What is the normal arterial partial pressure of carbon dioxide (PaCO₂) range in kilopascals?

Normal Arterial CO₂ Range in kPa

The normal arterial partial pressure of carbon dioxide (PaCO₂) is 4.6–6.1 kPa (34–46 mm Hg) for healthy adults. 1

Reference Range Details

• The established reference range of 4.6–6.1 kPa is based on data from healthy adult men aged 18–38 years and remains consistent with modern clinical practice and laboratory reference values 1

• Different laboratories and textbooks may report slightly different values, but all fall within 0.2 kPa of this reference range 1

Clinical Interpretation Thresholds

Abnormal values:

• Any PaCO₂ >6.1 kPa (45 mm Hg) may be considered abnormal 1
• Values up to 6.7 kPa warrant clinical consideration but may not require immediate intervention 1

Hypercapnia definition:

• Hypercapnia is definitively present when PaCO₂ exceeds the normal range of 4.6–6.1 kPa 1
• The BTS/ICS guidelines use ≥6.0 kPa as the threshold for defining hypercapnia in acute respiratory failure contexts 1
• For NIV initiation decisions, a higher threshold of ≥6.5 kPa is recommended when combined with pH <7.35 and respiratory rate >23 breaths/min 1

Important Clinical Caveats

Age considerations:

• The normal PaCO₂ range remains independent of age in elderly populations 2
• Unlike PaO₂, which declines with age, PaCO₂ does not require age-adjusted reference ranges 2

Disease states:

• In COPD patients with chronic airways obstruction, mean PaCO₂ was 5.4 ± 0.8 kPa (40.5 ± 6.1 mmHg), which falls within the normal range despite their lung disease 2
• PaCO₂ correlates negatively with FEV₁ in obstructive lung disease 2

Relationship to pH:

• The normal physiological range of 4.6–6.1 kPa represents the range where the relationship between PaCO₂ and carbon dioxide content is linear 1
• Acute respiratory acidosis occurs when pH falls below 7.35 in the presence of elevated CO₂ 1