What’s The Difference Between Oxygen Saturation And PaO2?The Airway Jedi
of chronic lung disease. disclosed no relevant financial relationships with Relationship between PaO2 and SaO2. The OHDC represents the relation-. Jul 28, The patient's pulse oximetry reading is SpO2 98% on air. . That is not a technical issue, just a reflection of the underlying physiology. hemoglobinopathy will have different SpO2 and SaO2 at a given PaO2 according to the. PaO2 and SaO2 are both measures of oxygen content in the blood. this site is for your information only and is not a substitute for professional medical advice.
And if I'm looking at just the arterial side, I could write, S little a O2. And I could compare to the partial pressure in the arterial side of oxygen. And remember, we have these little S-shaped curves. And all I want to point out is that, for any increase in my PaO2, in the partial pressure of oxygen, I'm going to have an increase in the O2 saturation. So there's an actual relationship there. And we usually measure this in percentage.
Percentage of oxygen that is bound to hemoglobin. And so this is the same thing here, as a certain percentage. So this whole top part of the formula, then, this whole bit in my brackets really is telling me about hemoglobin bound to oxygen. Now remember, that's not the only way that oxygen actually travels in the blood. Let me write out this second way that oxygen likes to get around. And the second way is when it dissolves in the blood.
So this is all going to be plus. And the second part of the equation is the partial pressure of oxygen. And this is measured in millimeters of mercury. So that's the unit. And this is times, now this is another constant, 0. And then, keep track of the units here because we have to end up with these units.
So you know everything has to cancel out to end up with that. So I have milliliters of oxygen on top. And I'm going to want to cancel my millimeters of mercury. So take that times milliliters of blood. So these are the units on the bottom. And they end up the same as we just worked through. We've got this crosses out with that. And my units are going to end up perfect. And this bottom bit, that I'm going to put in purple brackets. This bit tells me about dissolved oxygen.
So I have my oxygen bound to hemoglobin. And I have my dissolved oxygen. These are the two parts of my formula. So let me actually just quickly, before I move on, circle in blue, then, the important parts that I want you keep your eyeballs on.
There is the total O2 content, hemoglobin, oxygen saturation, and partial pressure of oxygen. And remember, this guy influences this guy. And we saw that on the O2 curve that I just drew. Let me just bring it up again, so I can remind you what I'm talking about.
In this graph, you can see how the two are related. There's a very nice relationship between the two. So this is my formula for calculating the total oxygen content. So let's actually use this formula. Let's think through this. And when I think through it, I always go through all of my four variables. Let me just jot them down here. So we keep track of them. Let's do PaO2, SaO2, and then hemoglobin and the total oxygen content.
These are my four variables. Now, let's do a little problem together. Let me make a little bit of space. And let's say I have two little containers. And the first container, this first one is full of blood.
Here's a B for blood. And here's a second container full of plasma. Remember, plasma is a part of the blood. But it's not all of the blood. Plasma specifically does not have any red blood cells or any hemoglobin. So let me just write that down. No hemoglobin in the plasma side. Just to make sure we don't lose track of that fact.
- Oxygen content
- Dissociated Oxygen Saturations
Now, plasma is yellow colored. So let me just make it yellow colored here. Make sure we clearly see that that's plasma. And blood I'm going to keep as a red color. So now, we have our two containers full of plasma and blood. So now, let's say, I decide to increase the partial pressure of oxygen in the air. So it's going to diffuse in here. And it's going to diffuse in here.
So I increase the partial pressure oxygen in the air. And it's going to diffuse into those two liquids. It's going to dissolve into those liquids. So my question is, as we go through one by one, each of these four variables, I want you to think through if they go up, if they go down, or if they stay the same.
So let's start with the first one, PaO2. Well, if the oxygen is going to diffuse into those liquids, then I would say the partial pressure of oxygen in the liquid would go up. Because we're not really talking about arterial blood here. We're just talking about blood. And we're not talking about arterial plasma. We're just talking a plasma because there's no artery connected to these two tanks of fluid. But the concept is the same.
So the partial pressure of oxygen is going to go up in the blood. And it's going to go up in the plasma because it just dissolves into those liquids. As PaO2 falls, the Hgb saturation also falls as Hgb releases oxygen to the tissues in the areas of lower oxygen supply. This is because Hgb binding sites become less attracted to oxygen as it is bound to fewer oxygen molecules. This property allows Hgb to rapidly release oxygen to the tissues. Deoxygenated blood returns to the heart to be pumped to the lungs and the cycle repeats.
Since a normal PaO2 is between mmHg, some people may think that an O2 saturation of 90 is normal as well — after all 90 was a pretty good grade to get in school. However, this interpretation is very wrong. This is the minimum oxygen concentration providing enough oxygen to prevent ischemia in tissues. As good as they are they can have problems. Movement can cause inaccurate readings. This is especially common in small children. Another problem is that poor perfusion from extreme vasoconstriction, hypotension, hypovolemia, and septic shock can all decrease peripheral blood flow.
What’s The Difference Between Oxygen Saturation And PaO2?
This sometimes makes it impossible for the sensor to measure the concentration correctly, or at all. You can often put the sensor on the ear lobe and get a more accurate reading. The presence of CO fools the monitor into reading high. The patient with CO poisoning appears flushed and pink. Children should have sensors appropriate to their size. The monitor is small, portable for use in the field, operating rooms, and in patient hospital rooms to provide continuous, real time monitoring of the patient.
Discrepancy between oxygen saturations on ABG an pulse oximetry
Measurement of PaO2 requires drawing and testing an arterial blood sample —something that requires a trained provider, a lab, and time. I have been practicing anesthesia for 35 years and the use of pulse oximetry revolutionized patient safety when we started using it. Oxygen saturation is one of the most valuable tools I have.