PaO2

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skyemt

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Ok...

so, i understand that an SPO2 reading of 90 correlates with a PaO2 reading of 60... and that this is a critical level to remain above...

i want to check my understanding of why... is this the point on the oxygen desaturation curve, where below this point, desaturation occurs swiftly?

is this level of PaO2 the proverbial cliff?

i would like to expand my understanding here, if i am off...
as i learn about this, it seems that the focus would be to do everything you can to prevent that PaO2 from dropping below 60...

i realize my understanding may be rudimentary, but i would like to learn more...
comments welcome...
 
SpO2 isn't quite that simple. It's more of a measure of the saturation of RBCs than a measure of PaO2. That said, the level of O2 carried in solution is very small.

(bracketed units need number. Unbracket units are for book keeping)
Code: 
PaO2=((0.003mL O2/100mmHg PO2)[PO2]) + (([g Hb]/100mL)(1.34mL O2/g HB)SaO2).
So, each gram/100mL of hemeglobin carries 400 times the amount of oxygen as each 100mL of plasma. A normal patient has somewhere around 15g Hb/100ml (12-18 is normal range).

So, a anemic patient can have an honest saturation of 100% yet still be hypoxic.
 
JPINFV said:
SpO2 isn't quite that simple. It's more of a measure of the saturation of RBCs than a measure of PaO2. That said, the level of O2 carried in solution is very small.

(bracketed units need number. Unbracket units are for book keeping)
Code: 
PaO2=((0.003mL O2/100mmHg PO2)[PO2]) + (([g Hb]/100mL)(1.34mL O2/g HB)SaO2).
So, each gram/100mL of hemeglobin carries 400 times the amount of oxygen as each 100mL of plasma. A normal patient has somewhere around 15g Hb/100ml (12-18 is normal range).

So, a anemic patient can have an honest saturation of 100% yet still be hypoxic.
Click to expand...

true, but an SpO2 reading of 90 does correlate to a PaO2 reading of 60, if you take out the rare exceptions (anemia, sickle cell, CO, etc..)
 
skyemt said:
true, but an SpO2 reading of 90 does correlate to a PaO2 reading of 60, if you take out the rare exceptions (anemia, sickle cell, CO, etc..)
Click to expand...

For the SpO2 to be 90% and the PaO2 to be 60 mmHg, you are assuming the oxyhemoglobin curve is in normal position. The position of this curve may shift rightwards (lower saturation for given PaO2) or leftwards (higher saturation for a given PaO2).

http://www.utmb.edu/ERC/selfstud/pulseoximetry/curve.htm

http://www.ccmtutorials.com/rs/oxygen/page06.htm

To further this discussion you would have to get into tissue oxygenation and SvO2 monitoring to adequately grasp the big picture.

http://www.ccmtutorials.com/rs/oxygen/page04.htm

ALL ABOUT OXYGEN
http://www.ccmtutorials.com/rs/oxygen/index.htm

http://dwb.unl.edu/Teacher/NSF/C14/C14Links/www.mtsinai.org/pulmonary/ABG/PO2.htm

http://kcweb.nhmccd.edu/programs/respcare/O2.ppt

Good schematic : Pathophysiology of Oxygen Delivery in Respiratory Failure
http://www.chestjournal.org/cgi/content/full/128/5_suppl_2/547S/F1

SvO2
http://www.lakesidepress.com/pulmonary/books/physiology/chap6_3.htm

http://signavitae.com/index.php?option=com_content&task=view&id=19&Itemid=6
 
One more very important thing to consider is A-a gradient.

If delivering 100% O2 via NRBM or ventilator and the SpO2 is 90%, the PaO2 could be 60 mmHg and the PAO2 may calculate to >450 depending on the barometric pressure. This gives you a very large A-a gradient. The SpO2 could even be 100% with a large A-a gradient and considerable work of breathing.

You will still have a very sick patient even with 100% SpO2.

See V/Q mismatching in ALL ABOUT OXYGEN from previous post.

Similar principles will also apply when using ETCO2.
 
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Man, it's people like Vent that make me look like someone who belongs on the short bus;). Just look at all the research Vent provides you with just about every possible question.

Now I know you're in school, and I do not want to contradict anything your instructor(s) may or may not teach you.

I say that because I'm more of a philosophical teacher, student, and provider. Vent's right. Roughly the majority of pts you will encounter with a curve that is not in midline position would probably be your medicals with advanced age and/or disease. Your young, healthy trauma's might present with a more midline curve.

But can you garrantee that in the prehospital arena? Naw.

Generally speaking, when your PaO2 gets less than around 60 torr, the body's organs begin to feel the effects of a decreased oxygenation environment. If this gets bad enough for long enough, cells begin to die. When they die, they don't come back. We've all heard to term "anoxic brain injury". This is how it happens. You usually have about 4-6 minutes, according to the American College of Surgeons, to get them adequately oxygenated.

Because what are you going to do if the curve shifts to the left? Give 'em O2. What about when it shifts to the right? Give 'em O2. What about a 22 yo rollover ejection? You guessed it. Give 'em O2.

Now, sky, it sounds like you are getting a handle on all of this. Keep up the good work. Good luck.
 
MSDeltaFlt,

I know Egan and Shapiro are still close to your heart. :D

I am glad to see some asking about the "whys" and not just recipes.
 
VentMedic said:
MSDeltaFlt,

I know Egan and Shapiro are still close to your heart. :D

I am glad to see some asking about the "whys" and not just recipes.
Click to expand...

Thanks, Vent. There's a philosophical theorum written I don't know how many years ago by I can't remember who that goes something like this:

"If you understand the 'why', then the 'what' and the 'how' will make sense."

Ever since I started applying that, I started learning things a lot faster and more easily.
 
MS and Vent,

thank you very much for the replies!

i am trying to get a handle on this...

if i were to be honest, it is a bit like pandora's box... every time i grasp a concept, it leads to more questions, then i realize more how much i don't know!

i would like to understand more practical applications of this to prehospital care... obviously, and understand of this physiology can help in the critical time before the PaO2 drops below 60... of course, i won't know that in the field...

also, the shift in the curve equates to more time/less time basically?

i did not get a chance to go through all the links vent posted, i will, and then probably have more questions.

since i got my basic cert, i have spent much of the time learning, and learning that i didn't (and don't) very much about A/P...

but... one chapter at a time... lol

thanks again for the help!
 
JPINFV said:
SpO2 isn't quite that simple. It's more of a measure of the saturation of RBCs than a measure of PaO2. That said, the level of O2 carried in solution is very small.

(bracketed units need number. Unbracket units are for book keeping)
Code: 
PaO2=((0.003mL O2/100mmHg PO2)[PO2]) + (([g Hb]/100mL)(1.34mL O2/g HB)SaO2).
So, each gram/100mL of hemeglobin carries 400 times the amount of oxygen as each 100mL of plasma. A normal patient has somewhere around 15g Hb/100ml (12-18 is normal range).

So, a anemic patient can have an honest saturation of 100% yet still be hypoxic.
Click to expand...

as a corollary, it's my understanding that cyanosis (not sure if it goes for both peripheral and central, or just peripheral) develops when 5g hb/100ml desaturates...

if that is correct, that means that an adult could de-saturate 1/3 of there hemoglobin before becoming cyanotic, and a child closer to 1/2...

would this not make cyanosis a VERY late and ominous sign of hypoxia/hypoxemia?
 
There is not to much more that can be said, there is a lot of info here. Allow me to maybe add...

The O2 dissociation curve that the SPo2 monitor uses to spit out a value that you can read from the display is obtained from tables/values that are programmed into the monitor. The clever guys with the white coats got these values from performing test on healthy adults. As mentioned, your patient , will most likely not fall into this category. Is it then wrong to use the SPo2?? Not at all, it the best we've got and adds value to the overall presentation of the patient to you.

It is also important to know all the false positives and negatives, as well as the mechanical working of the device. This will help you to to truely interpret the value you see on the display. Something you are highly unlikely to find in the manual of textbook: "Treat the patient, not the monitor" (Not saying you aren't)!!
 
OK guys, i am actually starting to get a grasp of this! i think!

so, with regards to the disassociation curve...

the problem is twofold... as the partial pressure drops, and the patient begins to de-saturate... the disassociation curve also moves to the right, due to work of breathing, hypercarbia, and acidosis... leading to a crash of the patient! IF I AM UNDERSTANDING THIS CORRECTLY...

a main goal of prehospital care would then be slow the shifting of the curve rightwards as much as possible...

a shift in the curve leftwards is desirable, and achieved with lower metabolic rates and lower core temperatures... then, probably no accident that hypothermia treatments for cardiac resuscitation are proving beneficial... basically, a big shift in the curve to the left!

sorry for the punctuation marks, but seems like light bulbs going on all around me...

if i am off track, please advise!
 
ok... a bit more...

when we give someone O2 via NRB, basically, we are increasing the partial pressure of O2 at the alveolar level...

in a healthy person, due to the flatness of the curve at high partial pressures, you will not see a big increase in SaO2...

however, in a diseased person who is experiencing a "rightward shift" of the disassociation curve, there will be a greater improvement in the SaO2...

is that correct?

also, with regards to JP's earlier statements... if someone is anemic, therefore less Hb... what Hb is present will still bind O2 normally, leading to the high SaO2 readings... however, due to there being less quantity of oxyhemoglobin in the blood, the overall O2 available to offload will be lower, hence the lower PaO2 for that given higher SaO2...

am i understanding this correctly?

thanks...
 
The pulse oximeter doesn't know about the Oxyhemoglobin curve. The pulse ox measures the hemoglobin and gives the percentage of oxygenated Hb. The principle of pulse oximetry is based on the red and infrared light absorption characteristics of oxygenated and deoxygenated hemoglobin. Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through. Deoxygenated hemoglobin absorbs more red light and allows more infrared light to pass through.

Of course PaO2 is just a one part of the picture. Notice also that the units is pressure as in mmHg. That confuses many people.

The lakepress website has some of the most useful information since it is also exerts from a book for ABG interpretation.
http://www.lakesidepress.com/pulmonary/ABG/PREFACEnew.htm

Sky, the link below will answer the questions from you last posts. You are on the right track. For practical example: CaO2 is a big factor athletes strive for by increasing their Hb by illegal and/or dangerous ways. Altitude training is better but takes times. Babies with congenital cyanotic heart defects will have a higher Hb/HCT level as will some patients who have chronic lung diseases with very low PaO2 levels.

http://www.lakesidepress.com/pulmonary/ABG/PO2.htm

http://www.lakesidepress.com/pulmonary/ABG/MixedAB.htm

http://www.lakesidepress.com/pulmonary/ABG/bicarbgap.98.htm

http://www.lakesidepress.com/pulmonary/ABG/bicarbgapx.98.htm

Good site with advice for new doctors:
http://www.lakesidepress.com/pulmonary/books/house/rules-a.html

EXAMPLE:

15 Rules on Oxygen Therapy -- What Every House Officer
Should Know

http://www.lakesidepress.com/pulmonary/books/house/rules-a.html#15rules

12. High FIO2 doesn't affect COPD hypoxic drive.
The reason a high FIO2 may raise PaCO2 in a patient with COPD is not because the extra oxygen cuts off the hypoxic drive. Modest rise in PaCO2 occurs mainly because the extra oxygen alters V/Q relationships within the lungs, creating more physiologic dead space.
Click to expand...

14. Face masks cannot deliver 100% oxygen unless there is a tight seal.
So-called non-rebreather face masks can deliver an FIO2 up to around 80%. It is a mistake to label a patient with any loose-fitting face mask as receiving an "FIO2 of 100%." (Again, 100% oxygen can only be delivered with a ventilator or tight-fitting face mask.)
Click to expand...

(Yes, doctors commonly use the FiO2 and Percentage of FiO2. interchangeably. New computer charting systems help them out when they forget the decimal point in some documentation.)

And now my favorite: ARDS

http://www.ardsnet.org/

http://www.aafp.org/afp/20020501/1823.html
 
Sky, I hope you're using this for class. To be totally honest with you, I do not even contemplate the dissociation curve whatsoever prehospital. It's pretty cool in the ER when you are trying to get a game plan for the ICU, great in the ICU when their oxygenation doesn't want to work right despite conventional means, and makes you look pretty smart to the cute nurses you want to impress.

You don't want the curve to shift at all. You want it where it belongs: in the middle.

But if someone's critical on a 911 call due to either trauma or severe SOB, I'm going to give them pretty much the same thing. It also happens to be the same thing my protocols tell me to give: Oxygen. I don't care what the curve is doing.

Vent may agree. Vent may not agree. That's just what's going on in my mind when I'm critically thinking.
 
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I agree on the "well, it really doesn't matter street level due to education, environment, tools, end goal (reach the hospital with a patient who is hopefully in a better situation than when they were found). On the other hand, its rather refreshing to see people who want to increase their education past "blood goes round and round and you need oxygen, so all patients get a NRB at 15LPM regardless of their complaint (stubed toe, needs ride to the store next to the hospital, etc). Oh, look, here's the short bus to take me to work!" (we've all worked with that provider)

I'm all for increased education, be it formal (class room) or informal (internet forum) as long as the provider understands the limitations of its application to the environment and restrictions that we find ourselves in. It's also helpful to understand that a pink patient with an SpO2 of 100 is not necessarily not hypoxic.
 
MSDeltaFlt said:
But if someone's critical on a 911 call due to either trauma or severe SOB, I'm going to give them pretty much the same thing. It also happens to be the same thing my protocols tell me to give: Oxygen. I don't care what the curve is doing.
Click to expand...

It is sad when physical assessment and listening to what the patient is saying takes the back seat to a pulse oximeter that will give a teddy bear a 99% SpO2 if the ceiling light catches it just right.

Not all patients will need a NRBM but again that depends on the assessment.


If a patient has a minute volume requirement of 22 L/Min and the NRBM is providing 15 L/Min, how well are you meeting that pt's demands?

Is the NRBM actually "high flow"?
 
This thread is on the go!

Now add Maternal Hb to the picture, and that our aim/use for the SPo2 monitor might be to measure oxygenated/deoxygenated Hb, it measures the percentage of saturated Hb. Thus reminding you of Carbon monoxide which has a higher affintiy for Hb than O2, yet you can still get a good (Negative) reading, in the presence of hypoxemia.

To vent: I often come across people refering to Fi02 as a percentage, i think it is more a lack of understanding. So you say it is a new thing?? I can't grasp the concept, as Fio2 is: Fraction of inspired oxygen. I am by no means a mathematician (and stand to be corrected), but how can a fraction be denoted as a percentage?? Its most probibly a podantic concept to try and get across, as we understand in anycase what is meant when Fio2 is refered to as a percentage!!
 
Vent and others, i will study further and be back with another post...
thank you!

MSDelta, you have provided me with a first on this site!!

of course, my protocols say to give O2 and get to the hospital with a better patient as well... so am i likely to consciously consider the disassociation curve in the field? maybe, maybe not...

after countless posts saying to me that the education standard is too low, and that if i don't really fully understand the physiology i have no real business doing this or that, you actually bring it back and say that the knowledge doesn't matter, and that i should just give O2 and go!!!!!

right on brother, there will be many who are pleased to see your post!

i, however, am not one of them... if i understand exactly what is going on on the physiologic level, then i am better able to assess and treat the patient... who knows, it may even help me treat a patient one day...
 
VentMedic said:
One more very important thing to consider is A-a gradient.

If delivering 100% O2 via NRBM or ventilator and the SpO2 is 90%, the PaO2 could be 60 mmHg and the PAO2 may calculate to >450 depending on the barometric pressure. This gives you a very large A-a gradient. The SpO2 could even be 100% with a large A-a gradient and considerable work of breathing.

You will still have a very sick patient even with 100% SpO2.

See V/Q mismatching in ALL ABOUT OXYGEN from previous post.

Similar principles will also apply when using ETCO2.
Click to expand...

i assume then, conversely, that up in the mountains the A-a gradient is inherently lower, leading to mild hypoxia in even healthy individuals??

we are using NRB or other mechanical ventilations to artificially raise the A-a gradient to saturate the hemoglobin with O2... however, this does nothing to fix the underlying pathology of the patient... is that what you are saying Vent??
 
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