Findings: Large right-sided pleural effusion was seen on chest radiograph. A ventilation-perfusion scan with Tc-99m-DTPA (aerosolized) and Tc-99m-MAA demonstrated two matching defects in the right lower lung fields, with triple match due to the effusion. Another new mismatch was seen in the right lower lobe. Overall probability of pulmonary embolism was considered intermediate, but based on the clinical suspicion, higher probability was suspected. The patient was not a candidate for anticoagulation with heparin, and pulmonary angiogram was further obtained. The angiogram clearly confirmed the findings of V/Q scan to demonstrate pulmonary embolus in the right upper lobe.

Diagnosis: Pulmonary embolism (PE) in the right lower lobe.

Discussion: The most common indication for V/Q scans is diagnosis of suspected PE. This type of scan comprises perfusion and ventilation imaging. Perfusion lung scanning is based on the principle of capillary blockade. With Tc-99m-Macroaggregated particles slightly larger than the pulmonary capillaries, pulmonary segments with decreased or absent blood flow show diminished radioactivity. Normally, capillary perfusion and alveolar ventilation are matched in order to maximize gas exchange. Amended PIOPED criteria are adopted for classifying the probability of pulmonary embolism in a patient into classes such as high, intermediate, low, very low, and normal. Patients with intermediate probability scans have a significant risk of having PE. (20-79% probability, 35% prevalence). Patients with intermediate probability scans, and multiple risk factors or clinical findings suggestive of DVT should undergo Doppler ultrasound, which if negative, should lead to pulmonary angiography as the next step in management.

The current patient in consideration, having multiple risk factors, and strong clinical suspicion or PE, underwent V/Q scan. The chest x-ray showed large new right pleural effusion. Compared to his older V/Q scan, the presented V/Q scan demonstrated new large mismatch in the right lower lobe. Chest x-ray in corresponding areas did not offer an explanation for this mismatch. Given the other findings on chest x-ray and V/Q scan, the scan was considered difficult to classify as high, low, or normal, and was considered intermediate. Doppler ultrasound was negative for DVT. A pulmonary angiogram demonstrates a large segmental branch to the right lower lobe. Additional small thrombus in a subsegmental branch to the right lower lobe was also suspected. The catheter was lowered into the inferior venacava and a filter placed under fluoroscopy. Following the appropriate algorithm in this patient changed the clinical management in an efficient way.

References:
Gottschalk A, Sostman HD, Coleman RE, et al. Ventilation-perfusion scintigraphy
in the PIOPED study. Part II. Evaluation of the scintigraphic criteria and interpretations.
J Nucl Med 1993; 34:1119-1126.

Juni JE, Alavi A. Lung scanning in the diagnosis of pulmonary embolism: the emperor
redressed. Semin Nucl Med 1991: 21:281-296.

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