title: Use of the alveolar dead space fraction (Vd/Vt) and plasma D-dimers to exclude acute pulmonary embolism in ambulatory patients.
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 ultrasonography, 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., < 0.2) in 3/26 PE+ patients and D-dimer concentrations were normal (< 0.5 microgram/L) in 4/26 patients in the PE+ group. The combination of a normal Vd/Vt and D-dimer concentration was 100% sensitive (95% CI = 88-100%) in excluding PE. False-positive testing (either test positive) occurred in 49/144 subjects (specificity 65%, 95% CI = 52-73%). The age-adjusted alveolar-arterial O2 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 < 0.2 and D-dimers < 0.5 microgram/L lowers the probability of acute PE.
authors.affiliation: Department of Emergency Medicine, Carolinas Medical Center, Charlotte, NC 28232-2861, USA. jkline@carolinas.org
authors.forename: J A
authors.initials: JA
authors.lastname: Kline
authors.forename: S
authors.initials: S
authors.lastname: Meek
authors.forename: D
authors.initials: D
authors.lastname: Boudrow
authors.forename: D
authors.initials: D
authors.lastname: Warner
authors.forename: S
authors.initials: S
authors.lastname: Colucciello
mesh_terms: D000208:Acute Disease; D000328:Adult; D000368:Aged; D000553:Ambulatory Care; D001784:Blood Gas Analysis; D003430:Cross-Sectional Studies; D003937:Diagnosis, Differential; D005260:Female; D005338:Fibrin Fibrinogen Degradation Products / Q000032:analysis*; D006801:Humans; D008168:Lung / Q000000981:diagnostic imaging; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D011655:Pulmonary Embolism / Q000175:diagnosis*; D012372:ROC Curve; D011877:Radionuclide Imaging; D012126:Respiratory Dead Space; D012129:Respiratory Function Tests; D012680:Sensitivity and Specificity; D013668:Technetium Tc 99m Aggregated Albumin; D016284:Technetium Tc 99m Pentetate; D014692:Ventilation-Perfusion Ratio
chemical_list: D005338:Fibrin Fibrinogen Degradation Products; D013668:Technetium Tc 99m Aggregated Albumin; C036309:fibrin fragment D; D016284:Technetium Tc 99m Pentetate