Integration of Capnography and Continuous Positive Airway Pressure (CPAP) in the Prehospital Setting

BACKGROUND Capnography is one of the most important respiratory monitoring tools used in EMS because changes to end-tidal CO2 (ETCO2) generally precede blood oxygen desaturation, and waveform morphology can be used to assess bronchial patency. Many of capnography’s indications overlap with those of continuous positive airway pressure (CPAP) therapy. However, there are currently no convenient methods to administer CPAP while using capnography, and no peer-reviewed studies have examined combining the two technologies in the prehospital setting. METHODS & FINDINGS Two types of capnography sampler were used to investigate how ETCO2, capnogram shape, and respiration rate (RR) are affected by applying a CPAP mask both with and without oxygen flowing. A traditional nasal-oral cannula (NC) sampler, as well as a novel sampler that does not break the seal between the CPAP mask and the patient’s face (called the CPAP-Capnography Adapter, or CCA), were evaluated on a Human Patient Simulator (HPS). No significant differences between the NC and CCA were found for ETCO2 and RR percent error. Placement of a CPAP mask without turning on O2 increased ETCO2. During CPAP therapy, the two samplers maintained their RR measurement accuracy, but ETCO2 values were significantly reduced and the capnogram depicted an increased β-angle with severe blunting of the inspiratory downstroke. CONCLUSIONS The HPS had set-point ETCO2 and PaCO2 values, indicating that the local CO2 concentration at the nares was diluted during CPAP and no longer accurately reflected the simulated subject’s breathing. The compromise in waveform morphology could be overcome by further research into interpreting capnogram differences during CPAP between healthy patients and those with respiratory pathologies. However, at this time, our results show that ETCO2 values and capnogram shape should be interpreted with caution during CPAP. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This work was funded by the Abrams Scholarship from the UNC/NCSU Joint Department of Biomedical Engineering, as well as significant in-kind support from the UNC School of Nursing Simulation Center, UNC/NCSU Joint Department of Biomedical Engineering, and Durham Technical Community College EMS Education program. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors. * EMS : Emergency Medical Services CO2 : Carbon Dioxide CPAP : Continuous Positive Airway Pressure ETCO2 : End-Tidal Carbon Dioxide (mmHg) PaCO2 : Partial Pressure of Arterial Carbon Dioxide (mmHg) SpO2 : Blood Oxygen Saturation RR : Respiration Rate (breaths/min) NC : Nasal-Oral Cannula CCA : CPAP-Capnography Adapter MAP : Mean Arterial Pressure HPS : Human Patient Simulator PEEP : Positive End Expiratory Pressure