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Arterial Blood Gas (ABG) Analysis: Basic to Clinical

What Is an ABG?

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The Henderson-Hasselbalch Equation (Foundation)

• A change in HCO3- (metabolic)
• A change in PaCO2 (respiratory)
• Or both simultaneously (mixed disorder)

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Step-by-Step ABG Interpretation

STEP 1 - Assess Oxygenation

• PaO2 < 80 mmHg = hypoxemia
• PaO2 < 60 mmHg = significant hypoxemia (SpO2 ~90%, threshold for O2 supplementation)

Alveolar Gas Equation: $$P_AO_2 = [FiO_2 \times (P_{atm} - P_{H_2O})] - \frac{PaCO_2}{R}$$ At room air (FiO2=0.21): PAO2 = 150 - (PaCO2 / 0.8)

A-a Gradient = PAO2 - PaO2

• Normal: 5-20 mmHg on room air; 20-65 mmHg on 100% O2
• Normal A-a gradient + hypoxia = pure hypoventilation (e.g., sedation, NMJ disease)
• Elevated A-a gradient = V/Q mismatch, shunt, or diffusion limitation (e.g., PE, pneumonia, pulmonary edema)

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STEP 2 - Determine the Primary Acid-Base Disorder

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STEP 3 - Assess Compensation

HCO3- + 15 ≈ last two digits of pH (and ≈ expected PaCO2) If PaCO2 matches: simple metabolic acidosis with respiratory compensation If PaCO2 > expected: also has respiratory acidosis If PaCO2 < expected: also has respiratory alkalosis

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STEP 4 - Calculate the Anion Gap (for metabolic acidosis)

Corrected AG = Measured AG + 2.5 × (4 - albumin in g/dL)

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STEP 5 - Delta-Delta Ratio (in high-AG metabolic acidosis)

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STEP 6 - Identify Mixed Disorders

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Mixed Disorder Examples (Harrison's, 22nd ed.)

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Causes of Each Primary Disorder

Metabolic Acidosis (High-AG)

Metabolic Acidosis (Normal-AG / Hyperchloremic)

Metabolic Alkalosis

Respiratory Acidosis

• Primary lung disease (COPD, pneumonia, pulmonary edema)
• Chest wall disease (flail chest, obesity hypoventilation)
• Respiratory muscle weakness (Guillain-Barré, myopathies, hypokalemia, hypophosphatemia)
• Decreased respiratory drive (CNS lesions, opioids, sedatives)
• Mechanical ventilation (iatrogenic hypoventilation)

Respiratory Alkalosis

• Anxiety/hyperventilation (most common ER presentation)
• Hypoxemia-induced hyperventilation (PE, pneumonia at altitude)
• Salicylate toxicity (early), hepatic encephalopathy, sepsis
• CNS disease (stroke, tumor), pregnancy
• Mechanical ventilation (iatrogenic hyperventilation)

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Worked Clinical Examples

Example 1 - Diarrhea

1. pH < 7.35 → acidemia
2. HCO3- low → metabolic acidosis
3. Check compensation: ΔPaCO2 = 1.3 × (25-5) = 26; predicted PaCO2 = 40-26 = 14 ≈ measured 13 ✓
4. AG = 133 - (118+5) = 10 → normal anion gap (hyperchloremic from diarrhea - GI HCO3- loss)

• Diagnosis: Simple metabolic acidosis with appropriate respiratory compensation (diarrhea)

Example 2 - Sepsis (Respiratory Alkalosis)

1. pH > 7.45 → alkalemia
2. PaCO2 low → respiratory alkalosis
3. Acute compensation: ΔHCO3- = -2 × (40-25)/10 = -3; expected HCO3- = 24-3 = 21 ≈ 22 ✓

• Diagnosis: Acute respiratory alkalosis (typical of early sepsis/anxiety)

Example 3 - Mixed: Lactic Acidosis + Respiratory Alkalosis (Sepsis ICU)

1. pH near normal but AG = 20 (elevated) → high-AG metabolic acidosis present
2. PaCO2 < predicted for metabolic acidosis alone → additional respiratory alkalosis
3. The near-normal pH is a false reassurance - two disorders are fighting each other

• Diagnosis: Mixed high-AG metabolic acidosis + respiratory alkalosis (sepsis)

Example 4 - COPD + Diuretics (Mixed Metabolic Alkalosis + Respiratory Acidosis)

1. pH essentially normal but PaCO2 markedly elevated and HCO3- markedly elevated
2. Normal pH in this setting indicates two opposing processes

• Diagnosis: Mixed metabolic alkalosis (diuretics) + respiratory acidosis (COPD)

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Osmolar Gap (Extra Tool in Toxic Alcohol Ingestion)

• Normal osmolar gap < 10 mOsm/kg
• Elevated gap + high-AG metabolic acidosis → suspect methanol or ethylene glycol
• Estimated ethylene glycol concentration (mg/dL) ≈ osmolar gap × 6

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Venous vs. Arterial Blood Gas

• Tintinalli's Emergency Medicine, p. 122

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ABG Management Principles

1. Identify the type of disorder(s) present
2. Identify the underlying cause
3. Treat the cause - not just the pH

• For DKA: insulin (not just bicarbonate)
• For lactic acidosis from sepsis: restore perfusion
• For toxic alcohol: specific antidotes (fomepizole)
• NaHCO3 is a reasonable adjunct in severe metabolic acidosis from bicarbonate loss or renal failure, while the underlying cause is addressed

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Quick Reference Summary Card

Normal ABG: pH 7.35-7.45 | PaCO2 35-45 | HCO3- 22-26 | PaO2 80-100

STEP 1: Oxygenate? → PaO2, SaO2, A-a gradient
STEP 2: pH → acidemia or alkalemia?
STEP 3: Primary disorder? → PaCO2 (resp) vs HCO3- (metabolic)
STEP 4: Compensation adequate? → Use formulas / Rule of 15
STEP 5: Anion gap? → AG = Na - (Cl + HCO3-)  [normal 8-12]
STEP 6: Delta-delta? → Hidden 2nd metabolic disorder?
STEP 7: Osmolar gap? → If toxic ingestion suspected

If compensation ≠ expected → MIXED DISORDER

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