Normal PCO2 Values

The normal reference range for arterial carbon dioxide tension (PaCO2) is 35-45 mmHg (4.6-6.1 kPa) in healthy adults. 1

Standard Reference Values

The American Thoracic Society and international consensus guidelines define normocapnia as 35-45 mmHg (approximately 4.7-6.0 kPa). 1
Minor variations between laboratories exist but typically remain within 0.2 kPa of this range, though the core reference of 34-46 mmHg (4.6-6.1 kPa) is consistent across most modern laboratory standards. 1
A typical reference point of 40 mmHg is commonly used as the midpoint of the normal range. 2

Hypercapnia is defined as PaCO2 >45 mmHg (>6.1 kPa), though some sources consider values above 43 mmHg as indicative of hypercapnia. 1, 2
Values up to 50 mmHg (6.7 kPa) may be considered acceptable in certain clinical contexts, particularly in patients with chronic lung disease. 1
In COPD patients, PaCO2 is typically elevated to 45-55 mmHg (6.0-7.3 kPa), with extreme values occasionally reaching 70-80 mmHg (9.3-10.6 kPa) in stable outpatients. 1

Hypocapnia is defined as PaCO2 <34 mmHg (<4.6 kPa). 1
Low PCO2 values of 30-35 mmHg (4.0-4.7 kPa) are typically seen in interstitial lung disease and pulmonary vascular disease as a compensatory mechanism due to increased respiratory drive. 1
Hypocapnia commonly occurs in hyperventilation states, anxiety, high altitude exposure, and certain metabolic disorders. 1

A normal or elevated PaCO2 in a breathless asthmatic patient indicates a very severe, life-threatening attack requiring immediate ICU consideration. 1
This represents a critical pitfall: a "normal" PaCO2 reading during an asthma exacerbation is NOT reassuring and actually indicates severe respiratory compromise. 1

In post-cardiac arrest patients and trauma settings, target PaCO2 should be maintained at 35-40 mmHg (5.0-5.5 kPa) to avoid adverse effects of hypocapnia including vasoconstriction and decreased cerebral blood flow. 1
In COPD patients receiving oxygen therapy, target saturations of 88-92% are recommended to avoid further CO2 retention and acidosis. 2

PaCO2 must be interpreted alongside pH and bicarbonate levels to properly assess acid-base status. 1
Respiratory acidosis occurs when pH falls below 7.35 in the presence of elevated PaCO2. 1
Chronic hypercapnia may lead to compensated respiratory acidosis with normal pH due to renal bicarbonate retention. 1
PaCO2 measurement requires arterial blood gas analysis for accurate assessment, though transcutaneous CO2 monitoring may be useful in certain settings. 1