ABG questions

kitsune01, BSN, RN · Jul 4, 2008

Okay, I just learned this, but I think I understand the basics, so here goes:

1) It's full compensation because the pH is within normal range of 7.35-7.45. I think you can tell it's respiratory acidosis because the pH is on the low end and because the 02 is low.

2) It is without compensation because the PaCO2 is in the normal range, showing that the lungs aren't attempting to compensate by retaining extra CO2

3) It's respiratory alkalosis because the pH is high and the CO2 is low. Those are the two parts of the ABG that "match" If the pH was low then it would be metabolic acidosis because then the pH and the low bicarb would "match". There is compensation because the bicarb is low, indicating that the kidneys are dumping bicarb to bring the pH back down. It is partial compensation because the pH is still high.

4) pH=7.28, so it's an acidosis. You look at the paCO2 and Bicarb to see which one would lead to acidosis. The paC02 is low (which by itself would suggest alkalosis), and the bicarb is low (indicating acidosis) So the bicarb is the one that matches and it is metabolic acidosis. There is compensation because the CO2 is low, indicating the body is trying to blow off the extra CO2 to shift the CO2 + H20 - H2CO3 - H+ + HCO3- equation to the left. It is partial compensation because the pH is still low.

5) Yep, it is complete compensation because the pH is back up to the bottom of the normal range.

Hopefully I got those right, and I hope that helps

Hands and Heart · Jul 4, 2008

Ok I thought I understood metabolic/respiratory acidosis/alkalosis but the more examples I look at the more confused I get.
1) pH 7.35, PaCO2 66, bicarbonate 38, PaO2 70 = respiratory acidosis w/ complete compensation. And why is it respiratory instead of metabolic? Is it because of the low O2 value?
Now, it's complete compensation because both values, 66 & 70, are out of range correct?
2) pH 7.52 PaCO2 45, bicarbonate 36, PaO2 95 = Metabolic alkalosis w/o compensation.
I understand this example, the bicarbonate is too high, therefore its metabolic, but again, why is it without compensation? I thought full compensation was when the lungs or kidneys are able to correct for changes in pH when the other system is unable to and partial compensation was when the lungs or kidneys are unable to correct completely for pH change and the pH is not in normal range?
3) pH 7.55 PaCO2 24, bicarbonate 20, PaO2 95 = resp. alkalosis w/ partial compensation.
Ok, so in this example both the CO2 and the bicarbonate values are out of range,so why is it respiratory and not metabolic? And why is it partial compensation?
4) pH 7.28 PaCO2 24, bicarbonate 15, PaO2 95 = metabolic acidosis w/ partial compensation.
Again, I don't understand this one at all.
5) pH 7.35 PaCO2 24, bicarbonate 15, PaO2 95 = Metabolic acidosis w/ complete compensation.
This example is just like the one above, so basically the complete/partial compensation depends on the pH value?
By the way, I am getting these questions from my med/surge workbook by IGNAtavicius, chapter 19, p. 130.

Steps to interpreting ABGs:

1. Determine if the pH is acidotic or alkalotic based on the midpoint value of the range 7.35-7.45. Therefore, anything below 7.4 would be considered acidotic and anything above 7.4 would be considered alkalotic.

2. Determine whether the determined acidotic/alkalotic state is caused by respiratory or metabolic issues. Which value matches the pH imbalance? CO2 changes reflect respiratory and HCO3 changes reflect metabolic.

The normal range for CO2 is 35-45. CO2 is the acid. So if the value is below 35, the person is losing CO2 from the body (through the respiratory tract), leaving a respiratory alkalotic state. If the CO2 is above 45, the person is retaining the acid CO2, causing a respiratory acidotic state.

The normal range for HCO3 is 22-26. HCO3 is the base. If the ABG value is below 22, the body is losing the HCO3 from the body (through the kidneys), leaving a metabolic acidotic state. If the HCO3 value is above 26, the body is retaining HCO3, causing a metabolic alkalotic state.

3. To determine if the body is trying to compensate, look at the other value that was not involved in your determination of respiratory or metabolic status. So if in step 2, you determined that the ABG represented respiratory acidosis, look at the HCO3 value. Is it out of its normal range? If it is the body is trying to compensate. Same goes for the CO2 if you determined that the cause of the state is metabolic.

4. To determine if the compensation is partial or complete, look back to the pH. If the pH is back in its normal range of 7.35-7.45, then the compensation is complete. If it is not, the compensation is partial.

Let's look at question #1:

pH 7.35, PaCO2 66, bicarbonate 38, PaO2 70

Step 1: Using the value of 7.4 as our desired pH, we find our given value on the acidotic side.

Step 2: PaCO2 is high. Retaining CO2 leads to acidosis. You can double check this by looking at the HCO3. Its value is 38, high. If the bicarbonate was the cause of the imbalance, the pH would be high indicating alkalosis, so we know the cause of the imbalance is respiratory acidosis.

Step 3: The bicarbonate is high. The body is trying to compensate.

Step 4: The pH is back within the normal range. Compensation is complete.

Hope this helps. Try it out on the other problems. Good luck.

Daytonite, BSN, RN · Jul 4, 2008

1) ph 7.35 (low normal), paco2 66 (high), bicarbonate 38(high), pao2 70 = respiratory acidosis w/ complete compensation. and why is it respiratory instead of metabolic? is it because of the low o2 value?

the ph is at the low end of normal. both the carbon dioxide and bicarb are out of whack. respiratory is in trouble because o2 is only 70. because the patient's ph is closer to the acid end of the ph and the co2 out of whack, this is

respiratory acidosis

.

2) ph 7.52 (high-alkalosis) paco2 45 (normal), bicarbonate 36 (high), pao2 95 = metabolic alkalosis w/o compensation.

metabolic alkalosis.

co2 is normal and bicarb is out of whack making this a metabolic problem. not compensated because the ph has not adjusted back to a normal range.

3) ph 7.55 (high-alkalosis) paco2 24 (low), bicarbonate 20 (low), pao2 95 = resp. alkalosis w/ partial compensation.

actually, according to my references the bicarb is at the low end of normal. it is

respiratory alkalosis

because the co2 is out of whack. this can happen when people hyperventilate.

4) ph 7.28 (low-acidosis) paco2 24 (low), bicarbonate 15 (low), pao2 95 = metabolic acidosis w/ partial compensation.

metabolic acidosis.

both bicarb and carbon dioxide are out of whack, but o2 is 95, so respiratory system is doing ok. problem is metabolic. ph is really low, so patient is in acidosis.

5) ph 7.35 (low end of normal) paco2 24 (low), bicarbonate 15 (low), pao2 95 = metabolic acidosis w/ complete compensation.

metabolic acidosis

. both bicarb and carbon dioxide are out of whack, but o2 is 95, so respiratory system is doing ok. problem is metabolic. ph is at low end of normal, so acidosis.

do not make this more difficult than it is.

first, you need to know that normal ph is 7.35-7.45.

acidosis is anything below 7.35
alkalosis is anything above 7.45

second, you need to know what your normal bicarb (hco3) and carbon dioxide (co2) levels are. whichever one is out of whack determines whether you have a respiratory or metabolic situation.

carbon dioxide is
always
the
respiratory
component; if it is out of whack along with the ph you have a respiratory acidosis or alkalosis situation
bicarb is
always
the metabolic component; it it is out of whack along with the ph you have a
metabolic
acidosis or alkalosis situation
when you have
both
carbon dioxide and bicarb out of whack the o2 will help you judge if it is respiratory or metabolic. if one value can clearly be seen to be more out of whack than the other, then that is the component to focus on.

when diagnosing, focus on the ph, bicarb and carbon dioxide levels.

there are links to acid/base websites on post #24 and abg tutorials on post #46 of this sticky thread in the nursing student assistance forum:

https://allnurses.com/forums/f205/pathophysiology-p-fluid-electrolyte-resources-145201.html - pathophysiology/ a & p/ microbiology/ fluid & electrolyte resources

kitsune01, BSN, RN · Jul 5, 2008

3) ph 7.55 (high-alkalosis) paco2 24 (low), bicarbonate 20 (low), pao2 95 = resp. alkalosis w/ partial compensation.
actually, according to my references the bicarb is at the low end of normal. it is
respiratory alkalosis
because the co2 is out of whack. this can happen when people hyperventilate.

in my path class we were given the normal values for bicarb as 22-26, with the usual caveat that normal values will vary slightly depending on which reference you or your hospital uses. the book the op got those problems from may be using 22 as the bottom of normal.

Bortaz, MSN, RN · Jul 5, 2008

https://allnurses.com/forums/1536277-post2.html

ROME was helpful to me, when we were studying it. The post I linked explained it very well.

This is how I learned it...
ROME:
Respiratory= Opposite:
- pH is high, PCO2 is down (Alkalosis).
- pH is low, PCO2 is up (Acidosis).
Metabolic= Equal:
- pH is high, HCO3 is high (Alkalosis).
- pH is low, HCO3 is low (Acidosis).
HTH,
B.

Daytonite, BSN, RN · Jul 5, 2008

rome was helpful to me, when we were studying it. the post i linked explained it very well.

the rome mnemonic is also posted on:

https://allnurses.com/forums/f205/pathophysiology-p-fluid-electrolyte-resources-145201.html - pathophysiology/ a & p/ microbiology/ fluid & electrolyte resources - post #25