The effect of Tc-99m-macroaggregated albumin administration on pulmonary vascular resistance

For patients with pulmonary hypertension, reduced administered activity of Tc-99m-macroaggregated albumin (MAA) is recommended for ventilation/perfusion scintigraphy. Although it is stated that MAA particles embolise in pulmonary capillaries, of which there are ~300 billion, they embolise in pulmonary arterioles, of which there are far fewer. This aim of this study, therefore, is to address the recommendation's validity. Using morphometric data of Horsfield, we estimated pulmonary vascular resistance (PVR) increments resulting from the administration of 100, 200, and 400 k MAA particles. We assumed two particle size distributions: one skewed left toward small particles and the other skewed right. MAA-induced PVR increments were also estimated for pulmonary vascular beds depleted by disease. Administration of 100, 200, and 400 k particles with left-skewed size distribution increases PVR by 2.7, 5.7, and 12.2%, respectively. Corresponding right-skewed values are 6.2, 13.5, and 32.4%. Following 25, 50, and 75% pulmonary vascular ablation, 200 k left-skewed particles increase PVR by 7.8, 12.2, and 29%, respectively, and right-skewed by 19, 32, and 109%, on top of PVR already increased from disease. Particle size distribution is important. Less than 200 k MAA particles should be administered to patients with pulmonary hypertension.