PulmCrit- Top 10 reasons pulse oximetry beats ABG for assessing oxygenation
The arterial blood gases on 2 L-min -1 of 02 were: Pao2 = 65 mm Hg, Paco2 = 51 mm Hg, In patients with an acute episode, restricted mobility should be minimized to The difference is due lo the large cross-sectional area of the pulmonary. On a Friday night shift, an ambulance brings you a 52 year-old man who had an episode of Low SpO2 readings occur because pulse oximeters utilize light nm to calculate the ratio of oxy-hemoglobin to deoxy-hemoglobin in blood. Dosing is mg/kg of methylene blue administered intravenously. B R O'Driscoll,1 L S Howard,2 A G Davison3 on behalf of the British Thoracic Society. 1 Department of . vulnerable to repeated episodes of hypercapnic respiratory failure. In these However, the arterial oxygen tension (PaO2) is less accurate in .. established a working party in association with 21 other societies and.
One of those numbers that I really dislike nurses putting all of their faith in is SpO2, okay. Code blue room 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.
If we dive in here to our lungs. 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.
Correlation between the levels of SpO2 and PaO2
Those alveoli will come in contact with blood passing through 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. Other portions of that blood is just going to sit in that pulmonary artery and flow through the blood.
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. 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. One thing we look at with here is we look at our delivery of oxygen, okay. Delivery of oxygen is made up of a couple of things. And the units on this are milliliters of oxygen per, I said, milliliters of blood. So these are the units here. And this is going to equal-- to figure this out, I need to know the hemoglobin concentration.
And there it's the grams of hemoglobin per milliliters of blood. And then, I have to multiply this by a constant. And the constant is 1. And what that number is, is it's telling me the milliliters of oxygen that I can expect to bind for each gram of hemoglobin. So that's actually quite a nice little number to have handy because now you can see that the units are about to cancel. This will cancel with this.
And I end up with our correct units. But there's one more thing I have to add in here which is the oxygen saturation.
Methemoglobinemia: Not the Usual Blue Man With Low SpO2
Remember, this O2 saturation. And if I know the O2 saturation, remember, there's this nice little curve. This is O2 saturation. 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.
Ep171: A&P: Never Trust SpO2 and Oxygen Delivery DO2 Video (cardiac SaO2, SpO2, PaO2))
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. PaO2, the oxygen tension in arterial blood, is the best way to determine how well the lungs are working. However, oxygen saturation is a better measurement of the systemic oxygen delivery to the tissues DO2 7: ABG is an invasive, painful, and expensive procedure.
An ABG is painful for the wrist and the wallet. In contrast, pulse oximetry is noninvasive, painless, and free 2. Occasionally, an arterial catheter might even be placed for the purpose of measuring frequent ABGs.
This is generally a terrible idea. The availability of an easy source of arterial blood encourages frequent ABGs and other labs as well. For example, one study found that the presence of an arterial catheter correlated with a four-fold greater volume of phlebotomy Tarpey Thus, it may not be obvious that the sample was venous.
These devices typically measure PaO2 and subsequently use this to calculate the oxygen saturation assuming a normal PaO2 vs. For patients with abnormal hemoglobin dissociation curves, this calculated saturation will be wrong.
ABG measurement may delay critical decisions. Occasionally, physicians may feel obligated to check an ABG before calling for help, to exercise due diligence.
Regardless, the practice of delaying treatment to obtain an ABG is usually unnecessary, particularly when oxygenation is concerned 3. PaO2 values are frequently misinterpreted.
We are constantly exposed to oxygen saturation values, leading to the development of a good sense about what they mean. Meanwhile, we are exposed to PaO2 values far less often, so we may struggle to interpret them.
Oxygen content (video) | Khan Academy
The most common error is panicking about a low PaO2 value. PaO2 values are always much lower than oxygen saturation values. This is simply a reflection of the oxygen saturation curve figure above. The lower number is scarier.
This cognitive bias is often seen when ABGs are obtained in patients on mechanical ventilation. For a patient with mild hypoxemia, the PaO2 value will often be surprisingly low.