Hypoxemia

[cfdavid]

So, during my MICU month, it's becoming obvious that there really is NO definition of hypoxemia.

A-a gradients seem very unreliable (since they can vary greatly depending on age, and also within any given age range) and not really used very often.

My problem is that how can we diagnose someone with "hypoxemia" when in fact, it's ill-defined?? (I know it's a broad Dx, and it needs to be worked up from there but it seems SO loosely thrown around in the ICU......)

I've heard one could use a generalization of Pa02<80 (or some say <60) and/or an Sa02<90.

I've also heard it described as a Pa02<50% on 50% Fi02......


*****What definition do you guys use to classify someone as hypoxemic???

cf

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[bullard]

I've heard one could use a generalization of Pa02<60 and/or an Sa02<90%

That's correct. I don't know who uses PaO2 < 80 as a cutoff.

Just don't conflate hypoxemia and hypoxia (like half your medicine attendings will) and you'll be all good!

 

[bullard]

Furthermore, you can use A-a gradients to diagnose V/Q mismatch (AKA shunt effect, which is not the same thing as true shunt), which you know is a cause of hypoxemia. But A-a gradient isn't going to show up in the definition of hypoxemia because there are causes of hypoxemia that have normal A-a gradients, like hypoventilation.

Hope that helped.

 

[cfdavid]

Furthermore, you can use A-a gradients to diagnose V/Q mismatch (AKA shunt effect, which is not the same thing as true shunt), which you know is a cause of hypoxemia. But A-a gradient isn't going to show up in the definition of hypoxemia because there are causes of hypoxemia that have normal A-a gradients, like hypoventilation.

Hope that helped.

Yes, that helps clarify things.

Any other input?

 

[seinfeld]

terms hypoxia/hypoxemic are confusing in the clinical setting. We really care about whether the cells are aerobic or anaerobic, check a lactate. Learn to report SaO2 or PaO2 with the FIo2 and the PEEP the pt is requiring. I prefer to hear things such as " pt has a high Oxygen requirement, 92% on 60%" (92% sat on 2 LNC is a ton different than 92% on 100% and Mean airway pressure of 30).

In ARDS i get nervous when the Sao2 is less than 88% or PaO2 less than 55.

 

[aredoubleyou]

So, during my MICU month, it's becoming obvious that there really is NO definition of hypoxemia.

A-a gradients seem very unreliable (since they can vary greatly depending on age, and also within any given age range) and not really used very often.

My problem is that how can we diagnose someone with "hypoxemia" when in fact, it's ill-defined?? (I know it's a broad Dx, and it needs to be worked up from there but it seems SO loosely thrown around in the ICU......)

I've heard one could use a generalization of Pa02<80 (or some say <60) and/or an Sa02<90.

I've also heard it described as a Pa02<50% on 50% Fi02......


*****What definition do you guys use to classify someone as hypoxemic???

cf

youre spending too much time with the internists...pondering the big questions. When oxygen consumption > delivery, the pt's gunna do poorly.

Generally when the sats are below 90, yoiu should be worried....especially if your cardiac output or Hb is low (less o2 delivery). Its been suggested that a mixed venous sat is the best measure of hypoxia....but theres plenty of problems with that (not including the need for central access, or a pa cath if you want to be really academic by the book).

A-a gradients & PF ratios help you diagnose the problem. THe med docs love to catogorize things, just realize evrything in actuality lies within a spectrum, and someones lungs with a p/f of 190 and 210 are the same, even if we call it diferent names

 

[leviathan]

Another noobie question; hopefully the OP's question was answered adequately so this isn't a thread hijack:

If you've got a good PaCO2 / ETCO2 but patient's still hypoxic (SaO2 in the 80s), is it fair to say that the patient is being ventilated fine (because of CO2) and there's only an oxygenation problem? And if that is true, if the patient is already on an FiO2 of 100%, then what could that problem be where O2 is selectively having trouble exchanging but CO2 is fine? Shunting? ARDS?

 

[leviathan]

CO2 is much more diffusible than O2, so you need a much larger V/Q mismatch to see a change in PaCO2, than to see a change in PaO2. In the instance you describe, the V/Q mismatch is large enough to cause changes in PaO2, but not large enough to cause changes in PaCO2.

All right. This is what confuses me from what I've read, because they say it ISN'T a V/Q mismatch if you can't improve the A-a gradient with increasing FiO2. If the A-a gradient isn't improving with higher O2, they call it a "shunt". If things do improve with higher O2, they call it a V/Q mismatch. But I thought a V/Q mismatch could ALSO be classified as a "shunt" if you have a problem with V (ventilation).

So what is this 'shunt' they are referring to that doesn't get better with oxygen, and how does it differ from a V/Q mismatch that causes shunting? Check this flowchart if you want to see where I'm coming from (pg. 10):
http://www.docstoc.com/docs/519288/4-Causes-of-Hypoxemia/

 

[leviathan]

In your example, substitute dead space for V/Q mismatch. V/Q mismatch is not specific enough and includes both shunt and deadspace.

Yeah I got that, but then why a separate 'shunt' category in the flowchart which is unresponsive to O2 therapy? What's the difference between that shunt (and what kind of shunt are they talking about) and a V/Q mismatch (which they say responds to O2 therapy) where the V/Q ratio is 0?

 

[aredoubleyou]

Yeah I got that, but then why a separate 'shunt' category in the flowchart which is unresponsive to O2 therapy? What's the difference between that shunt (and what kind of shunt are they talking about) and a V/Q mismatch (which they say responds to O2 therapy) where the V/Q ratio is 0?

v/q =infinity is pure deadspace
v/q= 0 is shunt

everything inbetween are shades of gray (hence people just say v/q mismatch b/c its either mixed or unknown).

when a certain amount of blood is not oxygenated it will bring the pO2 down, when it reaches about 20-30%, supposedly, no matter how well you oxygenate the unshunted blood, your po2 will still be low.

 

[fakin' the funk]

Agree with above.

I use the definition of hypoxemia as PaO2 < 60 mmHg. Hypoxemia is just like it says, "not enough oxygen in the blood." Similarly you can have hypoxemia from pure anemia...but I'm not aware of a ml O2/dl blood value that's used for anemic hypoxemia.

Hypoxia is a vague term that usually describes PATIENTS or TISSUES, and like others have said, most attendings mistake hypoxia for hypoxemia and vice versa all the freaking time. Patients (and their tissues) can be floridly hypoxic in septic shock with great oxygen content/PaO2's.

Lastly, I ALWAYS pipe up when people mention "shunt" and "V/Q mismatch" as causes of hypoxemia with the reminder that, as mentioned above, shunt and deadspace are both specific types of V/Q mismatch.

 

[cfdavid]

Agree with above.

I use the definition of hypoxemia as PaO2 < 60 mmHg. Hypoxemia is just like it says, "not enough oxygen in the blood." Similarly you can have hypoxemia from pure anemia...but I'm not aware of a ml O2/dl blood value that's used for anemic hypoxemia.

Hypoxia is a vague term that usually describes PATIENTS or TISSUES, and like others have said, most attendings mistake hypoxia for hypoxemia and vice versa all the freaking time. Patients (and their tissues) can be floridly hypoxic in septic shock with great oxygen content/PaO2's.

Lastly, I ALWAYS pipe up when people mention "shunt" and "V/Q mismatch" as causes of hypoxemia with the reminder that, as mentioned above, shunt and deadspace are both specific types of V/Q mismatch.

Thanks for all the input everyone. Good stuff, and yeah, hypoxia and hypoxemia are, frusteratingly, too often intertwined. It's annoying, especially from those same medicine residents/attendings that love definitions (which I agree with mostly).



cf

 

[tracheatoedoc]

Hypoxemia is a lower partial pressure of O2 than one should normally have. I don't think it's a 'Diagnosis', it's a sign.
The diagnosis would be ARDS, or LV failure etc.
I've never really noticed that there isn't a standard definition of what constitutes hypoxemia, and thinking about it, I don't know if there needs to be.

 

[jetproppilot]

*****What definition do you guys use to classify someone as hypoxemic???

cf

As scientists we are incessantly looking for definitions. Cuz thats science, right?

Data. Thats been replicated. Over and over.

With the same answer.

Over and over.

Makes it

science.

I'm a believer.

I'm also a skeptic sometimes. I believe our well intended science gets in the way sometimes.

Whats the definition of hypoxemia, academically speaking, CF?

I appreciate your question. Its important for tests. For boards.

By far, though, the MOST IMPORTANT tool of a clinician identifying hypoxemia is

YOUR EYES.

We become caught up with definitions. Yeah, memorize the s hi t, CF, so you can get the board certification.

Hypoxemia when dealing with a real, live patient doesn't need a book/paper with numerical definitions of the disease process for you to make the diagnosis.

What that patient needs from you, the clinician, to make the diagnosis, is for you to walk into the PACU/ICU/whatever/, and observe...LOOK...at the patient...for thirty seconds. OK..OK...OK...I'm exaggerating....EIGHT SECONDS....and an astute critical care physician will be able to make the diagnosis most times.

From a clinician's point of view, the academic definition is rhetorical.

Is the academic definition important?

Yeah, kinda.

Whats more important is that you hone your skills during your residency so in EIGHT SECONDS FLAT you can observe a patient's pathophysiologic state, make a diagnosis, and demand a treatment path.

IMHO

THATS WHAT SETS US APART FROM THE PARAPROFESSIONALS IMPEDING ON OUR TURF, SELLING TO THE PUBLIC THAT THEY'RE DELIVERING THE SAME PRODUCT.

Uhhhhhh,

NOPE.

I don't have to follow a protocol to figure out what the f u ck is going on.

I just use

MY EYES.

My M.D. TRAINING

gave that to me.

That kinda judgement is not found in nursing curriculum, even if they choose to move their curriculum to the "doctorate" laugh level.

 

[jetproppilot]

Agree with above.

I use the definition of hypoxemia as PaO2 < 60 mmHg. Hypoxemia is just like it says, "not enough oxygen in the blood." Similarly you can have hypoxemia from pure anemia...but I'm not aware of a ml O2/dl blood value that's used for anemic hypoxemia.

Hypoxia is a vague term that usually describes PATIENTS or TISSUES, and like others have said, most attendings mistake hypoxia for hypoxemia and vice versa all the freaking time. Patients (and their tissues) can be floridly hypoxic in septic shock with great oxygen content/PaO2's.

Lastly, I ALWAYS pipe up when people mention "shunt" and "V/Q mismatch" as causes of hypoxemia with the reminder that, as mentioned above, shunt and deadspace are both specific types of V/Q mismatch.

When you pipe up,

how is your intervention different for shunt and/or V/Q mismatch in the hypoxemic patient over more run of the mill causes of hypoxemia?

And, if in fact, theres no difference in intervention,

why are you

PIPING UP?

Hypoxemia is hypoxemia.

There are many causes, no one more important than another.

 

[sevoflurane]

MY EYES.

Funny you say that.... I was in the middle of a radical cystectomy today when I heard the two urologist mumbling under their breaths. They thought they had found a mass/mets to the small bowel and wanted to call in a general surgery to weigh in on options. I took a peak into the field and saw this pinkish mass that looked like a condom bulging out of the intestinal wall. I'm no expert, but I was pretty sure it was not a malignant mass... "Don't you think that's a Meckel's Dr. XXX?" The look on their face upon realization = Priceless. Anesthesia home run.

Nothing wrong with being skeptical or using your eyes... I would say advantageous.

 

[jetproppilot]

Funny you say that.... I was in the middle of a radical cystectomy today when I heard the two urologist mumbling under their breaths. They thought they had found a mass/mets to the small bowel and wanted to call in a general surgery to weigh in on options. I took a peak into the field and saw this pinkish mass that looked like a condom bulging out of the intestinal wall. I'm no expert, but I was pretty sure it was not a malignant mass... "Don't you think that's a Meckel's Dr. XXX?" The look on their face upon realization = Priceless. Anesthesia home run.

Nothing wrong with being skeptical or using your eyes... I would say advantageous.

I'm just sayin', Dude.

I'm just sayin".

 

[pgg]

how is your intervention different for shunt and/or V/Q mismatch in the hypoxemic patient over more run of the mill causes of hypoxemia?

There's a big difference in the treatment for hypoxemia caused by, for example, hypoventilation vs mainstem intubation.


I suspect your point is that getting bogged down in diagnostic hairsplitting that doesn't impact treatment is what milmd would call mental masturbation ... but we care about things like A-a gradients, shunt, and dead space because they DO impact what we do. Even if it's almost subconscious most of the time.

 

[jetproppilot]

There's a big difference in the treatment for hypoxemia caused by, for example, hypoventilation vs mainstem intubation.


I suspect your point is that getting bogged down in diagnostic hairsplitting that doesn't impact treatment is what milmd would call mental masturbation ... but we care about things like A-a gradients, shunt, and dead space because they DO impact what we do. Even if it's almost subconscious most of the time.

BIG DIFFERENCE, YOU SAID THERES A BIG DIFFERENCE. DUDE, REALLY? THERES A BIG DIFFERENCE BETWEEN HOPPIN' UP YOUR MINUTE VENTILATION A LITTLE TO MAKE UP FOR HYPOVENTILATION, AND PULLIN' YOUR TUBE BACK A LITTLE TO COMPENSATE FOR MAINSTEM INTUBATION?

REALLY?

seriously.

I fly airplanes.

I can tell you something about flying on instruments, where theres no visual reference to the ground.

Instrument flying is a series of SMALL ADJUSTMENTS.

Small, Dude.

Small.

Adjustments.

Talking about keeping a KING AIR flying. In the clouds.

BIG, COOL AIRPLANE.

STEADY, SMOOTH,

small adjustments.

keeps it flying, even when you can't see anything.

Hypoventilation and mainstem intubations are

SMALL ADJUSTMENTS.

Not big.

Requires a FLIP of the ventilation frequency on the ventilator or a PULL on the tube back to 22cm at the lips.

Theres no BIG DIFFERENCE IN TREATMENT, SLICK.

Just SMALL ADJUSTMENTS.

Like instrument flying.

In a King Air.

 

[pgg]

BIG DIFFERENCE, YOU SAID THERES A BIG DIFFERENCE. DUDE, REALLY? THERES A BIG DIFFERENCE BETWEEN HOPPIN' UP YOUR MINUTE VENTILATION A LITTLE TO MAKE UP FOR HYPOVENTILATION, AND PULLIN' YOUR TUBE BACK A LITTLE TO COMPENSATE FOR MAINSTEM INTUBATION?

REALLY?

seriously.

Uh, yeah, seriously. They're completely different interventions.

Simple, easy, quick, yes. But they're completely different interventions, and if you do the wrong one, the patient's not going to improve. No, you're not going to get an ABG (I hope) or halt the lipoma excision mid-case for an emergent V/Q scan to figure it out, but you asked

how is your intervention different for shunt and/or V/Q mismatch in the hypoxemic patient over more run of the mill causes of hypoxemia?

Hypoventilation and mainstem intubation are two common run of the mill causes of hypoxemia, one due to V/Q mismatch and one not, and the intervention is different. If you want a different answer, ask a different question!


FWIW, I used to fly airplanes too. Haven't done so in 12 or 13 years, and I never owned anything slick like a King Air. I was in the rent-the-beat-up-POS-by-the-hour club. So here's my analogy retort ... can't mimic your style, but you get the idea ...

Final approach.

In the clouds.

Dropping a little bit faster than you'd like. Not getting up close and personal with the trees and power lines yet, but you need to level off the approach a bit.

Small adjustment needed!

Small, dude.

Power or attitude?

Dude, a crude attitude and you're screwed. If you're shrewd and clued, you'd conclude more power is the key to not being worm food. Dude.

Simple actions. Simple choices. But pick the wrong one and maybe you'll be the next feature in that NTSB database.



C'mon jet - arguing that the diagnosis doesn't matter 'cause you're going to fly by the seat of your pants is the kind of thing SRNAs do when they're deer-eying a brewing crisis and just doing random ****. Diagnosis may be mostly subconscious when your anesthesia-fu is strong, but you're still doing it.


PS - One of my Marine pilot friends liked to call Bonanzas "split tail doctor killers" in honor of all the weekend physician pilots who taxi onto the wrong runway or try to beat the storm without an instrument rating ... he flew Harriers so my response was always to ask if he liked to play lawn darts at home, too. Fly safe, and watch out for those Bonanzas ...

 

[jetproppilot]

Uh, yeah, seriously. They're completely different interventions.

Simple, easy, quick, yes. But they're completely different interventions, and if you do the wrong one, the patient's not going to improve. No, you're not going to get an ABG (I hope) or halt the lipoma excision mid-case for an emergent V/Q scan to figure it out, but you asked



Hypoventilation and mainstem intubation are two common run of the mill causes of hypoxemia, one due to V/Q mismatch and one not, and the intervention is different. If you want a different answer, ask a different question!


FWIW, I used to fly airplanes too. Haven't done so in 12 or 13 years, and I never owned anything slick like a King Air. I was in the rent-the-beat-up-POS-by-the-hour club. So here's my analogy retort ... can't mimic your style, but you get the idea ...

Final approach.

In the clouds.

Dropping a little bit faster than you'd like. Not getting up close and personal with the trees and power lines yet, but you need to level off the approach a bit.

Small adjustment needed!

Small, dude.

Power or attitude?

Dude, a crude attitude and you're screwed. If you're shrewd and clued, you'd conclude more power is the key to not being worm food. Dude.

Simple actions. Simple choices. But pick the wrong one and maybe you'll be the next feature in that NTSB database.



C'mon jet - arguing that the diagnosis doesn't matter 'cause you're going to fly by the seat of your pants is the kind of thing SRNAs do when they're deer-eying a brewing crisis and just doing random ****. Diagnosis may be mostly subconscious when your anesthesia-fu is strong, but you're still doing it.


PS - One of my Marine pilot friends liked to call Bonanzas "split tail doctor killers" in honor of all the weekend physician pilots who taxi onto the wrong runway or try to beat the storm without an instrument rating ... he flew Harriers so my response was always to ask if he liked to play lawn darts at home, too. Fly safe, and watch out for those Bonanzas ...

Well said.

My bad.