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acidosis vs. alkalosis

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by sonias sonias (New) New

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This is my topic for this week in nursing school, respiratory & metabolic acidosis/ alkalosis. I am having trouble breaking it down. Can someone please help me understand this please? Any and all help is greatly appreciated.

Edited by Joe V

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Esme12 is a ASN, BSN, RN and specializes in Critical Care, ED, Cath lab, CTPAC,Trauma.

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Normal values:

PH = 7.35 - 7.45

C02 = 35 - 45

HC03 = 21-26

Respiratory acidosis = low ph and high C02

hypoventilation (eg: COPD, narcs or sedatives, atelectasis)

*Compensated by metabolic alkalosis (increased HC03)

For example:

ph 7.20 C02 60 HC03 24 (uncompensated respiratory acidosis)

ph 7.33 C02 55 HC03 29 (partially compensated respiratory acidosis)

ph 7.37 C02 60 HC03 37 (compensated respiratory acidosis)

Respiratory alkalosis : high ph and low C02

hyperventilation (eg: anxiety, PE, pain, sepsis, brain injury)

*Compensated by metabolic acidosis (decreased HC03)


ph 7.51 C02 26 HC03 25 (uncompensated respiratory alkalosis)

ph 7.47 C02 32 HC03 20 (partially compensated respiratory alkalosis)

ph 7.43 C02 30 HC03 19 (compensated respiratory alkalosis)

Metabolic acidosis : low ph and low HC03

diabetic ketoacidosis, starvation, severe diarrhea

*Compensated by respiratory alkalosis (decreased C02)


ph 7.23 C02 36 HC03 14 (uncompensated metabolic acidosis)

ph 7.31 C02 30 HC03 17 (partially compensated metabolic acidosis)

ph 7.38 C02 26 HC03 20 (compensated metabolic acidosis)

Metabloic alkalosis = high ph and high HC03

severe vomiting, potassium deficit, diuretics

*Compensated by respiratory acidosis (increased C02)


ph 7.54 C02 44 HC03 29 (uncompensated metabolic alkalosis)

ph 7.50 C02 49 HC03 32 (partially compensated metabolic alkalosis)

ph 7.44 C02 52 HC02 35 (compensated metabolic alkalosis)

*Remember that compensation corrects the ph.

Now a simple way to remember this......

CO2 = acid, makes things acidic

HCO3 = base, makes things alkalotic


Remember ROME





Ok always look at the pH first...


pH>7.45 = alkalosis

Then, if the CO2 is high or low, then it is respiratory...If the HCO3 is high or low then it is metabolic. How you remember that is that the respiratory system is involved with CO2 (blowing air off or slowing RR), and the kidneys (metabolic) are involved with HCO3 (excreting or not excreting).

Here is how you think thru it: pH = 7.25 CO2 = 40 HCO3 = 17

Ok, first, the pH is low so think acidosis. CO2 is WNL. HCO3 is low. Draw arrows if it helps. The abnormal values are both low (think Equal). Metabolic imbalances are equal. So, this must be metabolic acidosis!

Now, for compensation...If you have a metabolic imbalance, the respiratory system is going to try to compensate. Respiratory = CO2. If the CO2 is normal in the ABG, then there is no compensation going on. Compensation in acidosis will decrease the CO2 because you want to get rid of the acid (CO2). In alkalosis, it will increase because you want to add more acid (CO2)

If you have a respiratory imbalance, the kidneys will try to compensate. Kidneys = HCO3. If the HCO3 is normal in the ABG, then there is no compensation going on. Compensation in acidosis will increase HCO3 because you want to hold on to the base to make it more alkalotic. In alkalosis, it will decrease because you want to excrete the base to make it more acidic.

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Pink Magnolia is a BSN, RN and specializes in LAD.

314 Posts; 9,377 Profile Views


pH 7.35-7.45

CO2 (carbon dioxide-acidic) 35-45

HCO3 (bicarbonate-alkaline) 22-26

Always look at your pH first. If its high, then it's alkaline. If it's low, it's acidic.

Then look for another odd value.

If you see an increased pH, increased CO2, and increased HCO3  we at least know the pH is alkaline. But is it respiratory or metabolic? Using the ROME method we know Respiratory is Opposite (look at pH & CO2) & Metabolic is Equal (look at pH & HCO3). Ok, so pH is high & CO2 is high. It can't be respiratory because the values should be opposite. If it were respiratory then CO2 would be lower. Our pH is high & our HCO3 is high, so it must be metabolic because values are both higher than normal. Why is the CO2 high as well? Because it's compensating! The body is more alkaline, so the lungs will retain CO2 to lower the pH.

Rules for compensation:

2 odd values= uncompensated

3 odd values= partially compensated

when pH returns to normal value= fully compensated

Respiratory acidosis examples:

pH 45 HCO3 22-26 (uncompensated)

pH 45 HCO3 >26 (partially compensated)

pH 7.35-7.45 PCO2 >45 HCO3 >26 (fully compensated)

PH was low & CO2 was high, so HCO3 increases to compensate. The pH will rise in response to bicarb release from the kidneys. When the bicarb is enough to bring the pH within normal levels the HCO3 is said to be fully compensating the pH. The pH level is within normal limits, but CO2 and HCO3 are not until things eventually normalize.

Respiratory alkalosis examples:

pH >7.45 PCO2

pH >7.45 PCO2

pH 7.35-7.45 PCO2 >45 HCO3

Metabolic acidosis examples:



pH 7.35-7.45 PCO2

Metabolic alkalosis examples:

pH >7.45 PCO2 35-45 HCO3 >26 (uncompensated)

pH >7.45 PCO2 >45 HCO3 >26 (partially compensated)

pH 7.35-7.45 PCO2 >45 HCO3 >26 (fully compensated)

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4 Posts; 1,185 Profile Views

Thank you Esme12.

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4 Posts; 1,185 Profile Views

Thank you pink magnolia 611.

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Esme12 is a ASN, BSN, RN and specializes in Critical Care, ED, Cath lab, CTPAC,Trauma.

5 Followers; 4 Articles; 20,908 Posts; 147,621 Profile Views

Is this what you were looking for?

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Pink Magnolia is a BSN, RN and specializes in LAD.

314 Posts; 9,377 Profile Views

No problem! I had an exam over this stuff a few months ago, and this same concept is what I used to study. I hope it's useful for you. 🙂

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la_chica_suerte85 is a BSN, RN and specializes in Pediatric Hematology/Oncology.

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OMG Esme and pink magnolia thank you! We just started this topic this week, too, and it's always been a bit of a mystery to me especially when we start taking about partial compensation and what not.

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4 Posts; 1,185 Profile Views

It sure is.

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While some of this appears in other places on the net, I wrote it first :), and I hope it is as helpful to you as it has been for many others.

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



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:




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 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 : ASA 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 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. 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?

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105 Posts; 4,206 Profile Views

Thank you Esme for your example! That really helped me out, I kept wondering where the bicarbonate buffer came into play.

It's basically the opposite for each one...So for respiratory acidosis the compensatory mechanism is increasing the bicarbonate buffer & the compensatory mechanism for the metabolic acidosis is decreasing the amount of CO2 in the body. Also, I was reading in my book, do you happen to know why in metabolic alkalosis my book said the body increases the amount of CO2 which I understand & retain hydrogen ions but why does it secrete excess H2O? When I learned this system I learned it by the basic formula: H2O + CO2 H2CO3, & so if the body is secreting excess H2o, then your not creating carbonic acid but it is retaining H+ ions, so I guess that's another method.

Now I just gotta drill the lab values into my head.

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