Hi all, this is Jessica, and welcome to CritIC!
No medical YouTube channel would be complete without an acid base series.
Since there are already so many videos out there, I wasn't sure whether I should make
one.
However, since I will be covering other topics like diabetic ketoacidosis and lactic acidosis
in the future, I feel like I can't go without.
In the next four videos, you'll go through the different respiratory and metabolic acid-base
disorders.
In this video, we'll cover the steps to acid-base interpretation using the arterial blood gas
analysis.
Here goes!
Let's go back to high school.
Remember the Henderson-Hasselbalch equation?
It relates CO2 to hydrogen and bicarbonate like so.
CO2 plus H2O becomes H2CO3
However this disintegrates immediately into hydrogen (H+) and bicarbonate (HCO3-)
So what happens if we add a bucket o acids to our body?
Well the equation shifts to the left.
So we're stuck with more CO2.
Fortunately, our lungs will work extra hard to get rid of this.
That brings me to our first rule: you're blowing out your assess.. ehh acids.
You're blowing out your acids.
Hyperventilation brings down CO2, in which necessarily follows that hydrogen is lowered,
and pH goes up.
So the left side of this equation is the respiratory part.
It is a fast responding system for the compensation of the acid-base disturbance.
If the primary problem resides on the left side of the equation, we call that problem
a "respiratory" alkalosis or acidosis.
The right side of the equation is the metabolic part, since bicarbonate is handled mostly
by the kidneys.
It is a slow responding system for the compensation of the acid-base disturbance.
If the primary problem resides on the right side of the equation, we call that problem
a "metabolic" alkalosis or acidosis.
Remember that confusing equation that I quickly showed you with all the numbers?
Well don't.
But it shows that you can predict the change in pH if you know the change in either bicarbonate
or pCO2.
If this prediction doesn't add up, there's probably a combined acid-base disorder.
Please note the second rule: the body does not overcompensate.
If, for instance, there is a metabolic acidosis present, but CO2 is even lower than expected
so that pH is high, there is also a respiratory alkalosis, which deserves a separate workup.
It's not that that patient is overcompensating.
Let's go through the steps of the interpretation of a blood gas sample
Step 1.
Check pH and name the disorder.
Low pH is an acidosis, high pH is an alkalosis.
If pH is normal, you're not done yet.
It could be a compensated acid-base disorder, go to step 2.
Step 2.
Check pCO2.
As stated, pCO2 is representing our fast respiratory system.
If pH was normal and pCO2 is also normal, there's no acid base disorder.
You're done.
Now if you did have an abnormal pH, look again and ask yourself "does this pCO2 explain my pH?"
Remember that you're blowing out your acids.
So if your pH was high (alkalosis) and pCO2 is low, that does explain it.
And it's a respiratory alkalosis.
If it doesn't explain it, it's probably a metabolic alkalosis.
Check in step 3.
Vice versa for acidosis: if pH was low, and pCO2 is high, your CO2 explains the pH, so
a respiratory acidosis.
If it doesn't, it's probably metabolic.
Next step!
Step 3.
Check bicarbonate.
Check to see if your thoughts in step 2 were correct or not.
Like I said before, sometimes there are combined acid-base disturbances, and you'll develop
a feel for when your bicarb level corresponds with pH and CO2.
When in doubt, use the nomogram or a calculator.
Step 4.
Check for combined disorders.
I might get more into this in a future video if you're interested.
Step 5.
Check oxygen.
Huh?
But that's not part of the Henderson-Hasselbalch.
You're correct!
But it could explain causes for a respiratory alkalosis.
As explained in the next video.
Step 6.
Work out the disorder.
We'll go through the different acid base disorders in the next couple of videos.
Go through these steps each time your see a blood gas analysis.
If you're interested in more videos on combined acid-base disturbances, stuff like the delta-delta
ratio or the Stewart approach, let me know in the comments.
Now let's hop into the series.