Not as good as one would hope!
In an often quoted study involving 768 patients with suspected PE who underwent angiography, a combination of normal A-a gradient (<20 mm Hg ), normal PaO2 (>80 mm Hg), and normal PaCO2 (>35 mm Hg) was examined to help exclude PE (1). Among patients with no known cardiopulmonary disease and normal values in all 3 parameters, over 30% still had PE, while among those with cardiopulmonary disease and normal parameters 17% had PE.
In short, normal arterial blood gases may make PE less likely, they do not by any means exclude the possibility of PE.
Reference
- Stein PD, Goldhaber SZ, Henry JW, et al. Arterial blood gas analysis in the assessment of suspected acute pulmonary embolism. CHEST 1996; 109:78-81. https://www.ncbi.nlm.nih.gov/pubmed/8549223
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