Need help with ABG Analysis - page 2

by lheadRN

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pH- 7.36 - Normal PaCO2-58-High PaO2-50- Low HCO3-33- High What would the history and assessment of a patient with ABGs like this look like and what is the treatment? What do I do when the pH is normal? Since that is... Read More


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    http://www.rnceus.com/abgs/abgmethod.html
    http://www.scribd.com/doc/6442924/ABG-Interpretation

    Than the following, I got from a website that I thought may help you understand better. I hope so anyway.

    Normal ABG values are as follows:
    1. pH (acid base balance) = 7.35 to 7.45
    2. CO2 (carbon dioxice) = 35 to 45
    3. HcO3 (bicarbonate) = 22 to 26
    You also must note the following:

    CO2 greater than 45 is acidotic

    1. HcO3 less than 22 is acidotic
    2. Co2 less than 35 is alkalotic
    3. HcO3 greater than 26 is alkalotic
    How to interpret ABGs:

    All you have to do is memorize four basic questions and then answer them in order:

    A. Is the ABG normal? If all of them are, then you have a normal ABG and you can stop here. If any one of the values is out of the normal range, then you must move on to the next question.

    B. Is the pH Acidotic or Alkalotic?: To determine this you look only at the pH.


    1. Alkalotic: If the pH is greater than 7.41 the patient is Alkalotic.
    2. Acidotic: If the pH is below 7.39 the patient is acidotic.
    C. Is the cause respiratory or metabolic?: To determine this, you look at pH and compare it with HcO3 and CO2. If the pH is acidotic, you look for whichever value (HcO3 or CO2) that is also acidotic. If the pH is alkalotic, you look for whichever value (HcO3 or CO2) is also alkalotic.

    In this sence, you match the pH with HcO3 and CO2. If the pH matches with the CO2, you have respiratory. If the pH matches with the HcO3, you have metabolic.

    Or, put more simply:


    1. Metabolic Alkalosis: If the pH is alkatotic and the HcO3 alkalotic.
    2. Respiratory Alkalosis: If the pH is alkalotic and the HcO3 is alkalotic
    3. Metabolic Acidosis: If the pH is acidotic and the HcO3 acidotic.
    4. Respiratory Acidisis: If the pH is acidotic and the CO2 is acidotic.
    D. Is the cause compensated or uncompensated?

    1. Uncompensated: if the pH is anywhere outside the normal ranges (greater than 7.35 or less than 7.45)
    2. Compensated: pH is anywhere inside the normal ranges (Anything between 7.35 to 7.45)
    So, here are some examples:

    1. Ph 7.40, CO2 37, HcO3 23

    What do you have here? All the number are within normal range, so you have a normal ABG.

    That was easy enough. You need to go no further in analyzing this ABG.

    2. ph 7.23, CO2 50, HcO3 24

    What do you have here?

    A. Is the ABG normal? You can see right away that all the values are out of the normal range, so you must move on to the next question.

    B. Is the pH acidotic or alkalotic? Since the pH is less than 7.39 it is acidotic.

    C. Is is metabolic or respiratory? Since the pH is acidotic and the CO2 also acidotic, then you have respiratory acidosis.

    D. Is it compensated or uncompensated? Well, the pH is outside the normal range of 7.35 to 7.45, so it is uncompensated. You don't have to look at any other values. You are done.

    The ABG is uncompensated respiratory acidosis

    2. pH 7.36, CO2 50, HcO3 29,

    A. Is the ABG normal? You can see right away that both CO2 and HcO3 are out of the normal range, so you move on to the next question.

    B. Is is acidotic or alkalotic: The pH is less than 7.39, so it is acidotic

    C. Is the cause respiratory or metabolic?: The pH is acidotic and the CO2 is also acidotic, so you have respiratory acidosis.

    D. Is it compensated or uncompensated? Since the pH is within normal limits, it is compensated.

    In this example you have compensated respiratory acidosis.

    3. pH 7.50, CO2 42, HcO3 33

    A. Is the ABG normal? No. Some of the values are outside the normal ranges.

    B. Is it acidotic or alkalotic? The pH is greater than 7.41, so it is alkalotic.

    C. Is the cause respiratory or metabolic?: You know the pH is alkalotic, so you look for the matching value. The HcO3 is alkalotic, so it matches the pH. So, what you have is a metabolic problem.

    D. Is it compensated or uncompensated? Since the pH is outside the normal range of 7.35 to 7.45, it is uncompensated.

    Thus, you have uncompensated metabolic alkalosis.

    4. pH 7.50, CO2 18, HcO3 24

    A. Is the ABG normal? No, pH and CO2 are both out of the normal range.

    B. Is it acidosis or alkalosis? Since the pH is greater than 7.41 it is alkalosis

    C. Is is respiratory or metabolic? Since the pH is alkalotic and the CO2 is also alkalotic, you have a respiratory problem

    D. Is is compensated or uncompensated: It is uncompensated because the pH is outside the normal range of 7.35 to 7.45.

    What you have here is uncompensated respiratory alkalosis.






    People tend to get confused on the difference between CO2 and HCO3-. Remember that the job of the lungs is to expel the gas carbon dioxide (CO2) and bring in O2, and that bicarbonate (HCO3-) is an alkaline agent (sodium bicarbonate is regularly used in the hospital to raise patients' pH).
    The trick seems to be to think of CO2 as an acid. In looking at the above equation, you can see that if you were to increase CO2, (by hypoventilating, for example), you would drive the equation towards carbonic acid. Obviously, this increases the acidity of the blood (lowers the pH).
    The carbonic acid dissociates into a strong acid (H+) and a weak base (HCO3-).
    Similarly, if you were to hyperventilate and blow off lots of CO2, you would drive the equation to the left, using up acidic molecules, and thereby raising the pH.
    So the lungs can control pH by controlling CO2 - they retain it to lower pH and make blood more acidic, and they expel CO2 to raise pH and make the blood alkaline. Those are the fundamental points to remember about respiratory disorders.
    The kidneys have different mechanisms. They control the excretion of the ions on the RIGHT side of the equation - H+ and HCO3-.
    Obviously, excreting H+ raises the pH (less acid - more alkaline blood) and retaining H+ lowers the pH (more acid = more acidid blood).
    Similarly, excreting HCO3- lowers the pH (less base = more acidic blood), and retaining HCO3- raises the pH (more base = more alkaline blood).
    That's it for the basics of metabolic disorders - the kidney modifies the concentrations of H+ and HCO3-. There are many other metabolic reasons why the blood becomes alkaline or acidic, but always keep these basics in mind.
    The final thing to consider is that when one system (say the lungs) tips the balance, the other system (say the kidneys) tries to compensate. A good rule of thumb is that the second system can NEVER overcompensate. In simple acid-base disorders, you can assume that if the blood is acidic, the primary disorder is an acidosis (DON'T assume, therefore, that the primary disorder is an alkalosis that has overcompensated).
    lheadRN and SDChargersGirl#31 like this.
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    chenoaspirit,

    Compensation does not necessarily mean that the pH has returned to it's normal range. You can have partial compensation, where the pH has is still outside its normal range, however, compensatory mechanisms are evident.
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    Well, I copied/pasted this from an education website for nurses.
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    Compensated means compensated and yes the pH has returned to normal range. Yes there is also a partial compansation. I dont really know what you are meaning or trying to infer.
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    Quote from chenoaspirit
    Well, I copied/pasted this from an education website for nurses.
    It does an excellent job at explaining the basics, but it doesn't adequately explain compensation. Yes, being "fully compensated" means a return to a normal pH, but this is simplifying it.

    For instance, imagine the following ABG values:

    pH 7.32
    CO2 59
    HCO3 32

    It is clear that there is a respiratory acidosis from the pH and CO2 values, however, the HCO3 is also elevated. This is the compensatory mechanism. However, the pH is still acidotic, indicating that it isn't fully compensated.

    However, if we went solely by your copy/pasted example, it would be called "uncompensated", which is wrong.

    eta: ABG values are typical of a 50-50 club COPDer.
    Last edit by hypocaffeinemia on Feb 1, '09
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    Well, I am guessing it will be easier for her to learn the basics first, THEN she can go into the partial compensation issues. If someone had thrown every angle of compensation/partial into learning it I would have been confused. I first had to learn about compensation. Once I learned about that, then I learned about the partial compensation. But thanks for your additional input, Im sure she will appreciate it.
    OP, I also listed a couple websites to help you learn. But dont try to learn everything at once or you will not understand it. First, memorize the values and how they affect pH and such. Its a confusing thing to learn.
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    Guys! There is an ABGs analysis sheet on the weblinks that you merely need to print out and refer to when analyzing these things. All you need is to know is normal pH, CO2, HCO3 values, the ROME mnemonic and the partially compensated tables chart and you are good to go. These things don't even fill up a page.
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    Where is the weblinks you are referring to?
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    post #45 on http://allnurses.com/nursing-student...gy-145201.html - pathophysiology/ a & p/ microbiology/ fluid & electrolyte resources in the nursing student assistance forum

    i already listed this on post #2 of this thread.
  10. 0
    Thanks to everybody that replied to my question!!!


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