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Background End-tidal carbon dioxide pressure (etCO 2 ) is widely used in anaesthesia and critical care in intubated patients. The aim of our preliminary study was to evaluate the feasibility of a simple device to predict capnia in spontaneously breathing patients in an emergency department (ED). Patients and methods This study was a prospective, nonblind study performed in our teaching hospital ED. We included nonintubated patients with dyspnea (≥18 years) requiring measurement of arterial blood gases, as ordered by the emergency physician in charge. There were no exclusion criteria. End-tidal CO 2 was measured by an easy-to-use device connected to a microstream capnometer, which gave a continuous measurement and graphical display of the etCO 2 level of a patient's exhaled breath. Results A total of 43 patients (48 measurements) were included, and the majority had pneumonia (n = 12), acute cardiac failure (n = 8), asthma (n = 7), or chronic obstructive pulmonary disease exacerbation (n = 6). Using simple linear regression, the correlation between etCO 2 and Paco 2 was good ( R = 0.82). However, 18 measurements (38%) had a difference between etCO 2 and Paco 2 of 10 mm Hg or more. The mean difference between the Paco 2 and etCO 2 levels was 8 mm Hg. Using the Bland and Altman matrix, the limits of agreement were −10 to +26 mm Hg. Conclusion In our preliminary study, etCO 2 using a microstream method does not seem to accurately predict Paco 2 in patients presenting to an ED for acute dyspnea.