What is a normal PaO2 (partial pressure of oxygen) level in an arterial blood gas (ABG)?

Normal PaO2 in Arterial Blood Gas

The normal PaO2 range in healthy adults is 80-110 mmHg (10.9-14.8 kPa), with values declining progressively with age. 1, 2

Age-Specific Normal Values

The normal PaO2 varies significantly by age group, and this must be considered when interpreting ABG results:

• Ages 18-24 years: Mean 100.5 mmHg (13.4 kPa), normal range 90-111 mmHg (12.0-14.8 kPa) 1, 2
• Ages 25-34 years: Mean 100.5 mmHg (13.4 kPa), normal range 91-110 mmHg (12.1-14.7 kPa) 1, 2
• Ages 35-44 years: Mean 99 mmHg (13.2 kPa), normal range 83-114 mmHg (11.1-15.2 kPa) 1, 2
• Ages 45-54 years: Mean 97.5 mmHg (13.0 kPa), normal range 82-113 mmHg (10.9-15.1 kPa) 1, 2
• Ages 55-64 years: Mean 90.7 mmHg (12.1 kPa), normal range 82-100 mmHg (10.9-13.3 kPa) 1, 2
• Ages >64 years: Mean 89.2 mmHg (11.9 kPa), normal range 68-111 mmHg (9.0-14.8 kPa) 1, 2

The decline in PaO2 becomes more pronounced after age 55, with wider standard deviations in elderly patients reflecting greater physiologic variability. 1, 2

Clinical Thresholds for Hypoxemia

PaO2 below 80 mmHg warrants further investigation in adults under 65 years. 1 For patients 65 years and older, the threshold should be adjusted downward given the normal age-related decline. 1, 2

PaO2 below 60 mmHg (8.0 kPa) defines clinically significant hypoxemia with increased risk of tissue hypoxia, regardless of age. 2, 3 This represents the critical threshold where supplemental oxygen becomes necessary to prevent organ dysfunction.

Factors Affecting PaO2 Interpretation

When evaluating PaO2 values, consider these key variables:

• Alveolar-arterial oxygen gradient (P[A-a]O2): Normal is ≤15 mmHg in adults under 65 years, and ≤20 mmHg in those ≥65 years while breathing room air at sea level 1
• Altitude: Barometric pressure decreases with elevation, directly reducing inspired oxygen partial pressure and thus PaO2 2
• Body position: Can affect PaO2 in certain pathologic conditions 2
• Underlying cardiopulmonary disease: Pre-existing conditions alter baseline PaO2 and its interpretation 4

Critical Clinical Pitfall

PaO2 alone does not determine tissue oxygenation. 2, 5 Adequate oxygen delivery to tissues depends on the interaction of PaO2, hemoglobin concentration, cardiac output, and tissue oxygen extraction. 2, 5

A patient with normal PaO2 but severe anemia may have inadequate oxygen delivery (anemic hypoxemia), while a patient with carbon monoxide poisoning may have normal PaO2 but critically reduced oxygen content due to COHb formation (toxic hypoxemia). 5 Always interpret PaO2 in conjunction with oxygen saturation, hemoglobin level, and clinical context.

Relationship to Oxygen Saturation

Normal oxygen saturation (SaO2) measured from arterial blood correlates with PaO2 values:

• Ages 18-24 years: Mean SaO2 96.9%, range 96.1-97.7% 1
• Ages >64 years: Mean SaO2 95.5%, range 92.7-98.3% 1

Pulse oximetry (SpO2) <96% should prompt ABG analysis to directly measure PaO2, particularly when screening for conditions like hepatopulmonary syndrome. 1