Arterial Blood Gases: The Tic-Tac-Toe Game
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Example Case
A male patient with a history of COPD presents in respiratory distress. He is placed on BIPAP on arrival, and administered breathing treatments and steroids.
His blood gas shows a pH of 7.28, PaCO2 of 74, and HCO3 of 28. You begin to interpret the results to determine his acid base status.
Introduction
An arterial blood gas (ABG) is a fairly rapid blood test that measures the pH, oxygen (O2), carbon dioxide (CO2), bicarbonate (HCO3), and oxygen saturation levels from an artery.
ABGs can also measure glucose, lactate, electrolytes, hemoglobin, and various toxicology levels to name a few.
Knowing how to interpret blood gases is a crucial skill to have, as it can help you determine the patient’s acid base status, severity of their condition, and potential underlying pathophysiology of their presentation.
Unfortunately, analyzing blood gases can be confusing to learn at first.
Thankfully you have come to the right place!
Medical topics are made easy with every EZmed blog post, and today you will learn a simple trick to interpret blood gases using the tic-tac-toe method!
What is Normal?
First, we need to know the normal values of a blood gas before we can start to interpret it.
The main components of a blood gas used to determine a patient’s acid base status is the pH, the CO2 level, and the bicarbonate (HCO3) level.
The normal pH range is 7.35-7.45.
A normal PaCO2 range is 35-45. This is easy to remember - simply take the decimal numbers from the pH (0.35 - 0.45) and now you have your 35-45 CO2 range.
The normal values for HCO3 is 22-26.
3 Questions to Answer
There are 3 main questions to ask yourself when determining a patient’s acid base status.
Is it an acid (acidosis) or base (alkalosis)?
Is it metabolic or respiratory?
Is compensation present?
We are going to use the tic-tac-toe method to easily answer all 3 questions.
Tic-Tac-Toe Method
So what is the tic-tac-toe method?
Let’s use an example blood gas to demonstrate how this works.
Example blood gas: pH 7.53; PaCO2 41; HCO3 32
The first step is to draw out the tic-tac-toe board labeling the first column acid, the second column normal, and the third column base as seen below.
Now that the tic-tac-toe board has been drawn, we simply plug in the blood gas numbers.
Beginning with the pH, this will be used to determine if the patient’s acid base status reflects acidosis or alkalosis.
A pH less than 7.35 is consistent with acidosis (acid).
A pH greater than 7.45 is consistent with alkalosis (base).
The pH is 7.53 in our example. Since this is greater than 7.45, we will place the pH under the base column as seen below.
Next, we will plug in the CO2 and HCO3 values.
Let’s first remind ourselves that bicarbonate is a base and CO2 is an acid.
This is simple to remember because bicarbonate and base both start with the letter “B”, and CO2 starts with “C” and there is a “C” in acid.
The PaCO2 in our example is 41. This is normal, so we will place the PaCO2 under the normal column.
Then we take a look at HCO3. We have a value of 32 which is greater than the normal value of 22-26.
Remember that bicarb is a base. If we have more base than normal, then excess base is present. Therefore, HCO3 will go under the base column.
Had HCO3 been normal, then it would have gone under the normal column.
If HCO3 had been low ( < 22 ), then it would have gone under the acid column since there would have been a base deficit present.
Now that we have plugged in all of our values, it is time to play tic-tac-toe and circle 3 in a row which will give us our acid base status.
To make it even simpler, you always circle vertically and never diagonally.
In our example, we are going to circle “base, pH, and HCO3”.
Our acid base status will include the circled components.
Just a reminder that HCO3 is associated with the metabolic component of a blood gas, and CO2 is associated with the respiratory component.
Since HCO3 is circled, we have a metabolic disorder.
The circled pH and base tell us this is alkalosis.
Therefore, our example blood gas demonstrates a metabolic alkalosis.
Of note, calculating an anion gap becomes important, particularly with metabolic acidosis. If you want more information on this, be sure to check out the EZmed anion gap post for simple mnemonics and explanations!
Now we have to determine if there is compensation present.
We do this by looking at the remaining unused value, and in this case it is CO2.
We can see that CO2 is under the normal column.
Therefore, it is not doing anything to compensate for the metabolic disorder (otherwise CO2 would have been under the acid column).
The acid base status in our example is metabolic alkalosis without respiratory compensation.
One More Example…..Compensation
Now let’s look at an example with compensation and see how it appears on the tic-tac-toe board.
Example blood gas: pH 7.27; PaCO2 65; HCO3 30
The tic-tac-toe board will be drawn like above, and we will simply input the numbers again.
A pH of 7.27 is less than 7.35 which reflects acidosis, so the pH will be placed under the acid column.
PaCO2 is 65 which is greater than the normal value. We know that CO2 is an acid, and since we have more acid than normal we will place CO2 under the acid column.
HCO3 is 30 which is greater than the normal value. We know that HCO3 is a base, and since we have excess base than normal it will go under the base column.
Now that all the values are plugged in, we’ll play tic-tac-toe and circle the 3 in a row to get our acid base status.
“Acid, pH, and PaCO2” are all circled in this example.
We know that CO2 is associated with the respiratory component of a blood gas, and HCO3 is associated with the metabolic component.
Since CO2 is circled, we know we have a respiratory disorder.
Acid and pH are circled informing us that acidosis is present.
Therefore, we have a respiratory acidosis.
Lastly, we’ll check for the presence of compensation by assessing the unused value, and in this case it is HCO3.
Rather than the HCO3 being under the normal column (which would have meant no metabolic compensation present), it is under the base column.
Therefore, there is a degree of metabolic compensation taking place.
The acid base status of this example is respiratory acidosis with metabolic compensation.
If compensation is present, then you can calculate if it is partially or fully compensated.
This will be discussed in a future blog. The focus of this post is how to use the tic-tac-toe method to determine acid base status.
Lastly, I will mention there can be mixed disorders (metabolic and respiratory acidosis, or metabolic and respiratory alkalosis).
When this occurs, then the pH, CO2, and HCO3 will all be under the same tic-tac-toe column.
For example using the blood gas above, if HCO3 had been < 22 then it would have been under the acid column rather than the base. This would have indicated a mixed metabolic and respiratory acidosis.
Conclusion
Hopefully the tic-tac-toe method gave you an easy way to interpret blood gases and acid base status!
Remember that a pH < 7.35 is acidosis and a pH > 7.45 is alkalosis.
Bicarb is a base and CO2 is an acid.
Depending if HCO3 or CO2 is circled, it will either be a metabolic, a respiratory, or a mixed disorder.
Lastly, assess for the presence or absence of compensation.
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