ALVEOLAR-ARTERIAL OXYGEN GRADIENT IN THE ASSESSMENT OF ACUTE PULMONARY-EMBOLISM

Purpose: The purpose of this investigation is to evaluate the utility of the alveolar-arterial (A-a) oxygen gradient in the diagnosis of acute pulmonary embolism (PE) among patients who participated in the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED). Methods: Pulmonary embolism was diagnosed (n=280) or excluded (n=499) by angiography in all patients. Patients were then categorized as (1) the entire cohort, (2) no prior cardiopulmonary disease and no prior PE, and (3) no prior PE or deep venous thrombosis. Normal values of the A-a gradient were defined in three ways: (1) values less than or equal to 20 mm Hg; (2) values less than or equal to age/4+4; and (3) values based on age from the literature. Results: When a normal A-a gradient was defined as less than or equal to 20 mm Hg, 11 to 14% of patients with PE in the three categories of patients had a normal A-a gradient. When the equation age/4+4 was used, 8 to 10% of patients with PE in the three categories of patients had a normal A-a gradient. With age-related values from the literature, 20 to 23% of patients with PE in the three categories of patients had a normal A-a gradient. The A-a gradient was normal in comparable percentages of patients who did not have PE. Conclusion: Normal values of the A-a gradient did not exclude the diagnosis of acute PE.