Determining ABG results!

allnurses Guide · May 19, 2012

Some basics to help:

Remember that the body does not (normally) overcompensate. If its in the high end of the range think compensated alkalosis, low end think compensated acidosis. If pH and co2 are in opposite directions, think respiratory.

nurseprnRN, BSN, RN · May 19, 2012

abgs made simple

you want simple abgs? piece o' cake. people who have seen this before, well, just scroll on by. newbies who want a brief abg's refresher, take out your pencils and a piece of paper, cuz you'll need to do a bit of drawing .

i taught abg interpretation for yrs in a way that made it pretty foolproof. you will make your own key to interpret abg's, and will be able to reproduce it from memory any time you need to with very little trouble if you learn a very few **key concepts**, labeled **thus**..

take a piece of paper. make a big box on it, then draw vertical and horizontal lines on it so you have four boxes. i will try to make this come out, but...you should have

ab

cd

where the four boxes a,b,c,d are such that a is above c and b is above d. you don't need to label the boxes a,b,c,d, just get them in the right alignment. (this is way easier with a whiteboard bear with me).

*inside* each of the 4 boxes write the following, down the left edge:

ph

co2

bic

now, outside the big box do the following: above the "a" box write "resp"; above the "b" box write "metabolic"

to the left of the "a" box write "acidosis" and to the left of the "c" box write "alkalosis"

now you have a "resp" column and a "metabolic" column, an "acidosis" row and an "alkalosis" row. so you have respiratory acidosis and alkalosis boxes, metabolic acidosis and alkalosis boxes.

with me so far?

now, you're going to label the primary derangements, so later you can tell what's the derangement and what's the compensation. ok? in the respiratory column, underline co2's. in the metabolic column, underline the bicarb's. that's because in **respiratory disorders, the co2 gets messed up**, and in **metabolic disorders, the bicarb is messed up**. you knew that, or could figure it out pretty quick if you thought about it, right? thought so.

now. you are going to put upward-pointing and downward-pointing arrows next to the ph, co2, and bicarb labels inside every box. ready?

ph first. in the "alkalosis" row, make up arrows next to ph, because **ph is elevated in alkalosis (by definition)**. put down arrows in the acidosis row's phs, because **acidosis means a lower that nl ph**.

remember that **co2 is (for purposes of this discussion and general clinical use) acid** and **bicarb is alkaline** (this is the end of the key concepts. not too bad, huh?). (oops, i forgot: **nls are generally accepted as ph 7.35-7.45, co2 35-45 (nice symmetry there), bic 19-26**)

now go to the box that is in the respiratory column and the acidosis row. figured out that co2 must be elevated? good. put an up arrow next to that co2. go to the respiratory alkalosis box. figures that co2 must be low to cause this, right? put a down arrow next to that co2.

ok, now go to the next column, the metabolic one. i think you can figure out what happens here: in the metabolic alkalosis box, put an up arrow next to the bic, because high bicarb makes for metabolic alkalosis. put a down arrow next to the bic in the metabolic acidosis box, because in metabolic acidosis the bicarb is consumed by the acids (like, oh, asa) and is low.

you are now going to put arrows next to the blank spots in your boxes that show compensatory movements. ready? ok, what does your body want to do if it has too much acid? right, retain base. yes, of course if your body has too much acid it would like to get rid of it...but if it can't do that, then retaining bicarb is the compensation. so for every elevated co2 you see, put an up arrow with its bicarb.( chronic co2 retainers always have elevated bicarbs, and this is why.) you will find an up arrow next to the co2 in the resp/acidosis box.

so if your body is short on acids, what does it do? right, excrete base. so put a down arrow next to the bicarb in the resp/alkalosis box, because chronic low co2 makes the body want to get back into balance by getting rid of bicarb. however, remember that it takes a day or two for the kidney to do this job, and if you have nonfunctioning kidneys they won't do it at all.

likewise in the metabolic/alkalosis box, a high bicarb makes your body want to retain acid, increasing co2 being the fastest way to do that because all you have to do is hypoventilate, to bring your ph back towards nl. put an up arrow next to the co2 in the met/alk box. see the pattern here? put a down arrow next to the co2 in the met/acidosis box, because if your body has too much acid in it (think : asa overdose? dka?) it will want to get rid of co2 to compensate, and the fastest way to do that is to hyperventilate. this is why patients in metabolic acidosis are doing that deep, rapid breathing thing (kussmaul's respirations).

ok, i hear you wailing: but how do i know whether that elevated or decreased co2 or bicarb in my abg report is primary or compensatory?

well, now you have your key. so take your abg reports and look at them. say, try these. (notice that o2 levels have nothing to do with acid-base balance abg interpretation) (ok, if you are very hypoxic you can get acidotic...but you see that in the metabolic component, not the o2 measurement, because it's lactic acid your body is making if it's working in an anaerobic way)

1) ph = 7.20, co2 = 60, bic = 40.

first thing to look at is the ph. 1) is acidosis, with a low ph. look at your acidosis choices (you have two). find the acidosis where both co2 and bicarb are elevated, and you find your answer: respiratory acidosis with metabolic compensation. this is what you see in chronic lungers who have had high co2's for so long their kidneys have adapted to things by retaining bicarb. (it takes about 24 hrs for your kidneys to make this compensatory effort, so you can tell if your resp acidosis is acute (no or little change in bicarb) or chronic)). (remember, your lungs' first and most important job is not getting oxygen in, it's getting co2 out, and when chronic lungers have co2 retention, they're really getting bad. people with acute bad lungs will often have low oxygens and low co2's , because their ability to gain o2 goes first, and while they're trying to deep breathe their way back to a decent pao2, they hyperventilate away their co2. ....but i digress....)

2) ph = 7.54, co2 = 60, bic = 40

ph here? this is alkalosis, with a high ph.

the only box where ph is high and co2 & bic are both elevated is metabolic alkalosis with respiratory compensation. sometimes you'll see this in people who have a bigtime antacid habit. really. (you can get a short-term metabolic alkalosis with rapid severe vomiting, because the body's nl balance between acid and base has been disrupted due to a sudden loss of acid. things will equilibrate pretty quickly, though, all things considered.)

so even though you have identical abnormal co2's and bicarbs, you can look in your boxes, find the match, and see what you have. remember you underlined the primary disorder in each box?

wanna try another one?

3) ph = 7.19, co2 = 24, bic = 12. bingo, you found it: an acidosis where the co2 and the bic are both abnormally low. only fits in the metabolic acidosis box, so you have a metabolic acidosis with a respiratory compensation effort. incidentally, this is what you see in diabetic ketoacidosis, when they come in huffing and puffing to blow out that co2 because their ketosis is so high. also you see this picture in asa od's, because this is acetylsalicylic acid they ate, and the fastest way to get rid of acid is to blow it off via hyperventilation. increasing your bicarb takes 24-48 hrs. another quick way to get a metabolic acidosis is to poop out a lot of diarrhea, because you lose a lot of bicarb that way. another classic place for this is in mesenteric artery thrombosis, in which you have a lot of ischemic bowel sitting in there screaming for oxygen and making lactic acid when it can't have any.

i know this is long, but trust me, you'll never go wrong with it, and you can recreate it anytime. it doesn't really even matter how you set up your boxes, so long as you have a metabolic and a respiratory axis and an acid/alkaline axis. rotate your paper and you'll see what i mean.

why don't i care about pao2 here? well, because abg's mostly tell you about a/b balance and co2 and bicarb, that's why. probs with them can be serious probs without any abnormality in oxygenation at all.

remember that pao2 (arterial oxygen, measured in torr or mmhg) is not the same as spo2,( hemoglobin saturation, a percentage of red cells carrying oxygen). if you think they are, your pt could be in serious trouble before you do anything. there is a nomogram that shows you the relationship between arterial oxygen and saturation, which i regret i cannot reproduce here. but you can sketch out a basic version...

draw a graph where sats are on the vertical (left) axis and pao2's are on the horizontal (bottom) axis. draw little shaded band across the top at the 95%-100% sat areas. that's your normal saturation. draw a few dots there indicating a line of pao2's of 80-100, because those are normal pao2's.

now draw a dot for spo2 of 90 and pao2 of about 75. now, another dot showing spo2 of 85 and pao2 of about 60. another dot: spo2 of about 80 and pao2 of about 55. connecting all these dots should give you a sort of s curve, indicating that while the top is pretty flat in the pao2 80-100, spo2 95-100 range, pao2 drops off like a shot at decreasing spo2 levels.

your pt with a sat of 85 is not doing ok, he's in big trouble. while a pao2 of 75 torr isn't too bad at all, a sat of 75% is heading for the undertaker unless dealt with.

here's my very favorite abg of all time: ph = 7.11, pao2 = 136, paco2 = 96, bicarb = 36.

what happened to this lady? what will happen next?

"call" if you have any questions...

Double-Helix, BSN, RN · May 20, 2012

A helpful acronym is ROME. It stands for "Respiratory-Opposite, Metabolic-Equal" and relates to the CO2 and the pH.

So what you have to do is compare your pH and your CO2.

Are they OPPOSITE? (pH is high and CO2 is low OR pH is low and CO2 is high) If so- then you're dealing with a RESPIRATORY problem. (Respiratory-Opposite)

Are they EQUAL? (pH is high and CO2 is high OR pH is low and CO2 is low OR pH is high/low and CO2 is normal) If so, then you're dealing with a METABOLIC problem. (Metabolic-Equal)

** Note that this also works with the HCO3.

If they are OPPOSITE (pH is high and HCO3 is low OR pH is low and HCO3 is high) then it's a RESPIRATORY problem.

If they are EQUAL (pH is low and HCO3 is low OR pH is high and HCO3 is high) then it's a METABOLIC problem.

**Now, in regard to when both the CO2 and HCO3 are abnormal- You just have to figure out which one caused the problem.

Remember ROME?

If you have a low pH- first look at the CO2. Is it high or low? If it's high, then they are OPPOSITE so you know it's a RESPIRATORY problem. If the CO2 is low as well then they are EQUAL and you know it's a METABOLIC problem.

Confirm by checking the HCO3. If the pH is low and the HCO3 is low, they are EQUAL and it's a METABOLIC problem. If the pH is low and the HCO3 is high then they are OPPOSITE and it's a RESPIRATORY problem.

** So let's talk about compensation. In simple terms, compensation is when the body tries to fix itself. CO2 is acid- so more CO2 equals more acid and lower pH. HCO3 is alkaline, so more HCO3 equals more basic and higher pH.

So the body recognizes the pH is abnormal. It tries to correct the pH by increasing/decreasing the HCO3 or the CO2. So that's why you'll see two abnormal values- because the body is trying to fix the problem. But in order to interpret the ABG as metabolic/respiratory, you only need to know which abnormal value caused the low pH (ROME).

** If you see two abnormal values then you know the body is trying to compensate. If only one value is abnormal then the body isn't trying to fix the problem and your ABG is uncompensated.

To tell whether it is fully compensated or not, look at the pH. Is the pH in the normal range? If it is, then it is fully compensated. The body has fixed the problem and the pH is normal again.

If the pH is not in the normal range, but you have two abnormal values, then the body is trying to fix the problem, but it hasn't finished yet. So it's partially compensated.

Double-Helix, BSN, RN · May 20, 2012

Let's try a few practice problems.

First, a reminder of normal values: pH = 7.35-7.45 CO2 = 35-45 HCO3 = 22-26

1. pH = 7.28 CO2 = 54 HCO3 = 30

Step 1. Is it acidosis or alkalosis? Remember lower pH = acidic and higher pH = alkalotic. The pH is less than 7.40, so it's ACIDOSIS.

Step 2. Is it respiratory or metabolic?

Let's look at ROME and compare pH and CO2:

pH is LOW (less than 7.35) and CO2 is HIGH (more than 45) so they are OPPOSITE. According to ROME: Respiratory-Opposite. So this is a RESPIRATORY problem.

Confirm this by looking at the HCO3. pH is LOW, HCO3 is above 26, so it's HIGH. They are OPPOSITE. OPPOSITE is a RESPIRATORY problem.

Step 3. Is it compensated or uncompensated?

Are both your CO2 and your HCO3 abnormal? Let's check.

CO2 is above 45, so that's abnormal. HCO3 is below 22, so that's abnormal as well. We have two abnormal values, so we know the body is trying to fix itself. In this case, the body is trying to hold onto HCO3 (more base) to balance the high acid due to the high CO2. So we can say it's either FULLY COMPENSATED or PARTIALLY COMPENSATED.

To find out which, let's look at the pH again. It's 7.28, which is not in the normal range, so it's only PARTIALLY COMPENSATED.

Let's put the answers together to complete the problem: Partially compensated respiratory acidosis.

Here's another one:

2. pH = 7.50 CO2 = 49 HCO3 = 30

Step 1. Is it acidosis or alkalosis? pH is above 7.40, so it's high = ALKALOSIS

Step 2: Is it respiratory or metabolic?

Let's look at ROME:

pH is HIGH and CO2 is above 45, so it's also HIGH. They are EQUAL. According to ROME: Metabolic-Equal. So this is a METABOLIC problem.

Confirm by looking at the HCO3:

pH is HIGH and HCO3 is above 26, so it's also HIGH. They are EQUAL = METABOLIC problem.

Step 3. Is it compensated or uncompensated? We've established that both CO2 and HCO3 are abnormal. So we know the body is trying to compensate. Let's look at the pH. pH is 7.50, which is not in the normal range. So this is PARTIALLY COMPENSATED.

Put it together: Partially compensated metabolic alkalosis.

One more:

3. pH = 7.35 CO2 = 28 HCO3 = 20

Step 1. Is it acidosis or alkalosis?

pH is less than 7.4, so it's ACIDOSIS

Step 2. Is it respiratory or metabolic?

ROME:

pH is LOW and CO2 is less than 35, so it's also LOW. They are EQUAL. EQUAL = METABOLIC problem

Confirm with the HCO3:

pH is LOW and HCO3 is below 22, so it's also LOW. They are EQUAL. EQUAL = METABOLIC

Step 3. Is it compensated or uncompensated?

We have two abnormal values, so we know the body is trying to fix the problem. So let's look at the pH. It's 7.35, which is within the normal range. The body has successfully fixed the pH! So this is FULLY COMPENSATED.

Put it together and we have: Fully Compensated Metabolic Acidosis

Feel free to Private Message me if you have further questions or need clarification.

Esme12, ASN, BSN, RN · May 20, 2012

from vicki assistant administrator at an

i've attached a "tic-tac-toe" type handout i made for my students when i teach abgs. hope this helps :) attached files

this has an interactive quiz with tons of examples. it gives rationales after you answer. abg interpretation quiz

acidic/ normal/ alkaline

ph 7.45

pco2 >45 35-45

hco3- 26

tactic

1 memorize normal lab values

2 check the ph........ is it acidic or alkaline

3 look at the pco2 and hco3 .. whats out of normal

4. look at patient's diagnosis, patients actual assessment .. how do they look when compared to the labs.....do they look "sick"? in distress?

causes of acid-base imbalance

metabolic acidosis: diabetic ketoacidosis, diarrhea, renal failure, shock, salicylate overdose, sepsis. body has less buffer base the bicarb then normal or keeps too much acid.

metabolic alkalosis: loss of gi secretions, overuse antacids, potassium wasting diuretics. body has body has deficit of carbonic acid and decrease in h+ concentration without loss in amount of the base of hco3.

respiratory acidosis: hypoventilation r/t drug overdose, chest trauma, pulmonary edema, airway obstruction, copd, neuromuscular disease. body has less buffer than normal with higher amount of h+ ions.

respiratory alkalosis: hypertension r/t anxiety, high altitude, pregnancy, fever, hypoxia, excess tidal volume in ventilator patients, initial stage of pulmonary embolism. body has deficit of carbonic acid and decrease h+ ion

compensated acidosis or alkalosis is when the ph is normal but either the hco3 or the pco2 is out of whack.

hopeless romantic · May 20, 2012

Hello. I am studying ABG's and, having a hard time trying to figure out how to read an ABG when BOTH CO2 and HCO3 levels are abnormal. I can figure out respiratory alkalosis/acidosis, metabolic alkalosis/acidosis, and compensated/uncompensated when one value is abnormal. But when both values are abnormal how can I tell whether it is respiratory or metabolic and acidosis or alkalosis? Thanks!

Thank you everyone so much! Ashley PICU, RN I understood your version the best and I liked the examples you gave. Thanks so much again everyone!

hopeless romantic · May 20, 2012

Wow thank you so much! Everyone explained it well but something about the way you explained it made it click more for me! Great examples also.

hopeless romantic · May 20, 2012

Hello. I am studying ABG's and, having a hard time trying to figure out how to read an ABG when BOTH CO2 and HCO3 levels are abnormal. I can figure out respiratory alkalosis/acidosis, metabolic alkalosis/acidosis, and compensated/uncompensated when one value is abnormal. But when both values are abnormal how can I tell whether it is respiratory or metabolic and acidosis or alkalosis? Thanks!

Thanks everyone

Esme12, ASN, BSN, RN · May 20, 2012

ashley is a smart cookie!!!!

abg’s—it’s all in the family