Findings: The patient's doppler ultrasound examination of left lower extremity demonstrated a thrombus in his left popliteal vein. Venous flow in his superficial femoral vein was compromised with minimal respiratory variation. About 12 hours later, while on intravenous heparin, the patient complained of respiratory distress and a Ventilation-perfusion (V-Q) lung scan demonstrated mismatched perfusion defects involving the lower and middle lobes of the right lung as well as the lingula. No corresponding ventilation defects were present.

Diagnosis: Deep Venous Thrombosis (DVT) and High Probability Pulmonary embolism (PE).

Discussion: Pulmonary emboli cause 120,000 deaths per year in the United States. Risk factors for PE include immobilization for greater than seventy-two hours, recent hip surgery, cardiac disease, malignancy, estrogen use, and a history of prior deep venous thrombosis (DVT). Possible clinical findings include chest pain, tachypnea, dyspnea, rales, cough, tachycardia, hemoptysis, fever, diaphoresis, cardiac gallop, syncope, and phlebitis. PE's are most commonly sequelae of DVTs. However, other types of embolism include fat embolism in skeletal trauma or surgery, tumor embolism, amniotic fluid embolism, talc embolism in intravenous drug abusers, mercury embolism in thermometer accidents, air embolism, and parasites, especially schistosomiasis.

The goal of diagnostic imaging is, of course, to identify pulmonary emboli when they are present and to direct therapy. Patients diagnosed with a PE undergo therapy to prevent subsequent PE. Chest radiographs are not accurate in diagnosing PE but serve in diagnosing conditions which may mimic PE such as pneumonia, rib fracture, and pneumothorax. They are also necessary for comparison when interpreting a lung scintigram. According to the PIOPED criteria, two or more large perfusion defects or the arithmetic equivalent with normal ventilation and no chest radiograph abnormalities in the corresponding areas are required to interpret a V-Q scan as high probability for the presence of pulmonary embolus. A large defect is defined as greater than 75% of a pulmonary segment. Two moderate defects (25 - 75% of a segment) equal one large defect. High probability V-Q scans have a probability of greater than or equal to 80% for the presence of a PE, regardless of clinical assessment. When there is a high clinical suspicion of PE and a high probability V-Q scan, the probability of a PE being present is 95. Current inclination in face of a documented DVT, is to obtain a V-Q scan as a "baseline" regardless of the patient's respiratory status, so that should it decline later, one can compare and be more confident about the probability of pulmonary embolism in the acute setting.

References:
Lowe VF, Sostman HD. Pulmonary Embolism. In: Murray IPC, Ell PJ (Eds):
Nuclear Medicine in Clinical Diagnosis and Treatment, Vol. 1. Edinburgh;
Churchill Livingstone; 1994:29-45.

Worsley DF, Alavi A. Comprehensive Analysis of the Results of the PIOPED Study.
Prospective Investigation of Pulmonary Embolism Diagnosis Study. J Nucl Med Dec 1995; 36(12):2380-2387.

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