From the Guidelines
In the normal patient, the EtCO2 reading will typically be 2-5 mmHg lower than the PaCO2 level. This gradient exists because not all alveoli in the lungs participate equally in gas exchange, with some being ventilated but not perfused, thus diluting the exhaled CO2 concentration 1. The normal PaCO2 range is approximately 35-45 mmHg, so the corresponding normal EtCO2 would typically be around 30-40 mmHg.
Key Points to Consider
- The relationship between EtCO2 and PaCO2 is important clinically because EtCO2 monitoring provides a non-invasive, continuous method to estimate PaCO2 levels, particularly useful during anesthesia, procedural sedation, and critical care management.
- The gradient between EtCO2 and PaCO2 can widen significantly in patients with pulmonary disease, decreased cardiac output, or ventilation-perfusion mismatching, making EtCO2 less reliable as a PaCO2 estimate in these situations.
- A study found that an increase in ETCO2 might be the only clue to hypoventilation and potential respiratory compromise, and that all patients with respiratory depression demonstrated an ETCO2 greater than 50 mm Hg, an absent waveform, or an absolute change from baseline in ETCO2 greater than 10 mm Hg 1.
Clinical Implications
- EtCO2 monitoring can be used to detect early cases of inadequate ventilation before oxygen desaturation takes place, and may allow more rapid identification of hypoventilation than pulse oximetry alone 1.
- However, there is no evidence that this benefit has an impact on patient outcome when used in procedural sedation and analgesia 1.
From the Research
EtCO2 and PaCO2 Relationship
- The relationship between EtCO2 and PaCO2 is complex and can be affected by various factors, including respiratory disease and ventilation methods 2, 3, 4, 5.
- Studies have shown that EtCO2 can be a poor estimate of PaCO2 in patients with respiratory failure, with large differences found between the two values in individual patients 2, 4.
- However, some studies have found a good correlation between EtCO2 and PaCO2 in mechanically ventilated patients, with correlation coefficients ranging from 0.62 to 0.97 3, 5.
- The difference between EtCO2 and PaCO2 can be influenced by factors such as ventilation-perfusion mismatching and dead space/tidal volume ratio 4.
EtCO2 as a Predictor of PaCO2
- EtCO2 can be used as a noninvasive modality to estimate PaCO2 in certain patient populations, such as those with COPD on invasive mechanical ventilation 3.
- However, the accuracy of EtCO2 as a predictor of PaCO2 can be limited in patients with respiratory failure, particularly those with severe ventilation-perfusion mismatching 2, 4.
- The use of EtCO2 monitoring may reduce the need for invasive monitoring and/or repeated arterial blood gas analyses in mechanically ventilated patients 5.
Normal Patient EtCO2 Reading
- Unfortunately, there is no clear answer to the question of what the normal EtCO2 reading is in relation to PaCO2, as the relationship between the two values can vary widely depending on the individual patient and their underlying condition.
- However, studies have reported mean differences between EtCO2 and PaCO2 ranging from -19.4 mmHg to -40.0 mmHg, suggesting that EtCO2 readings may be lower than PaCO2 values in some patients 3, 4.