Diagnostic value of gas exchange tests in patients with clinical suspicion of pulmonary embolism

Objective To assess the value of parameters derived from arterial blood gastests in the diagnosis of pulmonary embolism. Method We measured alveolar-arterial partial pressure of oxygen[P(A-a)O2] gradient, PaO2 and arterial partial pressureof carbon diaxide (PaCO2) in 773 consecutive patients with suspectedpulmonary embolism who were enrolled in the Prospective Investigative Study ofAcute Pulmonary Embolism. Diagnosis The study design required pulmonary angiography in all patientswith abnormal perfusion scans. Results Of 773 scans, 270 were classified as normal/near-normal and 503 asabnormal. Pulmonary embolism was diagnosed by pulmonary angiography in 312 of503 patients with abnormal scans. Of 312 patients with pulmonary embolism, 12,14 and 35% had normal P(A-a)O2, PaO2 andPaCO2, respectively. Of 191 patients with abnormal scans andnegative angiograms, 11, 13 and 55% had normal P(A-a)O2,PaO2 and PaCO2, respectively. The proportions of patientswith normal/near-normal scans who had normal P(A-a)O2,PaO2 and PaCO2 were 20, 25 and 37%, respectively. Nodifferences were observed in the mean values of arterial blood gas data betweenpatients with pulmonary embolism and those who had abnormal scans and negativeangiograms. Among the 773 patients with suspected pulmonary embolism, 364 (47%)had prior cardiopulmonary disease. Pulmonary embolism was diagnosed in 151(41%) of 364 patients with prior cardiopulmonary disease, and in 161 (39%) of409 patients without prior cardiopulmonary disease. Among patients withpulmonary embolism, there was no difference in arterial blood gas data betweenpatients with and those without prior CPD. Conclusion These data indicate that arterial blood gas tests are of limitedvalue in the diagnostic work-up of pulmonary embolism if they are notinterpreted in conjunction with clinical and other laboratory tests.