Arterial blood gas interpretation

Nursing Students General Students

Published

Specializes in Oncology.

hi,

just wondering if anyone has any tips as to how to interpret this. just when i think i understand i don't. :uhoh3: i have ordered some books but wanted to know if there are any hints to make this easier. i get confused when the ph is normal and both the paco2 and hco3 are both elevated or vice versa. can't figure out how to tell if respiratory or metabolic.

thanks,

jenn :banghead:

try this link:

http://realnurseed.com/abg.htm

Specializes in Cardiac.

Put up your numbers...

For ex.

pH 7.30 ------Acid

HCO3 23 -------normal

CO2 50 ------Acid

This is a Resp acidosis. We didn't learn it with the arrowns up and down thing. We learned it this way. So we know its acidosis by the pH, and we know its resp because CO2 is the resp parameter and HCO3 is the metabolic parameter.

pH 7.36---------------normal-but closer to acidic that alkalotic

HCO3 30 --------------basic

CO2 50 (35-45)--------acid

So this is a compensated Resp. Acidosis. When they are both elevated, pick which one is closer to what the pH is. The pH is normal, but closer to acidotic than alkalotic. So you have too many acids (CO2) and too many bases (bicarb) and your pH is low/normal. So fully compensated Resp acidosis. The pH is normal because the kidneys are doing their job of holding on to HCO3 and balancing out all thoses acids. But you have too many acids, low normal pH, and too many bases, go for the two parmenters that match. If the pH was 7.43, and the rest were the same, then you'd have fully compensated met. acidosis.....

Specializes in med/surg, telemetry, IV therapy, mgmt.

https://allnurses.com/forums/f205/easy-way-remember-abgs-144947.html

http://maagnursing.com/abg/ - this is an abg and acid/base balance tutorial. you do not have to register to go through the tutorial.

http://web.indstate.edu/mary/abgdemo.html - an abg and abg analysis tutorial by a nursing instructor at indiana state university. http://www-isu.indstate.edu/mary/abgpract.htm - this is a quiz to test what you learned (includes answers).

http://www2.nursingspectrum.com/ce/self-study_modules/course.html?id=392 - "knowing your abg's: the blood gas report" a ce article from nursing spectrum on what is being tested when abgs are drawn and problem that lead to respiratory and metabolic disturbances.

http://www.ekgusa.com/ceol/abgmap/abgindex.html - a ce offering on acid-base analysis of blood gas values. there is a pre-quiz and post quiz with explanations of metabolic or respiratory acidosis or alkalosis, combined and compensated. it is from ekg skillbuilders.

Specializes in Telemetry & Obs.

https://allnurses.com/forums/f205/easy-way-remember-abgs-144947.html

Vicky, RN's attachments are spot on!! I didn't miss the first question on our exam using the tic-tac-toe method :p

Specializes in Peds.

What timing! We touched on this earlier but we are going to start hitting abg's hard next week.... Thanks to everyone for all the links.....

Specializes in Oncology.

Thank you all soooooo much!!! I have a test on Tuesday about this so I am more confident now!

Jenn

the short answer is in understanding the physiology of acid-base, because it's less an oxygen question than a ventilation question. briefly, you and i breathe more deeply when we exercise not because we need more oxygen (there's no way to increase your blood oxygen by breathing more room air), but because we need to get rid of co2 , the extra we are making when we carry something up the stairs or run. that you can blow off the more you hyperventilate.

the way our respiratory center decides whether to do that or not is based on the blood ph it sees circulating by. co2 works like an acid, lowering the blood ph; lower ph (from whatever reason..we'll get back to that) says, "hey! more deep breathing needed here! get rid of this co2 and get the ph back to normal!"

this is important: the normal respiratory driver is not hypoxia (low oxygen), but hypercarbia (high co2). your (normal) lungs' first job is to manage co2 levels, not to manage oxygen. that comes second. really. remember that.

now, when lungs fail, their ability to manage co2 decreases. co2 rises in the blood, and eventually that little sensor wears out and stops working. fortunately, there's a back-up system, which is an oxygen sensor. in chronic co2 retainers, their respiratory driver is not elevated co2 levels, it's falling oxygen levels. so, if their ventilation is inadequate, the body doesn't kick in with an elevated respiratory rate and minute volume (that's the total volume of air moved in and out in one minute) until the blood oxygen drops below threshold.

what this means is that someone who is a chronic co2 retainer is always a bit on the acidotic side (see below). it also means that if you give him oxygen to raise his spo2, his body will say, "ahh, no need to breathe now, all is well" until he stops long enough to drop his o2 back to make-me-breathe levels. the problem is that some people will stop breathing long enough to raise their co2 to lethal levels before the oxygen level gets low enough to make them breathe again, and they die. thus "never give a chronic lunger (well, at least a chronic co2 retainer, the blue bloater) a lot of oxygen, it's bad for them." now you know why.

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 :D 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 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? dka?) 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 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 co2's and bicarbs, you can look in your boxes, find ph, 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. (if you feel lousy after prolonged diarrhea, take a little bicarb in a glass of water; you'll be amazed how much better you feel and how fast!) 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?

Wow! That's a lot of typing. I like your method.

i confess that because this comes up so often i cut-and-paste it. but thanks. it was. :D

Specializes in Pediatric Critical Care.
the short answer is in understanding the physiology of acid-base, because it's less an oxygen question than a ventilation question. briefly, you and i breathe more deeply when we exercise not because we need more oxygen (there's no way to increase your blood oxygen by breathing more room air), but because we need to get rid of co2 , the extra we are making when we carry something up the stairs or run. that you can blow off the more you hyperventilate.

the way our respiratory center decides whether to do that or not is based on the blood ph it sees circulating by. co2 works like an acid, lowering the blood ph; lower ph (from whatever reason..we'll get back to that) says, "hey! more deep breathing needed here! get rid of this co2 and get the ph back to normal!"

this is important: the normal respiratory driver is not hypoxia (low oxygen), but hypercarbia (high co2). your (normal) lungs' first job is to manage co2 levels, not to manage oxygen. that comes second. really. remember that.

now, when lungs fail, their ability to manage co2 decreases. co2 rises in the blood, and eventually that little sensor wears out and stops working. fortunately, there's a back-up system, which is an oxygen sensor. in chronic co2 retainers, their respiratory driver is not elevated co2 levels, it's falling oxygen levels. so, if their ventilation is inadequate, the body doesn't kick in with an elevated respiratory rate and minute volume (that's the total volume of air moved in and out in one minute) until the blood oxygen drops below threshold.

what this means is that someone who is a chronic co2 retainer is always a bit on the acidotic side (see below). it also means that if you give him oxygen to raise his spo2, his body will say, "ahh, no need to breathe now, all is well" until he stops long enough to drop his o2 back to make-me-breathe levels. the problem is that some people will stop breathing long enough to raise their co2 to lethal levels before the oxygen level gets low enough to make them breathe again, and they die. thus "never give a chronic lunger (well, at least a chronic co2 retainer, the blue bloater) a lot of oxygen, it's bad for them." now you know why.

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 :D 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 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? dka?) 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 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 co2's and bicarbs, you can look in your boxes, find ph, 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. (if you feel lousy after prolonged diarrhea, take a little bicarb in a glass of water; you'll be amazed how much better you feel and how fast!) 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?

thanks for this!

The following is from a blood gas interpretation PDF file one of our nursing instructors gave us to review. I can't compare to the great explanation offered by GrnTea, but I can reiterate the points made, and provide this resource. All credit for this powerpoint goes to Orlando Regional Medical Center and the education staff at that facility.

Introduction

Arterial blood gas analysis is an essential part of diagnosing and managing a patient's oxygenation status and acid-base balance. The usefulness of this diagnostic tool is dependent on being able to correctly interpret the results. This self-learning packet will examine the components of an arterial blood gas and what each component represents and interpret these values in order to determine the patient's condition and treatment.

Overview

The pH is a measurement of the acidity or alkalinity of the blood. It is inversely proportional to the number of hydrogen ions (H+) in the blood. The more H+ present, the lower the pH will be. Likewise, the fewer H+ present, the higher the pH will be. The pH of a solution is measured on a scale from 1 (very acidic) to 14 (very alkalotic). A liquid with a pH of 7, such as water, is neutral (neither acidic nor alkalotic).

The normal blood pH range is 7.35 to 7.45. In order for normal metabolism to take place, the body must maintain this narrow range at all times. When the pH is below 7.35, the blood is said to be acidic. Changes in body system functions that occur in an acidic state include a decrease in the force of cardiac contractions, a decrease in the vascular response to catecholamines, and a diminished response to the effects and actions of certain medications. When the pH is above 7.45, the blood is said to be alkalotic. An alkalotic state interferes with tissue oxygenation and normal neurological and muscular functioning. Significant changes in the blood pH above 7.8 or below 6.8 will interfere with cellular functioning, and if uncorrected, will lead to death.

So how is the body able to self-regulate acid-base balance in order to maintain pH within the normal range? It is accomplished using delicate buffer mechanisms between the respiratory and renal systems. Let's examine each system separately.

The Respiratory Buffer Response

A normal by-product of cellular metabolism is carbon dioxide (CO2). CO2 is carried in the blood to the lungs, where excess CO2 combines with water (H2O) to form carbonic acid (H2CO3). The blood pH will change according to the level of carbonic acid present. This triggers the lungs to either increase or decrease the rate and depth of ventilation until the appropriate amount of CO2 has been re-established. Activation of the lungs to compensate for an imbalance starts to occur within 1 to 3 minutes.

The Renal Buffer Response

In an effort to maintain the pH of the blood within its normal range, the kidneys excrete or retain bicarbonate (HCO3-). As the blood pH decreases, the kidneys will compensate by retaining HCO3- and as the pH rises, the kidneys excrete HCO3- through the urine. Although the kidneys provide an excellent means of regulating acid-base balance, the system may take from hours to days to correct the imbalance. When the respiratory and renal systems are working together, they are able to keep the blood pH balanced by maintaining 1 part acid to 20 parts base.

Respiratory Acidosis

Respiratory acidosis is defined as a pH less than 7.35 with a PaCO2 greater than 45 mm Hg. Acidosis is caused by an accumulation of CO2 which combines with water in the body to produce carbonic acid, thus, lowering the pH of the blood. Any condition that results in hypoventilation can cause respiratory acidosis. These conditions include:

*Central nervous system depression related to head injury *Central nervous system depression related to medications such as narcotics, sedatives, or

anesthesia

*Impaired respiratory muscle function related to spinal cord injury, neuromuscular diseases, or neuromuscular blocking drugs

*Pulmonary disorders such as atelectasis, pneumonia, pneumothorax, pulmonary edema, or bronchial obstruction

*Massive pulmonary embolus *Hypoventilation due to pain, chest wall injury/deformity, or abdominal distension

The signs and symptoms of respiratory acidosis are centered within the pulmonary, nervous, and cardiovascular systems. Pulmonary symptoms include dyspnea, respiratory distress, and/or shallow respirations. Nervous system manifestations include headache, restlessness, and confusion. If CO2 levels become extremely high, drowsiness and unresponsiveness may be noted. Cardiovascular symptoms include tachycardia and dysrhythmias.

Increasing ventilation will correct respiratory acidosis. The method for achieving this will vary with the cause of hypoventilation. If the patient is unstable, manual ventilation with a bag- valve-mask (BVM) is indicated until the underlying problem can be addressed. After stabilization, rapidly resolvable causes are addressed immediately. Causes that can be treated rapidly include pneumothorax, pain, and CNS depression related to medications. If the cause cannot be readily resolved, the patient may require mechanical ventilation while treatment is rendered. Although patients with hypoventilation often require supplemental oxygen, it is important to remember that oxygen alone will not correct the problem.

Respiratory Alkalosis

Respiratory alkalosis is defined as a pH greater than 7.45 with a PaCO2 less than 35 mm Hg. Any condition that causes hyperventilation can result in respiratory alkalosis. These conditions include:

*Psychological responses, such as anxiety or fear *Pain *Increased metabolic demands, such as fever, sepsis, pregnancy, or thyrotoxicosis *Medications, such as respiratory stimulants. *Central nervous system lesions

Signs and symptoms of respiratory alkalosis are largely associated with the nervous and cardiovascular systems. Nervous system alterations include light-headedness, numbness and tingling, confusion, inability to concentrate, and blurred vision. Cardiac symptoms include dysrhythmias and palpitations. Additionally, the patient may experience dry mouth, diaphoresis, and tetanic spasms of the arms and legs.

Treatment of respiratory alkalosis centers on resolving the underlying problem. Patients presenting with respiratory alkalosis have dramatically increased work of breathing and must be monitored closely for respiratory muscle fatigue. When the respiratory muscles become exhausted, acute respiratory failure may ensue.

Metabolic Acidosis

Metabolic acidosis is defined as a bicarbonate level of less than 22 mEq/L with a pH of less than 7.35. Metabolic acidosis is caused by either a deficit of base in the bloodstream or an excess of acids, other than CO2. Diarrhea and intestinal fistulas may cause decreased levels of base. Causes of increased acids include:

*Renal failure *Diabetic ketoacidosis *Anaerobic metabolism *Starvation *Salicylate intoxication

Symptoms of metabolic acidosis center around the central nervous system, cardiovascular, pulmonary and GI systems. Nervous system manifestations include headache, confusion, and restlessness progressing to lethargy, then stupor or coma. Cardiac dysrhythmias are common and Kussmaul respirations occur in an effort to compensate for the pH by blowing off more CO2. Warm, flushed skin, as well as nausea and vomiting are commonly noted.

As with most acid-base imbalances, the treatment of metabolic acidosis is dependent upon the cause. The presence of metabolic acidosis should spur a search for hypoxic tissue somewhere in the body. Hypoxemia can lead to anaerobic metabolism system-wide, but hypoxia of any tissue bed will produce metabolic acids as a result of anaerobic metabolism even if the PaO2 is normal. The only appropriate way to treat this source of acidosis is to restore tissue perfusion to the hypoxic tissues. Other causes of metabolic acidosis should be considered after the possibility of tissue hypoxia has been addressed.

Current research has shown that the use of sodium bicarbonate is indicated only for known bicarbonate-responsive acidosis, such as that seen with renal failure. Routine use of sodium bicarbonate to treat metabolic acidosis results in subsequent metabolic alkalosis with hypernatremia and should be avoided.

Metabolic Alkalosis

Metabolic alkalosis is defined as a bicarbonate level greater than 26 mEq/liter with a pH greater than 7.45. Either an excess of base or a loss of acid within the body can cause metabolic alkalosis. Excess base occurs from ingestion of antacids, excess use of bicarbonate, or use of lactate in dialysis. Loss of acids can occur secondary to protracted vomiting, gastric suction, hypochloremia, excess administration of diuretics, or high levels of aldosterone.

Symptoms of metabolic alkalosis are mainly neurological and musculoskeletal. Neurologic symptoms include dizziness, lethargy, disorientation, seizures and coma. Musculoskeletal symptoms include weakness, muscle twitching, muscle cramps and tetany. The patient may also experience nausea, vomiting, and respiratory depression.

Metabolic alkalosis is one of the most difficult acid-base imbalances to treat. Bicarbonate excretion through the kidneys can be stimulated with drugs such as acetazolamide (DiamoxTM), but resolution of the imbalance will be slow. In severe cases, IV administration of acids may be used. It is significant to note that metabolic alkalosis in hospitalized patients is usually iatrogenic in nature.

Components of the Arterial Blood Gas

The arterial blood gas provides the following values:

pH

Measurement of acidity or alkalinity, based on the hydrogen (H+) ions present. The normal range is 7.35 to 7.45

PaO2

The partial pressure of oxygen that is dissolved in arterial blood. The normal range is 80 to 100 mm Hg.

SaO2

The arterial oxygen saturation. The normal range is 95% to 100%.

PaCO2

The amount of carbon dioxide dissolved in arterial blood. The normal range is 35 to 45 mm Hg.

HCO3

The calculated value of the amount of bicarbonate in the bloodstream. The normal range is 22 to 26 mEq/liter

B.E.

The base excess indicates the amount of excess or insufficient level of bicarbonate in the system.

The normal range is -2 to +2 mEq/liter. (A negative base excess indicates a base deficit in the blood.)

Steps to an Arterial Blood Gas Interpretation

The arterial blood gas is used to evaluate both acid-base balance and oxygenation, each representing separate conditions. Acid-base evaluation requires a focus on three of the reported components: pH, PaCO2 and HCO3. This process involves three steps.

Step One

Assess the pH to determine if the blood is within normal range, alkalotic or acidotic. If it is above 7.45, the blood is alkalotic. If it is below 7.35, the blood is acidotic.

Step Two

If the blood is alkalotic or acidotic, we now need to determine if it is caused primarily by a respiratory or metabolic problem. To do this, assess the PaCO2 level. Remember that with a respiratory problem, as the pH decreases below 7.35, the PaCO2 should rise. If the pH rises above 7.45, the PaCO2 should fall. Compare the pH and the PaCO2 values. If pH and PaCO2 are indeed moving in opposite directions, then the problem is primarily respiratory in nature.

Step Three

Finally, assess the HCO3 value. Recall that with a metabolic problem, normally as the pH increases, the HCO3 should also increase. Likewise, as the pH decreases, so should the HCO3. Compare the two values. If they are moving in the same direction, then the problem is primarily metabolic in nature. T

Compensation

Thus far we have looked at simple arterial blood gas values without any evidence of compensation occurring. Now see what happens when an acid-base imbalance exists over a period of time.

When a patient develops an acid-base imbalance, the body attempts to compensate. Remember that the lungs and the kidneys are the primary buffer response systems in the body. The body tries to overcome either a respiratory or metabolic dysfunction in an attempt to return the pH into the normal range.

A patient can be uncompensated, partially compensated, or fully compensated. When an acid- base disorder is either uncompensated or partially compensated, the pH remains outside the normal range. In fully compensated states, the pH has returned to within the normal range, although the other values may still be abnormal. Be aware that neither system has the ability to overcompensate.

In our first two examples, the patients were uncompensated. In both cases, the pH was outside of the normal range, the primary source of the acid-base imbalance was readily identified, but the compensatory buffering system values remained in the normal range.

Now let's look at arterial blood gas results when there is evidence of partial compensation.

In order to look for evidence of partial compensation, review the following three steps:

1.Assess the pH. This step remains the same and allows us to determine if an acidotic or alkalotic state exists.

2.Assess the PaCO2. In an uncompensated state, we have already seen that the pH and PaCO2 move in opposite directions when indicating that the primary problem is respiratory. But what if the pH and PaCO2 are moving in the same direction? That is not what we would expect to see happen. We would then conclude that the primary problem was metabolic. In this case, the decreasing PaCO2 indicates that the lungs, acting as a buffer response, are attempting to correct the pH back into its normal range by decreasing the PaCO2 ("blowing off the excess CO2"). If evidence of compensation is present, but the pH has not yet been corrected to within its normal range, this would be described as a metabolic disorder with a partial respiratory compensation.

3. Assess the HCO3. In our original uncompensated examples, the pH and HCO3 move in the same direction, indicating that the primary problem was metabolic. But what if our results show the pH and HCO3 moving in opposite directions? That is not what we would expect to see. We would conclude that the primary acid-base disorder is respiratory, and that the kidneys, again acting as a buffer response system, are compensating by retaining HCO3, ultimately attempting to return the pH back towards the normal range.

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