Normal 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
Understanding PaCO2
PaCO2 represents the partial pressure of carbon dioxide in arterial blood and is a critical parameter for:
- Assessing ventilatory status and acid-base balance in patients 1, 2
- Diagnosing respiratory disorders including respiratory failure 1
- Guiding ventilator management in critically ill patients 1
Clinical Significance of PaCO2 Values
Normal Range Interpretation
- The reference range of 4.6-6.1 kPa (34-46 mmHg) is consistent across most modern laboratory reference values 1
- Minor variations between laboratories exist but typically remain within 0.2 kPa of this range 1
- The 2024 international consensus guidelines similarly define normocapnia as 35-45 mmHg (≈4.7-6.0 kPa) 1
Abnormal PaCO2 Values
Hypercapnia
- Defined as PaCO2 > 6.1 kPa (45 mmHg) 1
- Values up to 6.7 kPa may be considered in certain clinical contexts 1
- Associated with type 2 respiratory failure, even if oxygen saturation remains normal 1
- Common in conditions like COPD, neuromuscular disorders, and respiratory depression 1
Hypocapnia
- Defined as PaCO2 < 4.6 kPa (34 mmHg) 1
- Often seen in hyperventilation states, anxiety, high altitude, and certain metabolic disorders 1
- May be associated with worse clinical outcomes in post-cardiac arrest patients compared to normocapnia 3
Clinical Applications
- In critical care settings, targeting normocapnia (35-45 mmHg or ≈4.7-6.0 kPa) 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 4, 5
- End-tidal CO2 (PETCO2) measurements may not always accurately reflect PaCO2 in critically ill patients 6
Important Considerations
- PaCO2 should be interpreted in conjunction with pH and bicarbonate levels to 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
- In certain clinical scenarios (severe lung injury, metabolic acidosis), target PaCO2 values may need adjustment outside the normal range 1
Remember that while PaCO2 is a valuable clinical parameter, it should be interpreted within the overall clinical context and alongside other relevant laboratory and clinical findings.