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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.

      Key words

      Abbreviations:

      CI (confidence interval), ELISA (enzyme-linked immunosorbent assay), EtCO2 (end-tidal CO2), Fdlate (late dead space fraction ExpCO2 15%), TLC (extrapolated CO2 partial pressure at an exhaled volume of 15% of the estimated total lung capacity), PE (pulmonary embolism), ROC (receiver operating characteristic), TLC (total lung capacity), VCap (volumetric capnography), VDalv (alveolar dead space), VDalv/Vtalv (alveolar dead space fraction), VDaw (airway dead space), VDphys (physiologic dead space), / (ventilation-perfusion ratio), Vt (tidal volume)
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