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Arterial Blood Gas (ABG) - Basics to Clinicals

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1. Why pH Matters

• pH < 7.35 = acidemia
• pH > 7.45 = alkalemia

The -emia suffix describes the blood state; the -osis suffix describes the process causing it.

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2. The Henderson-Hasselbalch Framework

CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺

• PaCO₂ - controlled by the lungs (respiratory)
• HCO₃⁻ - controlled by the kidneys (metabolic)

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3. The Three Buffer Systems

1. Bicarbonate buffer (extracellular, dominant) - responds within minutes via respiratory adjustment
2. Protein buffers (hemoglobin, albumin, intracellular proteins) - immediate buffering
3. Phosphate buffer (intracellular and renal) - key for urinary acid excretion

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4. Normal Compensatory Responses

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5. Stepwise ABG Interpretation

Step 1 - Assess the pH

• pH < 7.35 → Acidosis
• pH > 7.45 → Alkalosis
• pH = 7.35-7.45 → May still have a mixed disorder

Step 2 - Identify the Primary Disorder

Step 3 - Calculate the Anion Gap (for metabolic acidosis)

Anion Gap (AG) = Na⁺ - (HCO₃⁻ + Cl⁻) Normal = 12 ± 4 mEq/L (some labs: 7-9 mEq/L)

• Corrected AG = Measured AG + 2.5 × (4.4 - serum albumin in g/dL)
• Failure to correct will miss high-AG acidosis in critically ill patients with low albumin.

High AG metabolic acidosis - mnemonic MUDPILES (or GOLD MARK):

Normal AG (hyperchloremic) metabolic acidosis - mnemonic HARDUP:

• Hyperalimentation
• Acetazolamide / carbonic anhydrase inhibitors
• Renal tubular acidosis (RTA)
• Diarrhea (GI HCO₃⁻ loss - most common)
• Ureteral diversion
• Pancreatic fistula

Step 4 - Check Compensation (is it adequate?)

• If measured PaCO₂ > expected → additional respiratory acidosis
• If measured PaCO₂ < expected → additional respiratory alkalosis (e.g., salicylate poisoning)

Step 5 - Delta-Delta Ratio (for high AG metabolic acidosis)

Delta ratio = ΔAG / ΔHCO₃⁻ = (Measured AG - 12) / (24 - Measured HCO₃⁻)

Step 6 - Assess Oxygenation

• PaO₂ normal: 80-100 mm Hg on room air
• A-a gradient = PAO₂ - PaO₂
  • PAO₂ = (FiO₂ × 713) - (PaCO₂ / 0.8)
  • Normal A-a gradient ≈ age/4 mm Hg (or < 10-15 on room air)
  • Elevated A-a gradient → intrinsic lung disease (V/Q mismatch, shunt, diffusion defect)
  • Normal A-a gradient + hypoxemia → hypoventilation (pure PaCO₂ problem)

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6. The Acid-Base Map

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7. The Four Primary Disorders - Clinical Details

A. Metabolic Acidosis

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B. Metabolic Alkalosis

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C. Respiratory Acidosis

• Acute: HCO₃⁻ rises only ~1 mEq/L per 10 mm Hg PaCO₂ rise (buffering only)
• Chronic: HCO₃⁻ rises ~4 mEq/L per 10 mm Hg (full renal compensation)

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D. Respiratory Alkalosis

• Anxiety / pain / fever
• Pulmonary embolism (common, reflex hyperventilation)
• Pregnancy (progesterone stimulates breathing)
• Hepatic failure / cirrhosis
• Salicylate toxicity (early - before acidosis dominates)
• Mechanical ventilation (iatrogenic)
• CNS insult (meningitis, stroke)

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8. Mixed Disorders - Clinical Scenarios

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9. A Worked Clinical Example

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10. Quick Reference Card

ABG Normal Values
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pH       7.35 – 7.45
PaCO₂    35 – 45 mm Hg
HCO₃⁻   22 – 26 mEq/L
PaO₂    80 – 100 mm Hg
BE       -2 to +2 mEq/L

STEP-BY-STEP
1. pH → acidemia / alkalemia?
2. Match PaCO₂ or HCO₃⁻ to direction → primary disorder
3. If met acidosis: AG = Na - (Cl + HCO₃)
   Correct for albumin: +2.5 per g/dL below 4.4
4. Check compensation (Winter's, tables above)
   Mismatch = second disorder
5. If high AG: Delta ratio = ΔAG / ΔHCO₃
   <0.4 = NAGMA; 1-2 = pure HAGMA; >2 = HAGMA + met alk
6. A-a gradient for oxygenation assessment

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• Medical Physiology (Boron & Boulpaep) - acid-base physiology
• Symptom to Diagnosis: An Evidence-Based Guide, 4th Ed - stepwise approach
• Roberts and Hedges' Clinical Procedures in Emergency Medicine - compensation tables and worked examples
• Miller's Anesthesia, 10th Ed - delta ratio and base excess
• Comprehensive Clinical Nephrology, 7th Ed - mixed disorders