5 Easy Steps to Handle Arterial Blood Gases (ABGs) on the ABIM Board Exam

One of the seemingly challenging concepts to master for the ABIM Board Exam are arterial blood gases (ABGs). However, by following a simple yet systematic approach, we can tackle these questions without difficulty.

• Normal pH is 7.35 – 7.45
• If pH < 7.35 : Acidemia is present
• If pH > 7.45 : Alkalemia is present

• If pCO₂ is high and pH is low : Primary problem is Respiratory Acidosis
• If pCO₂ is low and pH is high : Primary problem is Respiratory Alkalosis
• If HCO₃ is low and pH is low : Primary problem is Metabolic Acidosis
• If HCO₃ is high and pH is high : Primary problem is Metabolic Alkalosis

• If the primary problem is Metabolic Acidosis, determine if the body is appropriately compensating with a Respiratory Alkalosis using Winter’s Formula:
  pCO₂ = 1.5 (HCO₃) + 8 +/- 2
• If the primary problem is Metabolic Alkalosis, determine if the body is appropriately compensating with a Respiratory Acidosis using the equation:
  pCO₂ = 40 + 0.7 (Δ HCO₃)
• If the primary problem is Respiratory Acidosis, determine if the body is appropriately compensating with a Metabolic Alkalosis using the following rules.
• Acute: 1 unit increase in HCO₃ for every 10 unit increase in pCO₂
• Chronic: 3.5 unit increase in HCO₃ for every 10 unit increase in pCO₂
• If the primary problem is Respiratory Alkalosis, determine if the body is appropriately compensating with a Metabolic Acidosis using the following rules.
• Acute: 2 unit decrease in HCO₃ for every 10 unit decrease in pCO₂
• Chronic: 5 unit decrease in HCO₃ for every 10 unit decrease in pCO₂

 

Step #4: Calculate the Anion Gap
• Anion Gap = Na – (Cl + HCO₃)
• Normal Anion Gap = 8 – 12 mEq/L
• If Anion Gap is high and the patient has Metabolic Acidosis, the patient has Widened Anion Gap Metabolic Acidosis (WAGMA).
Causes of WAGMA can be remembered by the mnemonic “MUD PILES”
• Methanol
• Uremia
• Diabetic Ketoacidosis
• Paraldehyde
• Isoniazid
• Lactic Acidosis
• Ethylene Glycol
• Salicylates

NOTE: Salicyclates can also cause Respiratory Alkalosis

 

If the patient has Metabolic Acidosis and Anion Gap is normal, then the patient has a Normal Anion Gap Metabolic Acidosis (NAGMA).
Two main causes of NAGMA are:
• Diarrhea, which will have a negative urine anion gap

(Urine Na + Urine Potassium) – Urine Chloride

• Renal tubular acidosis, which will have a positive urine anion gap

 

Step #5: Check a Delta Gap, if applicable
• If the Anion Gap is greater than 20mEq/L, then a Delta Gap needs to be calculated to determine if additional metabolic disorders are present.
• Delta Gap = Anion Gap – 12
• If Delta Gap + HCO₃ > 30, the patient also has Metabolic Alkalosis present
• If Delta Gap + HCO₃ < 23, the patient also has NAGMA present

 

This is a general approach for tackling ABGs, as relevant for your preparation of the ABIM Internal Medicine board exam.