CLINICAL SCENARIO
You are a medical consultant asked by a surgical colleague to see a 78-year-old woman, now 10 days after abdominal surgery, who has become increasingly short of breath over the last 24 hours. She has also been experiencing what she describes as chest discomfort, which is sometimes made worse by taking a deep breath (but sometimes not). Abnormal findings on physical examination are restricted to residual tenderness in the abdomen and scattered crackles at both lung bases. Chest roentgenogram reveals a small right pleural effusion, but this is the first roentgenogram since the operation. Arterial blood gases show a Po2 of 70 mm Hg, with a saturation of 92%. The electrocardiogram shows only nonspecific changes.You suspect that the patient, despite receiving 5000 U of heparin twice a day,