ABG analysis
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ABG (arterial blood gas) analysis is a systematic process of interpreting pH, PaCO2, PaO2, HCO3, and base excess to identify acid-base disorders and oxygenation status. The key steps are: determine if acidemic or alkalemic (pH), identify primary disorder (respiratory vs metabolic), check for compensation, and identify mixed disorders.
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Here is a complete, structured guide to ABG analysis.
Arterial Blood Gas (ABG) Analysis
Normal Values
| Parameter | Normal Range |
|---|---|
| pH | 7.35 - 7.45 |
| PaCO2 | 35 - 45 mmHg |
| PaO2 | 70 - 95 mmHg (age-dependent) |
| HCO3- | 22 - 26 mEq/L |
| Base Excess (BE) | -2 to +2 mEq/L |
| SaO2 | 95 - 100% |
Step-by-Step Approach
Step 1 - Check the pH
| pH | Interpretation |
|---|---|
| < 7.35 | Acidemia |
| 7.35 - 7.45 | Normal |
| > 7.45 | Alkalemia |
A normal pH does not rule out a mixed disorder.
Step 2 - Identify the Primary Disorder
Look at PaCO2 and HCO3- together with the pH:
| Disorder | pH | PaCO2 | HCO3- |
|---|---|---|---|
| Metabolic Acidosis | ↓ | ↓ (compensation) | ↓ (primary) |
| Metabolic Alkalosis | ↑ | ↑ (compensation) | ↑ (primary) |
| Respiratory Acidosis | ↓ | ↑ (primary) | ↑ (compensation) |
| Respiratory Alkalosis | ↑ | ↓ (primary) | ↓ (compensation) |
Key rule: If pH and PaCO2 move in opposite directions → primary respiratory disorder. If they move in the same direction → primary metabolic disorder.
Step 3 - Check for Appropriate Compensation
Compensation always moves in the same direction as the primary disturbance, but never fully corrects the pH.
Metabolic Acidosis - Respiratory Compensation (Winter's Formula)
Expected PaCO2 = (1.5 × HCO3-) + 8 ± 2
- PaCO2 above expected = concurrent respiratory acidosis
- PaCO2 below expected = concurrent respiratory alkalosis
Metabolic Alkalosis - Respiratory Compensation
Expected PaCO2 = (0.7 × HCO3-) + 21 ± 2
Respiratory Acidosis - Renal Compensation
- Acute: HCO3- rises by 1 mEq/L per 10 mmHg rise in PaCO2
- Chronic: HCO3- rises by 3.5 mEq/L per 10 mmHg rise in PaCO2
Respiratory Alkalosis - Renal Compensation
- Acute: HCO3- falls by 2 mEq/L per 10 mmHg fall in PaCO2
- Chronic: HCO3- falls by 5 mEq/L per 10 mmHg fall in PaCO2
If compensation falls outside the expected range, a mixed disorder is present.
Step 4 - Calculate the Anion Gap (if metabolic acidosis present)
Anion Gap = Na+ - (HCO3- + Cl-) Normal = 8-12 mEq/L (some sources cite 8-16)
The anion gap = unmeasured anions (plasma proteins, phosphate, citrate, sulfate).
High Anion Gap Metabolic Acidosis (HAGMA)
Mnemonic: MUDPILES (or GOLDMARK)
- M - Methanol
- U - Uremia (chronic renal failure)
- D - Diabetic ketoacidosis (DKA)
- P - Propylene glycol / Paracetamol
- I - Iron, Isoniazid
- L - Lactic acidosis
- E - Ethylene glycol
- S - Salicylates
Normal Anion Gap Metabolic Acidosis (NAGMA / Hyperchloremic)
Mnemonic: HARDUPS
- H - Hyperalimentation
- A - Addison's disease
- R - Renal tubular acidosis (RTA)
- D - Diarrhea
- U - Ureteroenteric fistula
- P - Pancreatic fistula
- S - Saline excess (dilutional)
Step 5 - Delta-Delta Ratio (for HAGMA)
If there is a high anion gap, use the delta-delta to check for a hidden concurrent metabolic disorder:
Delta-Delta = (AG - 12) / (24 - HCO3-)
| Ratio | Interpretation |
|---|---|
| < 0.4 | Concurrent normal AG metabolic acidosis |
| 0.4 - 1.0 | Mixed HAGMA + NAGMA |
| 1.0 - 2.0 | Pure HAGMA (expected) |
| > 2.0 | Concurrent metabolic alkalosis |
Step 6 - Assess Oxygenation
PaO2 normal: 70-95 mmHg (decreases with age)
PaO2 sitting = 104.2 - (0.27 × age in years)
Alveolar-Arterial (A-a) Gradient:
P(A-a)O2 = [FiO2 × (PB - 47)] - (PaCO2 / 0.8) - PaO2 Normal A-a gradient = (Age/4) + 4
| A-a Gradient | Mechanism of Hypoxemia |
|---|---|
| Normal | Hypoventilation, high altitude |
| Increased | V/Q mismatch, diffusion impairment, shunt |
Summary Table of Four Simple Disorders
| Disorder | pH | PaCO2 | HCO3- | Respiratory Compensation | Renal Compensation |
|---|---|---|---|---|---|
| Metabolic Acidosis | ↓ | ↓ | ↓ | Hyperventilation | ↑ HCO3- reabsorption |
| Metabolic Alkalosis | ↑ | ↑ | ↑ | Hypoventilation | ↑ HCO3- excretion |
| Respiratory Acidosis | ↓ | ↑ | ↑ | None | ↑ HCO3- reabsorption |
| Respiratory Alkalosis | ↑ | ↓ | ↓ | None | ↓ HCO3- reabsorption |
(Bold = primary disturbance)
A Quick Note on Venous Blood Gases (VBG)
VBGs are increasingly used as a safer alternative. Venous pH averages ~0.03 lower than arterial. PvCO2 runs 3-8 mmHg higher than PaCO2 and correlates less reliably. VBGs are unreliable in hypotensive patients or severe hypercapnia, but a normal PvCO2 effectively excludes hypercapnia.
Sources: Costanzo Physiology 7th Ed., Goldman-Cecil Medicine
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