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.