A 56-year-old man is evaluated for dyspnea and chest discomfort. Medical history includes hypertension, type 2 diabetes mellitus, and chronic kidney disease. Temperature is 37.8 C (100 F), blood pressure is 160/90 mm Hg, pulse is 98/min, and respirations are 24/min. BMI is 31 kg/m². The patient undergoes a 2-part nuclear medicine test. During the first part, he breathes in a radioactive tracer and a lung scan is performed. In the second part of the test, he is given an intravenous injection of a different radioactive material while breathing normal air, and the scan is repeated. When the images are compared, a large area of the lower right lung is not visualized in the second scan. Based on the observed findings, which of the following underlying disease processes is most likely present in this patient?

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11ec160b_3a19_98d6_9249_65c55e9dac96_MD0001_00 This patient, with dyspnea, chest discomfort, and a perfusion defect on ventilation-perfusion (V/Q) scan likely has a pulmonary embolism (PE). PEs are most commonly caused by deep vein thrombosis in the lower extremities that subsequently embolizes to the pulmonary vasculature. Signs and symptoms include acute-onset pleuritic chest pain, shortness of breath, tachycardia, and hypotension. CT pulmonary angiography is typically the imaging modality of choice; however, V/Q scans are helpful in evaluating patients in whom angiography is contraindicated (eg, contrast allergy, renal failure). V/Q scans are 2-part studies that compare regional ventilation and perfusion. The initial phase uses a radiolabeled aerosol that is inhaled and delivered throughout the tracheobronchial tree. The second phase uses an intravenous tracer that is distributed throughout the pulmonary vasculature. The images are then overlaid for comparison. Normal results show even distribution of both radionucleotide tracers throughout the lung (ie, V/Q match). V/Q mismatch occurs when alveoli are ventilated but not perfused (ie, a defect in the perfusion phase). This pattern can be seen in diseases that increase physiologic dead space, including pulmonary embolism (as in this patient) and malignancies that obstruct arterial blood flow. (Choices B, C, and E) Endobronchial neoplasms can cause obstruction of the airway. Left ventricular infarction can cause pulmonary edema. Exudative pleuritis causes a pleural effusion, often with compression and atelectasis of the adjacent lung. These diseases cause ventilation defects, typically with matched perfusion defects as the resulting alveolar hypoxia induces pulmonary vasoconstriction. However, this patient has isolated perfusion defects. (Choice D) Spontaneous pneumothorax would cause compression of the ipsilateral lung; V/Q scan would show decreased lung volume on ventilation and perfusion images compared to the normal lung. Educational objective: Ventilation-perfusion (V/Q) scans use radiotracers to compare the ventilation and blood perfusion of each area of the lung. V/Q mismatch with perfusion defects are often indicative of a pulmonary embolism, which are most commonly caused by deep vein thrombosis in the lower extremities that embolizes to the pulmonary vasculature. __________ References: Value of the ventilation/perfusion scan in acute pulmonary embolism. Results of the prospective investigation of pulmonary embolism diagnosis (PIOPED). (http://www.ncbi.nlm.nih.gov/pubmed/2332918) Computed tomographic pulmonary angiography vs ventilation-perfusion lung scanning in patients with suspected pulmonary embolism: a randomized controlled trial. (http://www.ncbi.nlm.nih.gov/pubmed/18165667)

A 56-Year-Old Man Is Evaluated for Dyspnea and Chest Discomfort