From the Guidelines
The normal PaCO2 range is 4.6–6.1 kPa (34–46 mm Hg), as defined by the British Thoracic Society guideline for oxygen use in adults in healthcare and emergency settings 1. This range is crucial in determining hypercapnia and type 2 respiratory failure. When interpreting PaCO2 values, it's essential to consider the following:
- PaCO2 above 6.1 kPa (46 mm Hg) indicates hypercapnia and potential type 2 respiratory failure, even if oxygen saturation is within the normal range 1
- PaCO2 below 4.6 kPa (34 mm Hg) may suggest respiratory alkalosis or hyperventilation
- PaCO2 within the normal range does not necessarily rule out respiratory or metabolic disorders, as compensatory mechanisms may be present Key points to consider when evaluating PaCO2 include:
- The patient's clinical condition, such as chronic respiratory disorders or metabolic diseases
- Other arterial blood gas parameters, like pH and bicarbonate levels, to assess acid-base status
- The need for regular monitoring of PaCO2 in patients with respiratory diseases, those on mechanical ventilation, or individuals with metabolic disorders affecting acid-base balance.
From the Research
Normal PaCO2 Range
The normal range for Partial Pressure of Carbon Dioxide (PaCO2) is generally considered to be between 35-45 mmHg.
- This range is supported by several studies, including 2, which found a mean PaCO2 of 40.44±2.70 mmHg in healthy volunteers, and 3, which defined normal PaCO2 as 35-45 mmHg.
- Another study, 4, found a mean PaCO2 of 5.4 +/- 0.8 kPa (40.5 +/- 6.1 mmHg) in elderly subjects with chronic obstructive pulmonary disease (COPD).
- A more recent study, 5, found that the nadir for in-hospital mortality risk was estimated to be at 39.5 mmHg, with a range of 35-45 mmHg being associated with the lowest death risk in most patients with craniocerebral diseases.
Factors Affecting PaCO2
PaCO2 levels can be affected by various factors, including:
- Respiratory disease, such as COPD or pneumonia, which can lead to hypercapnia or hypocapnia 3, 6
- Age, although some studies suggest that PaCO2 remains relatively constant with age 4
- Cerebral injury, which can affect the regulation of PaCO2 and lead to secondary injury 5
Measurement of PaCO2
PaCO2 can be measured using various methods, including:
- Arterial blood gas analysis, which is considered the gold standard 2, 3, 4
- Transcutaneous carbon dioxide devices, which have been shown to be accurate in certain settings 6
- End-tidal partial pressure of carbon dioxide (PETCO2) monitoring, which can be used as a non-invasive alternative to PaCO2 measurement 2