ABG interpretation made easy: acid base balance

So you made it this far.  Now you must interpret the results.  

Looking for some tips to ease your anxiety over an upcoming test that covers arterial blood gas (ABG) interpretation?  Well, look no further. The goal of this blog is to make your life easy.

ABG interpretation is as easy as remembering four basic questions, and then answering them in sequence.  Of course then you'll have to practice, practice, practice. By the time your test comes up you should be an ABG interpretation expert.

To make things simple, I will only refer to the three basic ABG values in this post

1. Acid Base Balance (pH)
2. Carbon Dioxide (CO2)
3. Bicarbonate (HcO3)

Normal ABG values are as follows:

1. pH  = 7.35 to 7.45  
2. CO2 = 35 to 45 
3. HcO3  = 22 to 26

You also must note the following:

1. CO2 greater than 45 is acidotic
2. HcO3 less than 22 is acidotic
3. Co2 less than 35 is alkalotic
4. HcO3 greater than 26 is alkalotic

To interpret these results, all you have to do is memorize these four basic questions, and then answer them in order.

A. Is the ABG normal?

1. If all the values fall within the normal parameters, then you have a normal ABG and you can stop here: The ABG is normal.  
2. If any one of the values is out of the normal range, then you must move on to the next question.

B. Is the pH Acidotic or Alkalotic? To determine this you look only at the pH.

1. Alkalotic: If the pH is greater than 7.45 the patient is Alkalotic.
2. Acidotic: If the pH is below 7.35 the patient is acidotic.

C. Is the cause respiratory or metabolic? To determine this you look at pH and compare it with HcO3 and CO2. If the pH is acidotic, you look for whichever value (HcO3 or CO2) is also acidotic. If the pH is alkalotic, you look for whichever value (HcO3 or CO2) is also alkalotic.

In this sense, you match the pH with HcO3 and CO2. If the pH matches with the CO2, you have respiratory. If the pH matches with the HcO3, you have metabolic.

Or, put more simply:

1. Metabolic Alkalosis: If the pH is alkatotic and the HcO3 alkalotic.
2. Respiratory Alkalosis: If the pH is alkalotic and the CO2 is alkalotic
3. Metabolic Acidosis: If the pH is acidotic and the HcO3 acidotic.
4. Respiratory Acidisis: If the pH is acidotic and the CO2 is acidotic.

A special case is when the pH, CO2 and HCO3 are all alkalosis or all acidotic.  In this case you have a case of combined alkalosis or combined acidosis.  

1. Combined Alkalosis:  If the pH is alkalotic, CO2 is alkalotic, and HCO3 is alkalotic
2. Combined Acidosis:  If the pH is acidotic, CO2 is acidotic, and HCO3 is acidotic

D. Is the cause compensated or uncompensated?

1. Compensated: pH is anywhere inside the normal ranges (Anything between 7.35 to 7.45)
2. Uncompensated: pH is anywhere outside the normal ranges (greater than 7.45 or less than 7.35).  Also, the value (CO2 or HCO3) that does not match the pH will still be in the normal range.  
3. Partially compensated: pH is anywhere outside the normal range, and the value that does not match the pH (CO2 or HCO3) will be outside its normal range.  This indicates the body is attempting to get the pH back to normal.  Example: A patient is in respiratory failure and his CO2 is 50 (acidotic) and pH is 7.24 (acidotic).  An HCO3 of 27 (alkalotic) means the body is attempting to get the pH back to normal, and this is considered compensation.   

Put A, B, C, and D together and you have your basic ABG interpretation.  That's it.  It's easy.

So, here are some examples:

1. Ph 7.40, CO2 37, HcO3 23

What do you have here? All the number are within normal range, so you have a normal ABG.

That was easy enough. You need to go no further in analyzing this ABG.

2. ph 7.23, CO2 50, HcO3 22

What do you have here?

A. Is the ABG normal? You can see right away that the pH and CO2 are out of the normal range, so you must move on to the next question.

B. Is the pH acidotic or alkalotic? Since the pH is less than 7.40 it is acidotic.

C. Is is metabolic or respiratory? Since the pH is acidotic and the CO2 also acidotic, then you have respiratory acidosis.

D. Is it compensated or uncompensated? Well, the pH is outside the normal range of 7.35 to 7.45, and the HCO3 is still in the normal range, the ABG is uncompensated. You don't have to look at any other values. You are done.

The ABG is uncompensated respiratory acidosis

2. pH 7.36, CO2 50, HcO3 29

A. Is the ABG normal? You can see right away that both CO2 and HcO3 are out of the normal range, so you move on to the next question.

B. Is is acidotic or alkalotic: The pH is less than 7.40, so it is acidotic

C. Is the cause respiratory or metabolic? The pH is acidotic and the CO2 is also acidotic, so you have respiratory acidosis.

D. Is it compensated or uncompensated? Since the pH is within normal limits, it is compensated.

In this example you have compensated respiratory acidosis.

3. pH 7.50, CO2 42, HcO3 33

A. Is the ABG normal? No. Some of the values are outside the normal ranges.

B. Is it acidotic or alkalotic? The pH is greater than 7.40, so it is alkalotic.

C. Is the cause respiratory or metabolic? You know the pH is alkalotic, so you look for the matching value. The HcO3 is alkalotic, so it matches the pH. So, what you have is a metabolic problem.

D. Is it compensated or uncompensated? Since the pH is outside the normal range of 7.35 to 7.45, it is uncompensated.

Thus, you have uncompensated metabolic alkalosis.

4. pH 7.50, CO2 18, HcO3 24

A. Is the ABG normal? No, pH and CO2 are both out of the normal range.

B. Is it acidosis or alkalosis? Since the pH is greater than 7.40 it is alkalosis

C. Is is respiratory or metabolic? Since the pH is alkalotic and the CO2 is also alkalotic, you have a respiratory problem

D. Is is compensated or uncompensated? It is uncompensated because the pH is outside the normal range of 7.35 to 7.45.

What you have here is uncompensated respiratory alkalosis.

5.  pH 7.07, CO2 89.3, HcO3 26

A.  Is the ABG normal?  No, all the numbers are out of the normal range

B.  Is it acidosis or alkalosis?  Since the pH is less than 7.40 it is acidotic

C.  Is it respiratory or metabolic?  Since the pH, CO2, and HCO3 are all acidotic, you have a special case called combined acidosis.

D.  Is is compensated or uncompensated? Since the pH is outside the 7.35 to 7.45 range, and the HCO3 is inside its normal range, the ABG is uncompensated.

What you have here is a case of uncompensated combined acidosis. Now, had the HCO3 in this example been on the alkalotic side of its normal range (say 27) this ABG would have been partially uncompensated.

Once you practice these you will be able to do these automatically in your head in only a few seconds just by looking at the numbers. Now you will want to move on to ABG interpretation made easy part II and, once you have oxygenation mastered, now it's time for some practice.

This post was originally published on 8/11/10 at respiratorytherapycave.blogspot.com  It has been read and approved by respiratory therapists, nurses, professors, and physicians. 

Further reading: 

1. 6 easy steps to ABG analysis
2. The ABG lexicon
3. More RT tips
4. ABG interpretation made easy part 2: interpreting level of oxygenation 
5. Learn about the 4-5-6, 7-8-9 rule
6. Oxyhemoglobin Dissociation Curve
7. How to know if your patient is a CO2 retainer
8. Cord Blood Gases made easy
9. The argument against ABGs
10. What is an ABG
11. Respriatory Therapy Formulas
12. A history of the hypoxic drive hoax
13. RT Cave Facebook Page
14. RT Cave on Twitter
15. 

Tips for drawing ABGs

I like the way he is facing the patient, although I can find many things
I would do different.  For instance, I would use the left middle finger to
hold the skin taut (the two finger technique).  It holds the artery stable
and almost guarantees you'll hit it.  I would also prop the wrist
up by using a towel.  However, do whatever works for  you.  The
student (or teacher) in the lab coat looks bored here. 

Your question: I'm a first  year student and I'm 0 for 3 during clinicals drawing ABGS so far.  Can you give some tips to help.  Putting the patients in pain with no results is pretty discouraging for me. Maybe you can teach me a few tips you've heard of (or use) if you can. 

My answer:  There are a couple things you can try.  I could probably show you faster than I can tell you.  Yet the following advice was given to me when I was a student in your same situation, having missed my first several ABGs.  I give this same advice to all students I have to supervise doing ABGs, and it seems to work well for them too.  

1. Have all your stuff ready before you enter the room.  Make sure the cap easily comes off the syringe, label the syringe, have the bandage ready, etc.  Also have a towel that you can ball up under the wrist if needed.  Wear gloves too, but make sure the the rubber is taught over your left pointer finger (right if you're right handed). Also pull the syringe back to 1cc (most syringes are self filling).  Do this prior to entering the room so you're not fumbling in front of the patient.



Here is a good picture of the two finger technique I love.  I would poke right
under the tip of that finger, over where I feel the pulse, as opposed to above
 the finger.  The angle of the syringe here is perfect.  Likewise, he (or she)has
 the wrist properly propped up using a towel. Of course this is my personal
preference, you may have (or develop) your own preferences.  

2. Identify pt.  Introduce yourself and say you're going to draw some blood.  Do not give out any further information unless the patient asks.  Do not say, "I'm a student," or "I have never poked a patient before," or "it's going to be a big poke," or "It's going to hurt more than a normal blood draw."  Be honest if asked a question, though.  I know this goes against contrary wisdom, but it works.  The less you say the less scared the patient will be.  Trust me.  It also makes you look more confident than you actually are.  Oh, and don't say "I'm nervous."  Chances are, no one will know you are nervous unless you say so.
3. Feel both radial arteries to decide which one has a better pulse, or which one is easier to access.  Always take time to do this, because many times one artery is easier to feel than the other.
4. Cock the patient's wrist up on either a pillow or on a rolled up towel.  The radial artery should be easy to access then.  You can either do this on the side of the bed if the patient is lying down, or on a table if the patient's sitting up (a table is preferable)
5. Position yourself facing the patient.  You can sit in a chair, alongside the bed, or even stand.  Just get comfortable.  If you're comfortable with it, you can even spark a conversation at this point to help the patient relax.  Ask the patient a question or something like that.  Or not. 
6. Uncap the syringe and hold it like a pencil 
7. I'm right handed, so I palpate the pulse with my left pointer finger.  I use the my left middle finger to draw back any loose skin so the skin around the artery is taught.  Sometimes I use whatever fingers aren't holding the syringe to draw back skin on that side too, if the skin is really wrinkly.  This secures the artery so it will not move when you're poking.  This is important.  This is called the 2-finger technique.  
8. Your left middle finger should stay put (the one holding the skin taut), but you can move your left pointer finger to find the artery using the tip of the finger.  Try to find a spot as close to the hand as possible because the artery is more stable here (although some say they like to go higher). 
9. Poke at a 30-60 degree angle toward the patient. Make sure you poke right under your pointer finger where you feel the pulse. You'll want to poke as close to your finger as you can.  (some people teach to poke to the left or right of their pointer finger, but I find this to be less effective for me.  It's better to poke right in front of your pointer finger, which should be right over the artery if you're feeling a pulse)  This is a key tip here, and greatly improved my success rate, thus improving my confidence. 
10. Insert the syringe slowly (very slowly). Stop as soon as you get blood flow. If you don't get blood and can still feel a pulse, (make sure you went deep enough first, especially if the patient has thick wrists) pull the syringe out slightly (and very slowly in case you went through it to begin with) and re-insert toward the direction the pulse feels strongest. 
11. Once you have blood flow do not move anything.  Allow the syringe to fill completely before moving any fingers or the syringe.  
12. Cap the syringe immediately, and do whatever else you've been trained to do to prepare the sample.
13. Make sure you check what oxygen the patient is on, as you'll need to report it.  

If you do steps 7-11 correctly you will succeed at 90% of your ABG draws.  Note, however, that you must always listen to your preceptor, and do as he says (even if he disagrees with me).   

I checked out the videos on youtube and none are as good as the way I teach it. Actually, they all make it look harder than it really is (IMO of course).  They all skipped my step #1 too and fumbled with the kit in the patient's room.  Oh, and make sure you wear goggles like the lady in this video (just kidding).  

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ABG errors and how to fix them

Most ABG machines are so sophisticated that they rarely make errors.  Most studies seem to confirm this, noting that as many as 68 percent of ABG analytical errors are handling errors that occur between the time of the draw and insertion of the syringe in the ABG machine, according to John J. Ancy in his RTmagazine.com article, "Blood Gases and Preanalytical Error Prevention." 

The following are possible preanalytical errors:

1.  Exposure to air:  

• Problem:  Whole blood continues to metabolize after the draw, and for this reason it's important to have proper handling of the sample. There is a bubble in the syringe.  
• Error:  Trapped air in the syringe causes the PO2 to move toward 150, which is the PO2 of room air.  So if the actual PO2 is less than 150,  the PO2 reading may be inaccurately high as it moves up towards 150.  If the actual PO2 is greater than 150, the PO2 reading may be inaccurately low as it moves down toward 150.   CO2 become slightly lower with a slight rise in pH.  
• Solution:  Tap bubbles from syringe and aspirate air into a filter immediately after the draw.

2.  Improper mixing of heparin:

• Problem:  If dry heparin is not mixed with the blood clots may form and readings may not be accurate
• Error:  Clotting cannot be reversed.  Clots may cause to machine to break down.  
• Solution:  A small amount of dry heparin will prevent clots.  Make sure to mix the syringe, roll it between your fingers, for about one minute, and then expelling a few drops into a gauze pad, both prior to inserting the syringe into machine.  The flea in capillary blood gases should be used to mix the sample for five seconds.  

3.  Ice storage

• Problem: Past policies recommended storing the post draw ABG in a slurry of ice, although new recommendations frown upon this.  It's difficult to get outdated hospital policies changed.  
• Error:   It reduces metabolism of the blood in the syringe, but the new plastic syringes are permeable to outside oxygen molecules.  Cooling increases hemoglobin's affinity for oxygen, and this may attract oxygen molecules from ambient air to these hemoglobin molecules.  This may artificially inflate PO2.  
• Solution:  Do not use an ice slurry.  Instead, assure that the blood is inserted into the analyser within 30 minutes of the draw.  New recommendations suggest only placing the ABG on ice if the time from draw to ABG analyzer is longer than 30 minutes.  I wrote more about this here. 

4.  Art line draw:  

• Problem:  Heparin is needed to keep blood from clotting in the art line system
• Error:  Heparin in the syringe may cause inaccurate results
• Solution:  You must waste two times the dead space in the system.  How do you know the dead space volume.  Ancy explains: "If vascular line dead space is unknown, turn the stopcock to the sampling port and withdraw flush solution until blood appears in the hub of waster syringe.  The volume in the syringe at that point will be equal to the dead space, double that volume for the waste draw."

5.  Changes:  

• Problem:  Changes in patient settings may affect ABG results:  Peep changes, oxygen changes, suctioning, ventilator settings, etc.
• Error:  ABG results may be artificially high or low
• Solution:  Most recommendations suggest waiting 20-30 minutes after changing oxygen, ventilator settings, or PEEP/CPAP.  

6.  Temperature Correction:  

• Problem:  Some experts recommend if a patient has a fever the temperature on the analyzer should be adjusted to match the patient temperature.  It's difficult to change past hospital policies, and it's difficult to get doctors to understand that methods of interpreting corrected and uncorrected temperature readings may be different.  For this reason, most medical experts recommend NOT making any corrections in temperature. In other words, there are no reference ranges for other temperatures other than room temperature.  I wrote about this here.  
• Error:  Temperature can inadvertently affect ABG results by speeding up metabolism.  
• Solution:  At present, the recommendation is not to make any temperature correction.  Ideally, there should be reference ranges for ABGs at all temperatures.  Temperatures should be corrected only at the recommendation of the physician, and any changes in temperature should be reported in the comments.  Interpretation of the results is to be done by the physician who requests them.  

Related:

ABGs interpretation made easy

The argument against ABGs

What is a blood gas?

A blood gas is a test we use to determine how much oxygen and CO2 are in the patient's blood.  It's a blood draw where you insert a needle into the patient's radial artery in the wrist area, bracheal artery in the antecubital area (the backside of the elbow) or the femoral artery in the groin (thankfully we don't use this area too often).

About 90 percent of the time we draw this blood from the wrist.  We draw arterial blood because this blood is  freshly oxygenated blood from the lungs on its way to tissues.  We want to know how much oxygen is in this blood.  If oxygen is low then we may choose to supply the patient with supplemental oxygen.  We can do this with a nasal cannula or a variety of masks.

If the CO2 is high we may need to assist the patient with his ventilations in order to help the patient blow off this CO2.  The reason CO2 gets high is because the patient is not taking good enough breaths.  He may be pooping out because his lungs are diseases.  In this case, we use his CO2 level to help us determine what we can do to help him.

Another thing an ABG does is help us determine the acidity (pH) of the blood.  If a patient is in severe respiratory distress his blood may become very acidotic.  If this happens, we may need to help the patient breath so that we can get his pH back to normal.

This blood test can also help a doctor diagnose some diseases. For example, if the CO2 is chronically elevated this can be a classic sign of chronic bronchitis or emphysema.  Too see a video of an ABG being drawn you can click here.

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The argument against serial ABGs, or, Cheerios cereal is better than serial ABGs

Why do doctors order serial ABGs anyway?  It's not like he could possibly know the patient is going to be in respiratory or metabolic distress every morning?  Wouldn't Alpha Bits be a better option for the patient?  Or how about Spaghetti O's?  Better yet, Fruity Pebbles or Fruit Loop Cereals would be much more enjoyable by the patient and therefore useful.  Yet simply ordering serial ABGs isn't of much use to the patient nor the doctor unless by luck of some crap shoot they so happen to be out of whack.  Yet one would hope that if the AM ABG two days from the time they were ordered are out of whack that the doctor would have picked up on it long before that time.  So that's why I'm proposing that Corn Flakes or even Cheerios would be a better option.  At least the patient would benefit from the added nutrients. How about Oat Meal?  At least Oat meal is proven to lower cholesterol.  Now, we won't get into the fact Oat meal doesn't even have any cholesterol in it to begin with, yet Oat meal would be much more beneficial to everyone involved than Serial ABGS.  So what's up with serial ABGs anyway?  Why the doctor fascination with them?  Why not just use the free and painless pulse oximetry or end tidal CO2 monitoring?  What do you think?

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The argument against ABGs

The argument for venous blood gases instead of arterial blood gases

Something we've been discussing at Shoreline Medical recently is the possibility of doing fewer
arterial blood gases (ABG) and more venous blood gases.

Sure an ABG is necessary when monitoring a patient in respiratory distress, although for the most part, there really isn't any more information you can obtain in an ABG that you can't simply obtain from a venous blood gas, coupled with end tital CO2 monitoring (ETCO2) and oxygen saturation (SpO2) monitoring.

Consider you have a patient with diabetes. The doctor wants to determine pH. If a pH is all the doctor wants, then a venous blood gas will work just great, as venous and arterial pHs are basically the same.

A 1998 study (as noted in this article from Emergency Medicine) found that in patients with diabetic keto acidosis, the venous pH was remarkably similar to the arterial pH.

Our Sepsis protocol calls for any patient suspected of having Sepsis to have an ABG. The reason is to get a baseline pH. Since this is the only reason, a venous blood gas would suffice.

The same with overdose patients. Poison control wants an ABG to be drawn when certain medicines are overdosed on. The reason is to check for pH. This is another example when a venous poke would suffice.

Think about it though. A venous poke is much less invasive and risky as an arterial poke, and the lab is in the room of the patient anyway drawing all the other labs. So, then it would be much better on the patient just to have all labs, including ABG, on just one poke.

In fact, according to Emergency Medicine, "When Is Venous Blood Gas Analysis Enough?" (38(12):44-48, 2006), revealed that a study performed in 1996 determined that most patients said a venous poke was about half as painful as an arterial poke.

You have a patient on a ventilator. Currently our protocol calls for daily ABGs. Our medical director is presently trying to convince the medical staff where I work that serial ABGs are not needed. What is needed is a continuous pulse oximeter and an end tidal CO2 monitor.

Then, 30 minutes to an hour after intubation, an ABG should be drawn just to get your baseline pH and to make sure the ETCO2 monitor and SpO2 correlate with the actual PO2 and CO2. That's it. From then forth all you need is daily venous pH. To monitor PO2, all you have to do is monitor the SpO2 and ETCO2.

A normal SpO2 is 90 or better. According to the oxyhemoglobin association curve, the PO2 is 30 less than the SPO2, therefore an SPO2 of 90% is equivelent to an SpO2 of 60, and an SpO2 of 80 correlates to a PO2 of 50. So there you have it.

Thus, according to the oxyhemoglobin disassociation curve, the formula goes like this (SPO2 minus 30 = PO2):

SPO2 of 90 = a PO2 of 60
SPO2 of 80 = a PO2 of 50
SPO2 of 70 = a PO2 of 40

It's basically called the 4-5-6-7-8-9 rule.

As far as monitoring CO2, all you have to do is monitor ETCO2. As the ETCO2 rises and falls, so to does the PCO2. A normal ETCO2 would be 30 to 50. Unless the patient is a CO2 retainer, all ETCO2 results greater or less than that should be reported to the physician.

Basically, if you have a patient who is not in respiratory distress, an ABG is never needed.

The proof

The toughest argument I've had is convincing doctors that a venous pH is basically the same as arterial pH. I remember once being called to do a stat ABG on a patient, and the pH on the patient was 6.78 and the CO2 was 75. The doctor was convinced I got venous blood and wanted me to redraw.

I took the gas to a second doctor, and he too was convinced I had obtained venous blood. I knew I had arterial not just by how forcefully the blood entered the syringe, but because the bicarb was 33 which shows the patient was probably a CO2 retainer to start with. Yet the doctors made me redraw the ABG. Of course it came out exactly the same, and they once again were convinced I had venous blood.

So the debate is ongoing. Yet, as you can tell by the picture, venous and arterial pH are essentially the same in a healthy patient. Why doctors are so convinced there is a major difference between the two is beyond me. Likewise, the bicarb (HCO3) is essentially the same too.

The only major difference is PO2, which can be monitored by saturations, which is completely non-invasive. There is no reason to ever draw an ABG just to prove that the PO2 is low. Again, all you have to do is subtract 30 from the SpO2 and you have your PO2.

The Emergency Medicine article notes a study done way back in 1985 that basically proved that "a venous pH of 7.25 or higher predicted an arterial pH of 7.2 or higher in 98% of all cases, which makes VBG testing valuable as a screening procedure.If the results are normal, ABG analysis should not be necessary. Conversely, abnormal venous levels predicted abnormal arterial values, but again in a nonlinear fashion. A venous pH of 7 or lower, for example, predicted an arterial pH of 7.2 or lower in 98% of cases. "

Here's another catcher the Emergency Medicine article notes. I have tried to convince doctors for years that an ABG is not needed during a code because if a patient is not breathing you already know the pH is low. Likewise, regardless of what the ABG shows, it's not going to alter what you do to try to save the life of the patient. So there is no need to rush to do an ABG.

In other words, you do not need to do an ABG to diagnose hypoxemic respiratory failure when the patient is showing obvious signs of hypoxemic respiratory failure.  Attempts to draw blood only delay treatment, and this can only increase morbidity and mortality.

The article is the first I've found that attempts to prove my point. It notes the following:

In cardiac arrest victims, the disparity between arterial and venous values is even greater. During cardiac arrest, tissue hypoxia is all but a certainty and is reflected by the lower pH and higher PCO₂ on the venous side. A 1986 study by Weil demonstrated a significantly lower pH in venous samples (mean, 7.15 vs 7.41 in arterial samples) and a significantly elevated PCO₂ (mean, 74 mm Hg vs 32 mm Hg) in these patients. In clinical practice, however, knowledge of either the arterial or venous pH or PCO₂ during cardiac arrest does not alter management, making the debate less relevant.

What's most interesting is my point was proven way back in 1986. Why is this information not translated in medical school? Yet, regardless, the argument is simple, that ABGs are needed sometimes to help a physician manage the care of a patient, yet more often than not a VBG will suffice.

ABG temperature correction

I was asked to do some research to determine if and when we should enter the correct temperature of the patient on hypothermic or hyperthermic patients. My research brought me to a great article written by Wesley Granger in Focus: Journal for Respiratory Care & Sleep Medicine, titled, "ABG temperature correction: to correct or not to correct; that is the question."

It appears he had the same interest as myself, and he did the research.

Here's the basics:

Research that goes back to the 1960s confirms that as the patient's temperature changes and the temperature in the ABG machine is not corrected:

1. The pH increases
2. The CO2 decreases
3. The PO2 decreases

The mechanism of the changes is to maintain a normal pH of 7.40, which, if you do not enter the corrected temperature into the ABG machine, you will note the pH, CO2 and PO2 will result as normal.

However, if you correct the temperature, the pH, CO2 and PO2 will change.

Should the temperature be corrected?

While it is known that the above values will change as the temperature is corrected in the ABG machine, most experts have trouble deciding on what are the normal ranges of pH, CO2 and PO2 at corrected temperatures.

Therefore, the general consensus is that, as Granger notes:

"We cannot use the traditional 'normal' ranges except at 37 degrees Celsius. Therefore, several articles I reviewed recommend that assessment of acid-base and oxygenation status be carried out on non-corrected (37 degrees Celsius) ABG values regardless of the patient's actual temperature. The 37 (degrees) Celsius ABG results will show if and what kind of acid-base disorder is present and the assessment is conducted in the same manner using the well recognized 'normal' values.

There are three exceptions. First, he notes, is when a patient is being intentionally cooled during surgery, in which case the physician will want to know the corrected pH because the goal is to maintain a normal pH.

The second exception is that if you want to calculate oxygen tension based on the A-a Gradient, PAO2/ PaO2 ratio or PaO2/FiO2 ratio (for a refresher on these formulas, click here), you will have to use the corrected ABG results.

The third exception is if you want to correlate the end tidal CO2 results and pulse oximeter results with the ABG. In this case, you will want to use the corrected ABG results.

So, as you can see, for some patients you may actually need to run both a corrected and a non corrected ABG.

But, for the most part, and for most situations, most experts do not at this time recommend that you enter the patient's correct temperature into the ABG machine, and run all ABG at the normal 37 degrees. This is recommended so you can use your normal ABG ranges to make therapeutic decisions.

I imagine, though, that once scientists are able to determine normal ranges at the corrected temperatures, new recommendations will be determined as to whether or not to correct.

So if your hospital is seeking wisdom on whether or not to enter the patient's correct temperature into the ABG machine, now you have the latest wisdom.

Here's how to know if patient is a CO2 retainer

Question: How do you know how to tell someone is a PaCO2 retainer by looking at the blood gas?

My humble answer: It's easy. First, the CO2 will be chronically elevated. Usually, you'll find it in the 50s, but it can be as high as 80 on a good day. Second, look at the pH. If the pH is normal and the PaCO2 is elevated, you have a PaCO2 retainer.

Question: So, what if you have a patient who is laboring and has a pH of 7.20. Can you still tell if this patient is a retainer, when his PaCO2 is 80 or even 100?

My humble answer: Great question. Yes. What you do here is look at the BiCarb. If a patient is a CO2 retainer, his Bicarb will be chronically elevated. So, if you have a BiCarb greater than 30, chances are the patient is a retainer.

Question: Can you give a sample blood gas?

My humble answer: Sure. Consider ph 7.38, PaCO2 50, Po2 50, HCO3 35. That's a blood gas of a Co2 retainer on a good day. Note the CO2 is elevated and the HCO3 is elevated to compensate to keep the pH normal. This is a normal homeostatic procedure. So, if you had a patient in respiratory acidosis, his gases might look like this on room air: pH 7.20, PaCO2 80, Po2 36, HCO3 38.

Question: So what if the doctor sees that low Pao2 and high PaCO2 and thinks you got venous blood. How can you prove to him it was not venous?

My humble answer: Sometimes you can't. However, you know if you got the artery by how well it filled the syringe. Still, a doctor might not believe you. You can just tell him that the HCO3 is high, so chances are the PaCO2 is chronically elevated. If the patient is in acidosis, chances are the PaCO2 is elevated even more. It's worth trying anyway.  Another way to tell if this is arterial or venous is to place the patient on 100% non-rebreather. If the oxygen goes up, you know you had arterial. This is the best approach either way, considering a PaO2 that low is life threatening, and requires oxygen anyway.

Edited on July 5, 2016

Some things should not be the job of the RT

I see the respiratory therapist as a member of the overall team of medical professionals who does his part in making a patient more comfortable or, if it comes to it, providing his expertise and skill in an attempt to save the life or improve the quality of a patent's life.

I suppose it's for that reason that I do not enjoy doing procedures just because a doctor orders it. I flinch when a bronchodilator breathing treatment is ordered on someone just because he or she is short of breath, or just out of a routine of the doctor -- or per his protocol.

Likewise, I flinch when I'm asked to do cord blood gases. The only reason this procedure is done is after a difficult birth because the doctor wants it documented that the gases were normal in case of a law suit. I do not see the RT as someone who does services just to prevent the doctor from being sued.

Thus, if a doctor wants a cord blood gas, he should draw it himself. After all, the RT had to be taken from the bedside of a person who was having difficulty breathing to draw the cord gas.

This is also why I'm anti doing EKGs on patients just because the doctor wants to make sure he covered all his grounds just in case the patient decides to sue.

That's also why I think doing Holter Monitors in the ER is not the job of the RT.

I'm not saying these things don't need to be done. What I'm saying is it should not be the job of the RT on duty.

Now, say, the doctor asked the RT kindly if he'd do these things, I'm sure he would oblige if he wasn't overly busy.

Yet, be it as it may, we do as we are told. We do things we do not approve and we do it with a smile. And then we blog about it in a wry or flippant way.

ABGs made easy: The Lexicon

Here are the basic terms you will need to understand in order to get a more detailed grasp of arterial blood gases. The explanation here is the pithy version, and this is to keep it easy for you.

Hyperventilation:  Rapid breathing (will blow off PaCO2)

Hypoventilation:  Slow breathing (will cause PaCO2 to rise)

Metabolic: The breakdown of foods within the cells of the body and its transformation to energy.

Respiratory: The exchange of oxygen (PaO2) and carbon dioxide (PaCO2) by means of the lungs. Humans breathe in oxygen and exhale carbon dioxide.

Homeostasis: The human body has the ability to maintain stability within the body. The human body is constantly trying to maintain homeostasis. When it comes to ABGs, the body alters PaCO2 and HCO3 to constantly work to keep pH within its normal range.

pH: A measure of the acidity or alkalinity of a fluid. In the human body this measure is normal between 7.35 and 7.45. The body is constantly trying to maintain pH homeostasis. A level of 7.30 to 7.50 is generally considered acceptable.

HCO3: (Bicarbonate) This acts as a buffer to maintain a normal pH in blood and other body fluids. The acidity is affected by foods or medications that we ingest and the function of the kidneys and lungs. A normal HCO3 is between 22 and 26. Bicarb has a symbiotic relationship with pH. When HCO3 increases, pH increases and becomes more alkalotic. When HCO3 decreases, pH decreases and becomes more acidotic.

PaCO2: (carbon dioxide): It's a byproduct of cellular metabolism released into the arterial bloodstream, carried on hemoglobin to the lungs where it is excreted from the body during respiration. PaCO2 has an inverse relationship with pH. When PaCO2 increases, arterial pH will decrease (become more acidic). When PaCO2 decreases, arterial pH will increase (become more alkalotic).

Oxygen: A colorless, odorless gas that makes up about 21% of the air we breathe. It is needed by the body for metabolism to occur. It is inhaled by the lungs and then carried through the bloodstream on hemoglobin to the cells, where metabolism occurs.

Acidosis: This occurs when there is too much acid in the body or not enough buffers (HCO3) in the blood to balance out the pH. This occurs when pH is abnormally low (less than 7.35). This can be caused by the lungs (not blowing off enough much PaCO2) or by the body's metabolic system (severe kidney disease, diabetic ketoacidosis,).

Alkalosis: This occurs when there is not enough acid in the body, or too many buffers (HCO3). This can be caused by hyperventilation (blowing off PaCO2), or excessive vomiting or diarrhea.

Respiratory Acidosis:  This is acidosis caused by the inability of the lungs to excrete PaCO2, and therefore PaCO2 levels in the blood rise, resulting in a decrease in pH.  The PaCO2 will be higher than 45 and the pH less than 7.35 (CO2 rises = pH to drop).  It is caused by hypoventilation. Treatment involves things that will ventilation (increase rate and depth of breathing), such as beta adrenergic medicine and positive pressure breaths.  Lacking intervention, the patient's body will try to compensate by the metabolic system increasing buffers to absorb the acid.  If the body is unable to compensate, intervention will be necessary. Possible resolution may include beta adrenergics, diuretics, or positive pressure breaths.

Respiratory Alkalosis:  This is alkalosis caused by the lungs blowing of too much PaCO2, and therefore PaCO2 levels in the blood drop, resulting in a rise in pH.  The pH will be less than 35 and the pH will be higher than 7.45 (CO2 drops = Ph rises).  It is caused by hyperventilation.  Treatment involves slowing the respiratory rate down, or decreasing the depth.   The body may try to compensate by slowing the rate of breathing to increase CO2.

Metabolic Acidosis: This is acidosis caused by the metabolic system. This can occur if a person excretes too many buffers (HCO3), and the kidneys are unable to generate enough HCO3, and therefore there aren't enough buffers in the body to balance out the acid.  HCO3 will be less than 22 and pH will be less than 7.35 (HCO3 drops = pH drops).  It can result from kidney failure, vomiting, diahrrea, or the administration of diuretics. The consequences of this can be severe, and may even result in coma or death if not treated. The respiratory system will try to compensate by increasing rate and depth of breathing.  This often results in kussmaul's breathing pattern (rapid and deep).

Ketoacidosis:  This is a special form of metabolic acidosis caused when a diabetic patient is unable to generate enough insulin.  Insulin is used to draw glucose into cells, and glucose is used for energy.  Lacking insulin, the body breaks down fat tissue for energy, and the result is the release of ketones, which is an acid.  The buildup of ketones in the blood causes acidosis that can be treated with the administration of insulin.  This often results in kussmaul's breathing pattern (rapid and deep).

Metabolic Alkalosis:  This is alkalosis caused by the metabolic system storing up too many buffers (HCO3).  The kidneys are producing too many buffers, or not excreting enough.  It is measured by HCO3 greater than 26 and pH greater than 7.45 (HCO3 rises = pH rises). This can result from poor kidney function (kidney failure).  It can also be caused by hypovolemia (loss of blood, shock), chloride depletion, hypokalemia, etc. It can be resolved by fixing the underlying problem (transfusion, administer chloride or calcium, or administering of magnesium).

Compensation: The body is constantly trying to maintain homeostasis. Compensation occurs when the pH is within the normal range of 7.35 to 7.45.

• If respiratory acidosis occurs, the kidneys will work to excrete HCO3 into the bloodstream until the pH is within normal range.
• If metabolic acidosis occurs, the lungs will try to increase the respiratory rate to blow off CO2 to balance the pH. Usually here you will see your rapid respiratory rate, almost like they are panting for air.
• If respiratory alkalosis occurs (Co2 too low and pH too high), the kidneys will work to excrete HCO3 from the body through the urinary tract.
• If metabolic alkalosis occurs, the lungs will try to compensate by slowing down the respiratory drive to increase CO2

Hypoxemia: A low level of oxygen in your blood. This means that less oxygen will be delivered to your tissues, thereby causing...

Hypoxia. A low level of oxygen in your tissues. The main symptoms here are increased heart rate, increased respiratory rate, shortness of breath, reduced capacity for exercise, fatigue, and confusion.

Hypoxemic Hypoxia. This is a condition where you have both hypoxemia and hypoxia.

Refractory Hypoxemia: This is when a patient has a low O2 that does not improve with increasing the oxygen. Generally, it is described as a PO2 of 60 torr or less with an FiO2 of 60% or greater.

SpO2: The percentage of oxygen in the inspired air that reaches the blood.

FiO2: Fraction of inspired oxygen. Room air is 21%. With supplemental oxygen, you can increase it up to 100%.

Further reading:

• ABG interpretation part I
• ABG interpretation part II