Abg analysis
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ABG (Arterial Blood Gas) analysis is a systematic interpretation of blood pH, PaCO2, PaO2, and HCO3 to identify acid-base disorders and oxygenation status. The basic steps are: check pH (acidosis vs. alkalosis), identify the primary disorder (respiratory vs. metabolic), assess compensation, and calculate the anion gap if metabolic acidosis is present.
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Here is a complete guide to ABG analysis, compiled from Goldman-Cecil Medicine, Costanzo Physiology, and related sources.
Arterial Blood Gas (ABG) Analysis
Normal Values
| Parameter | Normal Range |
|---|---|
| pH | 7.35 - 7.45 |
| PaCO2 | 35 - 45 mm Hg |
| PaO2 | 70 - 95 mm Hg (age-dependent) |
| HCO3- | 22 - 26 mEq/L |
| SaO2 | 95 - 100% |
Step-by-Step Interpretation
Step 1 - Assess pH
- pH < 7.35 = Acidemia
- pH > 7.45 = Alkalemia
- pH 7.35-7.45 = Normal (but a disorder may still exist if compensated)
Step 2 - Identify the Primary Disorder
Look at PaCO2 and HCO3- to find which one explains the pH change:
| Disorder | pH | PaCO2 | HCO3- |
|---|---|---|---|
| Metabolic Acidosis | ↓ | ↓ (compensation) | ↓ (primary) |
| Metabolic Alkalosis | ↑ | ↑ (compensation) | ↑ (primary) |
| Respiratory Acidosis | ↓ | ↑ (primary) | ↑ (compensation) |
| Respiratory Alkalosis | ↑ | ↓ (primary) | ↓ (compensation) |
Key rule: If the primary disturbance is metabolic (HCO3-), compensation is respiratory (CO2 adjusts). If the primary disturbance is respiratory (CO2), compensation is renal (HCO3- adjusts). The compensatory response always moves in the same direction as the primary disturbance.
Step 3 - Assess Compensation (Renal Rules)
Use these formulas to predict expected compensation and check for mixed disorders:
| Disorder | Formula |
|---|---|
| Metabolic Acidosis | For every 1 mEq/L ↓ HCO3- → ↓ PCO2 by 1.3 mm Hg |
| Metabolic Alkalosis | For every 1 mEq/L ↑ HCO3- → ↑ PCO2 by 0.7 mm Hg |
| Respiratory Acidosis (Acute) | 1 mm Hg ↑ PCO2 → ↑ HCO3- by 0.1 mEq/L |
| Respiratory Acidosis (Chronic) | 1 mm Hg ↑ PCO2 → ↑ HCO3- by 0.4 mEq/L |
| Respiratory Alkalosis (Acute) | 1 mm Hg ↓ PCO2 → ↓ HCO3- by 0.2 mEq/L |
| Respiratory Alkalosis (Chronic) | 1 mm Hg ↓ PCO2 → ↓ HCO3- by 0.4 mEq/L |
Winter's Equation (for metabolic acidosis): Expected PaCO2 = (1.5 × HCO3-) + 8 ± 2
- If measured PaCO2 > expected: additional respiratory acidosis
- If measured PaCO2 < expected: additional respiratory alkalosis
If a patient's actual values match predicted compensation = simple disorder. If they deviate = mixed disorder.
Step 4 - Calculate Anion Gap (if metabolic acidosis)
Anion Gap = Na+ - (HCO3- + Cl-)
Normal value: 8-16 mEq/L (unmeasured anions = proteins, phosphate, citrate, sulfate)
| Type | Anion Gap | Examples |
|---|---|---|
| High AG Metabolic Acidosis | Elevated (>16) | Diabetic ketoacidosis, lactic acidosis, salicylate poisoning, methanol, ethylene glycol, chronic renal failure ("MUDPILES" / "KULT") |
| Normal AG Metabolic Acidosis (Hyperchloremic) | Normal (8-16) | Diarrhea, renal tubular acidosis (RTA) |
In normal AG acidosis, lost HCO3- is replaced by Cl- (a measured anion), so the gap doesn't change. In high AG acidosis, HCO3- is replaced by an unmeasured organic anion.
Step 5 - Assess Oxygenation
Alveolar-Arterial (A-a) Gradient = FiO2(PB - PH2O) - (PaO2 + PaCO2/0.8)
Simplified: P(A-a)O2 ≈ Age/4 + 4
| A-a Gradient | Mechanism of Hypoxemia |
|---|---|
| Normal | Hypoventilation, decreased inspired O2 (altitude) |
| Elevated | V/Q mismatch (e.g., pneumonia), diffusion impairment (ILD), anatomic shunt, diffusion-perfusion impairment (hepatopulmonary syndrome) |
Normal PaO2 by age (sitting): PaO2 = 104.2 - (0.27 × age in years)
The Acid-Base Map
If a patient's pH/PCO2/HCO3- values fall within a shaded band = simple disorder. If they fall between bands = mixed disorder.
Quick Clinical Summary
| Finding | Think of |
|---|---|
| Low pH, low HCO3-, low PCO2 | Metabolic acidosis + respiratory compensation |
| Low pH, high PCO2, high HCO3- | Respiratory acidosis (acute = small HCO3- rise; chronic = larger rise) |
| High pH, high HCO3-, high PCO2 | Metabolic alkalosis + respiratory compensation |
| High pH, low PCO2, low HCO3- | Respiratory alkalosis (acute vs. chronic) |
| Normal pH but both PCO2 and HCO3- abnormal | Mixed disorder (e.g., metabolic acidosis + respiratory alkalosis) |
Venous Blood Gases (VBG) vs ABG
Venous pH averages ~0.03 lower than arterial. PvCO2 is 3-8 mm Hg higher and correlates less reliably with PaCO2. VBGs are unreliable in hypotensive patients or severe hypercapnia, but a normal PvCO2 can be used to exclude hypercapnia in stable patients. - Goldman-Cecil Medicine
Sources: Goldman-Cecil Medicine (International Edition); Costanzo Physiology, 7th Edition
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