The Acute Pulmonary Embolism 1 (APE 1) Trial: Prospective Investigation of Scintigraphic Diagnosis and Pathophysiology of Right Heart Strain

INTRODUCTION This project assesses both scintigraphic diagnosis and the pathophysiology of right heart strain in acute pulmonary embolism. The primary purposes are to validate various scintigraphic methods with pulmonary angiography as the standard of reference (purpose A). Furthermore, this project examines the relative contributions of 1) vascular obstruction, 2) vasoactive mediators, and 3) co-morbidity to the generation of right heart strain (purpose B).

BACKGROUND FOR PURPOSE A Acute pulmonary embolism is a life threatening condition. Early intervention can be lifesaving but treatment is associated with severe side effects. Thus, establishing accurate diagnosis is necessary. Clinical presentation and physical examination have proven insufficient for this purpose. This is also true for basic diagnostic investigations, e.g. electrocardiography, arterial blood gas analysis and chest x-ray, which are considered standard tests in patients with symptoms consistent with cardiopulmonary disease.

For several decades lung scintigraphy has been considered a cornerstone in the diagnosis of acute pulmonary embolism. Planar two-dimensional images of the pulmonary perfusion are obtained using a gamma camera after intravenous injection of radiolabelled microparticles, depicting perfusion defects caused by e.g. a pulmonary embolus. Similarly, ventilatory function can be assessed two-dimensionally by planar scintigraphic imaging following inhalation of a radioactive gas or aerosol. Lung scintigraphy has certain limitations, primarily related to image interpretation. The PIOPED criteria, based on the combined ventilation-perfusion scintigraphy, have gained global acceptance, but are often inconclusive and give rise to misunderstandings. A different set of interpretation criteria, provided by the PISA-PED study group, are based solely on perfusion imaging. Opposite the PIOPED criteria, PISA-PED criteria are always conclusive and exhibits both high sensitivity and specificity. The latter have, however, not gained the same widespread acceptance as the PIOPED criteria. It remains uncertain which set of criteria is the better to confirm or exclude the diagnosis of acute pulmonary. A prospective trial evaluating both PIOPED criteria (original and revised) and PISA-PED criteria against a valid standard of reference therefore seems appropriate.

The more resource demanding single photon emission computed tomography (SPECT) has recently been subject to renewed interest in the diagnosis of acute pulmonary embolism, but it remains unclear whether this technique offers additional information compared to conventional planar imaging. SPECT offers three-dimensional images, but otherwise the technique is similar to the methods described for planar images. Combining the PISA-PED criteria and SPECT imaging has not yet been tested in a prospective trial, but it seems highly relevant.

Since the diagnosis is often suspected several times in the same patient, the ability to distinguish acute and chronic changes in the scintigrams are essential. It is, however, unclear to what extend and how fast the scintigraphic changes resolve. This has great implications in scintigraphic control of patients recovering from acute pulmonary embolism.