Normal Arterial Blood Gas Values
For a healthy adult breathing room air at sea level, normal arterial blood gas values are: PaO2 80-110 mmHg (10.6-14.8 kPa), PaCO2 34-46 mmHg (4.6-6.1 kPa), pH 7.35-7.45, SaO2 95-98%, and HCO3 22-26 mmol/L. 1
Age-Specific PaO2 Values
Normal PaO2 values decline progressively with age, requiring age-adjusted interpretation 2, 1:
- Ages 18-24 years: Mean PaO2 13.4 kPa (100.5 mmHg), range 12.0-14.8 kPa (90-111 mmHg) 1
- Ages 25-34 years: Mean PaO2 13.4 kPa (100.5 mmHg), range 12.1-14.7 kPa (91-110 mmHg) 1
- Ages 35-44 years: Mean PaO2 13.2 kPa (99 mmHg), range 11.1-15.2 kPa (83-114 mmHg) 1
- Ages 45-54 years: Mean PaO2 13.0 kPa (97.5 mmHg), range 10.9-15.1 kPa (82-113 mmHg) 1
- Ages 55-64 years: Mean PaO2 12.1 kPa (90.7 mmHg), range 10.9-13.3 kPa (82-100 mmHg) 1
- Ages >64 years: Mean PaO2 11.9 kPa (89.2 mmHg), range 9.0-14.8 kPa (68-111 mmHg) 1
The decline in PaO2 with age is approximately 0.12-0.16 mmHg per year after age 20 3, 1.
Age-Specific Oxygen Saturation Values
Normal SaO2 ranges also vary by age 2:
- Ages 18-24 years: Mean 96.9%, range 96.1-97.7% 2
- Ages 25-34 years: Mean 96.7%, range 95.3-98.1% 2
- Ages 35-44 years: Mean 96.7%, range 95.5-97.9% 2
- Ages 45-54 years: Mean 96.5%, range 94.4-98.5% 2
- Ages 55-64 years: Mean 95.1%, range 94.5-97.3% 2
- Ages >64 years: Mean 95.5%, range 92.7-98.3% 2
The British Thoracic Society emphasizes that the precise normal SaO2 in healthy adults at sea level falls within a narrow range of approximately 95-98% 2.
Carbon Dioxide and pH Values
Normal PaCO2 is tightly regulated between 4.6-6.1 kPa (34-46 mmHg) 1. The normal pH range is 7.35-7.45, with carbon dioxide levels more tightly controlled than oxygen 2.
Clinical Significance Thresholds
Hypoxemia is defined as PaO2 below 8.0 kPa (60 mmHg), which corresponds to SaO2 below 90% 1, 2. The British Thoracic Society states that most experts emphasize maintaining SaO2 above 90% for acutely ill patients, with a desirable target range of 94-98% for most clinical situations 2.
Sudden exposure to SaO2 levels below 80% can cause impaired mental functioning even in healthy individuals 2. The brain is the most sensitive organ to hypoxia, though other organs may be vulnerable at higher oxygen levels during critical illness 2.
Important Clinical Caveats
The alveolar-arterial oxygen gradient (A-a gradient) increases with age, rising approximately 0.12 mmHg per year 3, 1. This reflects age-related changes in ventilation-perfusion matching rather than disease 1.
Gender differences exist: Women show a greater age-related increase in PaCO2 (0.075 mmHg/year) compared to men, associated with greater reduction in minute ventilation with aging 4, 3.
Body mass index affects oxygenation: In women, each kg/m² increase in BMI decreases SaO2 by 0.14% and PaO2 by 0.52 mmHg 3.
Altitude Considerations
These values apply specifically to sea level conditions 1. At altitude, normal values differ substantially—for example, at 2,240 meters, mean PaO2 is approximately 71 mmHg with PaCO2 around 30 mmHg due to compensatory hyperventilation 3, 5.