A-a gradient questions

[Kazu]

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Can you have a normal A-a gradient and still be hypoxic? Whats the relationship between pulse ox and the abg. why do you need an ABG if you have the pulse ox. Can you be 100% on pulse ox and still be hypoxic?

 

[12R34Y]

Yes you can still be hypoxic and have an o2 sat of 100%.

All pulse oximetry measures is BOUND hemoglobin. so ANYTHING that is binding hemoglobin (ie. Carbon monoxide) will also be measured.

People with CO poisoning can have a pulse ox of 100%, but there O2 carrying capacity is greatly impaired and thus they can be hypoxic.

later

 

[12R34Y]

oh, forgot to answer your other question.

If you have an elevated A-a gradient then that suggests a diffusion problem in the lung causing a difference in alveolar oxygen concentration v.s. the concentration of oxygen in the arteries.

If you have a normal Aa gradient, but still have hypoxemia you are probably looking at hypoventilation as a cause. NOT a lung problem (diffusion problem).

Hypoventilation increases C02 and will displace O2 in the alveolus causing hypoxemia.

i think this is right, but i'm sure if i'm wrong people will correct me.

later

 

[Kazu]

if there is no Co poisoning or other poisoning and you have a 100% pulse ox, can you still be hypoxic.

 

[Homunculus]

if there is no Co poisoning or other poisoning and you have a 100% pulse ox, can you still be hypoxic.

in septic shock you will be hypoxic without being hypoxemic

-- your friendly neighborhood bacteria avoiding caveman

 

[docB]

How about severe anemia? Your Hb will be 100% saturated but you will still be hypoxic on a gas.

 

[Homunculus]

How about severe anemia? Your Hb will be 100% saturated but you will still be hypoxic on a gas.

yeah, that and when your partial pressure of O2 drops i.e. very high altitude

 

[Kazu]

I would think in high O2 the drop in partial pressure should not be an issue until you start dropping in your percent saturation, since most O2 is carried by hemoglobin.

 

[Kazu]

I meant high altitude, not high O2

 

[Midwest Medic]

Also, at a PaO2 of 60, your pulse ox is still pretty good... >92%. So yes, you can have a god pulse ox and be hypoxemic. Our ICU attending is pounding that into our heads right now. So never rely on a nurse teling you the pt. is having some trouble breathing, but their pulse ox is fine doctor......