The chest radiograph is normal. Perfusion imaging demonstrates near-absence of perfusion to the entire right lung, except for a small area in the region of the anterior basal segment. Additionally the left lung has absent perfusion to the superior/anterior basal/lateral basal segments of the lower lobe, and decreased perfusion to the posterior basal segment. The ventilation images are normal (with the exception of xenon activity within the liver, an indication of steatosis).

These findings certainly meet PIOPED criteria for a high probability of pulmonary embolism (discussed separately), and such extensive mismatched perfusion defects are highly suspicious for central pulmonary embolism.

3 days after the exams above, this patient developed severe chest pain and underwent CTA of chest, abdomen, and pelvis (due to concern for aortic dissection).

CT images demonstrated saddle pulmonary embolus.

In the case of central pulmonary embolism, perfusion abnormalities on V/Q scan are often more variable than the peripheral wedge-shaped areas of decreased perfusion encountered with small emboli. These findings (such as areas of completely absent perfusion or decreased activity within an entire lobe/lung) may make orientation and localization of defects more difficult, and close correlation with ventilation images is necessary.

The diagnosis of massive pulmonary embolism is based on clinical/physiologic parameters (hypotension, cardiogenic shock, cardiac arrest), while submassive pulmonary embolism is seen in patients without these presentations but with evidence of right ventricular dysfunction. Piazza G, Goldhaber SZ. Circulation 2010;122:1124-1129. The ability to identify right heart dysfunction by CT parameters (e.g. RV/LV diameter ratio) is an advantage of CTA over V/Q scan in this setting. Lu MT, et al. AJR 2012;198:1353-1360.

See topic discussion: Diagnosis of Pulmonary Embolism