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Here is a complete reference on ABG normal values and their clinical significance, sourced from authoritative medical textbooks.
Arterial Blood Gas (ABG) - Normal Values & Clinical Significance
Normal Reference Ranges
| Parameter | Normal Value | Normal Range |
|---|---|---|
| pH | 7.40 | 7.35 - 7.45 |
| PaCO₂ | 40 mm Hg | 35 - 45 mm Hg (4.5 - 6.0 kPa) |
| PaO₂ | ~95-100 mm Hg | 75 - 100 mm Hg (10.6 - 13.3 kPa) |
| HCO₃⁻ (bicarbonate) | 24 mEq/L | 22 - 26 mEq/L |
| Base Excess/Deficit | 0 | 0 ± 3 mmol/L |
| O₂ Saturation (SaO₂) | ~98% | 95 - 100% |
- Current Surgical Therapy 14e, p.1608 | Pye's Surgical Handicraft
What Each Parameter Tells You
1. pH (7.35 - 7.45)
- < 7.35 = Acidemia
- > 7.45 = Alkalemia
- The pH is your first and most important number - it tells you the net acid-base state of the blood.
2. PaCO₂ (35 - 45 mm Hg) - The "Respiratory Component"
Reflects alveolar ventilation. In respiratory disorders, PaCO₂ moves opposite to pH:
- ↑ PaCO₂ (hypercapnia) + ↓ pH = Respiratory acidosis (hypoventilation)
- ↓ PaCO₂ (hypocapnia) + ↑ pH = Respiratory alkalosis (hyperventilation)
In metabolic disorders, PaCO₂ moves in the same direction as pH (compensatory).
3. PaO₂ (75 - 100 mm Hg) - Oxygenation
- The gold standard measure of oxygenation (better than pulse oximetry)
- Affected by altitude: PaO₂ in Denver (~1500 m altitude) is ~20 mm Hg lower than at sea level
- Decreases with age: PaO₂ = 109 - 0.43 × (age in years)
- Decreases in supine position due to diaphragm displacement and V/Q mismatch
- Causes of low PaO₂: hypoventilation, V/Q mismatch, diffusion impairment, shunt
- Fishman's Pulmonary Diseases and Disorders
4. HCO₃⁻ (22 - 26 mEq/L) - The "Metabolic Component"
- ↓ HCO₃⁻ = Metabolic acidosis
- ↑ HCO₃⁻ = Metabolic alkalosis
- Note: The HCO₃⁻ on an ABG report is a calculated value, not directly measured. Always confirm with a simultaneous electrolyte panel.
5. Base Excess / Base Deficit (0 ± 3 mmol/L)
- Defined as the amount of base needed to titrate 1 L of blood to pH 7.4 at 37°C
- > +3 = Metabolic alkalosis
- < -3 (base deficit) = Metabolic acidosis
- Purely reflects the metabolic component of acid-base balance
Calculating base deficit:
ABG vs. Other Blood Gas Methods
| Test | Use | Key Difference |
|---|---|---|
| ABG (arterial) | Most accurate - oxygenation, ventilation, acid-base | Gold standard |
| VBG (venous) | Quick screen for acid-base | PvCO₂ is ~6-8 mmHg higher; pH slightly lower than arterial |
| CBG (capillary) | Pediatric/bedside use | Correlates best for pH, moderate for PCO₂, worst for PO₂ |
- Harriet Lane Handbook, 23rd ed.
In Vitro Changes (Sample Handling)
If the sample is not iced and analyzed immediately, the values drift over time at 37°C:
| Parameter | Change per 10 min |
|---|---|
| pH | -0.01 |
| PCO₂ | +1.0 mm Hg |
| O₂ content | -0.001 vol% |
Blood cell metabolism (especially WBCs) consumes O₂ and produces CO₂, so samples must be kept on ice or analyzed promptly. Air bubbles in the syringe will also artificially raise PaO₂ and lower PaCO₂.
- Fishman's Pulmonary Diseases and Disorders
Quick Clinical Significance Summary
| Finding | Meaning | Common Causes |
|---|---|---|
| pH < 7.35, ↑ PaCO₂ | Respiratory acidosis | COPD, opioids, neuromuscular disease |
| pH > 7.45, ↓ PaCO₂ | Respiratory alkalosis | Anxiety, PE, pain, mechanical overventilation |
| pH < 7.35, ↓ HCO₃⁻ | Metabolic acidosis | MUDPILES: Methanol, Uremia, DKA, Propylene glycol, INH, Lactic acidosis, Ethylene glycol, Salicylates |
| pH > 7.45, ↑ HCO₃⁻ | Metabolic alkalosis | Vomiting, diuretics, hyperaldosteronism |
| PaO₂ < 60 mm Hg | Hypoxemic respiratory failure | Pneumonia, ARDS, PE, pulmonary edema |
Compensation Formulas
When a primary disorder is present, the body compensates. Use these to check whether the compensation is appropriate or if a mixed disorder is present:
| Primary Disorder | Expected Compensation |
|---|---|
| Metabolic acidosis | PaCO₂ = 1.5 × [HCO₃⁻] + 8 ± 2 (Winter's Formula) |
| Metabolic alkalosis | PaCO₂ = 0.7 × [HCO₃⁻] + 20 ± 5 |
| Acute respiratory acidosis | ↑ HCO₃⁻ = ΔPaCO₂ / 10 |
| Chronic respiratory acidosis | ↑ HCO₃⁻ = 4 × (ΔPaCO₂ / 10) |
| Acute respiratory alkalosis | ↓ HCO₃⁻ = 2 × (ΔPaCO₂ / 10) |
| Chronic respiratory alkalosis | ↓ HCO₃⁻ = 4 × (ΔPaCO₂ / 10) |
- Current Surgical Therapy 14e
Key takeaway: The ABG gives you four core values - pH, PaCO₂, PaO₂, and HCO₃⁻. Use pH to identify acidemia vs. alkalemia, PaCO₂ for the respiratory component, and HCO₃⁻/base excess for the metabolic component. Always check PaO₂ for concurrent hypoxemia, and apply compensation formulas to detect mixed disorders.
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