Comparison of Capnography and Blood Gas PaCO₂ Measurements

Overview

Capnography provides continuous, non-invasive assessment of ventilation by measuring end-tidal carbon dioxide (EtCO₂). Different capnographic technologies, including mainstream, sidestream, and microstream capnography, may produce different EtCO₂ measurements depending on their sampling and measurement techniques. This prospective, non-randomized observational study was designed to compare EtCO₂ values obtained using mainstream, sidestream, and microstream capnography with carbon dioxide partial pressure (PCO₂) values obtained from clinically indicated blood gas analysis in adult patients presenting to the emergency department. For each participant, EtCO₂ measurements were obtained using the three capnographic methods in close temporal proximity to blood gas sampling. The relationship and agreement between blood gas PCO₂ and EtCO₂ measurements were planned to be evaluated using correlation analyses and Bland-Altman analysis. Additional analyses were planned according to whether the blood gas sample was arterial or venous.

This prospective, non-randomized, cross-sectional observational study was conducted in the Emergency Department of the University of Health Sciences Sultan II. Abdülhamid Han Training and Research Hospital. The study population consisted of adult patients presenting to the emergency department for whom blood gas analysis was requested by the treating physician as part of routine clinical care. The primary objective of the study was to evaluate the agreement between blood gas PCO₂ measurements and end-tidal carbon dioxide (EtCO₂) values obtained using three different capnographic technologies: mainstream, sidestream, and microstream capnography. A secondary objective was to compare the agreement of each capnographic method with blood gas PCO₂ measurements and to evaluate whether the relationship differed according to the type of blood gas sample, arterial or venous. Eligible participants were adults aged 18 years or older whose level of consciousness and clinical condition were suitable for EtCO₂ measurement and who provided informed consent. Exclusion criteria included age younger than 18 years, pregnancy, refusal or inability to provide informed consent, Glasgow Coma Scale score below 15, and clinical or anatomical conditions preventing appropriate capnographic measurement. Patients with sepsis, diabetic ketoacidosis, hypovolemic shock, acute major bleeding, or severe dehydration were also excluded because these conditions could potentially affect the relationship between EtCO₂ and blood gas PCO₂. For each participant, EtCO₂ measurements were obtained using mainstream, sidestream, and microstream capnography. Blood gas sampling was performed simultaneously with or in close temporal proximity to the capnographic measurements. For mainstream capnography, participants breathed through an airway interface connected directly to the capnography sensor, and EtCO₂ measurements were recorded after a stable capnogram had been obtained over consecutive respiratory cycles. Sidestream and microstream EtCO₂ measurements were obtained using the corresponding sampling interfaces connected to the respective capnography monitors. Demographic and clinical variables, including age, sex, height, weight, smoking status, relevant chronic diseases, and blood gas parameters, were recorded. The association between blood gas PCO₂ and EtCO₂ values obtained using each capnographic method was planned to be evaluated using correlation analysis. Agreement between blood gas PCO₂ and EtCO₂ measurements was planned to be assessed using Bland-Altman analysis. Prespecified subgroup analyses were planned according to whether the blood gas sample was arterial or venous.