Transient right-to-left shunt in massive pulmonary embolism

A 56-yr-old man, two months after an operation for an acoustic neurinoma, g radually developed dyspnea. Massive pulmonary embolism (MPE), with a signif icant right-to-left (R-L) shunt, was seen in a perfusion scan of the lungs with Tc-99m MAA. Radioactivity was noted in the thyroid, spleen, kidneys an d brain. A cardiac ultrasound study did not reveal intracardiac shunting. A few days later, when the patient's condition improved, another perfusion s can of the lungs did not show the shunt, whereas a subsequent digital subtr action angiographic study confirmed the diagnosis of MPE but failed to reve al the cause of the shunt. In the absence of any possible pathophysiologica l mechanism, to explain the observed R-L shunt, we deduce that the particle s of Tc-99m MAA might have passed through the precapillary pulmonary arteri ovenous anastomoses and/or through dilated pulmonary capillaries, as a resu lt of highly increased pulmonary vascular pressure due to MPE.