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Arterial Blood Gas (ABG) Analysis - Complete Guide

What is an ABG?

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Normal ABG Values

IMP Note: The HCO₃⁻ on ABG is calculated from the Henderson-Hasselbalch equation. It should match the HCO₃⁻ measured on the electrolyte panel within ±2 mmol/L. If they don't match, suspect a lab or sampling error.

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Step-by-Step ABG Interpretation (7 Steps)

STEP 1 - Determine the pH (Acid or Alkali?)

• pH < 7.35 = Acidemia
• pH > 7.45 = Alkalemia
• pH 7.35 - 7.45 = "Normal" - but a disturbance may still exist with compensation, or two opposing disorders may be cancelling each other out!

IMP Note: A normal pH never rules out an acid-base disorder.

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STEP 2 - Identify the Primary Disturbance (Respiratory or Metabolic?)

• In respiratory disorders: pH and PaCO₂ move in opposite directions
• In metabolic disorders: pH and PaCO₂ move in the same direction
• Rosen's Emergency Medicine, p. 2505

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STEP 3 - Assess for Expected Compensation

Metabolic Acidosis:

• Winter's Formula: Expected PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2
• Or: PaCO₂ decreases 1.2 mmHg per 1 mEq/L drop in HCO₃⁻

Metabolic Alkalosis:

• Expected PaCO₂ increases 6 mmHg per 10 mEq/L rise in HCO₃⁻

Acute Respiratory Acidosis:

• HCO₃⁻ rises 1 mEq/L per 10 mmHg rise in PaCO₂

Chronic Respiratory Acidosis:

• HCO₃⁻ rises 4 mEq/L per 10 mmHg rise in PaCO₂

Acute Respiratory Alkalosis:

• HCO₃⁻ falls 2 mEq/L per 10 mmHg drop in PaCO₂

Chronic Respiratory Alkalosis:

• HCO₃⁻ falls 4 mEq/L per 10 mmHg drop in PaCO₂

IMP Note - Acute vs Chronic Respiratory Rule: In acute respiratory changes, the kidney hasn't had time to compensate (takes 24+ hours). Acute compensation is buffering only; chronic compensation involves renal retention/excretion of HCO₃⁻.

• Swanson's Family Medicine Review, p. 6424

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STEP 4 - Calculate the Anion Gap (AG)

• Normal AG: ~10 mEq/L (range 6-12 mEq/L)
• Elevated AG (>12): indicates accumulation of unmeasured anions

IMP - Correct AG for Albumin (if hypoalbuminemia):

Example: Albumin = 2.5 g/dL → Corrected AG = Measured AG + (2 × 2.5) = Measured AG + 5

Causes of High AG Metabolic Acidosis - MUDPILES Mnemonic:

Causes of Normal AG (Hyperchloremic) Metabolic Acidosis:

• Diarrhea (GI bicarbonate loss)
• Renal Tubular Acidosis (RTA)
• Hyperalimentation
• Saline infusion
• Harrison's Principles of Internal Medicine 22E, p. 412

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STEP 5 - Delta-Delta Ratio (ΔAG / ΔHCO₃⁻) for Mixed Disorders

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STEP 6 - Assess Oxygenation (PaO₂ and A-a Gradient)

• At room air (FiO₂ = 0.21): PAO₂ ≈ 150 - (PaCO₂/0.8)

• Normal A-a gradient: < 15 mmHg (increases with age, up to ~20 mmHg)
• Rule of thumb for age: A-a gradient = Age/4 + 4

IMP Note: A normal A-a gradient with hypoxemia = hypoventilation (or high altitude). An elevated A-a gradient = V/Q mismatch, shunt, or diffusion defect.

• Costanzo Physiology 7th Edition, p. 246

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STEP 7 - Check the Osmolar Gap (if toxic alcohol suspected)

• Calculated Osmolality = (2 × Na) + (Glucose/18) + (BUN/2.8) + (EtOH/3.7)
• Normal Osmolar Gap: ≤ 10 mOsm/kg

Methanol concentration (mg/dL) ≈ Osmolar gap × 3 Ethylene glycol concentration (mg/dL) ≈ Osmolar gap × 6

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Quick Summary - The 4 Primary Disorders

1. Metabolic Acidosis

• pH ↓, HCO₃⁻ ↓, PaCO₂ ↓ (compensatory)
• Causes: MUDPILES (high AG) / Diarrhea, RTA (normal AG)
• Treatment: Treat underlying cause; NaHCO₃ only if pH < 7.0

2. Metabolic Alkalosis

• pH ↑, HCO₃⁻ ↑, PaCO₂ ↑ (compensatory, rarely exceeds 55 mmHg)
• Causes: Vomiting, NG suction, diuretics, antacids, Conn's syndrome
• Note: Compensation for metabolic alkalosis via CO₂ retention is limited

3. Respiratory Acidosis

• pH ↓, PaCO₂ ↑, HCO₃⁻ ↑ (compensatory)
• Causes: COPD, acute respiratory failure, neuromuscular disease, CNS depression, obesity hypoventilation
• Acute: HCO₃⁻ rises 1/10; Chronic: HCO₃⁻ rises 4/10

4. Respiratory Alkalosis

• pH ↑, PaCO₂ ↓, HCO₃⁻ ↓ (compensatory)
• Causes: Anxiety/pain, pregnancy, salicylate toxicity (early), sepsis, CNS events, high altitude
• Acute: HCO₃⁻ falls 2/10; Chronic: HCO₃⁻ falls 4/10

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Clinical Example - Salicylate Toxicity (Mixed Disorder)

1. pH 7.47 → Alkalemia
2. PaCO₂ 25 mmHg + pH 7.47 → pH and CO₂ move opposite = Respiratory Alkalosis (primary)
3. Check for concurrent metabolic disorder:
   • ΔPaCO₂ = 40 - 25 = 15
   • Predicted pH = 7.40 + (15/10 × 0.08) = 7.40 + 0.12 = 7.52
   • Measured pH (7.47) is lower than predicted (7.52)
   • → Concurrent Metabolic Acidosis present

• Rosen's Emergency Medicine, Box 113.2

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IMP High-Yield Notes for Exams

1. Henderson-Hasselbalch equation: pH = 6.1 + log [HCO₃⁻ / (0.03 × PaCO₂)]
2. Never overcorrect: Compensation never returns pH to normal AND never overcorrects past normal
3. Albumin correction for AG is mandatory - easily missed!
4. Lactic acidosis is the most common cause of high AG metabolic acidosis
5. Winter's formula is for metabolic acidosis compensation check only
6. Rule of 10:0.08 - every 10 mmHg change in PaCO₂ = 0.08 change in pH (opposite direction) in pure respiratory disorders
7. Chronic respiratory acidosis → HCO₃⁻ can exceed 30 mEq/L (renal compensation); pH may be near-normal
8. Right-to-left shunt is the only cause of hypoxemia that does NOT respond well to supplemental O₂
9. Simultaneous ABG + electrolytes before therapy - critical for accurate interpretation
10. Triple acid-base disturbances exist - e.g., alcoholic ketoacidosis (metabolic acidosis) + vomiting (metabolic alkalosis) + hepatic hyperventilation (respiratory alkalosis)

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Sources:

• Harrison's Principles of Internal Medicine 22E (2025), Steps in Acid-Base Diagnosis
• Rosen's Emergency Medicine, Three-Step Focused Algorithm for ABG (Box 113.2)
• Swanson's Family Medicine Review, Approach to Arterial Blood Gas Analysis
• Costanzo Physiology 7th Ed., Hypoxemia and A-a Gradient
• Fishman's Pulmonary Diseases, Classification of Respiratory Disorders