Blood Component and Pharmacologic Therapy for Hemostatic Disorders

Red Blood Cells

RBCs, commonly referred to as packed cells , may be prepared by whole blood donation or through apheresis procedures. Although not typically considered a hemostatic component, RBCs may contribute to normal clot formation, or perhaps more accurately, a decrease in RBCs may contribute to a bleeding tendency. Template bleeding time is prolonged as hematocrit falls, and RBC transfusions alone have been shown to improve hemostasis in patients with uremia and chronic anemia. The mechanism for this effect is not known but is likely multifactorial and may involve RBC mechanical properties affecting viscosity and blood flow, hemostatic activation by RBC microparticles, ability of RBCs to aggregate and adhere to each other and to vascular endothelium, molecular signaling via RBC surface proteins, including blood group antigens, participation in nitric oxide metabolism, and movement of platelets toward the vessel wall with increasing intravascular RBC mass.

A meta-analysis of studies comparing liberal with restrictive transfusion practices reported no benefit for a higher hemoglobin threshold to improve hemostasis. However, the adverse association of hematocrit with bleeding is greatest at hemoglobin concentrations of less than 6 g/dL, a level hardly ever accepted in these trials. RBC transfusions may produce rapid hemostatic effects while restoring oxygen-carrying capacity; however, the liberal transfusion thresholds may be too low to demonstrate these effects. Administration of the erythroid lineage cytokine erythropoietin (EPO) has become standard therapy for treating anemia and raising hemoglobin concentration in patients with renal disease and chronic conditions. Although EPO administration may have some effects on platelet reactivity, the improvement in baseline hematocrit that occurs during EPO administration may account for the reduced incidence of bleeding in patients with uremia. RBC transfusions provided to maintain hemoglobin concentrations higher than 9 g/dL produce no significant increment in hemostasis. At the other end of the spectrum, both hemorrhage and thromboembolic disease have been reported with elevated red cell mass. Little is currently understood as to the exact mechanism, risk factors, and best approach to prevent them. For example, about 8% of patients with polycythemia vera present with, and a similar percentage eventually develop, bleeding episodes. Whereas hemostatic defects may result from clones of abnormal platelets and leukocytes, elevated blood viscosity and sluggish blood flow likely play a role as well, since hemostatic defects are not uncommon in other syndromes of erythrocytosis such as high-altitude polycythemia and cyanotic heart disease; phlebotomy applied judiciously is standard management.

Exclusive reliance on RBC replacement in the rapidly bleeding patient may induce a hemostatic defect. Stored RBCs contain few functional platelets and as little as 5% to 10% plasma, depending on the preservative solution and method of collection used. Goal-directed therapy using serial testing for platelet count and screening coagulation assays such as prothrombin time (PT), partial thromboplastin time (PTT), and in some centers viscoelastometry are used to guide component replacement during “massive transfusion,” conventionally defined as transfusion that exceeds one blood volume within 24 hours. However, the approach to massive transfusion/fluid resuscitation has been changing, particularly in the setting of trauma where an unpredictable and highly lethal hemostatic disorder termed “trauma-associated coagulopathy” (TAC) may develop. Early balanced transfusion therapy is currently recommended for the massively bleeding trauma patient, as it may prevent the onset of microvascular hemorrhage. A proactive approach on the part of the transfusion service as part of “damage control resuscitation,” early administration of blood components in a balanced ratio such as 1 : 1 : 1 for units of plasma to platelets to RBCs, that closely approximates whole blood, can improve survival even if such strategies do not prevent or correct TAC, and minimize the need for blood and crystalloid fluids (see Chapter 40 ).

Platelets

Platelet concentrates may be prepared by pooling platelets obtained through centrifugation from individual units of whole blood. A “unit” of platelets has been defined as containing at least 5.5 × 10 ¹⁰ platelets—hence the term “six pack,” which describes an often prescribed dose of 3.3 × 10 ¹¹ . Recent trends in blood center collection procedures have resulted in the creation of most platelet products by single-donor apheresis, with the “dose” expressed by dividing the measured platelet content of the product by 3.0 × 10 ¹¹ . These are arbitrary designations. Assessment of clinical response according to the dose of platelets administered and a posttransfusion platelet count measured ideally within 1 hour of transfusion should be performed in all patients to guide further therapy (see later discussion).

Apheresis or single-donor collections result in fewer donor exposures for a given dose of platelets. Apheresis platelets may contain 200 to 250 mL of donor plasma, but heat-labile clotting factors decay rapidly at a storage temperature of 22°C (71.6°F). It has been surprisingly difficult to document an advantage of single-donor apheresis platelets over pooled random donor components, although one suspects that alloimmunization, bacterial contamination, and virus transmission all should be reduced. Pathogen-inactivated apheresis platelets have been licensed in the United States to reduce the risk of transfusion-transmitted infections.

Platelets are transfused for prophylactic and therapeutic indications. Prophylactic transfusion triggers remain controversial. Previous recommendations using a trigger of 20,000 platelets/µL for patients in stable condition were based on estimates of bleeding in children with leukemia, many of whom had received aspirin before recognition that aspirin is a powerful antiplatelet agent. Subsequent studies, although avoiding aspirin, indicate that far lower platelet numbers are not dangerous. However, relying on a therapeutic transfusion strategy alone may not be safe, at least for some patients, and may result in an increased overall risk of hemorrhage and of cerebral bleeding. Most clinicians now administer platelet transfusions prophylactically to nonbleeding patients in stable condition who have amegakaryocytic thrombocytopenia caused by chemotherapy and/or leukemia at platelet counts of less than 5000/µL, the number calculated necessary to maintain the integrity of the vascular endothelium. A trigger of 20,000 platelets/µL may be more prudent for patients who are febrile, have rapidly falling counts, or have evidence of additional hemostatic defects. Platelet counts of at least 50,000/µL may be more reassuring to the clinician when invasive procedures such as endoscopy, lumbar puncture, and bronchoscopy are anticipated. Meager evidence suggests that higher counts reduce morbidity and mortality. Clinical indications for therapeutic platelet transfusions are controversial and should be based on the patient's clinical condition, the cause of bleeding, and the number and function of circulating platelets (see Chapter 7 ).

Bleeding caused by platelet defects acquired after cardiopulmonary bypass (CPB) surgery or aspirin ingestion often responds to platelet transfusion; oozing related to uremia does not respond in this way, because transfused platelets rapidly acquire the uremic defect. Evolving algorithms for platelet transfusion in surgical settings based on point-of-care testing of platelet count and function have the potential to improve patient care and blood product utilization.

Most platelet transfusions are administered to patients with defects in platelet production or function. However, some patients with immune-mediated thrombocytopenia may have satisfactory responses to platelet transfusions. Platelet survival generally is no longer than a few hours. Therefore patients rarely benefit from prophylactic transfusions, although therapeutic transfusions may be lifesaving. Platelet transfusions are ordinarily considered to be contraindicated in patients with thrombotic platelet destruction, such as those with TTP, heparin-induced thrombocytopenia (HIT), and possibly disseminated intravascular coagulation (DIC). However, a retrospective analysis of 110 patients with the clinical diagnosis of TTP, 55 of whom had ADAMTS13-deficient TTP, and 23 of whom received platelet transfusions, showed no evidence of increased adverse events associated with these transfusions. Whereas an analysis of a nationally representative hospital discharge database suggested an increased frequency of arterial thrombosis and in-hospital mortality when platelets are transfused to patients with such consumptive disorders as HIT and TTP, the classical teaching that platelet transfusions are contraindicated in these circumstances should not prevent their administration when patients are at risk of life-threatening hemorrhage.

Platelet transfusions should be monitored through baseline and posttransfusion platelet counts. Some physicians prefer to standardize this evaluation by calculating a “corrected count increment” (CCI) as follows:

where (platelet count) _pre and (platelet count) _post are the platelet counts before and after transfusion, respectively.

A CCI above 4000 to 5000 platelets/µL suggests an adequate response to platelet transfusion, although two consecutive poor CCIs in the absence of fever, splenomegaly, active bleeding, consumption, ABO incompatibility, or other causes associated with increased platelet destruction suggest refractoriness to therapy. Development of platelet immune refractoriness presents a vexing clinical problem. Many patients can be treated well with platelets from human leukocyte antigen (HLA)–compatible relatives or even unrelated matched donors. When such donors are unavailable, treatment by HLA “best matching” or platelet cross-matches should be tried. These strategies may be cumbersome and expensive, and are not uniformly effective. A computer-assisted matching program that can identify epitope-matched platelets is proving increasingly helpful. Repeated platelet transfusion in the absence of documented count increments exposes patients to the potential risks of transfusion without evidence of any benefit and depletes an often scarce and costly blood resource.

Pathogen-reduced platelet concentrates are now licensed and available, although increased cost of this component has slowed widespread introduction. This component all but eliminates the risk of bacterial contamination of platelets stored at room temperature and dramatically reduces the risk of most viral agents—most notably human immunodeficiency virus (HIV), hepatitis B virus (HBV), and hepatitis C virus (HCV). Although platelet increments from these components may be modestly reduced, clinical studies have confirmed equivalent hemostasis and safety.

Fresh Frozen Plasma

FFP is prepared by freezing the plasma component of a unit of whole blood within 6 to 8 hours of collection. Hemostatic activity of the coagulation factors is maintained even after storage for 1 year or longer, depending on the storage temperature. Once thawed, the plasma can be stored at a refrigeration temperature for no longer than 24 hours. On average, 1 mL of FFP contains 1 unit of each coagulation factor. However, the volume of a “unit” of FFP, the concentration of the citrate anticoagulant, and the concentration of individual donor coagulation factors are variable, depending on the donor blood composition, the method of collection, and the anticoagulant solution used. Although for an individual donor the factor concentrate levels may vary from 50% to 150% of 1 unit per mL, the differences are usually unimportant except for infants and small children, where single units may be transfused.

FFP remains the blood component of choice for treating patients with deficiency of factor II, V, X, or XI who require treatment. FFP is commonly used to treat bleeding patients with acquired deficiencies of multiple coagulation factors, such as those with DIC, dilutional coagulopathy, and TTP. The rationale and use of FFP for patients with prolonged PT and PTT assays appears to be decreasing with the increased understanding of the coagulopathy of liver disease, lack of proven efficacy, and growing appreciation of the risks of plasma infusion (see Chapter 36 ). FFP may also be used for rapid yet temporary reversal of the coagulation defects induced by vitamin K antagonists, although administration is limited by the large volumes required (see later discussion); these patients may require vitamin K therapy for long-term control, and those with cerebral hemorrhage may benefit from the use of rFVIIa or the higher concentration of vitamin K–dependent factors contained in PCCs. Plasma is not effective in reversing anticoagulation induced by heparin or the direct oral anticoagulants (DOACs).

Depending on the cause, mild prolongations of PT and PTT (<1.5 times the midpoint of the normal laboratory range) do not mandate FFP prophylaxis in trauma patients or in those scheduled to undergo elective invasive procedures (see Chapter 34 ). FFP transfusion in critically ill patients has limited efficacy and is associated with significant morbidity—in particular, pulmonary edema and acute lung injury. Indiscriminate transfusion of FFP for mild, clinically insignificant prolonged PT may result in unnecessary allergic reactions and delays in diagnostic procedures. No clinical benefit has ever been documented.

Several other plasma components are available. Plasma stored for 24 hours before freezing (SP-24) is technically not FFP, but in practice is used interchangeably. A psoralen and ultraviolet light treated pathogen-inactivated single donor FFP has been licensed, but is not yet widely used in the United States. Solvent detergent (SD) plasma is FFP that has been pooled and treated to inactivate lipid-encapsulated viruses such as HIV, HBV, and HCV. Both inactivated products are several-fold more expensive than FFP. SD plasma is now available in the United States and Canada (Octaplas), and has been widely used in Europe for years. SD plasma has a more uniform concentrations of coagulation factors and reduced concentrations of high-molecular-weight von Willebrand factor (VWF)—a potential advantage for treatment of TTP but a potential disadvantage in other hemostatic disorders. SD plasma also has reduced concentrations of protein S and α ₂ -plasmin inhibitor (α ₂ -PI). Use of SD plasma in patients in stable condition, in critically ill neonates, in women with obstetric and gynecologic emergencies, and in patients with liver disease appears safe and improves abnormal coagulation test results. Consensus recommendations for SD plasma include patients who require a high volume of plasma transfusions, such as those with TTP or hemolytic uremic syndrome (HUS) with associated factor H deficiency; patients with clotting factor deficiencies for which specific licensed concentrates may not be readily available (factors V, XI, XIII) who have experienced allergic reactions to FFP; and patients who require plasma infusion but for whom blood group compatible products are not available in a timely manner. The advantages of a product that is free of the major class of transfusion-transmitted viruses must be weighed against the potential disadvantages of trading a relatively safe single-donor component for one made from larger pools that may contain nonencapsulated viruses such as hepatitis A virus and parvovirus B19. Transfusion related acute lung injury (TRALI) has not been reported after SD plasma administration.

Cryoprecipitate

Cryoprecipitate is the cold insoluble fraction formed when FFP is thawed at 4°C (39.2°F). “Cryo” is rich in factor VIII, factor XIII, VWF, and fibrinogen. The product can be stored frozen at −20°C (−4°F) for up to a year. When resuspended in a plasma volume of 10 to 20 mL after preparation from single-donor plasma, cryoprecipitate contains 80 to 100 U of factor VIII/VWF, which represents 40% to 70% of the original amount in the plasma; 100 to 250 mg of fibrinogen; and approximately 30% of the original amount of factor XIII. Because of its higher concentration of these factors compared with FFP, cryoprecipitate served for decades as the primary replacement therapy for patients with hemophilia A and VWD. Cryoprecipitate has been replaced by DDAVP (see later discussion) and by virus-inactivated or recombinant preparations for first-line therapy for most mild cases of either disease, and it should no longer be used for these purposes unless other therapies are not available. Similarly, the use of cryoprecipitate to treat uremic bleeding (10 units per treatment) has largely been supplanted by treatment with DDAVP, estrogen, RBC transfusion, or EPO. The use of cryoprecipitate as a source of fibrinogen for “home-brewed” fibrin glue preparations has largely and thankfully been supplanted by standardized commercial products that contain virus-inactivated human fibrinogen.

Currently, cryoprecipitate is used primarily as replacement therapy in patients with hypofibrinogenemia that is congenital (rare) or acquired (i.e., after thrombolytic therapy, DIC, plasmapheresis, or massive transfusions), although a commercial virus-inactivated concentrate (Riastap) is available for the former application. When cryoprecipitate is used for these indications, 10 to 20 units are normally pooled and infused, with repeat doses administered every 6 to 8 hours depending on clinical status and results of laboratory tests of fibrinogen concentration. Fibrinogen levels of more than 50 mg/dL are considered sufficient to support physiologic hemostasis and produce normal PT or PTT results. The commercial concentrate is labeled for potency in milligrams; a vial contains approximately 1 g fibrinogen. Cryoprecipitate may also be useful for treatment of patients with factor XIII deficiency, a rare congenital deficiency, or an acquired disorder resulting from autoantibody formation. A recombinant concentrate (Tretten, catridecacog) is now available and is administered in doses ranging from 20 to 75 U/kg. A plasma-derived concentrate (Corifact) suitable for management of patients with isolated factor XIII subunit B deficiency has been licensed. Cryoprecipitate still enjoys widespread use because of the increased cost and limited availability of the commercial products.

Commercial Plasma Fractions

A variety of products prepared through special processing of plasma pools are used to treat patients with hemostatic disorders. Among the earliest preparations were the so-called PCCs, which are impure mixtures of vitamin K–dependent proteins that are isolated by ion exchange chromatography from the cryoprecipitate supernatant of large plasma pools after removal of ATIII and factor XI. Various processing techniques involving ion exchangers permit production of four-factor concentrates (4F-PCC), which include factor VII, or three-factor concentrates, which consist mainly of factors II, IX, and X. The PCCs are standardized according to their factor IX content. During production, activated clotting factors are produced that are later inactivated through a variety of processes, including manipulation of pH and addition of heparin and/or ATIII. These products may be further adjusted to produce activated PCCs used for the treatment of patients with acquired factor VIII or factor IX inhibitors (see Chapter 3 ). PCCs are now treated to inactivate transfusion-transmitted viruses. Adverse events associated with PCC transfusion include immediate allergic reactions, HIT (for preparations containing heparin), and thromboembolic complications such as DIC, arguably the most important adverse effect. PCC administration is indicated only when the desired increase in factor activity cannot be achieved through other therapeutic measures (e.g., life-threatening bleeding in cases of excessive oral anticoagulation with vitamin K; see later discussion).

When rapid reversal of warfarin is indicated, for example in a bleeding patient with a prolonged PT/international normalized ratio (INR) or one requiring emergency surgery, 4F-PCC infusion is now considered the treatment of choice as compared with FFP. 4F-PCC has a safety profile similar to that of plasma, but a lower risk of fewer fluid overload events. Because some studies have reported an increase in thromboembolic events, particularly in patients with malignancies, PCCs should not be used for warfarin reversal for elective invasive procedures. Discontinuation of warfarin is generally sufficient. Rapid PCC infusion in volunteers produces rapid and sustained increases in coagulation factors II, VII, IX, and X and anticoagulant proteins with no clinical evidence of thrombosis or viral transmission.

Virus-inactivated, intermediate-purity factor VIII preparations with high VWF content have replaced cryoprecipitate as the primary therapy for VWD. One of these products, Humate-P, has been approved by the FDA for the treatment of patients with VWD, and the package insert now provides the concentration of ristocetin cofactor activity required to facilitate dosing.

Recombinant Factor VIIa

Mechanism of Action

Recombinant FVIIa was developed to produce hemostatic bypassing activity without the adverse events associated with the use of PCCs in the treatment of hemophilic patients with inhibitors. Although the full spectrum of activity and the interrelated mechanisms responsible for its clinical activity have not been fully elucidated, evidence suggests that the effects of this agent extend beyond those that are measurable through traditional in vitro assessments of hemostasis. In a cell-based model of coagulation described by Roberts and colleagues ( Fig. 28.1 ), rFVIIa, in combination with tissue factor (TF) exposed locally on the surface of a damaged cell and factors V and X, generates thrombin, which catalyzes a series of reactions on the platelet membrane involving factors XI, VIII, and IX to cause platelet activation, release of VWF, and a still larger burst of thrombin that further augments hemostasis. In addition to exerting coagulation and platelet activation effects, thrombin activates factor XIII, which results in fibrin cross-linking. The high local concentrations of thrombin produce a stable clot structure that is relatively resistant to fibrinolysis and further counteracts fibrinolysis through activation of thrombin-activatable fibrinolysis inhibitor (TAFI).

Additional considerations for the clinical use of rFVIIa are hypothermia and acidosis. Hypothermia inhibits platelet adhesion at mild temperatures (33°C to 37°C [91.4°F to 98.6°F) and both platelet adhesion and coagulation at lower temperatures (<33°C); acidosis further inhibits coagulation and can worsen hemorrhage. Clinical anecdotal experience suggests that rFVIIa is effective in some hypothermic patients with acidosis in vitro, and clinical studies indicate that activity is more significantly reduced by acidosis than by hypothermia.