Volumetric capnography as a screening test for pulmonary embolism in the emergency department

Study objective: To compare the diagnostic performance of volumetric capnography (VCap), which is the plot of the expired CO2 partial pressure against the expired volume during a single breath, with the PaCO2 to end-tidal CO2 (EtCO2) gradient, in the case of suspected pulmonary embolism (PE). Design: Single-center, prospective study. Setting: Emergency department of a teaching hospital. Patients: A total of 45 outpatients with positive enzyme-linked immunosorbent assay d-dimer levels of > 500 ng/mL. The diagnosis of PE was confirmed in 18 outpatients according to a validated procedure based on the ventilation-perfusion lung scan and/or spiral CT scanning. Interventions: Curves of VCap were obtained from a compact monitor connected to a computer. A sequence of four to six stable breaths allowed the calculation of the following several variables: alveolar dead space fraction; the ratio of alveolar dead space (VDalv) to airway dead space (VDaw); the VDalv to physiologic dead space (VDphys) fraction; the slope of phase 3; and the late dead space fraction (Fdlate) corresponding to the extrapolation of the capnographic curve to a volume of 15% of the predicted total lung capacity. Results: The mean ( SD) PaCO2-EtCO2 gradient was 5.3 0.7 mm Hg in the PE-positive group and 2.8 0.7 mm Hg in the PE-negative group (p 0.019). Four variables of the VCap exhibited a statistical difference between both groups, as follows: the VDalv/VDaw fraction; the slope of phase 3; the VDalv/VDphys fraction; and the Fdlate, which was 8.2 3.3% vs 7.7 2.8%, respectively (p 0.000011). The diagnostic performance expressed as the mean area under a receiver operating characteristic curve comparison was 75.9 7.4% for the PaCO2-EtCO2 gradient and 87.6 4.9% for the Fdlate (p 0.02). Conclusion: Fdlate, a variable of VCap, had a statistically better diagnostic performance in suspected PE than the PaCO2-EtCO2 gradient. VCap is a promising computer-assisted bedside application of pulmonary pathophysiology. Future research should define the place of this technique in the diagnostic workup of PE, especially in the presence of positive d-dimers. (CHEST 2004; 125:841– 850)