Why is it safe to block a portion of the pulmonary circulation with MAA particles in patients with suspected pulmonary emboli?

Blocking a portion of the pulmonary circulation with MAA (macroaggregated albumin) particles is considered safe primarily because this procedure typically obstructs only a small number of pre-capillary arterioles. MAA particles are administered during scanning for diagnosing pulmonary embolism, where a limited area of blood flow is intentionally occluded to visualize the perfusion of the lungs.

The occlusion occurs mainly in the terminal arterioles, and this localized blockage allows for a diagnostic image without significantly compromising lung function. Since the lungs have a dual blood supply (from both the pulmonary and bronchial circulations), this means that blocking small arterioles does not lead to substantial respiratory distress or significant impairment. The remaining circulating blood can still provide proper gas exchange in other areas of the lung.

This method is designed to provide critical imaging information while maintaining overall lung perfusion and function. The choice of using MAA particles and their size is meticulously calibrated to ensure that only a limited area is impacted, safeguarding patient health while aiding in accurate diagnosis.