What are possible causes of unilateral decreases in lung perfusion with relatively preserved lung ventilation on a ventilation perfusion scan?
The most common cause of unilaterally decreased lung perfusion is a central mass lesion such as lung cancer, which has a predilection for affecting lung perfusion given the more readily compressible pulmonary vasculature compared to the more rigid bronchi. Patients with fibrosing mediastinitis, congenital disease such as pulmonary artery atresia or congenital corrected heart disease, or uncommonly massive unilateral PE can also have a similar pattern on a V/Q scan. This can also be observed in patients who are status post unilateral lung transplantation. Interestingly, in patients with COPD, the perfusion pressure in the pulmonary arteries may be relatively normal, and therefore perfusion in the transplanted lung in the immediate postoperative period may be at only 50% to 60% but will increase with time. On the other hand, in patients with pulmonary hypertension, the lung transplant may receive up to 90% of the perfusion in the early postoperative period.