From the Guidelines
The difference of 15 mmHg between end-tidal carbon dioxide (EtCO2) and arterial carbon dioxide (PaCO2) is abnormal and warrants investigation, despite the EtCO2 appearing "normal." Typically, EtCO2 should be 2-5 mmHg lower than PaCO2, with a gradient greater than 10 mmHg suggesting ventilation-perfusion mismatch, as indicated by the concept of physiologic dead space-to-tidal volume ratio (Vd/VT) discussed in the ats/accp statement on cardiopulmonary exercise testing 1. This discrepancy could indicate conditions such as pulmonary embolism, severe COPD, significant atelectasis, or low cardiac output states. To address this, perform a thorough clinical assessment including:
- Vital signs
- Respiratory examination
- Review of the patient's history Consider additional diagnostic tests such as:
- Arterial blood gas analysis
- Chest imaging
- Possibly CT pulmonary angiography if pulmonary embolism is suspected While managing the underlying cause, ensure adequate ventilation by adjusting ventilator settings if the patient is mechanically ventilated, or providing appropriate respiratory support if not. The increased gradient occurs because areas of the lung receiving ventilation but inadequate perfusion (dead space) don't participate in gas exchange, causing exhaled CO2 (measured as EtCO2) to be significantly lower than arterial CO2 levels, which is also related to the concept that end-tidal Pco2 (PetCO2) should not be used as an index of Pa CO2, as it can be misleading, especially in patients with lung disease 1.
From the Research
EtCO2 and PaCO2 Variance
- The variance between EtCO2 and PaCO2 readings can be significant, with studies showing a range of differences [ 2, 3 ].
- A study published in the Indian Journal of Critical Care Medicine found a mean bias of -19.4 mmHg between PaCO2 and EtCO2 values in COPD patients on mechanical ventilation [ 2 ].
- Another study published in the Journal of Critical Care found a poor correlation between EtCO2 and PaCO2 in nonintubated ICU patients with acute respiratory failure, with a correlation coefficient of 0.62 [ 3 ].
Factors Affecting EtCO2 and PaCO2 Variance
- The variance between EtCO2 and PaCO2 can be affected by various factors, including the patient's underlying condition, ventilatory status, and the presence of dead space [ 3, 4 ].
- A study published in Prehospital and Disaster Medicine found that the sole use of EtCO2 to monitor ventilation in severely traumatized and burn patients may lead to avoidable respiratory acidosis [ 4 ].
- Another study published in Critical Care Medicine found that the difference between PaCO2 and EtCO2 decreased from 9.8 to 2.8 mmHg during thrombolytic therapy in patients with massive pulmonary embolism [ 5 ].
Clinical Implications
- The significant variance between EtCO2 and PaCO2 readings has important clinical implications, particularly in patients who require precise control of ventilatory settings [ 6 ].
- A study published in the Journal of the Intensive Care Society found that the PaCO2-ETCO2 gradient was significantly higher than expected in pre-hospital intubated patients, with a median gradient of 2.0 kPa [ 6 ].
- These findings suggest that EtCO2 may not be a suitable surrogate for PaCO2 in certain patient populations, and that arterial blood gas analysis may be necessary to ensure accurate monitoring of ventilatory status [ 2, 3 ].