Ventilation/perfusion mismatch caused by positive pressure ventilatory support.

In a patient with lobar atelectasis who was on positive pressure ventilatory support, ventilation and perfusion images showed absent ventilation and normal perfusion (reverse mismatch) in the region of the atelectasis and normal ventilation and decreased perfusion (true mismatch) not caused by pulmonary embolism in another lung zone. We report this case to emphasize that the lung scan findings in patients on positive pressure ventilatory support be carefully interpreted for the diagnosis of pulmonary emboli.