Teaching Rounds Day 11/15
Hypoxemia...
Beyond the Mechanism & Back to the Bedside
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Hypoxemia...
Beyond the Mechanism & Back to the Bedside
Join us, #medtwitter & #medstudenttwitter
I fondly remember learning about hypoxemia in medical school and breaking it down by mechanism
1. Low FiO2
2. Hypoventilation
3. Impaired diffusion
4. Shunt
5. V/Q mismatch - the go-to answer every time!
1. Low FiO2
2. Hypoventilation
3. Impaired diffusion
4. Shunt
5. V/Q mismatch - the go-to answer every time!
This was a great way to understand hypoxemia, but I haven't found myself using this approach clinically.
For me, the diagnostic exercise boils down to the fundamental pulmonary unit -the alveolar/capillary interface.
Why?
With two bedside tools, we can rapidly localize the hypoxemia to:
1. Alveolar
2. Vascular
3. Other
Why?
With two bedside tools, we can rapidly localize the hypoxemia to:
1. Alveolar
2. Vascular
3. Other
What tools?
1. A stethoscope.
Vascular disorders - pulmonary embolism & shunting - tend to be silent.
Alveolar diseases - e.g, pneumonia - make their presence heard.
1. A stethoscope.
Vascular disorders - pulmonary embolism & shunting - tend to be silent.
Alveolar diseases - e.g, pneumonia - make their presence heard.
2. The A-a gradient
Assessed at the beside, without anyone leaving the room, not even to the lab?
YES
Rapid improvement of hypoxemia with minimal supplemental O2 tells us the highway between the alveolus and the vasculature is relatively patent =
A-a gradient isn't too high.
Assessed at the beside, without anyone leaving the room, not even to the lab?
YES
Rapid improvement of hypoxemia with minimal supplemental O2 tells us the highway between the alveolus and the vasculature is relatively patent =
A-a gradient isn't too high.
The more dramatic the correction of the hypoxemia (83% RA -> 96% 1L) the more confident you can be that A-a gradient is low.
This usually means that Alveolus and the Vasculature are working fine!
This usually means that Alveolus and the Vasculature are working fine!
Why bother when there is a MUCH more accurate way of measuring the A-a = an ABG?
1. It's easy to do
2. I don't get an ABG in everyone with hypoxemia
3. You might just have diagnosed a COPD exacerbation and may want to take it easy on the O2 supplementation
1. It's easy to do
2. I don't get an ABG in everyone with hypoxemia
3. You might just have diagnosed a COPD exacerbation and may want to take it easy on the O2 supplementation
These two tools - stethoscope & A-a gradient - are part of the 1 minute initial assessment.
They aren't precise enough to get a diagnosis, but helpful to begin the localization process.
They aren't precise enough to get a diagnosis, but helpful to begin the localization process.
Wait.
How did we know the patient was hypoxemic?
Well, we didn't really.
We saw that their O2 sat was low, and made the most common inference: PaO2 is low.
Is that always the case?
Nope.
How did we know the patient was hypoxemic?
Well, we didn't really.
We saw that their O2 sat was low, and made the most common inference: PaO2 is low.
Is that always the case?
Nope.
POCUS & CXR will take you far, and in most cases, to the answer.
Sometimes, we fall short of a clear Dx with these tests.
What next?
Sometimes, we fall short of a clear Dx with these tests.
What next?
The most fascinating cause of hypoxemia?
Easy.
Orthodeoxia!
Easy.
Orthodeoxia!
