Use of the Alveolar Dead Space Fraction (Vd/Vt) and Plasma D‐dimers to Exclude Acute Pulmonary Embolism in Ambulatory Patients

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Abstract
Objective: To evaluate the utility of a modified calculation of the alveolar dead space fraction (Vd/Vt), combined with plasma D‐dimers, to aid in the exclusion of acute pulmonary embolism (PE). Methods: A prospective comparison of screening modalities was performed in a metropolitan teaching ED. Ambulatory patients evaluated for PE underwent simultaneous end‐tidal CO2 and arterial blood gas determinations, as well as venous latex‐agglutination D‐dimer quantification. The modified Bohr equation was used to calculate Vd/Vt as an index of alveolar dead space. Acute PE was diagnosed or excluded using appropriate combinations of clinical suspicion, ventilation‐perfusion lung scanning, lower‐extremity venous Doppler ul‐trasonography, pulmonary angiography, and comprehensive follow‐up. Results: Of 170 subjects studied, PE was confirmed (PE+) in 26 (15%) and excluded (PE‐) in 144 (85%). In the PE+ group, Vd/Vt was 0.31 ± 0.13 (mean ± SD), and in the PE‐ group, Vd/Vt was 0.06 ± 0.10 (p< 0.05, t‐test). Regarding false‐negative rates, Vd/Vt was normal (i.e., 2 gradient was 33 ± 38 torr in the PE+ group vs 13 ± 37 torr in the PE— group (p = 0.11). Conclusions: In ambulatory patients, the finding of Vd/Vt and D‐dimers <0.5 μg/L lowers the probability of acute PE.