Volumetric or time-based capnography for excluding pulmonary embolism in outpatients?

Abstract

Background: Volumetric capnography is technically more demanding but theoretically better than the time-based alveolar deadspace fraction (Paco2 - Et. co2)/. Paco2 as a bedside diagnostic tool for excluding pulmonary embolism (PE) in outpatients. Objective: We compared both diagnostic accuracy in patients with a suspected PE and positive D-dimer enzyme-linked immunosorbent assay results. Patients and methods: In this clinical multicenter trial with prospective inclusion and 3-month follow-up, alveolar deadspace fraction was compared by receiver operating characteristic (ROC) analysis with other parameters derived from volumetric capnography. Results: Capnography was performed in 239 patients, and 205 tests (86%) were conclusive. The incidence of PE was 33%. The alveolar deadspace fraction accuracy expressed with ROC curve analysis was 0.73 ± 0.04. The diagnostic performances of parameters from volumetric capnography were not significantly better. Sixteen per cent [95% confidence interval (CI) 12-21%] of patients presented a (Paco2 - Et. co2)/. Paco2 ratio under the cut-off value of 0.15, with a low clinical probability. This combination excluded PE, with a sensitivity of 96% (95% CI 89-99%) and a negative likelihood ratio of 0.17 (95% CI 0.09-0.33%). Conclusion: Volumetric capnography failed to show superiority to alveolar deadspace fraction measurements [(Paco2 - Et. co2)/. Paco2] for exclusion of PE in outpatients with positive D-dimer test results. Future studies should clarify the safety of excluding PE in patients combining low clinical probability with positive D-dimer results and (Paco2 - Et. co2)/. Paco2 ratios below the cut-off value of 0.15. © 2009 International Society on Thrombosis and Haemostasis.