ABG question

belle005 · Oct 8, 2011

If you still need help understanding abg interpretation, this site will give you problem after problem until you really start to understand it. This is how I learned it. http://www.vectors.cx/med/apps/abg.cgi

loveslife · Oct 8, 2011

Dudette10 you need to look this up in your text or ask someone for help. Actually, take the question to your nursing professor to clarify. Or go online and look up the answers because acid/base is online and it is a very simple topic.

Jolie, BSN · Oct 8, 2011

I think that perhaps the point of this scenario is to teach the student that labs are meaningless without assessing the baby's condition.

I agree that the low normal pH with an increased bicarb would lead one to believe that the baby had been experiencing a respiratory acidosis, but that doesn't fit with a normal CO2. It is possible that the baby cried before and during the lab draw, temporarily decreasing the CO2. It may have been somewhat higher before the blood draw, which would be consistent with respiratory acidosis and metabolic compensation.

We need to know the baby's respiratory rate and work of breathing to round out this scenario.

With RSV, secretions may be copious, possibly blocking the airway and interfering with normal gas exchange. There is also the possibility that the baby is becoming fatigued from increased respiratory effort and may soon slow the respiratory rate to the point where it is insufficient for gas exchange.

We do know that the oxygenation is inadequate.

This kiddo may be in danger of experiencing respiratory failure, and bears very close observation.

Jolie, BSN · Oct 8, 2011

"I think it's partially compensated respiratory acidosis. Gas exchange is affected, as shown by the Dx and low O2.
The thing that throws me off is the normal range of CO2, but the baby might be breathing rapidly and shallowly in order to blow off the CO2 that caused the acidotic condition.
I believe you are confusing compensation for respiratory acidosis versus metabolic acidosis. If this baby had an elevated CO2 causing a respiratory acidosis, the compensatory mechanism would NOT be an increased respiratory rate. If the baby were capable of increasing his RR to blow off CO2, he wouldn't have experienced a respiratory acidosis in the first place. The compensation for a respiratory acidosis, which is generally caused by some degree of respiratory insufficiency, is metabolic, ie reserving bicarb to return the pH to normal.
Think about the compensation the body does the lungs(quick) and the kidneys(slow) BOTH try to compensate but the lungs compensate first with breathing, which is the quickest way to compensate, then the kidneys. The breathing you are talking about would be true for acidosis as this compensatory mechanism is quick and happens first.
Rapid breathing is a compensation for metabolic acidosis, not respiratory acidosis. Again, if a patient were capable of increasing his resp. rate to blow off CO2, he would not have experienced a respiratory acidosis to begin with.
Then the kidneys compensate and it is the reason for a high HCO3. The body is compensating by the kidneys retaining HCO3, a base, to counteract a low pH.
You have described here metabolic compensation, which is the body's reaction to respiratory acidosis.
Severe dehydration causes metabolic acidosis.
This patient is not experiencing metabolic acidosis. Metabolic acidosis occurs with a deficit of bicarb. This baby has a slight excess, not a deficit.

honeykrown, MSN, NP · Oct 8, 2011

im not an expert but since you guys are so sure about metabolic with the kidneys kicking it, why cant it be respiratory since the CO was suppose to be high but then rapid breathing to get rid of the CO and it lowered it becoming normal. just my two cents

dudette10, MSN, RN · Oct 8, 2011

Dudette10 you need to look this up in your text or ask someone for help. Actually, take the question to your nursing professor to clarify. Or go online and look up the answers because acid/base is online and it is a very simple topic.

ABGs are often one of the more difficult concepts for nursing students to grasp, imo. And, I would beware of some online calculators because they are only as good as the person who programmed it.

If it was programmed to ID acidosis/alkalosis based on the pH, and then look for the bicarb or CO2 value that is out of range (regardless if they are up or down), it would spit out "metabolic acidosis."

Indeed, a lot of them did. I think the same/similar code for the abg calculator is showing up on multiple sites, btw. Programmers borrow code all the time...

I still don't agree with metabolic acidosis, though, because it's clear that the kidneys are compensating with the high bicarb. So, the bicarb can't be the cause of the acidosis.

You pointed out in a couple of posts that uncompensated metabolic acidosis would require the bicarb to be low. I agree. Partially compensated or fully compensated metabolic acidosis would still show a low bicarb. No permutation of metabolic acidosis would show a high bicarb. So, I'm not sure how you got to metabolic acidosis as the answer to this question. :confused:

The CO2 is throwing everyone off, but Jolie's suggestion of why that might have happened sounds logical. I also think she's on target when she says that this question is intended to encourage students to look at the patient, not just the lab values.

This is a nursing school question, and the fact that the baby's diagnosis was included in it is important to the correct answer. The ABG values are respiratory in origin.

Lbarrien · Oct 8, 2011

Below is the rest of the assessment. I need to answer this "Discuss the meaning of the ABG results and implications for Mary Renee’s care."

General Information:

Mary Renee is a 3 month old with a 4 day history of upper respiratory infection. She was seen at a clinic today and diagnosed with (RSV). Due to severe respiratory distress she was admitted to PICU this morning.

Physical Examination:

On assessment the nurse notes:

General survey- Pale, in obvious acute distress- crying initially

Vital Signs:

T-100

P-166

R-44

BP- 82/44

O2Sats- 88% on 4 L Oxygen

Neurologic- crying, does not console

Cardiac- tachycardia

Respiratory- acute respiratory distress, wheezing, coorifice breath sounds, intercostals and supra-sternal retractions, nasal flaring, copious secretions

Integument- pale, moist, mottled

Musculoskeletal- MAE initially but later becomes lethargic with decreased movement

Gastrointestinal- abdomen soft, tympany on percussion from air

Genitourinary- voids in diaper- no problems

Psychosocial- inconsolable…

Physician’s Admission Orders

Continuous pulseoximetry

O2 to keep Sats above 93%

Chest x-ray PA and left lateral.

Notify M.D. for body temperature over 100°F.

NPO

Albuterol 0.083% q 2 hours via HHN

Laboratory Results :

White blood cells9.000/mm3

Red blood cells 8.54 mil/mm3

Hemoglobin 12.8 g/%

Hematocrit36%

Platelets243 mm3

Arterial Blood Gas:

PH- 7.33

PaO2- 60

PaCO2- 40

HCO3- 28

nurseprnRN, BSN, RN · Oct 9, 2011

"the thing that throws me off is the normal range of co2, but the baby might be breathing rapidly and shallowly in order to blow off the co2 that caused the acidotic condition. the bicarb is elevated to compensate for the lower ph."

poor little beggar has a pao2 of only 60, so he's hyperventilating to get more os. even at that, his lungs are lousy because their first function is to get co2 out, and hyperventilating isn't even doing that very well.

* 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 blackboard 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 bicarbs. 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 than nl ph**.

remember that **co2 is 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 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.

likewise in the metabolic/alkalosis box: a high bicarb makes your body want to retain acid, increasing co2 being the fastest cuz 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 : as**a** overdose? dk**a**?) it will want to get rid of co2 to compensate, and the fastest way to do that is to hyperventilate.

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 [color=#dd0000]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 betwreen 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 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 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.[color=#dd0000] 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.

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?

Jolie, BSN · Oct 9, 2011

lbarrien,

Thanks for the additional information. It confirms my suspicion that this baby indeed has a respiratory acidosos caused by ineffective respiratory effort (exhaustion from increased work of breathing coupled with secretions impairing gas exchange).

The reason the baby's CO2 seems normal is that she is crying, which enables her to blow off more CO2 than she otherwise would. This does not represent respiratory compensation. (Her body is not causing her to cry in order to blow off CO2), rather she is crying out of discomfort (pain, air hunger.) Eventually she will become exhausted and the crying will lessen and stop. At this point, she will begin to retain CO2 again, possibly to the point of requiring respiratory support. (Not just supplemental O2, but breathing support in the form of bag and mask ventilation, CPAP or intubation and ventilation.)

RSV is a self-limiting illness, in that if you can support the baby's vital body functions, the virus will run its course. What makes it especially dangerous is that young infants can appear to be compensating fairly well (as your initial blood gas indicates), but then go downhill very fast when they reach their limits of exhaustion, from the physical work of breathing coupled with dehydration and poor nutrition.

These babies also need IV fluid support to maintain hydration and calorie intake while they are unable to eat orally.

Lbarrien · Oct 9, 2011

Thank you Jolie!! I just got extremely confused with those labs and I'm not very familiar with peds, but again THANKS!! :)