ABG Analysis
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Here is a complete, structured reference on ABG (Arterial Blood Gas) Analysis drawn from Tintinalli's Emergency Medicine, Roberts & Hedges' Clinical Procedures in Emergency Medicine, Symptom to Diagnosis, and Miller's Anesthesia.
Arterial Blood Gas (ABG) Analysis
What an ABG Measures
An arterial blood sample is the reference standard for pH, oxygen (PaO2), carbon dioxide (PaCO2), and lactate. The bicarbonate (HCO3-) is a calculated (not directly measured) value derived from the measured pH and PaCO2. Modern analyzers may also report hemoglobin, electrolytes, and lactate.
| Parameter | Normal Value |
|---|---|
| pH | 7.35 - 7.45 (ideal: 7.40) |
| PaCO2 | 35 - 45 mm Hg (ideal: 40) |
| HCO3- | 22 - 26 mEq/L (ideal: 24) |
| PaO2 | 75 - 100 mm Hg |
| SaO2 | > 95% |
| Base Excess (BE) | -2 to +2 mEq/L |
Step-by-Step ABG Interpretation
Step 1 - Determine pH Status
- pH < 7.35 = Acidemia
- pH > 7.45 = Alkalemia
- pH 7.35-7.45 = Normal (but a primary disorder may still be present with full compensation)
Step 2 - Identify the Primary Disorder
The key relationship: CO2 + H2O ⇌ H2CO3 ⇌ HCO3- + H+
| Condition | pH | Primary Change | Disorder |
|---|---|---|---|
| Respiratory Acidosis | Low | PaCO2 high (> 45) | Hypoventilation |
| Respiratory Alkalosis | High | PaCO2 low (< 35) | Hyperventilation |
| Metabolic Acidosis | Low | HCO3- low (< 22) | Acid gain or base loss |
| Metabolic Alkalosis | High | HCO3- high (> 26) | Base gain or acid loss |
Quick Rule:
- If acidotic: HCO3- < 24 → metabolic acidosis; PaCO2 > 40 → respiratory acidosis
- If alkalotic: HCO3- > 24 → metabolic alkalosis; PaCO2 < 40 → respiratory alkalosis
Step 3 - Check for Appropriate Compensation
Compensation is never complete - it reduces the pH change but does not normalize it. If compensation exceeds predicted values, a mixed disorder is present.
| Primary Disorder | Compensatory Response | Formula |
|---|---|---|
| Metabolic Acidosis | ↓ PaCO2 (hyperventilation) | Expected PaCO2 = 1.3 × ΔHCO3- fall from 24 |
| Metabolic Alkalosis | ↑ PaCO2 (hypoventilation) | Expected PaCO2 = 0.6 × ΔHCO3- rise |
| Respiratory Acidosis (Acute) | ↑ HCO3- (renal, fast) | HCO3- ↑ 1 mEq/L per 10 mm Hg rise in PaCO2 |
| Respiratory Acidosis (Chronic) | ↑ HCO3- (renal, slow) | HCO3- ↑ 4 mEq/L per 10 mm Hg rise in PaCO2 |
| Respiratory Alkalosis (Acute) | ↓ HCO3- | HCO3- ↓ 2 mEq/L per 10 mm Hg fall in PaCO2 |
| Respiratory Alkalosis (Chronic) | ↓ HCO3- | HCO3- ↓ 5 mEq/L per 10 mm Hg fall in PaCO2 |
Respiratory compensation for metabolic disorders is rapid (minutes to hours). Metabolic compensation for respiratory disorders requires 3-5 days (renal adjustment of HCO3-).
Step 4 - For Metabolic Acidosis: Calculate the Anion Gap (AG)
AG = Na+ - (HCO3- + Cl-)
- Normal AG = 12 ± 4 mEq/L (some institutions use 7-9 mEq/L)
- Adjust for albumin: AG drops 2.5 mEq/L for every 1 g/dL fall in albumin below 4.4 g/dL
High Anion Gap (HAGMA) - mnemonic MUDPILES / KULT
| Cause | Notes |
|---|---|
| Ketoacidosis | DKA, alcoholic, starvation |
| Lactic acidosis | Hypoxia, shock (septic, cardiogenic, hypovolemic), seizures, CO poisoning |
| Uremia | Advanced renal failure |
| Toxins | Methanol, ethylene glycol, salicylate, D-lactic acid |
Normal Anion Gap (NAGMA) - mnemonic DURHAM
| Cause | Notes |
|---|---|
| Diarrhea | Loss of HCO3- in stool |
| Renal Tubular Acidosis (RTA) | Failure to excrete H+ or reabsorb HCO3- |
| Carbonic anhydrase inhibitors | e.g., acetazolamide |
| Dilutional | Large-volume normal saline infusion |
| Early renal failure |
Step 5 - For High AG Metabolic Acidosis: Apply the Delta-Delta (Δ/Δ) Ratio
This checks whether an additional metabolic disorder is hiding behind the anion gap acidosis.
Delta Ratio = (Measured AG - Normal AG) / (Normal HCO3- - Measured HCO3-)
= ΔAG / ΔHCO3-
| Delta Ratio | Interpretation |
|---|---|
| < 0.4 | Hyperchloremic (normal AG) metabolic acidosis |
| 0.4 - 1.0 | Mixed HAGMA + normal AG metabolic acidosis |
| 1.0 - 2.0 | Pure HAGMA with appropriate compensation |
| > 2.0 | HAGMA + concurrent metabolic alkalosis |
Oxygenation Assessment
A-a Gradient (Alveolar-Arterial Oxygen Difference)
P(A-a)O2 = [FiO2 × (Patm - PH2O)] - (PaCO2 × 1.25) - PaO2
Where: FiO2 = 0.21 on room air, Patm = 760 mm Hg, PH2O = 47 mm Hg
- Normal A-a gradient < 15 mm Hg in young adults
- Normal rises with age: estimated as Age/4 + 4 mm Hg
- Elevated A-a gradient indicates a V/Q mismatch, shunt, or diffusion defect (e.g., pneumonia, PE, pulmonary edema, ARDS)
- A normal A-a gradient with hypoxemia suggests hypoventilation as the sole cause
PaO2/FiO2 (P/F) Ratio
- Calculated as: PaO2 ÷ FiO2 (FiO2 as a decimal)
- Normal: ~400-600 mm Hg
- Used to define severity of lung failure (ARDS criteria):
- P/F ≥ 300 = Normal
- P/F 200-300 = Mild ARDS
- P/F 100-200 = Moderate ARDS
- P/F < 100 = Severe ARDS
Differential Diagnosis Summary
Acidoses (pH < 7.4)
| Metabolic Acidosis (HCO3- < 24) | Respiratory Acidosis (PaCO2 > 40) |
|---|---|
| DKA, alcoholic/starvation ketoacidosis | COPD, asthma, pulmonary edema, pneumonia |
| Lactic acidosis (shock, sepsis, hypoxia) | Pneumothorax, massive pleural effusion |
| Uremia | Stroke, CNS depression, intoxication |
| Methanol, ethylene glycol, salicylates | Neuromuscular disease (Guillain-Barré, MG, ALS) |
| Diarrhea, RTA, dilutional | Sleep apnea, chest wall deformity |
Alkaloses (pH > 7.4)
| Metabolic Alkalosis (HCO3- > 24) | Respiratory Alkalosis (PaCO2 < 40) |
|---|---|
| Vomiting, NG drainage | Anxiety, pain, fever |
| Diuretics, hypokalemia | Pregnancy |
| Primary hyperaldosteronism | Pulmonary embolism, pneumonia |
| Hypercortisolism | Mechanical over-ventilation |
| Excessive licorice | Cirrhosis, CNS insult |
Acid-Base Map
This map plots PaCO2 vs. pH to identify where a patient's values fall - inside a confidence band means a simple disorder, outside means a mixed disorder.
Venous Blood Gas (VBG) vs. ABG
| Parameter | VBG vs. ABG |
|---|---|
| pH | Venous is ~0.05 units lower - clinically reliable for most purposes |
| PaCO2 | Venous trends with arterial but can vary ±20 mm Hg - normal VBG CO2 predicts normal arterial CO2 |
| PaO2 | Venous values do NOT correlate - cannot substitute for arterial oxygenation assessment |
| Lactate | Normal and markedly elevated values correlate; mildly elevated venous lactate may not reflect arterial level |
Base Excess (BE)
Standard Base Excess (SBE) quantifies the metabolic component independent of respiratory changes:
SBE = [HCO3-]actual - 24.8 + (16.2 × (pH - 7.40))
- BE 0 ± 2 = Normal
- BE < -2 = Metabolic acidosis (base deficit)
- BE > +2 = Metabolic alkalosis
A negative BE is more precisely called a base deficit.
Worked Examples
Example 1 - Metabolic Acidosis with appropriate compensation:
- 58-year-old woman with 1 week of diarrhea
- Na 133, K 2.8, Cl 118, pH 7.26, PaCO2 13, HCO3- 5
- Step 1: pH 7.26 = acidosis
- Step 2: HCO3- low (5) = primary metabolic acidosis
- Step 3: Expected PaCO2 = 40 - (1.3 × (24-5)) = 40 - 24.7 = ~15 mm Hg; actual PaCO2 = 13 mm Hg → appropriate compensation (pure metabolic acidosis)
- AG = 133 - (5 + 118) = 10 → normal AG → loss of HCO3- from diarrhea
Example 2 - Respiratory Alkalosis:
- 74-year-old with urosepsis: pH 7.49, PaCO2 25, HCO3- 22
- Step 1: pH 7.49 = alkalosis
- Step 2: PaCO2 low (25) = primary respiratory alkalosis
- Step 3: Expected HCO3- drop = 2 mEq/L per 10 mm Hg fall × (15 mm Hg fall) = 3 mEq/L; actual drop = 2 mEq/L → appropriate compensation
Sources: Tintinalli's Emergency Medicine, p. 121-122 | Roberts & Hedges' Clinical Procedures in Emergency Medicine, p. 1733-1735 | Symptom to Diagnosis, p. 68-72 | Miller's Anesthesia, p. 6494-6496
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