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Arterial Blood Gas (ABG) Analysis and Clinical Correlation

Normal ABG Values

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

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What ABG Measures

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Step-by-Step ABG Interpretation

Step 1 - Determine pH Status

• pH < 7.35 → Acidemia
• pH > 7.45 → Alkalemia
• pH 7.35-7.45 → Normal (but a mixed disorder may still exist)

Step 2 - Identify the Primary Disorder

Step 3 - Assess Compensation

• Roberts and Hedges' Clinical Procedures in Emergency Medicine; Costanzo Physiology

• The last two digits of pH (±0.02)
• The measured PaCO2 (±2 mm Hg)

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The Four Primary Acid-Base Disorders

1. Metabolic Acidosis

AG = Na+ - (Cl- + HCO3-) Normal AG = 9-15 mEq/L (threshold for "wide" = >15 mEq/L)

Wide (High) Anion Gap Metabolic Acidosis - Mnemonic: MUDPILES

Normal Anion Gap (Hyperchloremic) Metabolic Acidosis - Mnemonic: HARDUP

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2. Metabolic Alkalosis

• GI loss of H+: Vomiting, nasogastric suctioning
• Renal loss of H+: Diuretics, hyperaldosteronism, Cushing's syndrome, Bartter/Gitelman syndrome
• Exogenous HCO3-: Bicarbonate infusion, antacid overuse
• Contraction alkalosis: Volume depletion concentrates HCO3-

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3. Respiratory Acidosis

• CNS depression: Opioids, sedatives, brainstem lesions
• Neuromuscular disease: Myasthenia gravis, Guillain-Barré, ALS
• Chest wall disorders: Severe kyphoscoliosis, flail chest
• Airway obstruction: COPD exacerbation, severe asthma, foreign body
• Lung disease: Pneumonia, pulmonary edema, ARDS

• Acute: HCO3- rises only slightly (buffering by tissues), pH drops significantly
• Chronic: Kidneys compensate by retaining HCO3-, pH is partially restored

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4. Respiratory Alkalosis

• Hypoxia-driven: High altitude, severe anemia, pulmonary embolism, pneumonia
• Stimulation of breathing: Anxiety/pain, salicylate toxicity (early), pregnancy, sepsis (early)
• CNS causes: Brainstem lesions, head injury
• Iatrogenic: Mechanical ventilation with excessive rate

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Mixed Acid-Base Disorders

1. The measured compensation does not match the predicted formula
2. pH is normal but PaCO2 and HCO3- are both abnormal
3. Clinical context does not fit a single disorder

• pH 7.47, PaCO2 25 mm Hg (alkalemia + low CO2 = respiratory alkalosis)
• Predicted pH for this PaCO2 = 7.40 + [(40-25)/10 × 0.08] = 7.52
• Measured pH of 7.47 < predicted 7.52 → concurrent metabolic acidosis (salicylate accumulation)
• Result: Mixed respiratory alkalosis + metabolic acidosis - Rosen's Emergency Medicine

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Oxygenation Assessment from ABG

A-a Gradient (Alveolar-arterial O2 difference)

PAO2 = FiO2 × (Patm - PH2O) - (PaCO2 / 0.8) On room air: PAO2 ≈ 150 - (PaCO2/0.8) A-a gradient = PAO2 - PaO2

PaO2/FiO2 (P/F) Ratio

• 300 = Normal

• 200-300 = Mild ARDS
• 100-200 = Moderate ARDS
• < 100 = Severe ARDS

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Venous Blood Gas (VBG) vs. ABG

• pH correlates closely with ABG (±0.05 units)
• PvCO2 is higher than PaCO2 by ~4-6 mm Hg but trends reliably
• PvO2 cannot substitute for PaO2 - cannot assess oxygenation
• Venous lactate correlates well when clearly normal or elevated; mildly elevated values should be confirmed arterially - Tintinalli's Emergency Medicine

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Clinical Correlation: Common Scenarios

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Quick Approach Summary (Bedside Algorithm)

1. Look at pH → Acidemia or Alkalemia?
2. Look at PaCO2 → Is the change in PaCO2 consistent with the pH change?
   - pH ↓ and PCO2 ↑ → Respiratory acidosis
   - pH ↑ and PCO2 ↓ → Respiratory alkalosis
   - pH ↓ and PCO2 ↓ → Metabolic acidosis (with respiratory compensation)
   - pH ↑ and PCO2 ↑ → Metabolic alkalosis (with respiratory compensation)
3. Check HCO3- to confirm
4. Apply compensation formulas → Is it appropriate? If not → MIXED disorder
5. If metabolic acidosis → Calculate Anion Gap
6. If high AG → Check osmolar gap (if toxin suspected)
7. If high AG → Check delta-delta ratio
8. Assess oxygenation → PaO2, A-a gradient, P/F ratio

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