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DR. CHINTAN DAVE
01 / 08
ABG CHEAT CODE
The 'Mountain Method' for ABGs
A visual way to identify the primary disorder, check compensation, and catch mixed acid-base problems fast.
Conquer ABGs once and for all.
Never confuse compensation again.
10:1
10:2
10:4
10:5
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DR. CHINTAN DAVE
02 / 08
THE ROADMAP
Read every ABG in 3 moves
Use the same sequence every time.
01
What is the pH?
Acidemia, alkalemia, or near normal?
02
What is the primary process?
PaCO2 or HCO3-?
03
Is compensation appropriate?
If not, think mixed disorder.
Normal values: pH 7.35-7.45 | PaCO2 35-45 | HCO3- 22-26
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03 / 08
STEP 1
Start with the pH
The pH tells you which side of the mountain you're on.
pH < 7.35
Acidemia
pH 7.35-7.45
Normal /
compensated /
mixed
pH > 7.45
Alkalemia
Always start here before looking at anything else.
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04 / 08
STEP 2
Find the primary disorder
Know what CO2 and HCO3- do to pH.
PaCO2 = respiratory
• PaCO2 up → pH down
• PaCO2 down → pH up
HCO3- = metabolic
• HCO3- down → pH down
• HCO3- up → pH up
Whichever value moves with the pH is usually the primary process.
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05 / 08
STEP 3
Use the Mountain Method
Start at normal: PaCO2 40, HCO3- 24.
Each mountain step = 10 mmHg change in PaCO2.
ACUTE
10:1
+10 PaCO2
+1 HCO3-
10:2
-10 PaCO2
-2 HCO3-
CHRONIC
10:4
+10 PaCO2
+4 HCO3-
10:5
-10 PaCO2
-5 HCO3-
Normal baseline = PaCO2 40 / HCO3- 24
Read it like a hike: uphill = acidosis, downhill = alkalosis.
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06 / 08
DON'T FORGET METABOLIC
Quick metabolic compensation rules
These help you decide whether compensation fits.
Metabolic acidosis
Expected PaCO2 = 1.5 x HCO3- + 8 ± 2
Winter's formula
Metabolic alkalosis
Expected PaCO2 ≈ 40 + 0.7 x (HCO3- - 24) ± 5
If the measured value falls outside the expected range, suspect a mixed disorder.
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07 / 08
QUICK CHEAT SHEET
Examples you can do in your head
ACUTE ACIDOSIS
PaCO2 60
Expected HCO3- ≈ 26
40 → 60 is +20, so add 2
CHRONIC ACIDOSIS
PaCO2 60
Expected HCO3- ≈ 32
40 → 60 is +20, so add 8
ACUTE ALKALOSIS
PaCO2 30
Expected HCO3- ≈ 22
40 → 30 is -10, so subtract 2
CHRONIC ALKALOSIS
PaCO2 30
Expected HCO3- ≈ 19
40 → 30 is -10, so subtract 5
If the actual HCO3- falls off the mountain, think mixed disorder.
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DR. CHINTAN DAVE
08 / 08
SAVE THIS CHECKLIST
My forever ABG checklist
Use this every time you read an ABG.
01
Look at the pH: acidosis, normal, or alkalosis?
02
Ask whether PaCO2 or HCO3- explains it.
03
If it's respiratory, use the Mountain Method.
04
If it's metabolic, check expected compensation.
05
If it doesn't fit, suspect a mixed disorder.
06
Then check anion gap, oxygenation, and the clinical context.
Always name the disorder completely: e.g., chronic respiratory acidosis with appropriate metabolic compensation.
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