Normal PCO2 Range
The normal range for arterial partial pressure of carbon dioxide (PaCO2) is 35-45 mmHg (4.6-6.1 kPa) in healthy adults. 1
Standard Reference Values
The American Thoracic Society and European Respiratory Society establish the normal PaCO2 range as 34-46 mmHg (4.6-6.1 kPa), with 40 mmHg representing the typical midpoint reference value 1, 2
The 2024 international consensus guidelines define normocapnia as 35-45 mmHg (approximately 4.7-6.0 kPa), which is consistent across most modern laboratory reference standards 1
Minor variations between laboratories exist but typically remain within 0.2 kPa (approximately 1.5 mmHg) of this established range 1
Clinical Context for Target Values
In trauma and critical care settings, target PaCO2 should be maintained at 35-40 mmHg (5.0-5.5 kPa) to avoid the adverse effects of hyperventilation-induced hypocapnia, including vasoconstriction, decreased cerebral blood flow, and impaired tissue perfusion 3
For post-cardiac arrest patients, targeting normocapnia (35-45 mmHg) is specifically recommended to optimize neurological outcomes 1
In patients on mechanical ventilation, PaCO2 monitoring is essential for guiding ventilator management and should be interpreted alongside pH and bicarbonate levels to assess complete acid-base status 1
Defining Abnormal Values
Hypercapnia is defined as PaCO2 >45 mmHg (>6.1 kPa), though some clinical contexts may tolerate values up to 50 mmHg (6.7 kPa) 1, 2
Hypocapnia is defined as PaCO2 <34 mmHg (<4.6 kPa), commonly seen in hyperventilation states, anxiety, high altitude exposure, and certain metabolic disorders 1
Important Clinical Pitfalls
In acute severe asthma, a "normal" PaCO2 (35-45 mmHg) in a breathless patient is actually a marker of severe, life-threatening respiratory compromise requiring immediate escalation of care, as these patients should be hypocapnic from tachypnea 1
End-tidal CO2 (EtCO2) should not be used as a reliable substitute for arterial PaCO2, particularly in critically ill patients, as studies show substantial differences with 61% of patients having delta PCO2 >10 mmHg 4
In chronic lung disease (particularly COPD), baseline PaCO2 is typically elevated to 45-55 mmHg (6.0-7.3 kPa), and values in the low 40s may actually represent improvement rather than normalcy for these patients 1
Transcutaneous CO2 monitoring may be useful for trending but has unacceptably wide limits of agreement (±10-22 mmHg) compared to arterial sampling, especially when PaCO2 exceeds 60 mmHg 5