Prospective, observational study investigating the level of agreement between transcutaneous and invasive carbon dioxide measurements in critically ill emergency department patients.

Background: Transcutaneous carbon dioxide (Ptcco) measurement is a non-invasive surrogate marker for arterial carbon dioxide (Paco), which requires invasive arterial blood sampling. Use of Ptcco has been examined in different clinical settings, however, most existing evidence in the adult emergency department (ED) setting shows insufficient agreement between the measurements. This study assessed the level of agreement between Ptcco and Paco in undifferentiated adult ED patients across multiple timepoints.

Methods: This prospective observational study (study period 2020-2021) assessed paired Ptcco and Paco measurements at four consecutive timepoints (0, 30, 60 and 90 min) in adult (aged 18 years or over) Australian ED patients requiring hospital admission and arterial catheter insertion. Agreement between the pairs was assessed using Bland-Altman analysis. It was prospectively determined by expert consensus that limits of ±4 mm Hg would be a clinically acceptable level of agreement between Ptcco and Paco.

Results: During the study period 168 paired Ptcco and Paco readings were taken from 42 adult ED patients. Bland-Altman analysis showed a mean Ptcco reading 3.85 mm Hg higher than Paco although at each timepoint the 95% CIs breached the limit of 4 mm Hg difference. In addition, only 66% (111/168) of results fell within the clinically acceptable range.

Conclusion: The level of agreement between Ptcco and Paco measurements may not be sufficiently precise for the adoption of Ptcco monitoring in patients presenting to the ED.