Other Pulmonary Lesions visible on V/Q Scan
Pulmonary Infarction:
Pulmonary infarction is an uncommon complication of PE because of the bronchial circulation. The perfusion scan defect associated with infarction is usually larger than the chest x-ray consolidation. Frank infarction must be differentiated from transient radiographic opacities, which are common, and are most likely due to intraparenchymal hemorrhage associated with the embolism.
Pulmonary Arterial Hypertension:
(See also discussion in Chest textbook)
Perfusion is typically heterogeneous, with small peripheral defects, however, the scan may also be normal. Recently, the presence of reverse mismatches have been described in patients with primary pulmonary hypertension. A reverse mismatch represents a failure of hypoxic vasoconstriction. A reverse mismatch is defined as an area of lung which has absent or diminished ventilation, but is perfused normally, or to a greater degree than the remainder of the lungs. If extensive, this finding is of great physiologic importance as blood shunted through poorly ventilated lung tissue can be a major cause of hypoxia. This finding is felt to reflect an inadequate hypoxic vasoconstriction reflex and may be related to lack of vessel responsiveness secondary to replacement of medial hypertrophy by intimal hyperplasia in late stages of the disorder. In patients with primary pulmonary hypertension HRCT demonstrates a characteristic mosaic attenuation pattern. Areas of reverse mismatch on the V/Q scan have been shown to correlate with areas of increased attenuation on HRCT [AJR, Apr 1995, p. 831-835].
Fat Emboli:
(See also discussion in Chest textbook)
In this condition, small fat globules from disrupted marrow lodge in the lungs and produce edema and alveolar hemorrhage. There is generally a heterogeneous perfusion pattern observed with multiple small to moderate perfusion defects scattered throughout both lungs, but segmental perfusion defects are rarely present. Unfortunately, the lung scan is not very useful in diagnosing fat emboli but can be used to exclude thromboemboli in these patients.
Hepatopulmonary Syndrome:
(See also discussion in the Chest textbook)
Hepatopulmonary syndrome is a cause of hypoxia in about one third of patients with decompensated cirrhosis. In the condition, there is dilatation of the pulmonary capillaries. MAA particles will pass through these dilated capillaries producing a pattern characteristic of a right to left shunt.
