ABG analysis Medicine
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I now have comprehensive information to compile a thorough ABG analysis guide. Let me put it all together:
Arterial Blood Gas (ABG) Analysis
Normal ABG Values
| Parameter | Normal Range |
|---|---|
| pH | 7.35 - 7.45 |
| PaCO2 | 35 - 45 mmHg |
| PaO2 | 80 - 100 mmHg |
| HCO3- | 22 - 26 mEq/L |
| SaO2 | 95 - 100% |
| Base Excess | -2 to +2 mEq/L |
Step-by-Step ABG Interpretation
Step 1 - Assess the pH
- pH < 7.35 = Acidosis
- pH > 7.45 = Alkalosis
- pH 7.35-7.45 = Normal (but a primary disorder may still exist with compensation)
Step 2 - Determine the Primary Process (Respiratory vs. Metabolic)
| Condition | pH | PaCO2 | HCO3- |
|---|---|---|---|
| Respiratory Acidosis | ↓ | ↑ (>45) | Normal/↑ (compensatory) |
| Respiratory Alkalosis | ↑ | ↓ (<35) | Normal/↓ (compensatory) |
| Metabolic Acidosis | ↓ | Normal/↓ (compensatory) | ↓ (<22) |
| Metabolic Alkalosis | ↑ | Normal/↑ (compensatory) | ↑ (>26) |
Rule: In a simple disorder, the pH and PaCO2 move in opposite directions for respiratory disorders; in metabolic disorders, the pH and HCO3- move in the same direction.
Step 3 - Assess Compensation
Compensation never fully corrects the pH. If the pH is normalized, suspect a mixed disorder.
| Primary Disorder | Expected Compensation |
|---|---|
| Metabolic Acidosis | ↓ PCO2 = 1.3 × ↓ HCO3- (Winter's formula below) |
| Metabolic Alkalosis | ↑ PCO2 = 0.6 × ↑ HCO3- |
| Respiratory Acidosis (Acute) | HCO3- ↑ by 1 mmol/L per 10 mmHg ↑ in PCO2 |
| Respiratory Acidosis (Chronic) | HCO3- ↑ by 4 mmol/L per 10 mmHg ↑ in PCO2 |
| Respiratory Alkalosis (Acute) | HCO3- ↓ by 2 mmol/L per 10 mmHg ↓ in PCO2 |
| Respiratory Alkalosis (Chronic) | HCO3- ↓ by 5 mmol/L per 10 mmHg ↓ in PCO2 |
Winter's Formula (Metabolic Acidosis Compensation Check)
Expected PCO2 = (1.5 × HCO3-) + 8 ± 2
- If measured PCO2 > expected → concurrent respiratory acidosis
- If measured PCO2 < expected → concurrent respiratory alkalosis
- If measured PCO2 = expected → pure metabolic acidosis with appropriate compensation
Useful trick: In metabolic acidosis, expected PCO2 approximately equals the last two digits of the pH. (e.g., pH 7.23 → expected PCO2 ≈ 23 mmHg)
Step 4 - Calculate the Anion Gap (AG)
AG = Na+ - (Cl- + HCO3-) Normal = 8-12 mEq/L (or up to 16 if albumin-uncorrected)
An elevated AG indicates the presence of an unmeasured anion.
High AG Metabolic Acidosis - Mnemonic: MUDPILES CAT (or GOLD MARK - newer)
| MUDPILES | |
|---|---|
| M | Methanol, Metformin |
| U | Uremia |
| D | DKA (Diabetic Ketoacidosis) |
| P | Paraldehyde, Phenformin |
| I | Isoniazid, Iron |
| L | Lactic acidosis |
| E | Ethylene glycol |
| S | Salicylates |
Normal AG (Hyperchloremic) Metabolic Acidosis - Mnemonic: HARD-UP / DURHAM
Common causes: diarrhea, RTA (renal tubular acidosis), acetazolamide, ureteroenteric fistula.
Step 5 - Delta-Delta Ratio (for High AG Metabolic Acidosis)
Used to detect a concurrent metabolic alkalosis or normal-AG acidosis hidden behind a high-AG acidosis.
Delta ratio = (AG - 12) / (24 - HCO3-)
| Delta Ratio | Interpretation |
|---|---|
| < 0.4 | Pure normal-AG (hyperchloremic) acidosis |
| 0.4 - 0.8 | Mixed high-AG + normal-AG acidosis |
| 1.0 - 2.0 | Pure high-AG metabolic acidosis |
| > 2.0 | High-AG acidosis + concurrent metabolic alkalosis |
Step 6 - Assess Oxygenation
A-a Gradient (Alveolar-Arterial PO2 Difference)
PAO2 = FiO2 × (Patm - PH2O) - (PaCO2/RQ) On room air: PAO2 = 150 - (PaCO2 / 0.8) A-a gradient = PAO2 - PaO2
Normal A-a gradient: < 10 mmHg (increases with age: ~2.5 + 0.21 × age)
| A-a Gradient | Cause of Hypoxemia |
|---|---|
| Normal | Hypoventilation (e.g., CNS depression, obesity) |
| Elevated | V/Q mismatch, shunt, diffusion defect |
Acid-Base Map
This map plots PCO2 (x-axis) against pH (y-axis) to classify simple and mixed disorders:
- Zone 1 (red): Mixed respiratory + metabolic acidosis
- Zone 2 (pink): Mixed respiratory + metabolic alkalosis
- Zone 3 (tan): Metabolic alkalosis + respiratory acidosis
- Zone 4 (gold): Metabolic acidosis + respiratory alkalosis
- N: Normal zone
Mixed Disorders (Harrison's, 22nd Ed.)
| Pattern | Key Finding | Classic Example |
|---|---|---|
| Met. Acidosis + Resp. Alkalosis | High AG; PaCO2 below predicted | Lactic acidosis + sepsis |
| Met. Acidosis + Resp. Acidosis | High AG; PaCO2 above predicted | Severe pneumonia/pulmonary edema |
| Met. Alkalosis + Resp. Alkalosis | PaCO2 does not rise as predicted | Liver disease + diuretics |
| Met. Alkalosis + Resp. Acidosis | PaCO2 higher than predicted; pH near normal | COPD + diuretics |
| High-AG Acidosis + Met. Alkalosis | ΔAG >> ΔHCO3- | Uremia + vomiting |
| Mixed High-AG + Normal-AG Acidosis | ΔHCO3- > ΔAG | Diarrhea + lactic acidosis |
Osmolal Gap (for Toxic Ingestions)
OsmCalc = 2×Na + (BUN/2.8) + (Glucose/18) Osmolal Gap = OsmMeasured - OsmCalc Normal = 5-10 mOsm/kg; gap >10 suggests unmeasured osmoles
Causes of elevated osmolal gap: methanol, ethylene glycol, isopropyl alcohol, ethanol, acetone.
Worked Clinical Examples
Example 1 - Diarrhea (from Roberts & Hedges):
Na 133 | K 2.8 | Cl 118 | pH 7.26 | PCO2 13 | HCO3- 5
- pH 7.26 = Acidosis; HCO3- low = Metabolic acidosis
- Winter's: Expected PCO2 = 1.5×5 + 8 = 15.5 → Actual = 13 ≈ within range
- Conclusion: Pure metabolic acidosis, appropriate respiratory compensation
Example 2 - Sepsis (Harrison's):
Na 140 | Cl 106 | HCO3- 14 | PaCO2 24 | pH 7.39
- AG = 140-(106+14) = 20 (elevated)
- pH near normal; HCO3- low; PCO2 low
- Delta ratio: (20-12)/(24-14) = 0.8 → approaching pure high-AG
- Conclusion: High-AG metabolic acidosis (lactic) + respiratory alkalosis (sepsis-driven hyperventilation)
Example 3 - COPD + Diuretics (Harrison's):
Na 140 | Cl 88 | HCO3- 42 | PaCO2 67 | pH 7.42
- pH 7.42 = near normal; both HCO3- and PCO2 elevated
- Conclusion: Metabolic alkalosis (diuretic-induced) + respiratory acidosis (COPD) - pH appears "normal" masking both
Sources
- Roberts and Hedges' Clinical Procedures in Emergency Medicine, Acid-Base Balance section
- Harrison's Principles of Internal Medicine, 22nd Ed. (2025), Ch. 58 - Mixed Acid-Base Disorders
- Tintinalli's Emergency Medicine - A Comprehensive Study, ABG Analysis
- Murray & Nadel's Textbook of Respiratory Medicine - Arterial Blood Gas
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