Normal Arterial Blood Gas Values
For a healthy adult breathing room air at sea level, normal ABG values are: pH 7.35–7.45, PaO₂ >90 mmHg (typically 90–110 mmHg in young adults), PaCO₂ 35–45 mmHg (4.6–6.1 kPa), bicarbonate 22–26 mEq/L, and oxygen saturation (SaO₂) 95–98%. 1, 2
Core Parameters
pH
- Normal range: 7.35–7.45 1, 2
- Values below 7.35 indicate acidemia; values above 7.45 indicate alkalemia 1, 2
Partial Pressure of Oxygen (PaO₂)
- Normal range: >90 mmHg, with typical values of 90–110 mmHg in young adults 1, 2
- Age-related decline occurs: In adults aged 18–24 years, mean PaO₂ is 13.4 kPa (100 mmHg) with a range of 11.98–14.82 kPa (90–111 mmHg) 1
- For adults >64 years, mean PaO₂ decreases to 11.89 kPa (89 mmHg) with a wider range of 9.02–14.76 kPa (68–111 mmHg) 1
- PaO₂ <60 mmHg represents life-threatening hypoxemia requiring immediate intervention 2, 3
Partial Pressure of Carbon Dioxide (PaCO₂)
- Normal range: 35–45 mmHg (4.6–6.1 kPa) 1, 2, 3
- Any value >45 mmHg (6.1 kPa) is considered abnormal, though values up to 49 mmHg (6.7 kPa) may be tolerated depending on clinical context 1
- Carbon dioxide levels are more tightly regulated than oxygen levels 1
Oxygen Saturation (SaO₂)
- Normal range: 95–98% in healthy adults at sea level 1
- Age-specific values: Adults aged 18–24 years have mean SaO₂ of 96.9% (range 96.1–97.7%), while those >64 years have mean SaO₂ of 95.5% (range 92.7–98.3%) 1
- The desirable target saturation range for acutely ill patients is 94–98% 1
Bicarbonate (HCO₃⁻)
- Normal range: 22–26 mEq/L 2
- Values <22 mEq/L suggest metabolic acidosis; values >26 mEq/L suggest metabolic alkalosis 2
Base Excess
Age-Related Changes
PaO₂ declines progressively with age at approximately 0.12–0.16 mmHg per year after age 20 4, 5. This physiologic decline means:
- Young adults (18–24 years): Mean PaO₂ 100 mmHg, SaO₂ 96.9% 1
- Middle-aged adults (45–54 years): Mean PaO₂ 97 mmHg, SaO₂ 96.5% 1
- Older adults (>64 years): Mean PaO₂ 89 mmHg, SaO₂ 95.5% 1
The alveolar-arterial oxygen gradient (A-a gradient) increases with age at approximately 0.12 mmHg per year 4.
Additional Parameters
Lactate
Ionized Calcium
- Normal range: 1.1–1.3 mmol/L 2
Anion Gap
- Normal range: 8–12 mEq/L (calculated as Na⁺ – [Cl⁻ + HCO₃⁻]) 2
Common Pitfalls
Do not assume abnormal values in older adults represent disease—the normal range widens with age, and mean values decline physiologically 1. A PaO₂ of 89 mmHg or SaO₂ of 93% may be normal in a patient >65 years old 1.
Pulse oximetry (SpO₂) alone cannot assess ventilation or acid-base status—arterial blood gas measurement remains essential for critically ill patients to evaluate PaCO₂ and pH 2, 3.
After initiating or changing oxygen therapy, repeat ABG within 30–60 minutes to verify adequate response without precipitating respiratory acidosis, particularly in patients at risk for CO₂ retention 2, 3.