ABG analysis
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Here is a thorough guide to Arterial Blood Gas (ABG) Analysis, synthesized from Roberts and Hedges' Clinical Procedures in Emergency Medicine, Murray & Nadel's Textbook of Respiratory Medicine, and Tintinalli's Emergency Medicine.
Arterial Blood Gas (ABG) Analysis
What an ABG Measures
An ABG directly measures:
- pH - hydrogen ion concentration
- PaCO2 - partial pressure of CO2 (reflects ventilation)
- PaO2 - partial pressure of oxygen (reflects oxygenation)
- HCO3- - calculated via the Henderson-Hasselbalch equation
- Modern analyzers also report lactate, hemoglobin, and electrolytes
Normal Values
| Parameter | Normal Range |
|---|---|
| pH | 7.35 - 7.45 |
| PaCO2 | 35 - 45 mmHg |
| PaO2 | 80 - 100 mmHg |
| HCO3- | 21 - 27 mEq/L |
| SaO2 | 95 - 100% |
Step-by-Step ABG Interpretation
Step 1 - Determine the pH
- pH < 7.35 = Acidemia
- pH > 7.45 = Alkalemia
- pH 7.35-7.45 = Normal (may still have a compensated or mixed disorder)
Step 2 - Identify the Primary Disorder
| Condition | pH | PaCO2 | HCO3- |
|---|---|---|---|
| Respiratory acidosis | Low | High (>45) | Normal/High |
| Respiratory alkalosis | High | Low (<35) | Normal/Low |
| Metabolic acidosis | Low | Normal/Low | Low (<22) |
| Metabolic alkalosis | High | Normal/High | High (>26) |
Key rule: If the pH and PaCO2 move in opposite directions, the primary disorder is respiratory. If the pH and HCO3- move in the same direction, the primary disorder is metabolic.
Step 3 - Assess Compensation
Compensation is never complete. It brings the pH toward normal but never normalizes it (unless a mixed disorder is present).
| Primary Disorder | Expected Compensation |
|---|---|
| Metabolic acidosis | PCO2 decreases: APCO2 = 1.3 x AHCO3- (Winter's formula) |
| Metabolic alkalosis | PCO2 increases: APCO2 = 0.6 x AHCO3- |
| Acute respiratory acidosis | HCO3- rises 1 mEq/L per 10 mmHg rise in PCO2 |
| Chronic respiratory acidosis | HCO3- rises 3.5-5 mEq/L per 10 mmHg rise in PCO2 |
| Acute respiratory alkalosis | HCO3- falls 2 mEq/L per 10 mmHg fall in PCO2 |
| Chronic respiratory alkalosis | HCO3- falls 5 mEq/L per 10 mmHg fall in PCO2 |
If the actual compensation does not match the predicted value, a mixed disorder is present.
Step 4 - Calculate the Anion Gap (for metabolic acidosis)
AG = Na+ - (Cl- + HCO3-)
- Normal AG = 8-12 mEq/L (some labs use 12 as upper limit)
| High AG Metabolic Acidosis (MUDPILES) | Normal AG Metabolic Acidosis (HARDASS/USED CARP) |
|---|---|
| Methanol | Hyperalimentation |
| Uremia | Addison's disease |
| DKA | Renal tubular acidosis (RTA) |
| Propylene glycol / Paracetamol | Diarrhea |
| Isoniazid / Iron | Acetazolamide |
| Lactic acidosis | Spironolactone / Saline excess |
| Ethylene glycol | |
| Salicylates |
Step 5 - Delta-Delta Ratio (for high AG metabolic acidosis)
The Delta-Delta (AAG/AHCO3-) ratio detects mixed metabolic disorders:
AAG = Calculated AG - 12
- Ratio ~1:1 = Pure AG metabolic acidosis (e.g., DKA, early lactic acidosis)
- Ratio > 2:1 = Concurrent metabolic alkalosis (HCO3- higher than expected)
- Ratio < 1:1 = Concurrent non-AG metabolic acidosis (HCO3- lower than expected)
Step 6 - Assess Oxygenation
A-a gradient = PAO2 - PaO2
Where: PAO2 = FiO2 x (Patm - PH2O) - PaCO2/0.8
- On room air at sea level: PAO2 = 0.21 x (760 - 47) - PaCO2/0.8 = ~150 - PaCO2/0.8
- Normal A-a gradient ~ 10 mmHg (increases with age; rough formula: age/4 + 4)
| A-a Gradient | Interpretation |
|---|---|
| Normal | Hypoventilation (CNS depression, NMD, chest wall) |
| Elevated | V/Q mismatch, shunt, diffusion impairment |
Acid-Base Map
This classic map plots pH vs. PaCO2 with HCO3- isopleths. Points falling within the labeled bands suggest a simple disorder; points falling outside (colored zones 1-4) indicate a mixed disorder.
- Zone 1 (red): Mixed respiratory + metabolic acidosis
- Zone 2 (pink): Mixed respiratory + metabolic alkalosis
- Zone 3 (yellow): Metabolic alkalosis + respiratory acidosis
- Zone 4 (orange): Metabolic acidosis + respiratory alkalosis
Worked Examples
Example 1 - Metabolic Acidosis with Diarrhea
Patient: Na+ 133, K+ 2.8, Cl- 118, pH 7.26, PCO2 13, HCO3- 5
- Acidemia (pH < 7.35)
- Metabolic acidosis (low HCO3-, PCO2 not elevated)
- Expected PCO2 = 40 - (1.3 x 20) = 40 - 26 = 14 mmHg - actual is 13 mmHg - appropriate compensation
- AG = 133 - (118 + 5) = 10 - normal AG acidosis (diarrhea - HCO3- loss)
Example 2 - Respiratory Alkalosis (Sepsis)
Patient: Na+ 138, K+ 3.2, Cl- 105, pH 7.49, PCO2 25, HCO3- 22
- Alkalemia (pH > 7.45)
- Respiratory alkalosis (low PCO2, HCO3- not elevated)
- PCO2 drop = 15 mmHg; expected HCO3- drop = 3 mEq/L - actual drop is 3 mEq/L - appropriate compensation
- No mixed disorder - consistent with early sepsis-driven hyperventilation
Arterial vs. Venous Blood Gas
| Parameter | ABG | VBG |
|---|---|---|
| pH | Gold standard | Correlates closely (±0.05 units) |
| PCO2 | Reference | Trends with arterial (±20 mmHg) |
| PO2 | Reference | Cannot substitute - do not use |
| Lactate | Reference | Useful for screening; confirm if mildly elevated |
A normal venous PCO2 is predictive of normal PaCO2, making VBG useful as a screening tool when arterial sampling is difficult.
Key Clinical Pearls
- Acute vs. chronic respiratory acidosis: In acute, pH falls 0.08 per 10 mmHg rise in PCO2 with only a 1 mEq/L rise in HCO3-. In chronic (3-5 days of renal compensation), HCO3- rises 3.5-5 mEq/L per 10 mmHg rise in PCO2 and pH is near-normal.
- Over-compensation does not occur physiologically - if pH is overcorrected, suspect a mixed disorder.
- Metabolic compensation for respiratory disorders takes 3-5 days (renal); respiratory compensation for metabolic disorders is rapid (minutes to hours).
- Always check the anion gap even when the pH and HCO3- appear normal - a masked AG acidosis can co-exist with metabolic alkalosis.
Sources: Roberts and Hedges' Clinical Procedures in Emergency Medicine; Murray & Nadel's Textbook of Respiratory Medicine; Tintinalli's Emergency Medicine: A Comprehensive Study.
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ABG Quick Reference
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What's included on the card:
| Section | Content |
|---|---|
| Normal Values | pH, PaCO₂, HCO₃⁻, PaO₂, SaO₂, Base Excess with acidosis/alkalosis thresholds |
| 6-Step Approach | Systematic interpretation method side-by-side with primary disorders table |
| Compensation Formulas | All 6 formulas including Winter's formula, acute vs chronic |
| Anion Gap | Formula, normal ranges, albumin correction note |
| Delta-Delta Ratio | Formula + interpretation of <1, 1-2, >2 |
| Causes | MUDPILES, HARDASS, respiratory acidosis, metabolic alkalosis - colour-coded boxes |
| Oxygenation | A-a gradient formula, PaO₂ classification, hypoxaemia causes |
| Respiratory Alkalosis | Full cause list |
| Clinical Pearls | 6 high-yield pearls on mixed disorders, hidden AG, VBG vs ABG, compensation timing |
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