c. Microaggregate Filters and Microemboli.
(1) Recently, there has been considerable interest in the particles that
develop in stored blood and are too small to be removed by standard blood filters. The
presence of these microaggregates can be measured by the screen filtration pressure
(SFP) which is the pressure required to force blood through a mesh filter with
20-micrometer pores. Patients who receive massive transfusions of older stored blood
have a progressive fail in SFP from central venous to arterial blood, suggesting that the
transfused microaggregates lodge in the pulmonary microcirculation. This has been
suggested as an etiologic mechanism of shock lung, with its attendant hypoxemia and
pulmonary microemboli; however, patients receiving massive transfusions usually have
many complicating factors and the exact role of these microaggregates in humans
(2) There are five different types of filters: (1) screen filtration, (2) dacron wool
filtration, (3) polyurethane foam filtration, (4) polyurethane foam plus nylon wool
filtration, and (5) screen filtration plus Dacron wool. Commercially available
microaggregate filters are composed of one or more of these materials. The relative
ability of these filters to trap microaggregates has not been established. As the filter
becomes saturated with microaggregates, the flow rate of blood through it decreases.
Thus, if a microaggregate filter is to be used, the selection of a specific brand is often a
matter of balancing the degree of microaggregate removal against the flow rate desired
through the filter and the frequency with which the filter can be changed. Most, if not all,
of these filters remove platelets and, therefore, are contraindicated for use in the
transfusion of fresh WB, platelet concentrates, or granulocyte concentrates. Other
hematologic parameters remain unchanged after passage of blood through
microaggregate filters; however, it seems reasonable that infusion of too many units
through the same filter, especially if pressure is used, may cause RBC hemoIysis.
(3) Microaggregate filters are not required for most transfusions. If the
blood bank physician feels that microaggregate filters are indicated, a general policy
can be established, such as instituting their use in a particular patient after a certain
number of transfusions within 12 or 24 hours. Microaggregate filters are routinely used
during cardiopulmonary bypass.
(1) Blood products should be administered intravenously, although other
routes (intra-peritoneal, intra-arterial, intrabone marrow) are possible. A vein should be
selected which will be large enough to accommodate the infusion needle, but is
comfortable for the patient. Veins In the anticubltal fossa are probably more accessible
and most widely used; however, infusion in these veins limits the patient's ability to flex
the elbow during transfusion. Veins on the forearm or hand are equally suitable for
infusion, although venipuncture in these areas is often more painful to the patient.