Background It is known that the arterial carbon dioxide pressure (PaCO₂) is useful for emergency physicians to assess the severity of dyspnoeic spontaneously breathing patients. Transcutaneous carbon dioxide pressure (PtcCO₂) measurements could be a non-invasive alternative to PaCO₂ measurements obtained by blood gas samples, as suggested in previous studies. This study evaluates the reliability of a new device in the emergency department (ED).

Methods We prospectively included patients presenting to the ED with respiratory distress who were breathing spontaneously or under non-invasive ventilation. We simultaneously performed arterial blood gas measurements and measurement of PtcCO₂ using a sensor placed either on the forearm or the side of the chest and connected to the TCM4 CombiM device. The agreement between PaCO₂ and PtcCO₂ was assessed using the Bland–Altman method.

Results Sixty-seven spontaneously breathing patients were prospectively included (mean age 70 years, 52% men) and 64 first measurements of PtcCO₂ (out of 67) were analysed out of the 97 performed. Nineteen patients (28%) had pneumonia, 19 (28%) had acute heart failure and 19 (28%) had an exacerbation of chronic obstructive pulmonary disease. Mean PaCO₂ was 49 mm Hg (range 22–103). The mean difference between PaCO₂ and PtcCO₂ was 9 mm Hg (range −47 to +54) with 95% limits of agreement of −21.8 mm Hg and 39.7 mm Hg. Only 36.3% of the measurement differences were within 5 mm Hg.

Conclusions Our results show that PtcCO₂ measured by the TCM4 device could not replace PaCO₂ obtained by arterial blood gas analysis.