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 gas
tests in the diagnosis of pulmonary embolism.

      Method:
      We measured alveolar-arterial partial pressure of oxygen
            [P(A–a)O2] gradient, PaO2 and arterial partial pressure
            of carbon diaxide (PaCO2) in 773 consecutive patients with suspected
            pulmonary embolism who were enrolled in the Prospective Investigative Study of
            Acute Pulmonary Embolism.


      Diagnosis:
      The study design required pulmonary angiography in all patients
            with abnormal perfusion scans.


      Results:
      Of 773 scans, 270 were classified as normal/near-normal and 503 as
            abnormal. Pulmonary embolism was diagnosed by pulmonary angiography in 312 of
            503 patients with abnormal scans. Of 312 patients with pulmonary embolism, 12,
            14 and 35% had normal P(A–a)O2, PaO2 and
            PaCO2, respectively. Of 191 patients with abnormal scans and
            negative angiograms, 11, 13 and 55% had normal P(A–a)O2,
            PaO2 and PaCO2, respectively. The proportions of patients
            with normal/near-normal scans who had normal P(A–a)O2,
            PaO2 and PaCO2 were 20, 25 and 37%, respectively. No
            differences were observed in the mean values of arterial blood gas data between
            patients with pulmonary embolism and those who had abnormal scans and negative
            angiograms. 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%) of
            409 patients without prior cardiopulmonary disease. Among patients with
            pulmonary embolism, there was no difference in arterial blood gas data between
            patients with and those without prior CPD.


      Conclusion:
      These data indicate that arterial blood gas tests are of limited
            value in the diagnostic work-up of pulmonary embolism if they are not
            interpreted in conjunction with clinical and other laboratory tests.