Study objective: To compare the diagnostic performance of volumetric capnography (VCap), which is the plot of the expired CO_{2} partial pressure against the expired volume during a single breath, with the Paco_{2} to end-tidal CO_{2} (EtCO_{2}) 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) Paco_{2}-EtCO_{2} 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 Paco_{2}-EtCO_{2} 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 Paco_{2}-EtCO_{2} 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.