Capillary blockade involves the intentional microembolization of a capillary bed with particles, permitting external visualization of the perfusion of this capillary bed. This is achieved by the IV injection of a radiolabeled, precipitated, biodegradable macroaggregate of human serum albumin commonly known as Tc-99m MAA 34. Compared to the 7 µm diameter of the average capillary, at least 90% of the MAA particles are between 10-90 µm in size; none are >150 µm in their longest aspect.These are the legal particle size requirements as listed in the manufacturers' package inserts and in the current USP.
For an adult without known pulmonary hypertension, the ideal number of particles to be injected is 350,000 with a suggested range of 200,000-700,000. Even though these appear to be very large numbers, there is a very large margin of safety since fewer than 1/1,000 capillaries are blocked by the typical injection. For a patient with known pulmonary hypertension, the number of particles should be limited to 150,000. Figure 6 (for patients without pulmonary hypertension) and Figure 7 (for patients with pulmonary hypertension) indicate the correct reconstitution volume and activity for a Sn-MAA kit whose average number of particles per vial is known. Use of the graph in Figure 6 produces a Tc-MAA preparation with 350,000 particles on a 3 mCi dose at t0 and 700,000 particles on a 3 mCi dose at 6 hr, the shelf life of most commercially available preparations. Use of the graph in Figure 7 produces a Tc-MAA preparation with 150,000 particles on a 3 mCi dose at t0, as required for patients with pulmonary hypertension. It is virtually impossible to draw a dose for a neonate or very small child from a vial prepared for an adult. The requirement of 50,000 particles containing 0.5 mCi in a reasonable volume for injection can only be achieved by a special preparative method: a full vial of Sn-MAA is reconstituted with 5 ml of 0.9% NaCl solution, then 1 ml is removed aseptically, placed in a sterile evacuated vial, and labeled with the appropriate volume and activity of pertechnetate solution (dependent entirely upon the number of particles in a vial). Children's should be scaled down from 350,000 for a 70 kg child to 50,000 for a neonate (refer to figure 8). The tbiol of Tc-MAA is 5-12 hr, depending upon the manufacturer.
|Stephen Karesh, PhD.||
Last Updated: August 14, 1996