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.