynamics and download that chart for free.

 

You can share it, print it, mark it up, do whatever you want with it. It’s going to be available in a PDF format so you can have it available in your notes and take to work or however you want.

 

  1. So what is hemodynamics? So hemodynamics essentially is – it’s blood flow. It’s the motion and equilibrium under the action of external forces.

 

So what we’re going to do today is we’re going to talk about hemodynamics and we’re going to understand that blood flow just a little bit better and the motion and what’s created on equilibrium. OK.

 

So when we discuss hemodynamics, there are a few key terms that we really need to know. The terms that we’re going to focus on today of course are going to be heart rate. We’re going to talk about cardiac output and cardiac index. We’re going to talk about stroke volume and we’re going to talk about preload, afterload, and contractility.

 

We’re going to talk about these several things and how they play in together and how they affect our hemodynamic equilibrium and how they all interplay together.

 

It’s pretty neat how they all work together and we’re going to discuss each of these as we go. So first of all, let’s – so it’s pretty amazing how all these kind of work together. Right now we will start with our heart rate.

 

So our normal heart rate is going to be 60 to 100 beats per minute. Our heart rate obviously is the rate at which our heart is actually beating. So it’s an important factor of hemodynamics because the way that which our heart beats is going to affect how much blood actually moves throughout the system.

 

So heart rate is going to directly affect cardiac output. So what is cardiac output? So, cardiac output is the volume of blood that leaves the left ventricle every minute. Volume from left ventricle every minute. The normal value for that is going to be four to eight liters per minute.

 

Now as people started assessing cardiac output more and more, they found that that volume that needs to leave the left ventricle every minute is going to vary from person to person. So, in order to understand better if someone was getting enough cardiac output or not, we also analyze cardiac index. What cardiac index does is it takes cardiac output and divides it by body surface area and what that does is that gives us a number based on how large the person is, if they’re getting enough cardiac output or not.

 

So cardiac index is cardiac output divided by body surface area. Our normal range for that is going to be 2.5 to 4 liters per minute per meter squared of body surface area. OK. So that’s going to really give us a better indication of cardiac output because it really bases it all off of the size of the individual.

 

So we have our cardiac output, which is going to be the four to eight liters per minute and that’s going to be just a generic number. We can use our cardiac index which is the 2.5 to 4 liters per minute per meter squared and that’s going to give us a better indication if this person is actually perfusing enough or not.

 

Now what is going to affect our cardiac output? The primary determinant of cardiac output is going to be our stroke volume. So what is stroke volume?

 

So, cardiac output is the volume that leaves the left ventricle per minute. Stroke volume on the other hand is the volume of blood that leaves the left ventricle every stroke. Volume from left ventricle per stroke or per heartbeat. Let’s say per beat.

 

So there’s the difference there. This is the volume per minute. This is our cardiac output. This is the volume per beat. So obviously our stroke volume is going to directly affect our cardiac output. The volume that leaves our left ventricle every beat is obviously going to affect the volume that leaves the heart every minute.

 

Stroke volume is measured in milliliters per beat and our normal value for that is going to be 60 to120 mills per beat. OK. So that’s kind of the easy part of this and now we kind of get on to the more complicated aspect of this.

 

But if we look back at the volume of blood that leaves our heart every beat, it’s going to directly affect the volume of blood that leaves our heart every minute. That’s going to directly affect our heart rate.

 

They would be affected by our heart rate. So as we work through this, we can adjust our heart rate, adjust our cardiac output. We can adjust our cardiac output with different things to adjust the heart rate and things like that.

 

So now we’re going to kind of move forward to something a little bit more complicated. We’re going to start up here with stroke volume.

 

  1. So we still have everything above what we just talked about, our cardiac output, cardiac index and heart rate. But there are three things that essentially affects stroke volume. Again, stroke volume is going to be the volume of blood that leaves the heart with each beat and our normal amount is going to be 60 to 120 milliliters per beat.

 

There are three things that affect our stroke volume. So we will draw the three things here. The first thing is going to be our contractility. So our contractility is the ability of the ventricles to squeeze those ventricles that exert with each beat. That can also be called inotropy and so when you hear about positive inotropes and things like that, what those drugs are doing is they’re directly affecting our contractility.

 

So stroke volume is affected by contractility. It’s also affected by preload and it’s affected by afterload. OK.

 

So what are preload and afterload? OK. We will just talk about this very briefly and very kind of superficially. So preload essentially is your end-diastolic volume. So you have your heart here. You have your atria on top, ventricles on the bottom.

 

So preload is going to be the amount of volume essentially within the ventricles at the end of diastole. So after the ventricles have filled, there’s going to be a volume in there and that volume is going to exert a pressure against those ventricles. That volume and that pressure, that is your preload.

 

So as these are stretched at the end of diastole, that’s going to be your preload. How much volume is in those ventricles at the end of diastole? So then what is afterload?

 

So we understand preload. Afterload is going to be the amount of pressure that these ventricles are going to have to exert in order to get blood out of the ventricles. So, here’s our left ventricle with full of blood. Now in order to actually open this aortic valve, we’re going to have to exert a certain amount of pressure.

Date Published - Oct 29, 2014
Date Modified - May 10, 2016

Jon Haws RN

Written by Jon Haws RN

Jon Haws RN began his nursing career at a Level I Trauma ICU in DFW working as a code team nurse, charge nurse, and preceptor. Frustrated with the nursing education process, Jon started NRSNG in 2014 with a desire to provide tools and confidence to nursing students around the globe. When he's not busting out content for NRSNG, Jon enjoys spending time with his two kids and wife.