Steady-state end-tidal alveolar dead space fraction and D-dimer: Bedside tests to exclude pulmonary embolism

Study objective: Less than 35% of patients suspected of having pulmonary embolism (PE) actually have PE. Safe bedside methods to exclude PE could save health-care resources and improve access to diagnostic testing for suspected PE. In patients with suspected PE, we sought to determine the sensitivity, specificity, and negative predictive value of (1) a steady-state end-tidal alveolar dead space fraction (AVDSf) of < 0.15, (2) a negative D-dimer result, and (3) the combination of a steady-state end-tidal AVDSf of < 0.15 and a negative D-dimer result. Study design: Prospective cohort study. Setting: Tertiary-care center in Ottawa, Ontario, Canada. Patients: Consecutive inpatients, outpatients, and emergency department patients with suspected PE referred to the Departments of Nuclear Medicine or Radiology for investigation of suspected PE. Interventions and measurements: All study patients had D-Dimer and alveolar dead space measurements prior to determining outcome (PE or no PE) with ventilation/perfusion scans and/or noninvasive leg vein imaging and/or pulmonary angiography. Results: Two hundred forty-six eligible and consenting patients underwent diagnostic imaging that excluded PE in 163 patients, diagnosed PE in 49 patients, and was indeterminant in 34 patients. A negative D-dimer result excluded PE with a sensitivity of 83.0% (95% confidence interval [CI], 69.2 to 92.4%), a negative predictive value of 91.2% (95% CI, 83.4 to 96.1%), and a specificity of 57.6%. A steady-state end-tidal AVDSf of < 0.15 excluded PE with a sensitivity of 79.5% (95% CI, 63.5 to 90.7%), a negative predictive value of 90.7% (95% CI, 82.5 to 95.9%), and a specificity of 70.3%. The combination of a negative D-dimer result and a steady-state end-tidal AVDSf of < 0.15 excluded PE with a sensitivity of 97.8% (95% CI, 88.5 to 99.9%), a negative predictive value of 98.0% (95% CI, 89.4 to 99.9%), and a specificity of 38.0%. Conclusion: This simple combination of bedside tests may safely rule out PE without further diagnostic testing in large numbers of patients with suspected PE.