Articles on Medical Diseases and Conditions

Tag: Platelets

  • Massive Blood Transfusion

    Massive blood transfusion is defined by the AABB as replacement of the patient’s blood volume (equivalent to 8-10 units of whole blood in a 70-kg person) within 12 hours (some define the time period as 24 hours). Transfusion of such volumes presents special difficulties, depending on the substance being transfused and the rate of administration. By the end of a transfusion sufficient to replace the equivalent quantity of one blood volume, roughly one third of any substance originally in the patient’s blood will remain (range 18%-40%), predominantly because of dilution with transfused material. The most common and serious complication of massive transfusion is bleeding; the most common identifiable cause is platelet related, with factor deficiency next most frequent. Massive transfusion is sometimes complicated by disseminated intravascular coagulation (reported in 5%-30% of severely traumatized patients).

    Citrate anticoagulant may cause difficulty when large volumes of whole blood are given in very short periods of time. Citrate has calcium-binding activity that may lead to hypocalcemia and also has a potential depressant effect on the myocardium. Ionized calcium measurement is much more helpful to assess this possibility than total serum calcium. Citrate is metabolized in the liver. Most patients are able to tolerate a large amount of citrate if liver function is adequate. However, some patients have poor liver function. One investigator states that the average person could receive one unit of whole blood every 5 minutes without requiring calcium supplements. At the other extreme, some have used 1 ampule (1 gm) of calcium gluconate for every five units of citrated whole blood. Calcium chloride has four times as much calcium as calcium gluconate, so that dosage using calcium chloride must be reduced proportionately. The majority of investigators currently deemphasize need for calcium supplements, and use of packed RBCs considerably reduces the amount of transfused citrate. Temperature of transfused blood is ordinarily not a problem. However, large amounts of rapidly administered cold blood increase the possibility of ventricular fibrillation. Ordinarily, blood can be allowed to warm in room air; if this is not possible, one can use a water bucket (30°C) for unopened blood containers or special warming devices (37°C) through which the blood passes via tubing while being administered. Transfusion of blood at ordinary speed does not require warming. Coagulation factor deficiency must be considered during massive transfusion. This involves primarily the labile factors V and VIII. Factor V has a storage half-life of about 10-14 days (range, 1-16 days), and levels in stored whole blood may be 30% (of normal) or even less in stored whole blood at 21 days. Factor VIII has a half-life of about 5-10 days (range, 1-16 days), and the level in stored whole blood is reported to be 15%-30% at 21 days. Lesser amounts would be present in packed RBCs due to the decreased amount of plasma. In either case, transfused material would probably contribute some factor V and VIII activity to residual patient activity. Whereas some have advocated transfusing 1 unit of fresh blood (or fresh frozen plasma or cryoprecipitate) for every 5 units or 10 units of stored whole blood during massive transfusion, a 1984 Consensus Development Conference on Fresh-Frozen Plasma stated that there is no evidence that the prophylactic use of fresh frozen plasma is necessary unless documented coagulation defects due to factor V or VIII deficiency are present. Coagulation defects are manifest by abnormal bleeding or oozing (not accounted for by failure to ligate damaged blood vessels). One relatively frequent contributing problem is concurrent disseminated intravascular coagulation due to tissue damage from trauma or tissue hypoxia from blood loss.

    Platelets. Platelets devitalize rapidly on storage if they are not separated from RBCs. In fresh whole blood, platelets are about 60% effective at 24 hours and almost completely ineffective after 48 hours. Ordinary bank blood or packed RBCs, therefore, essentially have no functioning platelets, even though the platelet count may be normal. This may produce difficulty in massive transfusions using stored bank blood or packed RBCs, although there is usually no problem when administration takes place over longer periods of time. After one total blood volume replacement, the majority of patients still have platelet counts of roughly 100,000/mm3 or more due to the 35%-40% of original platelet count remaining plus some mobilization of platelets from the spleen and bone marrow. However, the platelet count may reach 50,000/ cu mm or less with not all of these platelets being functional. Nevertheless, the 1986 Consensus Conference on Platelet Transfusion Therapy noted that most patients undergoing massive transfusion do not bleed because of dilutional thrombocytopenia alone and recommended that evidence of clinically abnormal bleeding or oozing plus thrombocytopenia be present before transfusing platelets rather than prophylactic administration of platelets.

  • Platelets

    Platelets are supplied in units that are equivalent to the number of platelets in one unit of whole blood (about 5.5 x 1010). These are obtained as single platelet units from random-donor whole blood units or as multiple platelet units from a single donor by means of platelet apheresis. Platelets are stored at room temperature up to 5 (sometimes 7) days. One single-donor unit ordinarily is expected to raise the platelet count 7,000-11,000/mm3 per square meter of body surface (equivalent to 5,000-10,000/mm3 [10-25 x 109/L] in an average-size adult). It has been suggested that platelets should be transfused as soon as possible after collection to retain maximum function and that infusions should be rapid (10 ml/min). Some also suggest a microaggregate filter (usually 40-µ size) if platelet storage has been more than 24 hours from time of collection. Platelet concentrates prepared by ordinary methods usually contain some plasma (about 50 ml/platelet unit) and some WBCs. Platelets from donors who have taken aspirin within 72 hours of donation may have deficient platelet function.

    Platelet antigens. Platelets themselves contain various antigens, including ABO and HLA. Single-donor platelets can be transfused without typing (similar to random-donor platelets) or can be typed (usually ABO and HLA) for recipient compatibility before administration. Platelet ABO incompatibility usually has only a minor effect on donor platelet survival. HLA incompatibility may become a more serious problem. After repeated transfusions with random-donor nonmatched platelets about 50%-70% of patients (range, 10%-100%) become sensitized to platelet HLA antigens (or sometimes, to platelet-specific antigens), and these patients usually become refractory to additional platelet transfusions. Most can still be transfused with HLA-matched platelets. Siblings have the best chance of providing HLA-compatible blood, although nonsiblings often match adequately. Some institutions administer only HLA-matched platelets when long-term need for platelets is anticipated. However, the AABB currently recommends that patients receive non-HLA-matched platelets initially, with HLA-matched platelets reserved for those who become refractory. Some reports suggest that leukocyte depletion helps delay platelet sensitization. Some investigators perform a 1-hour and a 24-hour platelet count after transfusion. Low 1-hour recovery is said to suggest platelet antigen sensitization. When conditions that decrease platelet survival (e.g., fever, infection, or disseminated intravascular coagulation [DIC]) are present, the 1-hour count shows normal recovery but the 24-hour count is low. One report suggests that a platelet count 10 minutes after transfusion provides the same platelet values as the 1-hour count.

    Indications for platelet transfusion. Platelet transfusion can be therapeutic or prophylactic. Therapeutic transfusions are indicated when severe acute bleeding occurs and the patient is severely thrombocytopenic (<50,000 platelets/mm3 or µL). When the patient has thrombocytopenia but has very minor bleeding or is not bleeding, the question of prophylactic platelet transfusion may arise. The decision is usually based on the degree of risk and the type of disorder being treated. Until 1993, most authorities considered patients to be high-risk if their platelet counts were less than 20,000/mm3 or µL (some used a cutoff value of 10,000/mm3); moderate-risk patients (transfusion only if clinically indicated) were those with counts of 20,000-50,000mm3; and low-risk patients included those with counts over 50,000mm3. Based on more recent studies, investigators are now proposing 5,000mm3 (or µL) as the threshold “trigger” value for prophylactic platelet transfusion (rather than 20,000 or even 10,000). The bleeding time has also been used as a guide to therapy; a bleeding time value of less than twice the upper limit of the reference range would not ordinarily need platelet transfusion. In patients with conditions that require multiple platelet transfusions over relatively long time periods, an additional consideration is the probability of developing antiplatelet antibodies that would interfere with therapy if actual bleeding developed later. In idiopathic thrombocytopenic purpura, antiplatelet antibodies that destroy donor platelets are already present, so that transfusion is useless unless the patient is actively bleeding. In drug-induced thrombocytopenia, transfusion is useless if the drug is still being given; after medication has been discontinued, transfusion can be helpful since a normal platelet count will usually return in about 1 week and transfused platelets survive about 1 week.

    Platelet concentrates given to bone marrow transplant patients, severely immunodeficient or immunosuppressed patients, and blood relatives of the donor should have the platelet unit irradiated with at least 25 Gy to avoid graft-vs.-host disease.