ABG
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I now have comprehensive content from multiple authoritative textbooks. Here is the full ABG overview:
Arterial Blood Gas (ABG) - Full Overview
What Is an ABG?
An ABG measures three values directly: pH, PaCO2, and PaO2. Serum bicarbonate (HCO3-) is then calculated from the Henderson-Hasselbalch equation:
pH = 6.1 + log10 ( [HCO3-] / 0.03 × PaCO2 )
The test is the gold standard for assessing ventilation, oxygenation, and acid-base status. - Murray & Nadel's Textbook of Respiratory Medicine
Normal ABG Values
| Parameter | Normal Range |
|---|---|
| pH | 7.35 - 7.45 |
| PaCO2 | 35 - 45 mmHg |
| HCO3- | 21 - 27 mEq/L |
| PaO2 | 75 - 100 mmHg |
| SpO2 | 95 - 100% |
| Base Excess | -2 to +2 mEq/L |
ABG Components Explained
| Component | Measured/Calculated | Reflects |
|---|---|---|
| pH | Directly measured | Overall acid-base status |
| PaCO2 | Directly measured | Respiratory component (ventilation) |
| PaO2 | Directly measured | Oxygenation |
| HCO3- | Calculated | Metabolic component (renal) |
| Base Excess | Calculated | Metabolic deviation from normal |
Step-by-Step Interpretation (Table 16-8 Approach)
(Barash, Cullen & Stoelting's Clinical Anesthesia, 9e)
Step 1 - Determine Acidemia or Alkalemia
- pH < 7.35 = Acidemia
- pH > 7.45 = Alkalemia
- pH 7.35-7.45 = Normal (but a mixed disorder may still exist)
Step 2 - Identify Primary Process
| pH | PaCO2 | HCO3- | Disorder |
|---|---|---|---|
| Low | High | High | Respiratory acidosis |
| Low | Low | Low | Metabolic acidosis |
| High | Low | Low | Respiratory alkalosis |
| High | High | High | Metabolic alkalosis |
Step 3 - Assess Compensation
Is compensation appropriate, or is there a second primary disorder?
Step 4 - Calculate the Anion Gap
AG = Na+ - (Cl- + HCO3-)
- Normal AG = 8-12 mEq/L (up to 16 if albumin-unadjusted)
- Albumin correction: add 2.5 × (4 - observed albumin) to the AG
- Always calculate the AG even if the pH is normal - it may unmask a mixed disorder.
Step 5 - Check for Appropriate Compensation
| Primary Disorder | Compensation Formula |
|---|---|
| Metabolic acidosis | Expected PaCO2 = (1.5 × HCO3) + 8 ± 2 (Winter's formula) |
| Metabolic alkalosis | Expected PaCO2 = 40 + 0.7 × (HCO3 - 24) |
| Acute resp. acidosis | HCO3 rises 1 mEq/L per 10 mmHg rise in PaCO2 |
| Chronic resp. acidosis | HCO3 rises 3.5-4 mEq/L per 10 mmHg rise in PaCO2 |
| Acute resp. alkalosis | HCO3 falls 2 mEq/L per 10 mmHg fall in PaCO2 |
| Chronic resp. alkalosis | HCO3 falls 5 mEq/L per 10 mmHg fall in PaCO2 |
Step 6 - Delta-Delta Ratio (if AG elevated)
Δ/Δ = (AG - 12) / (24 - HCO3)
| Ratio | Interpretation |
|---|---|
| < 1.0 | Concurrent non-AG metabolic acidosis |
| 1.0 - 2.0 | Pure anion gap metabolic acidosis |
| > 2.0 | Concurrent metabolic alkalosis OR compensated chronic resp. acidosis |
The Four Primary Acid-Base Disorders
1. Respiratory Acidosis
- Cause: Hypoventilation -> CO2 retention
- ABG: pH ↓, PaCO2 ↑
- Common causes: COPD exacerbation, opioid overdose, neuromuscular disease, obesity hypoventilation
- Compensation: Kidneys retain HCO3- over 3-5 days
2. Respiratory Alkalosis
- Cause: Hyperventilation -> CO2 blown off
- ABG: pH ↑, PaCO2 ↓
- Common causes: Anxiety, pain, fever, PE, early sepsis, pregnancy, altitude
3. Metabolic Acidosis
- Cause: HCO3- loss or acid accumulation
- ABG: pH ↓, HCO3- ↓
- Subclassify by anion gap:
- High AG (MUDPILES/GOLDMARK): DKA, lactic acidosis, uremia, toxins (methanol, ethylene glycol, salicylates)
- Normal AG (hyperchloremic): Diarrhea, RTA, saline administration, Addison's disease
4. Metabolic Alkalosis
- Cause: HCO3- gain or H+ loss
- ABG: pH ↑, HCO3- ↑
- Subclassify by urine Cl-:
- Chloride-responsive (urine Cl- < 20): Vomiting, NG suctioning, diuretics
- Chloride-resistant (urine Cl- > 20): Hyperaldosteronism, Cushing's, severe hypokalemia
Alveolar-Arterial (A-a) Gradient
A-a gradient = PAO2 - PaO2
Where: PAO2 = (FiO2 × [Patm - PH2O]) - (PaCO2 / 0.8)
- Normal A-a gradient: ~10 mmHg (increases with age; rough rule: age/4 + 4)
- Normal A-a gradient + hypercapnia: Central hypoventilation, neuromuscular disease (the lungs themselves are fine)
- Elevated A-a gradient + hypercapnia: Intrinsic lung disease (COPD, pneumonia, pulmonary edema, PE)
Clinical Consequences
Severe Acidemia (pH < 7.2):
- Cardiac: decreased contractility, arrhythmias, catecholamine resistance
- Metabolic: hyperkalemia, insulin resistance, increased protein degradation
- Neuro: altered mental status, coma
Severe Alkalemia:
- Cardiac: arrhythmias
- Neuro: seizures, tetany, perioral paresthesias
- Metabolic: hypokalemia, hypocalcemia
Arterial vs. Venous Blood Gas
| Parameter | ABG | VBG |
|---|---|---|
| pH | Direct, accurate | ~0.03-0.05 lower than ABG |
| PCO2 | Reference standard | Trends with ABG, up to ±20 mmHg difference |
| PO2 | Accurate | NOT useful for oxygenation assessment |
| Lactate | Accurate | Correlates with ABG except mildly elevated values |
A normal venous PCO2 reliably predicts a normal PaCO2; however, VBG cannot replace ABG for evaluating hypercarbia in critical scenarios. - Tintinalli's Emergency Medicine
Sources: Murray & Nadel's Textbook of Respiratory Medicine | Barash, Cullen & Stoelting's Clinical Anesthesia, 9e | Tintinalli's Emergency Medicine
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