The most appropriate diagnostic test to perform next is a ventilation-perfusion (V/Q) scan. This patient's presentation strongly suggests chronic thromboembolic pulmonary hypertension (CTEPH), for which V/Q scanning is the preferred and most sensitive diagnostic imaging study. CTEPH usually presents with progressive exertional dyspnea that correlates with the extent of hemodynamic derangement and right ventricular involvement. Patients with CTEPH may have findings of pulmonary hypertension and cor pulmonale and invariably have defects on V/Q scan. This patient's cardiac examination, with an accentuated pulmonic component of S₂, is compatible with pulmonary hypertension, as are the findings on echocardiography. Additionally, her clear lung fields, hypoxia, and findings on pulmonary function testing are consistent with a diffusion defect. If she has an abnormal V/Q scan, right heart catheterization with pulmonary angiography is required to confirm the diagnosis and characterize the degree of thrombosis, which may help guide therapy. The absence of deep venous thrombosis on venous studies of the legs is insufficient to rule out recurrent pulmonary emboli. V/Q scanning can also differentiate CTEPH (group 4 pulmonary hypertension) from pulmonary arterial hypertension (group 1 pulmonary hypertension), which is the other major diagnostic consideration in this patient.

Although CT angiography can identify proximal pulmonary arterial webs and luminal narrowing when present, it is less sensitive than V/Q scanning in detecting CTEPH. Furthermore, in this patient with clear lung fields on chest radiograph, CT scanning is unlikely to provide additional diagnostically useful information about the pulmonary parenchyma.

Pulmonary hypertension of this severity is typically associated with severe obstructive sleep apnea and marked sleep-related hypoxemia. There is little evidence suggesting sleep-disordered breathing in this nonobese woman; therefore, polysomnography is not indicated at this time.

In operative candidates, CTEPH is best managed surgically with pulmonary thromboendarterectomy. Pulmonary angiography is performed preoperatively to assess the extent and distribution of thrombus. It is not, however, an appropriate test during the initial evaluation.