What is the expected partial pressure of carbon dioxide (pco2) level?

Normal Arterial Carbon Dioxide Tension (PaCO2) Levels

The normal reference range for arterial carbon dioxide tension (PaCO2) is 4.6-6.1 kPa (34-46 mmHg) for healthy adults. 1

Normal PaCO2 Values and Clinical Significance

• The reference range of 4.6-6.1 kPa (34-46 mmHg) is consistent across most modern laboratory reference values and international consensus guidelines 1
• Minor variations between laboratories exist but typically remain within 0.2 kPa of this range 1
• PaCO2 represents the partial pressure of carbon dioxide in arterial blood and is a critical parameter for assessing ventilatory status and acid-base balance 1
• The 2024 international consensus guidelines similarly define normocapnia as 35-45 mmHg (≈4.7-6.0 kPa) 1

Abnormal PaCO2 Values

• Hypercapnia is defined as PaCO2 > 6.1 kPa (45 mmHg) 1, 2
• Values up to 6.7 kPa may be considered in certain clinical contexts 1
• Hypercapnia is associated with type 2 respiratory failure, even if oxygen saturation remains normal 1
• Hypocapnia is defined as PaCO2 < 4.6 kPa (34 mmHg) 1
• Hypocapnia is often seen in hyperventilation states, anxiety, high altitude, and certain metabolic disorders 1

PaCO2 in Different Clinical Conditions

Normal Respiratory Function

• In healthy individuals, PaCO2 is maintained between 35-45 mmHg through normal respiratory mechanisms 1
• Pa,CO2 is typically normal or slightly reduced in COPD patients with Pa,O2 in the 60–70 mmHg range 3

Chronic Obstructive Pulmonary Disease (COPD)

• Carbon dioxide retainers with COPD typically have an elevated Pa,CO2, usually in the range 45–55 mmHg (6.0–7.3 kPa) 3
• Extreme values as high as 70–80 mmHg (9.3–10.6 kPa) can occasionally be seen, even in stable outpatients 3
• BiPAP therapy should be considered for patients with a PaCO2 between 6.0 and 6.5 kPa (45-48.8 mmHg) 2

Interstitial Lung Disease (ILD) and Pulmonary Vascular Disease (PVD)

• In ILD and PVD, Pa,CO2 is typically 30–35 mmHg (4.0–4.7 kPa) 3
• This hypocapnia is often a compensatory mechanism due to increased respiratory drive 3

Clinical Applications and Monitoring

• Targeting normocapnia (35-45 mmHg) is recommended for post-cardiac arrest patients 1
• Monitoring PaCO2 is essential for managing patients on mechanical ventilation 1
• PaCO2 measurement requires arterial blood gas analysis, though non-invasive methods like transcutaneous CO2 monitoring may be useful in certain settings 1, 4
• In ventilated patients with head injuries, maintaining PaCO2 between 35-40 mmHg is recommended 5
• Mechanical ventilation is superior to hand-bagging for maintaining optimal PaCO2 levels during transport 5

Important Considerations and Pitfalls

• PaCO2 should be interpreted in conjunction with pH and bicarbonate levels to assess acid-base status 1, 6
• A normal or elevated PaCO2 in a breathless asthmatic patient is a marker of a very severe, life-threatening attack 1
• Do not assume that a "normal" PaCO2 reading in an asthmatic having an exacerbation is reassuring - it may actually indicate severe respiratory compromise 1
• In certain patients, lowering PaCO2 through hyperventilation may cause cerebral vasoconstriction and reduced cerebral circulation 7
• Winters' formula (pCO2 = 1.5*HCO3 + 8) can be used to predict the expected PaCO2 in metabolic acidosis 6