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ABG (Arterial Blood Gas) Analysis - How to Interpret & Treat

What ABG Measures

pH = 6.1 + log10 ([HCO3-] / 0.03 × PaCO2)

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Step-by-Step Interpretation (6-Step Method)

Step 1 - Identify pH Status

• pH < 7.35 = Acidemia
• pH > 7.45 = Alkalemia

Step 2 - Identify Primary Process

Step 3 - Assess Compensation

Step 4 - Calculate Anion Gap (AG)

• Correct for albumin: add 2.5 × (normal albumin - observed albumin) to the AG
• High AG acidosis: MUDPILES (Methanol, Uremia, DKA, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates)
• Normal AG acidosis: check urine AG

Step 5 - Urine Anion Gap (if non-AG acidosis)

• Positive urine AG = GI bicarbonate losses (e.g., diarrhea)
• Negative urine AG = Renal bicarbonate losses (e.g., RTA)

Step 6 - Delta-Delta Ratio (if high AG present)

• < 1.0 = mixed anion gap + non-anion gap acidosis
• 1.0-2.0 = pure anion gap metabolic acidosis

• 2.0 = mixed anion gap acidosis + metabolic alkalosis (or compensated chronic respiratory acidosis)

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Treatment by Disorder

1. Respiratory Acidosis (pH ↓, PaCO2 ↑)

• Treat underlying cause (e.g., bronchodilators in COPD, antibiotics in pneumonia)
• Supplemental oxygen to maintain PaO2 > 60 mmHg and SpO2 > 90%
• NIV (BiPAP/CPAP) if increased work of breathing, PaO2/FiO2 < 200, or PaCO2 > 45 - use if patient is conscious, hemodynamically stable, and able to protect airway
• Invasive mechanical ventilation if: NIV fails, unable to protect airway, severe shock, coma, seizures, hematemesis
• Permissive hypercapnia in ARDS (lung-protective ventilation with TV < 6 mL/kg to keep plateau pressure < 30 mmHg)
• Avoid sodium bicarbonate in respiratory acidosis - it worsens CO2 retention

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2. Metabolic Acidosis (pH ↓, PaCO2 ↓, HCO3- ↓)

• Mild metabolic acidosis (pH > 7.25) - often no emergent treatment needed
• Emergent treatment if: HCO3- < 15 mmol/L or pH < 7.15-7.20
• Sodium bicarbonate (IV): indicated for:
  • Renal failure with AKI/CKD metabolic acidosis
  • Hyperchloremic/non-AG acidosis
  • Renal tubular acidosis (long-term oral NaHCO3 + chloride restriction)
  • Controversial in lactic acidosis and DKA - treat the underlying cause primarily
• Monitor for bicarbonate complications: metabolic alkalosis, hypocalcemia, hypokalemia, hypernatremia, volume overload
• DKA: IV fluids, insulin, potassium replacement - bicarbonate rarely needed
• Lactic acidosis: treat the cause (sepsis, shock, hypoxia)

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3. Respiratory Alkalosis (pH ↑, PaCO2 ↓)

• Reassurance and removal of stressors
• Breathing retraining - diaphragmatic breathing
• Do NOT use paper bag rebreathing - can cause dangerous hypoxemia
• Short-acting benzodiazepines if conservative measures fail
• In mechanically ventilated patients: reduce tidal volume and/or respiratory rate
• Improve sedation and analgesia if patient is breathing over the set rate
• Long-term: cognitive behavioral therapy + treat precipitating conditions

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4. Metabolic Alkalosis (pH ↑, PaCO2 ↑, HCO3- ↑)

• Chloride-responsive alkalosis (urine Cl- < 25 mEq/L): give normal saline (0.9% NaCl) or Ringer's lactate to correct hypochloremia and volume contraction
• Contraction alkalosis: correct free water deficit using: Free water deficit = 0.6 × weight (kg) × ([Na+]/140 - 1)
• Chloride-resistant alkalosis (urine Cl- > 40 mEq/L - e.g., hyperaldosteronism, Cushing's): treat the underlying cause, potassium-sparing diuretics
• In mechanically ventilated patients who are chronic CO2 retainers - reduce minute ventilation cautiously to avoid unmasking metabolic alkalosis

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Quick Acute on Chronic Respiratory Acidosis Distinction

• Acute: PaCO2 > 45, pH < 7.35, HCO3- rises ~1 mEq/L per 10 mmHg ↑ CO2
• Chronic: PaCO2 > 45, pH near-normal (7.35-7.45), HCO3- rises ~3.5-5 mEq/L per 10 mmHg ↑ CO2 (renal compensation over 3-5 days)
• A-a gradient helps identify cause: normal A-a gradient with hypercapnia = hypoventilation/depressed drive; widened A-a gradient (> 20) = pulmonary disease

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Key Pearls

• Always draw ABG and venous electrolytes simultaneously - bicarbonate values should match within 2-3 mEq/L; a discrepancy means lab error or timing mismatch
• Always calculate the anion gap even when pH and PaCO2 are normal - a hidden high AG signals a mixed disorder
• For every acute rise in PaCO2 of 10 mmHg, pH falls by 0.08
• Sodium bicarbonate worsens respiratory acidosis - avoid it unless the primary disorder is metabolic