ABG Interpretation Made Easy

Normal values

1. pH: 7.35-7.45
2. PaCO2: 35-45 mmHg (4.7-6 kPa)
3. PaO2: 75-100 mmHg
4. HCO3-: 22-26 mEq/L
5. SaO2: 94-100%
6. Base excess: -2 to +2 mmol/L

Step 1: pH

1. >7.4 = Alkalemia
2. <7.4 = Acidemia

Step 2: pCO2

1. Increased = Respiratory acidosis
   • Determine chronicity by looking at metabolic compensation:
     • Acute: 10 Δ pCO2 = 0.08 Δ pH (opposite direction)
     • Chronic: 10 Δ pCO2 = 0.03 Δ pH
2. Decreased = Respiratory alkalosis

Step 3: HCO3-

1. Increased = Metabolic alkalosis (VOMED)
2. Decreased = Metabolic acidosis
   • Determine anion-gap (AG): Normal = 5-12 (or 2.5 X Albumin)
     • AG = Na – [Cl – HCO3]
       • AG increased = Anion gap acidosis (MUDPILES/GOLDMARK)
       • AG decreased = Non-anion gap (hyperchloremic) acidosis (GUT/HARDUP)

Step 4: Determine compensation

If there is metabolic acidosis or alkalosis, determine if there is appropriate respiratory compensation:

1. Expected pCO2 = 1.5 X [HCO3] + 8 +/- 2 (Winter’s formula) OR
2. Expected pCO2 = last 2 digits of pH

No respiratory compensation: Expected pCO2 = Measured pCO2

Respiratory compensation: Expected pCO2 ≠ Measure pCO2

Step 5: Delta ratio

ΔAG / ΔHCO3

• 1 = No other derangements
• >1.5 = Superimposed metabolic alkalosis
• <0.8 = Superimposed non-anion gap metabolic acidosis

Mnemonics

MUDPILES:

1. Methanol/Metformin
2. Uremia
3. DKA
4. Paraldehyde
5. INH
6. Lactic acidosis
7. Ethylene glycol
8. Salicylates

GOLDMARK:

1. Glycols
2. Oxoproline
3. L-lactate (standard lactate seen in lactic acidosis)
4. D-lactate (exogenous lactate produced by gut bacteria)
5. Methanol
6. Aspirin
7. Renal failure
8. Ketones (diabetic, alcoholic, starvation)

GUT:

1. Gastrointestinal losses (diarrhea, pancreatic fistula)
2. Urinary losses
3. Total parenteral nutrition

HARDCUP:

1. Hyperchloremia
2. Acetazolamide, Addison’s disease
3. Renal tubular acidosis
4. Diarrhea, iliostomies, fistula
5. Cholestyramine
6. Ureteroenterostomies
7. Pancreatoenterostomies

VOMED:

1. Vomiting or aspiration
2. Overcorrection of chronic hypercarbia or any acidosis
3. Mineralocorticoid excess
4. Ethylene glycol poisoning/Early sepsis
5. Diuretics/Diarrhea