Concordance and limits between transcutaneous and arterial carbon dioxide pressure in emergency department patients with acute respiratory failure: a single-center prospective observational study

INTRODUCTION Transcutaneous CO₂ (PtCO₂) is a continuous and non-invasive measure recommended by scientific societies in the management of respiratory distress. The objective of this study is to evaluate the correlation between PtCO₂ and blood pressure of CO₂ (PaCOv) by blood gas analysis in emergency patients with dyspnoea and to determine the factors that interfere in this correlation. METHODS From January to June 2014, all patients admitted to resuscitation room of the emergency department targeted for arterial blood gases were included prospectively. A sensor measuring the PtCO₂ was attached to the ear lobe of the patient before the gas analysis. Anamnesis, clinical and laboratory parameters were identified. RESULTS 90 patients with dyspnoea were included (with 104 pairs of measurements), the median age was 79 years [69-85]. The correlation between PtCO₂ and PaCO₂ was R(2)= 0.83 (p <0.001) but became lower for values of PaCO₂>60 mm Hg. The mean bias (±SD) between the two methods of measurement (Bland-Altman analysis) was -1.4 mm Hg (±7.7) with limits of agreement of -16.4 to 13.7 mm Hg. In univariate analysis, PaO₂ interfered in this correlation. After multivariate analysis, the temperature (OR = 3.01, 95% CI = 1.16-7.09) and the PaO 2 (OR = 1.22, 95% CI = 1.02-1.47) were found to be significant. CONCLUSIONS In patients admitted in emergency unit for acute respiratory failure, there is a significant correlation between PaCO₂ and PtCO₂, mainly for values below 60 mm Hg. The two limiting factors of use are hyperthermia and users training.