Arterial Blood Gas (ABG) Interpretation: A Guide to Acid-Base Disorders

Arterial Blood Gas (ABG) analysis is a critical diagnostic tool in medicine, used to evaluate a patient's oxygenation, ventilation, and acid-base balance. Understanding primary acid-base disorders is essential for medical students and healthcare professionals preparing for NCLEX, USMLE, and DAT exams.

This guide simplifies the interpretation of acid-base disorders and their compensatory mechanisms.

Step-by-Step ABG Interpretation

Step 1: Check the pH

• pH < 7.35 → Acidosis

• pH > 7.45 → Alkalosis

Once you determine if the patient has acidosis or alkalosis, the next step is to identify the primary cause.

Step 2: Identify the Primary Disorder

Acidosis (pH < 7.35)

🔹 Two possible causes:
1️⃣ Metabolic Acidosis:

• HCO₃⁻ < 24 mM (Low bicarbonate)

• Cause: Excess acid production (e.g., lactic acidosis, ketoacidosis, renal failure) or bicarbonate loss (e.g., diarrhea)

• Compensation: Respiratory compensation via hyperventilation → ↓ PCO₂ (blowing off CO₂ to reduce acidity)

2️⃣ Respiratory Acidosis:

• PCO₂ > 40 mmHg (High carbon dioxide)

• Cause: Hypoventilation due to lung disease, airway obstruction, neuromuscular disorders

• Compensation: Renal compensation → ↑ HCO₃⁻ reabsorption by kidneys to neutralize excess acid

Alkalosis (pH > 7.45)

🔹 Two possible causes:
1️⃣ Metabolic Alkalosis:

• HCO₃⁻ > 24 mM (High bicarbonate)

• Cause: Loss of acid (e.g., vomiting, diuretics) or excess bicarbonate (e.g., antacid overdose)

• Compensation: Respiratory compensation via hypoventilation → ↑ PCO₂ (retaining CO₂ to increase acidity)

2️⃣ Respiratory Alkalosis:

• PCO₂ < 40 mmHg (Low carbon dioxide)

• Cause: Hyperventilation due to anxiety, high altitude, pain, sepsis

• Compensation: Renal compensation → HCO₃⁻ excretion to reduce alkalinity

Step 3: Look for Compensation

• Respiratory compensation occurs rapidly (minutes to hours)

• Renal compensation takes longer (hours to days)

Example: Partially Compensated vs. Fully Compensated

✔ Partially compensated: pH remains abnormal, but compensatory mechanisms are active
✔ Fully compensated: pH normalizes, but PCO₂ and HCO₃⁻ remain abnormal

NCLEX/USMLE Exam Tip

A 55-year-old COPD patient presents with confusion and lethargy. ABG results:

• pH = 7.30

• PCO₂ = 50 mmHg

• HCO₃⁻ = 28 mM

What is the diagnosis?

A) Metabolic acidosis
B) Respiratory acidosis
C) Metabolic alkalosis
D) Respiratory alkalosis

💡 Correct Answer: B) Respiratory acidosis

• Low pH → Acidosis

• High PCO₂ → Respiratory cause

• Elevated HCO₃⁻ → Renal compensation in chronic COPD

Conclusion

ABG interpretation is essential for diagnosing acid-base imbalances in critically ill patients. Understanding pH, PCO₂, and HCO₃⁻ levels helps differentiate respiratory vs. metabolic disorders and assess compensatory responses.

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