Pulmonary embolism (PE) frequently remains undiagnosed. The partial pressure of carbon dioxide (PaCO2), a surrogate of dead-space ventilation, is useful in the evaluation of the degree of pulmonary artery occlusion. At present, there is no knowledge about the prognostic role of PaCΟ2 variations during the first hours of an acute PE. Transcutaneous measurement of CO2 (PtcCO2) is a simple, non-invasive method that correlates well with PaCO2 levels, evaluated in this study for the first time in patients with PE.