Nursing Student
New Grad

Never Trust SpO2 and Oxygen Delivery DO2 Video (cardiac SaO2, SpO2, PaO2))

spo2 sao2 difference

Struggling in nursing school? will skyrocket your grades 🚀

In this video episode we discuss why you shouldn’t place all your trust in SpO2 and how the oxygen deliver system works within the body.

It is easy to put all of your trust in monitors . . . I mean when was the last time a computer failed us right?

SpO2 is a poor indicator of tissue oxygenation and really doesn’t give the nurse the complete story.

Things we discuss:

  • What is the difference between SpO2 and SaO2
  • Why you shouldn’t fully trust SpO2
  • The importance of a good clinical assessment





Share this post:

Share on facebook
Share on twitter
Share on pinterest
Share on reddit
Share on whatsapp
Share on email

Podcast Transcription

All right, guys. One of the first things that I tell every new nurse and every student that I work with is not to simply trust your monitors, not to simply trust the numbers that you see, but to do some actual clinical investigation and trust your assessment of your patient. One of those numbers that I really dislike nurses putting all of their faith in is SpO2, okay. The pulse ox is helpful but let’s talk about oxygen delivery and why you shouldn’t just trust your pulse ox.


Code blue room 305.


Okay, so the number that we’re getting whenever we use a pulse ox is we’re getting our SpO2. What is SpO2? What SpO2 is really trying to give us is it’s trying to give us an indirect measure of SaO2. What SaO2 is is SaO2 is really the saturation of our arterial blood with oxygen, okay. Within our arteries we have our red blood cells. Within our red blood cells we have hemoglobin. Our hemoglobin can carry four oxygen molecules. What we’re really looking at is we’re looking at the saturation of that hemoglobin with oxygen. SpO2 is really just an indirect measure of that, and it’s kind of what we do as an easy way out.


Let me tell you why that’s not the best number to depend on. If we dive in here to our lungs. Here’s our lungs, here’s our trachea. As we breathe air in air will eventually, you know that has oxygen in it and that air will eventually make it down to the alveoli. Those alveoli will come in contact with blood passing through the pulmonary artery, okay. We’re breathing air in, blood is passing through our pulmonary artery, and oxygen that we breathe in is going to diffuse through the alveoli into the pulmonary artery, okay. We have all this blood sitting in that pulmonary artery. Some of that oxygen that we breathe in is going to go actually and attach to hemoglobin, okay. Some of it’s actually going to go and attach with hemoglobin. That’s call oxygenated hemoglobin, okay. Other portions of that blood is just going to sit in that pulmonary artery and flow through the blood. That’s oxygen that just stays in the plasma. The vast majority of our oxygen is going to bond with that hemoglobin. Then the blood will then pass through the body, go into the pulmonary vein and out through the body, okay.


The reason we need that oxygen, right, we have to have oxygen to carry out ATP which creates energy and allows us to do everything that we need to do. The blood is then going to go out to the body to the tissues. These tissues require oxygen, okay. These tissues have to have oxygen in order to carry out their metabolic processes, in order to carry out ATP, okay. As that blood passes through the lungs and becomes oxygenated. Remember we’re going to have oxygen in two places. We’re going to have oxygen bound to hemoglobin in our red blood cells, and then we’re also going to have some oxygen in the plasma, okay.


All that blood is going to come back into the left side of our heart to our left atria, and then from the left atria it’s going to go into the left ventricle up to the aorta, okay. From the aorta that blood with oxygen attached to hemoglobin and oxygen floating in the plasma is then going to go down into the tissues. Remember, these tissues have to have oxygen in order to carry out their metabolic process. In order for ATP to occur these tissues have to get that oxygen. What’s going to happen is these tissues are going to start extracting oxygen from those red blood cells in order to get their ATP going.


One thing we look at with here is we look at our delivery of oxygen, okay. That’s abbreviated as DO2, okay. Delivery of oxygen is made up of a couple of things. It’s made up of cardiac output. We’ve talked about cardiac output before. Cardiac output is our stroke volume times our heart rate, so how much blood is being pumped with each squeeze of the heart and how fast our heart is pumping, okay. That makes up our cardiac output, how much blood is going out of the heart every minute. It’s also made up of the O2 contents of arterial blood. We’re talking about our oxygen delivery, how much oxygen is being delivered to the tissues, how much is being taken to the tissues. That’s made up of how much blood is coming out of the heart and how much oxygen is in that blood.


What we’ve talked about so far is we’ve talked about the difference between SpO2 versus SaO2, all right. SaO2 is the percent saturation of hemoglobin in the arterial blood. We use SpO2 as kind of indirect measure of this. This is the number we want to get. In order to get this we need to do like an ABG or something, so we use our pulse oximeter and we get our SpO2, which isn’t as valid as this. What we’ve been talking about is why you should be trusting your instinct and your nursing assessment versus just the SpO2 monitor.


We discussed also … Let’s draw a little alveoli. Here’s are alveoli. What happened is little oxygen molecules will come in here as we breathe, right, and pulmonary artery’s going to pass by here, and as pulmonary artery passes by little oxygen molecules are going to come into the blood, okay. Now, some of those oxygen molecules are going to bind with hemoglobin in the RBCs. Each hemoglobin can take four oxygens. Most of the oxygen is going to go on and bind with the hemoglobin, okay. This oxygen that binds with our hemoglobin is our SaO2, okay.


This other oxygen that is sitting in the plasma is our PO2, okay. What we really are trying to figure out here is we’re looking at our delivery of oxygen. Our delivery of oxygen is made up of our cardiac output times the oxygen content of the blood, okay. Content of arterial blood, O2 content of arterial blood and cardiac output. This is made up of stroke volume and heart rate, okay. We’re gonna start piecing this together a little bit more, but this is why you can’t just simply trust your SpO2, okay.


We talked a little bit about how oxygen travels through the body in two ways. It travels through the body either bond to hemoglobin or in the plasma. As this blood is traveling around the body the hemoglobin is going to give up some of this oxygen to the body and to the tissues, okay. We have a hemoglobin with four oxygens on it. As it passes through the body it’s going to give up some of this oxygen to the body, and these tissues need this oxygen in order to complete ATP, so we’re going to give up some of this oxygen, allow these tissues to do their metabolic processes, and the blood is going to come back to the heart, receive more oxygen, and then carry out its whole deal. This association between SaO2 and SpO2 is known was the oxyhemoglobin disassociation curve. What it really does is just kind of shows us how readily hemoglobin is going to give up oxygen, okay.


We’re not going to dive into too much to oxyhemoglobin disassociation curve, but what I really want you to understand is that you can’t simply trust SpO2 of how well a patient is actually doing, because what SpO2 is not giving us doesn’t tell us about tissue oxygenation in any way, okay. Your patient could have a great SpO2 yet they’re not really oxygenating their tissues, okay. The tissue aren’t getting the oxygen they actually need. Where that comes in, that comes in with you having excellent assessment skills and being able to really determine exactly how a patients doing as you look at them and as you assess them and then possibly getting further assessments.


Again, really quickly SaO2 is talking about oxygen bound to hemoglobin. Pao2 is oxygen dissolved in plasma, okay. Most of our oxygen in our blood is going to be bound to the hemoglobin, okay, and our SpO2 is really an indirect measure of the arterial oxygen bound to hemoglobin, okay. There is a text that can be done to determine how much oxygen the tissues are actually extracting. That test is an SvO2 or an ScvO2, okay. What this does is … A lot of times now what’s really used is ScvO2. We can measure that through a central line where we get the superior vena cava, right. Our central line’s going to sit right in the superior vena cava. What we can do is we can draw some blood out of there, and when we draw blood out of there what we’re doing is we’re looking for our mixed venous O2. We want to know what’s the percentage, what’s the saturation, of this blood that is coming back into the heart. Why is that important? How does that work?


Here’s our heart. Blood goes out, goes around the body, comes back in. We’ve got a superior vena cava. We draw some blood out of our superior vena cava, okay. As this blood is traveling around the body it’s dropping off a little bit of oxygen here and there, right. We’re dropping off oxygen as we travel throughout the body. We want to test our patients ScvO2 to determine if the tissues are getting the oxygen they need or if they’re maybe kind of starving a little bit for oxygen, trying to get more oxygen. When we draw our ScvO2 we just draw from a central line, we draw out a little sample of blood. Our oxygen saturation is within that mixed venous O2, okay.


Our normal ScvO2 is greater than 70%, okay. As this blood leaves the heart we’re gonna be at a much higher percent and then as it travels and comes back to the heart before it gets reoxygenated this blood is going to be saturated, hopefully over 70%, okay. If that number starts to go below 70% what that’s telling us is the body’s really kind of starving to get the oxygen it needs, okay. The body’s really trying to extract more oxygen in order to get what it needs in order to carry out ATP. For example, one time this would actually drop would be in a situation like sepsis, okay. Our tissues are kind of starving for oxygen and so what they’re going to do is they’re going to start extracting additional oxygen from the hemoglobin as it passes by. When we draw our ScvO2 this number’s gonna be below 70. Some things that physicians can do is they can try to increase cardiac output and they can do different things to try to deliver the oxygen they need to the tissues, okay.


I just really want you guys to take from this … What I want you to really take is I want you to understand that SpO2 is really very indirect measure of what we’re looking for, okay. It kind of gives us an indirect measure of SaO2, but what it really fails to tell us is it fails to tell us about tissues, okay. It fails to tell us are our tissues getting oxygenation, okay. You’re gonna have a patient who has a perfect SpO2, but what’s happening is there tissue may still be starving and so it’s very important to use your clinical judgment. Put together all the pieces. Look at the monitor, look at the patient, look at everything that you need to be looking at in order to determine, is the patient actually doing well, okay. I don’t want you to ever really just rely on numbers, especially a number like SpO2 that can really give you a false sense of security, okay. I want you to be a nurse that’s thinking beyond that and really digging into everything that you need to know, all right.


I appreciate you guys so much. I appreciate you subscribing and rating the show and sharing it with your friends. If you have any questions, you need to get ahold of us, please find us on social media or reach us, contact us at Thank you so much. Please leave a comment, leave a review, share with a buddy. We will talk to you soon. You guys know what you need to do now, go out and be great. Happy nursing.