What is the cause of a discrepancy between end-tidal CO2 (EtCO2) and partial pressure of carbon dioxide in arterial blood (PaCO2) of 15?

From the Guidelines

The difference of 15 mmHg between end-tidal carbon dioxide (EtCO2) and arterial carbon dioxide (PaCO2) levels is abnormal and warrants further investigation to identify the underlying cause, which may include ventilation-perfusion mismatch, equipment malfunction, or severe hypotension, among others. The normal gradient between EtCO2 and PaCO2 is typically 2-5 mmHg, and a discrepancy of this magnitude suggests a significant issue that may impact patient care and outcomes, particularly in critically ill patients where accurate assessment of carbon dioxide levels is crucial for guiding ventilation management 1. Several conditions can lead to this discrepancy, including pulmonary embolism, severe COPD or asthma, low cardiac output states, and significant intrapulmonary shunting. Notably, the recent international consensus on cardiopulmonary resuscitation and emergency cardiovascular care science emphasizes the importance of targeting normocapnia, defined as a PaCO2 of 35 to 45 mm Hg, in adults with return of spontaneous circulation (ROSC) after cardiac arrest, highlighting the need for accurate carbon dioxide monitoring 1.

To address this issue, it is essential to:

• Verify equipment function and sampling technique to rule out any technical issues
• Assess for clinical signs of conditions that may cause ventilation-perfusion mismatch, such as pulmonary embolism or severe COPD
• Consider arterial blood gas analysis for accurate assessment of carbon dioxide levels, particularly if the patient is unstable
• Be aware of the potential for EtCO2 values to not accurately reflect PaCO2 values, as suggested by unpublished data and observational studies not included in the recent review 1.

Given the potential for significant clinical implications, it is crucial to prioritize the identification and management of the underlying cause of the discrepancy between EtCO2 and PaCO2 levels, rather than relying solely on EtCO2 monitoring, which may provide falsely reassuring readings in the presence of significant pulmonary pathology.

From the Research

Causes of Variation between EtCO2 and PaCO2

• The difference between EtCO2 and PaCO2 can be attributed to several factors, including ventilation-perfusion mismatching, dead space, and alveolar dead space 2, 3.
• In patients with respiratory failure, the correlation between EtCO2 and PaCO2 can be poor due to the presence of an alveolar dead space, particularly in those with chronic obstructive pulmonary disease (COPD) 2.
• The use of EtCO2 monitoring in nonintubated patients with acute respiratory failure may not provide accurate estimates of PaCO2, and its use may be limited in certain patient populations 2, 4.
• Factors such as severity of injury, acidosis, and higher base deficits can also affect the correlation between EtCO2 and PaCO2, leading to a greater discrepancy between the two values 4.

Clinical Implications

• The difference between EtCO2 and PaCO2 can have significant clinical implications, particularly in patients with severe hypercapnia undergoing noninvasive ventilation 5.
• A positive correlation between EtCO2 and PaCO2 values can be found in certain patient populations, such as those undergoing mechanical ventilation 6.
• The measurement of the PaCO2-EtCO2 gradient can be a reliable method for monitoring the effectiveness of noninvasive ventilation in acute hypercapnic respiratory failure 5.

Limitations of EtCO2 Monitoring

• EtCO2 monitoring may not be suitable for all patient populations, particularly those with severe respiratory failure or those who are nonintubated 2, 4.
• The accuracy of EtCO2 monitoring can be affected by various factors, including the presence of dead space, ventilation-perfusion mismatching, and alveolar dead space 3.
• Alternative methods, such as transcutaneous CO2 monitoring, may provide more accurate estimates of PaCO2 in certain patient populations 2.