Acute Hypoxemia: A Rapid-Response Refresher

Matthew F. Watto, MD; Paul N. Williams, MD


September 29, 2022

This transcript has been edited for clarity.

Matthew F. Watto, MD: Welcome back to The Curbsiders. I'm Dr Matthew Watto, here with my great friend, Dr Paul Nelson Williams. Today we're going to be talking about acute hypoxemia and how to approach it, from our conversation with Dr Nick Mark.

I want to start us off with some basic definitions, things that are going to get you made fun of by your pulmonary colleagues if you mess them up. So, hypoxemia. How do you pronounce it?

Paul N. Williams, MD: Hypoxemia.

Watto: Okay, thank you. Paul I should have asked you that before we started recording.

Hypoxemia. That's where you have a low oxygen saturation level. Usually that's what you're measuring on the pulse oximeter. That's the oxygen level in your blood. Hypoxia is how much oxygen is making it to the tissue level. The two do not always go together and you want to be precise in your language. If the pulse oximeter is measuring low, that's hypoxemia; and with hypoxia, you might start to see end-organ damage, problems with the heart, kidneys, the peripheral tissues.

I've mentioned the pulse oximeter a couple of times. That's a very reliable device, right? They did a great job developing it and there are no holes in what it's able to do for us now.

Williams: Dr Mark made the point a couple of times that it's very specific but it's not always accurate. It all hearkens back to how they figured out how these values worked in the first place. They derived the values that correspond to the pulse oximeter readings through a cohort of servicemen who were all predominately White. So there wasn't a whole lot of variation in pigmentation, and one of the issues that we run into with pulse oximetry is that the number can actually be inaccurate in our patients with darker skin. You can get inaccurate pulse oximeter readings. So you need to be mindful of that when looking at the measurements.

Watto: One of the pearls that we took home was that if precision and accuracy really matter in making a decision, you might want to think about getting an arterial blood gas just to make sure your pulse oximeter is giving you an accurate value. Until we have better equipment, that's what you might have to do.

Williams: It's a terrific point because while 3% doesn't sound like much, it can be the difference between someone getting home oxygen and not qualifying for medical equipment. So 3% can be a huge deal. The difference between 87% and 90% is bigger than you might think.

Watto: Paul, one more question for you. When I give someone supplemental oxygen, immediately their oxygen levels are going to shoot through the roof. Am I correct?

Williams: No, but I love the framing device, as usual. Two points about the pulse oximeter are worth mentioning. Because it takes time for the blood to get from the heart to the peripheral tissues, the pulse oxygen saturation is going to lag behind whatever intervention you place. Give it at least 30 seconds. And if the patient has a decreased cardiac output, it will probably take even longer before you actually see how the intervention affects the pulse oxygen saturation level. So, don't panic if you intubate or increase the oxygen or apply noninvasive ventilation and it doesn't change immediately, because it takes some time for the blood to get to where it's being measured.

And I'd like to make one more point. Going back to the cohort from whom the values were first derived, they had them breathe lower levels of inspired oxygen to get to the lower oxygen saturation levels. So the lower saturation levels are not going to be as accurate as the higher levels. But the difference between 60% and 50% is academic anyway; low is low once you get to a certain point.

Watto: Those are such great points, Paul.

When you go into the patient's room and you see that the patient has hypoxemia, aside from making sure that the waveform on the pulse oximeter is good and it's a true reading, you want to make that initial assessment.

Is this patient sick? Are they not sick? As you mentioned, the degree of hypoxemia is important. At those very low levels, you have to assume it's bad. Below 70%, we know it's bad, vs someone in the high 80s, which gives you a little more time.

Then we consider what's been happening, trying to think through, at the bedside, what might be causing the drop in oxygen saturation. We are point-of-care ultrasound (POCUS) enthusiasts. Dr Mark did mention that if you are facile in POCUS, you can look for B lines in the lungs for pulmonary edema. You can look for a collapsed lung with lung sliding. Is it there or not? And you can look at the heart to see whether there are signs of right ventricular overload, which could suggest pulmonary embolism or a low ejection fraction. There are all sorts of things you could potentially do with POCUS.

Testing is pretty basic. It's a chest x-ray, blood gases, and some basic labs.

The difference between the alveolar and the arterial oxygen pressure is what we're looking at to try to make our differential diagnosis.

Here are the six causes of hypoxemia, and he said some of these we can rule out right away, such as low inspired oxygen content. Unless you're at altitude, that's probably not the cause of acute hypoxemia.

Basically, three of the causes go along with a normal AA gradient and three go along with an elevated AA gradient or difference. Most of the causes we're going to be seeing in a rapid response in the hospital are associated with a V/Q mismatch, where there's a difference in ventilation and perfusion.

He also said that if you put supplemental oxygen on somebody and they're not getting better, then that's a sign that you may be dealing with a shunt. And something I had never thought about, Paul — did you ever diagnose a shunt because a patient was started on nitrates or a calcium channel blocker?

Williams: Nope. I wish I could say that I had, but no, that was the first time I heard of it.

Watto: You have to moonwalk out of the room if you make that diagnosis.

Williams: Just drop the pulse ox on the floor.

Watto: All right, Paul, anything else? This was another episode just deep and heavy with pearls. I'll give you the last word if there's anything else you wanted to talk about. I know we talked about oxygen delivery systems as well.

Williams: There is just so much wall-to-wall stuff. If all of this sounds appealing to you, I would encourage you to go and listen to Diagnosing & Managing Acute Hypoxemia with Nick Mark, MD. There's just too much to cram into a 5-minute video.

Watto: This has been another episode of The Curbsiders, bringing you a little knowledge food for your brain hole. Until next time, I've been Dr Matthew Frank Watto.

Williams: And I remain Dr Paul Nelson Williams. Thank you and goodbye.

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