The Impossible ABG Tutor

1. pH INTERPRETATION

Normal Range: 7.35 - 7.45

WHEN TO USE: Always - first step in every ABG

Classification:

• pH < 7.35 → Acidemia
• pH > 7.45 → Alkalemia
• pH 7.35-7.45 → Normal (does not exclude acid-base disorder)

2. PRIMARY DISORDER IDENTIFICATION

WHEN TO USE: After pH classification

Method: Determine which direction HCO₃ and PaCO₂ move the pH

• Metabolic Acidosis: Low HCO₃ (< 22) causes low pH
• Metabolic Alkalosis: High HCO₃ (> 26) causes high pH
• Respiratory Acidosis: High PaCO₂ (> 45) causes low pH
• Respiratory Alkalosis: Low PaCO₂ (< 35) causes high pH

3. ANION GAP (AG)

Equation: AG = Na − (Cl + HCO₃)

Corrected AG: AG + 2.5 × (4 − albumin)

Normal: 8-12 mEq/L (with albumin 4 g/dL)

WHEN TO USE: When metabolic acidosis is present (HCO₃ < 22)

⛔ DO NOT USE: In metabolic alkalosis, pure respiratory disorders, or normal HCO₃

Classification:

• AG > 12 → High Anion Gap Metabolic Acidosis (HAGMA)
• AG ≤ 12 → Normal Anion Gap Metabolic Acidosis (NAGMA)

4. WINTER'S FORMULA

Equation: Expected PaCO₂ = 1.5 × HCO₃ + 8 ± 2

STRICT GATE - WHEN TO USE: ONLY in metabolic acidosis (low HCO₃)

⛔ DO NOT USE: In metabolic alkalosis, respiratory disorders, or normal HCO₃

Purpose: Assess respiratory compensation adequacy for metabolic acidosis

Interpretation:

• Actual PaCO₂ within expected ± 2 → Appropriate compensation
• Actual PaCO₂ > expected + 2 → SUPERIMPOSED respiratory acidosis
• Actual PaCO₂ < expected − 2 → SUPERIMPOSED respiratory alkalosis

5. METABOLIC ALKALOSIS COMPENSATION

Equation: Expected PaCO₂ = 40 + 0.7 × (HCO₃ − 24) ± 5

STRICT GATE - WHEN TO USE: ONLY in metabolic alkalosis (high HCO₃)

⛔ DO NOT USE: In metabolic acidosis, respiratory disorders, or normal HCO₃

Interpretation:

• Actual PaCO₂ within expected ± 5 → Appropriate compensation
• Actual PaCO₂ > expected + 5 → Superimposed respiratory acidosis
• Actual PaCO₂ < expected − 5 → Superimposed respiratory alkalosis

6. RESPIRATORY ACIDOSIS COMPENSATION

Acute: For every 10 mmHg ↑ PaCO₂ → HCO₃ ↑ 1 mEq/L

Chronic: For every 10 mmHg ↑ PaCO₂ → HCO₃ ↑ 3-4 mEq/L

WHEN TO USE: When respiratory acidosis is the primary disorder (high PaCO₂)

Interpretation:

• Matches acute pattern → Acute respiratory acidosis
• Matches chronic pattern → Chronic respiratory acidosis
• HCO₃ higher than expected → Concomitant metabolic alkalosis
• HCO₃ lower than expected → Concomitant metabolic acidosis

Note: "Concomitant" is used here because the metabolic disorder exists independently, not as compensation

7. RESPIRATORY ALKALOSIS COMPENSATION

Acute: For every 10 mmHg ↓ PaCO₂ → HCO₃ ↓ 2 mEq/L

Chronic: For every 10 mmHg ↓ PaCO₂ → HCO₃ ↓ 4-5 mEq/L

WHEN TO USE: When respiratory alkalosis is the primary disorder (low PaCO₂)

Interpretation:

• Matches acute pattern → Acute respiratory alkalosis
• Matches chronic pattern → Chronic respiratory alkalosis
• HCO₃ lower than expected → Concomitant metabolic acidosis
• HCO₃ higher than expected → Concomitant metabolic alkalosis

8. ADJUSTED BICARBONATE

Equation: Adjusted HCO₃ = HCO₃ + (AG − 12)

STRICT GATE - WHEN TO USE: ONLY when HAGMA is confirmed (AG > 12 AND metabolic acidosis present)

⛔ DO NOT USE: In NAGMA, metabolic alkalosis, respiratory disorders, or normal AG

Purpose: Detect hidden metabolic disorders beneath HAGMA

Interpretation:

• Adjusted HCO₃ < 18 → Additional NAGMA present
• Adjusted HCO₃ 18-30 → Pure HAGMA (no hidden metabolic disorder)
• Adjusted HCO₃ > 30 → Hidden metabolic alkalosis present

Clinical Context: This reveals metabolic disorders that would exist if the HAGMA were corrected

9. DELTA RATIO

Equation: Delta Ratio = (AG − 12) ÷ (24 − HCO₃)

STRICT GATE - WHEN TO USE: ONLY when HAGMA is confirmed (AG > 12 AND metabolic acidosis present)

⛔ DO NOT USE: In NAGMA, metabolic alkalosis, respiratory disorders, or normal AG

Purpose: Pattern analysis of HAGMA to detect mixed metabolic disorders

Interpretation:

• Delta < 1 → HAGMA + additional NAGMA
• Delta 1-2 → Pure HAGMA
• Delta > 2 → HAGMA + metabolic alkalosis (or chronic respiratory acidosis with renal compensation)

Complementary Tool: Delta Ratio and Adjusted HCO₃ should yield consistent conclusions