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Now that we have run through the all of the possible acid base imbalances, let’s talk in more detail about the oxygenation levels that you’ll find on an arterial blood gas.
First I just want to do a quick review of gas exchange. Remember that venous blood enters the lungs deoxygenated period carbon dioxide exits the bloodstream and is replaced by oxygen. Then, the oxygenated blood leaves the lungs and goes out to the body via the arterial system. So, when we are obtaining arterial blood, this is the process that we are evaluating. Are the patients getting appropriate amounts of oxygen into their blood?
So, there are two main values that we will receive on an arterial blood gas. The first is our PaO2. This stands for the partial pressure of oxygen, or the amount of oxygen dissolved in arterial blood. Again, this tells us how well the gas exchange is occurring in our lungs. Are we getting appropriate amounts of oxygen into our arterial blood? The SaO2 tell us how much of our hemoglobin is saturated with oxygen. So it’s giving us an idea of the capacity of our arterial blood to actually carry the oxygen out to our tissues. In either case if these levels are too low you’ll probably see cyanosis, maybe cool skin, and probably shortness of breath or tachypnea as the patient tries to compensate for the lack of oxygen. Now I want to give you some key critical thinking points about each one of these values.
First let’s think about the PaO2. If you remember from the very first lesson, the normal PaO2 is 80 to 100 mmHg. But remember what was special about that…this value is assuming the patient is on Room Air, which is 21% oxygen. We write that 21% FiO2, which means “fraction of inspired oxygen”. So – if you have a patient who has a PaO2 of 120, you tend to think “sweet! this guy’s doing great!”. Except – maybe that patient is actually on 60% FiO2 so, in reality, it’s not that great. So – to really evaluate how well YOUR specific patient is actually doing, we use what’s called the P/F ratio. It stands for PaO2 to FiO2 ratio and you get it by dividing the PaO2 by the FiO2. The normal P/F ratio is about 400 or above. Think about it – a PaO2 of 90 divided by 21% which is 0.21 – that gets you about 430. That’s a GREAT oxygenation level. When we see lower and lower P/F ratios, it means they have a lung injury or are in respiratory distress. Less than 200 is severe Acute Respiratory Distress Syndrome, which we talk about in the Respiratory Course. So, let’s use the example I just gave. Your patient has a PaO2 of 120 – divided by their FiO2 of 60% or 0.6. That gives you a P/F ratio of 200. So even though you see the PaO2 is greater than 100, this is actually NOT a good oxygenation value for this patient. Okay? So it’s super important that you’re looking at the big picture and you understand that this normal value is on Room Air!
Now, what I want you to know about the SaO2 is that there are some things that will affect its accuracy or reliability. Because of these things, you may see a high SpO2 but actually the patient is not oxygenating well at all. Remember that the SaO2 measures the saturation of hemoglobin. Each hemoglobin has 4 heme groups, so it can hold 4 oxygen molecules. If I have 100 hemoglobin molecules and 95 of them are fully saturated, I’ll have an SaO2 of 95%, which is good. But, there’s something called the oxyhemoglobin dissociation curve that compares PaO2 to SaO2. As the PaO2 rises, so does the SaO2 just like this curve. But, changes in temperature or pH levels can cause this curve to shift to the right or to the left. So, your SaO2 may not change at all, but meanwhile your PaO2 has changed significantly. Another thing that affects the SaO2 is Anemia. If instead of having 100 hemoglobin molecules, I only have 50, and 48 of them are saturated, I’ll have an SaO2 of 96% – which you’d think is great, right? BUT – my actual capacity to carry oxygen out to the tissues is actually quite low because I have WAY less hemoglobin molecules, right? It’s like it fakes you out! Another thing that does that is carbon monoxide. Carbon monoxide will take the place of oxygen on the hemoglobin and these lab values will tell you that it is 100% saturated…except it’s 100% saturated with the WRONG thing! So you really can’t carry enough oxygen to the tissues! And, don’t forget, if we’re looking at peripheral oxygen saturation or pulse ox, things like cold fingers or poor perfusion will also affect how reliable that number is, okay? Again, these are just some things you need to be thinking about critically when it comes to your patient. It doesn’t mean we ignore the oxygen saturation, by any means, but make sure you’re looking at the big picture of what’s going on with your patient.
As far as therapeutic management for any kind of hypoxia or hypoxemia, we always want to treat the underlying cause administer supplemental oxygen. If anemia is part of the problem we may also administer blood transfusions. And if there’s any kind of airway or breathing issue of course we want to provide airway support or mechanical ventilation. I know that I hammered this home in the respiratory acidosis lesson, but I’m going to say it again. Remember that providing supplemental oxygen to someone who is not breathing correctly or has an airway obstruction is not beneficial. Make sure that their airway is open and that their breathing is appropriate, and then provide oxygen. Yes, in the real world it only takes 5 seconds to apply oxygen. However, it also only takes 5 seconds to apply an EKG lead, but that is not going to help the patient. It’s not always about how quickly you can do something, but about the impact it’s actually going to have. So make sure that your patient’s airway and breathing are taking care of first, okay?
Let’s recap. The PaO2 or the partial pressure of oxygen dissolved in arterial blood gives us a good picture of the ability of the lungs to perform gas exchange and to get oxygen into the arterial blood. The normal value is 80 to 100 on room air. The SaO2 Or arterial oxygen saturation tells us how much of the hemoglobin is saturated with oxygen and therefore gives us an idea of our capacity to carry oxygen to the tissues. Make sure that you’re looking for things like anemia or signs of carbon monoxide poisoning that may indicate that your SaO2 level is not as reliable. Always remember to look at the big picture. The P/F ratio will help you get an idea of how bad the situation is for this specific patient by comparing the PaO2 to the FiO2. And of course we always want to treat the cause first. Make sure they have airway or breathing support, give supplemental oxygen, and possibly even give a blood transfusion if anemia is significant issue.
So, those are the basics of evaluating the oxygenation values on an arterial blood gas. I have attached the ARDS case study to this lesson as well, so that you can practice calculating P/F ratios. Make sure you check out all the other resources attached to this lesson as well. Now, go out and be your best selves today. And, as always, happy nursing!!