Thread by @ChrisCarrollMD

A #Tweetorial on Blood Gas Analysis (adapted from my WKRP in Cincinnati style method of teaching blood gases).

Blood gas interpretation is something that is initially very confusing, is usually poorly taught, comes up hourly in the #pedsICU and #ICU and will be on every exam for the rest of your life.

But it doesn't have to be that complicated and this method will get you through 99.99% of blood gas interpretation and serves as a good foundation to learn those exceptions. (So don't @ me!)

QUICK & DIRTY BLOOD GAS INTERPRETATION
1. Does the patient have an acidosis or alkalosis?
2. What is the primary problem (metabolic or respiratory)?
3. Is there compensation?
4. How’s the oxygenation?

1. Does the patient have an acidosis or alkalosis?

Is the pH < 7.4? Acidotic
Is the pH > 7.4? Alkalotic

(Have yet to see a blood gas that is 7.400!)

1. Does the patient have an acidosis or alkalosis?

So depending on the answer the patient has a primary XXX acidosis or alkalosis.

Now you just need to figure out if the XXX is "Respiratory" or "Metabolic"

2. What is the primary problem (metabolic or respiratory)?

Look at the pCO2. Is it consistent with acidosis or alkalosis?

If pCO2 > 40, then acidosis
If pCO2 < 40, then alkalosis

2. What is the primary problem (metabolic or respiratory)?

Look at the Bicarb. Is it consistent with acidosis or alkalosis?

If Bicarb < 24, then acidosis
If Bicarb > 24 then alkalosis

Or look at the Base Excess/Deficit

If positive? Alkalosis
If negative? Acidosis

Now here's the key part, you can't correct past "zero", or in this case 7.400. So if the pCO2 is consistent with the pH, then it is a primary respiratory acidosis/alkalosis. And if the bicarb/BE is consistent with the pH, then it is a primary metabolic acidosis/alkalosis

3. Is there compensation?

If one that isn't primary is in the opposite direction, then there is compensation. If they are both in the same direction, then you have a combined respiratory and metabolic acidosis/alkalosis

Now, let's run through some cases.

You admit a 4 year old boy after an acute barbiturate ingestion. He suffers a significant depression of mental status and respiration. A blood gas is obtained that shows pH = 7.16, PCO2 = 70, HCO3 = 25

So what is the primary acid base disorder?

A 2 year old girl presents after a 3 day history of severe diarrhea and vomiting. According to history, she “can’t keep anything down” and has had one wet diaper in the last 24 hours. A blood gas reveals: pH=7.34, pCO2=26, HCO3=12.

So what is the acid base disorder?

A 3 month old previously healthy patient is brought into the emergency department with respiratory distress. You obtain the following arterial blood gas: 7.21/64/75/25/+1.

What is the acid base disorder

Here's another way of thinking about this

So what about Arterial vs. Venous vs. Capillary Gases?

You may have noticed that I’m ignoring the difference

Practically, it’s not very important
*when talking about acid-base balance
*is important when get to oxygenation!

For pH:
Venous vs. Arterial result in change of about 0.05
Capillary in the middle
So normal Arterial pH = 7.40, normal Venous = 7.35

For pCO2:
Venous vs. Arterial result in change of about 5
Capillary in the middle
So normal Arterial pCO2 = 40, normal Venous = 45

How about another case

Another 3 month old infant is brought into the ED with respiratory distress. You obtain the following arterial blood gas: 7.49/29/94/25/+1.

What is the primary acid base disorder?

A chronically ventilated patient in the #PedsICU has the following blood gas: 7.38/62/50/32/+8.

What is the primary acid base disorder?

You can also calculate expected compensation of blood gases

But unless you’re going into #ICU, #PedsICU or #Nephrology (or your attending is an asshole), you won’t be asked to do this outside the board exam setting.

4. How’s the oxygenation?

If have arterial gas, can calculate PaO2/FiO2 ratio
-The quickest way to assess oxygenation
- Normally, should be 500.
- On room air, normal PaO2 = 100. So, 100/0.21 = 500

4. How’s the oxygenation?

A PaO2/FiO2 between
200-300 = mildly diminished oxygenation
100-200 = moderately diminished oxygenation
<100 = severely diminished oxygenation

So let's do one more case,

Arterial Blood Gas (on 60% FiO2):
7.20/50/76/25/+2

What's the primary acid base disorder?

And how is the oxygenation? PaO2 76 on 60% FiO2

PaO2/FiO2 ratio =

The answer to this one is PaO2/FiO2 = 76/0.60 = 126 (so 100-200).

Hardly anyone gets this joke when I give the lecture live...

Hope you enjoyed this brief and hopefully well timed #Tweetorial. I'll do Respiratory Failure next!