Volumetric capnography no better than time-based methods for excluding PE
MedWire News: Volume-based capnographic parameters do not appear to improve the performance of the more readily obtained alveolar deadspace fraction for exclusion of suspected pulmonary embolism (PE) in outpatients with positive D-dimer test results, research shows.
Despite being technically more demanding, volumetric capnography is theoretically better at excluding PE in outpatients than the time-based alveolar deadspace fraction that corresponds to (CO2 partial pressure [PaCO2] – end-tidal CO2 [EtCO2])/PaCO2, say Frank Verschuren (Université catholique de Louvain, Brussels, Belgium) and colleagues.
In a prospective, multicenter trial with 3-month follow-up, Verschuren and team used receiver operating characteristic (ROC) analysis to compare the diagnostic accuracy of alveolar deadspace fraction with that of volumetric capnography in 239 patients with suspected PE and positive D-dimer enzyme-linked immunosorbent assay results.
The researchers report that the PE was diagnosed in 78 (33%) patients. The alveolar deadspace fraction accuracy, expressed as the area under the ROC curve (AUC), was 0.73. The diagnostic performance of volumetric capnography was not significantly better, with AUCs of between 0.69 and 0.74.
There were 38 (16%) patients who presented with a low clinical probability and a (PaCO2 – EtCO2)/PaCO2 ratio below the cutoff value of 0.15 for normal alveolar deadspace. This combination of findings excluded PE with a sensitivity of 96% and a negative likelihood ratio of 0.17.
From a clinical perspective, this finding suggests that capnography could decrease the need for spiral computed tomography or lung scintigraphy in about 16% of patients with suspected PE, remark Verschuren and co-authors in the Journal of Thrombosis and Haemostasis.
They conclude: “Volumetric capnography failed to show superiority to alveolar deadspace fraction measurements ([PaCO2 – EtCO2]/PaCO2) for exclusion of PE in outpatients with positive D-dimer test results.”
“The safety of ruling out PE in cases of low clinical probability and normal capnographic parameters in patients with positive D-dimer results needs further confirmation.”
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By Laura Dean