Using an EtCO₂ Cannula Under a CPAP Mask
You should not use a standard nasal EtCO₂ cannula under a CPAP mask because it compromises mask seal, causes undesired leak, and produces inaccurate measurements due to dilution from PAP device flow. 1, 2
Why This Doesn't Work
The American Academy of Sleep Medicine guidelines explicitly state that measurement of airflow using a nasal pressure cannula under the mask is not recommended during positive airway pressure titration 1. The problems are threefold:
- Mask seal disruption: The tubing passing underneath the mask to reach the nares prevents achieving a good seal, leading to excessive leak 1
- Measurement inaccuracy: The exhaled gas sample becomes diluted by the continuous flow from the PAP device, rendering EtCO₂ readings unreliable 2, 1
- Unvalidated approach: While some clinicians attempt to use a small nasal cannula under the mask to minimize dilution, the accuracy of this method has never been documented 2
What to Use Instead
Transcutaneous CO₂ (PtCO₂) monitoring is the preferred method for measuring CO₂ during CPAP or any mask ventilation 2. The 2012 AASM guidelines are clear: "PtCO₂ is the preferred technology in patients...who are using supplemental oxygen or mask ventilation" 2.
Advantages of Transcutaneous Monitoring:
- Accuracy is not degraded by mask ventilation 2
- No interference with mask seal
- Provides reliable trend monitoring of arterial CO₂
- Works regardless of mouth breathing or supplemental oxygen 2
Important Caveats:
- PtCO₂ has a slower response time (approximately 2-minute lag) compared to EtCO₂ 2
- Requires calibration and validation against arterial blood gas when possible 1
- Readings should be within 10 mm Hg of concurrent arterial PCO₂ for reliability 1
- Cannot detect breath-by-breath changes 2
Alternative Monitoring Approach
If you need breath-by-breath CO₂ monitoring and cannot use transcutaneous monitoring, use the airflow signal directly from the CPAP device rather than attempting EtCO₂ sampling 1. Modern CPAP devices have accurate internal flow sensors that can detect respiratory events without requiring additional cannulas under the mask.
Clinical Bottom Line
The evidence consistently shows that placing any type of cannula under a CPAP mask creates more problems than it solves. Transcutaneous CO₂ monitoring is the evidence-based solution when CO₂ monitoring is needed during CPAP therapy.