Red Blood Cell–Containing Components

Blood components containing RBCs are indicated for the treatment of symptomatic deficits of oxygen-carrying capacity.^20,21 PRBCs are the most widely available RBC-containing blood component, although in settings where the collection facilities do not have the capability of making components, whole blood may be the only component available. Donor whole blood is collected in a preservative-anticoagulant solution that contains citrate, phosphate, dextrose (glucose), and adenine (CPDA) or just citrate, phosphate, and dextrose. In the latter case, the platelet-rich plasma is removed after centrifugation of the whole blood unit, and a solution containing adenine, dextrose, and occasionally mannitol is added to the PRBCs. The additive-solution systems permit storage for 42 days (compared with 35 days for CPDA) and better preservation of 2,3-diphosphoglycerate (DPG) levels. The characteristics of the CPDA and additive-solution PRBCs and of whole blood are shown in Table 10-4. Although the hematocrit is reduced in the additive-solution PRBCs, the red cell mass is the same.

RBCs carry glycoconjugate antigens of the ABH histo-blood group system on the cell surface that are determined by six common alleles on chromosome 9.^19a During the first year of life, infants begin to elaborate alloantibodies to whichever A or B antigens they lack. These isoagglutinins are invariably present after a few months and constitute a formidable immunologic obstacle to transfusion or transplantation across this ABO barrier. The RBCs for transfusion must be compatible with the ABO isoagglutinins of the intended transfusion recipient. Similarly, components with a large volume of plasma (e.g., whole blood, FFP, apheresis platelets) must be compatible with the A or B surface antigens expressed on the recipient’s RBCs. PRBCs must be ABO compatible with the recipient, whereas whole blood must be ABO identical. Table 10-5 summarizes the permissible combinations.

Only RBCs express the Rh(D) antigen. Rh(D)-positive patients may receive Rh(D)-positive or Rh(D)-negative RBCs. Rh(D)-negative patients are routinely given Rh(D)-negative RBCs for any elective transfusions, but in the setting of massive transfusion it may be necessary to switch to Rh(D)-positive RBCs to preserve the supply of Rh(D)-negative RBCs. The blood bank usually determines when to make this substitution based on the inventory and does so more quickly for a patient who is a male or a postmenopausal female (Table 10-6). The objective is to avoid exposing a female with childbearing potential to Rh(D)-positive RBCs and possibly triggering the production of the anti-D alloantibody, which is responsible for the most severe forms of hemolytic disease of the newborn. Table 10-7 shows the common initial volume of PRBCs needed to increase the hemoglobin level by 2 to 3 g/dL.

TABLE 10-7 Common Initial Doses of Blood Components and Expected Effects in Children

^*This recommendation may be reduced pending the impact of the prophylactic platelet dose (PLADO) trial, as published for all age groups⁴⁰ and for the pediatric age range.^40a

The changes that occur to RBC during storage under conventional blood bank conditions has been well described.²² These observations have generated physiologically plausible hypotheses about how such changes may impair the function of the banked RBCs in vivo. The loss of intra-erythrocytic levels of 2,3-DPG and its corresponding decrease in the P₅₀ value may reduce the ability of stored RBCs to relinquish bound O₂ compared with 2,3-DPG–replete RBCs. The depletion of nitric oxide (NO) may reduce the vasodilatory properties of the RBCs, hence impairing their ability to maintain the patency of the small vessels in the microcirculation and blood flow to the tissues.²³ Numerous changes in the composition and behavior of the RBC plasma membrane, including the loss and oxidation of membrane lipids and proteins and the rearrangement of some membrane constituents,²⁴ correlate with changes in the shape and elasticity of the RBC membrane.^25,26 The loss of elasticity in particular can impede the rapid movement of the RBCs through the microcirculation.

These hypotheses and some supportive data from animal models²⁷ have led to a number of clinical studies (mostly in trauma, critical care, colorectal surgery, and cardiac surgery) of outcomes after using stored RBCs, but the results have been inconclusive.^28–30 About half of the studies found a statistical association between at an unfavorable clinical outcome measure and the transfusion of RBCs, which had been stored for a longer time. However, no association was seen in one half of the studies, including two that were extensions of previous studies with positive findings. A small number of randomized, controlled trials addressed this issue without statistically significant differences in outcomes between patients receiving RBCs stored for different amounts of time,³¹ although two of them were underpowered.^32,33 Knowing that RBCs change during storage raises the question of whether these changes affect the patients in a clinically meaningful way, a question that remains unanswered. Currently, there are four randomized, clinical trials being conducted in North America in populations of especially vulnerable patients: neonates in the intensive care unit, adults in the intensive care unit, and adults undergoing cardiac surgery.^34–37 The results of these trials should help to guide future transfusion practice.

Platelets

Platelets may be obtained from a whole blood donation or collected by apheresis. Whole blood–derived platelets are separated by centrifugation and suspended in 40 to 60 mL of plasma at a concentration that is two to four times greater than in the circulation. Each unit contains a minimum of 5.5 × 10¹⁰ platelets and is stored at 20° to 24° C with gentle continuous agitation for a maximum of 5 days. One unit of whole blood–derived platelets can be expected to increase the platelet count of a 70-kg adult by 5000 to 10,000/mm³ and increase the count in an 18-kg child by 15,000/mm³.^38,39 A unit of platelets obtained by apheresis contains at least 3 × 10¹¹ platelets in 200 to 400 mL of plasma, or the equivalent of approximately 6 units of whole blood–derived platelets. A common dose for pediatric patients is 0.1 to 0.3 unit/kg of body weight, or 10 to 15 mL/kg (see Table 10-7); this dose usually produces an increment of 30,000 to 90,000/mm³. However, a rigorous reassessment of platelet dosing was just carried out in the prophylactic platelet dose trial.^40,41 Doses equivalent to the standard dose of one pheresis pack (or 6 units) per m², half of this dose, and double this dose were compared in 1272 adult and pediatric patients who received at least one platelet transfusion. Blood losses were determined with the World Health Organization (WHO) bleeding scale: grade 0 = no bleeding, grade 1 = petechiae, grade 2 = mild blood loss, grade 3 = gross blood loss, and grade 4 = debilitating blood loss. No differences were observed in bleeding outcomes (WHO grade 2 or greater) among the three doses; this was also true for the subset of 200 pediatric patients. The recommended platelet dose may be reduced in the near future, but this change awaits further discussion by the blood transfusion and hemostasis community.^42–44 This trial did not involve patients undergoing surgical procedures with ongoing blood loss.

In the setting of dilutional thrombocytopenia with ongoing losses or a consumptive coagulopathy (e.g., disseminated intravascular coagulation), larger doses (≥0.3 unit/kg) may be required to boost the platelet count above 50,000/mm³. Because platelets are suspended in plasma that contains the anti-A and anti-B isoagglutinins, they should be ABO compatible with the recipient’s RBCs. Some blood donors have high-titer isoagglutinins that can produce hemolysis in transfusion recipients if a large enough volume of plasma is given.⁴⁵ The transfusion of plasma-incompatible, whole blood–derived platelets to adult recipients does not produce clinically significant hemolysis because the volume of plasma given is so small relative to the plasma volume of an adult. However, apheresis platelets (and whole blood–derived platelets for small children) should be ABO compatible with the recipient’s RBCs. Because platelets do not express Rh antigens, matching for Rh(D) antigen is not necessary for apheresis platelets because they contain virtually no RBCs. However, whole blood–derived platelets may contain enough RBCs to provoke Rh alloimmunization so platelets from Rh(D)-negative donors are given preferentially to Rh(D)-negative recipients with childbearing potential. If a premenopausal female receives whole blood–derived platelets from an Rh(D)-positive donor, Rh immune globulin can be administered within 72 hours to prevent alloimmunization. Platelets should never be withheld in an emergency situation because of Rh(D) incompatibility.

Platelets are essential to hemostasis associated with the vascular injury of surgery and are necessary for the control of surgical bleeding. Platelets are also required for the maintenance of an intact endothelial barrier to spontaneous blood loss. The number of platelets required to provide adequate hemostasis in the surgical setting is much greater than the level needed to provide prophylaxis against spontaneous hemorrhage. A platelet count of 10,000/mm³ is considered adequate to prevent spontaneous bleeding or bleeding from minor invasive procedures (e.g., lumbar puncture, line placement) in an otherwise stable child. If overt signs of bleeding are present or a more significant hemostatic challenge in the form of a surgical procedure is imminent, a level of 30,000 to 50,000/mm³ may be required.^46–50 A target level of 50,000/mm³ is appropriate in the setting of massive transfusion.^47,51–53 Platelets may also be required for children with adequate counts but in whom platelet function is impaired. Many medications (e.g., aspirin; nonsteroidal antiinflammatory agents; dipyridamole; platelet P2Y12 receptor blockers such as clopidogrel or prasugrel; or glycoprotein IIa/IIIb receptor inhibitors such as abciximab, eptifibatide, or tirofiban; serotonin uptake antagonists such as Zoloft) and some conditions (e.g., renal failure with blood urea nitrogen levels above 60 mg/dL) cause abnormal platelet function, which may interfere with surgical hemostasis, in which case it may be necessary to maintain the platelet count at a somewhat greater level, at least until the effect of the medication dissipates or the child’s platelets have largely been replaced by banked platelets.^54,55 In a few settings, such as intracranial, ophthalmic, and otologic surgery, even greater levels (100,000/mm³) are sought.

There is no clear-cut threshold below which the platelet count predicts clinical bleeding in the perioperative period. Each child must be individually assessed by constantly observing the surgical field for evidence of abnormal bleeding.⁵⁶ Unfortunately, we lack a well-validated bedside tool to assess platelet function. The utility of the thromboelastogram and other devices to measure platelet function under controlled flow conditions, such as the platelet function analyzer (PFA-100), are being investigated.^57–59 The standard technique for diagnosis and evaluation of thrombocytopathies remains Born-O’Brien platelet aggregometry, but it is not useful in the intraoperative setting.⁶⁰ Most commonly, thrombocytopenia rather than a newly acquired platelet function defect is the problem in the operative setting and massive transfusion.

A child occasionally presents for surgery with a previously characterized platelet dysfunction that may be associated with bleeding. If the child has a normal platelet count, it is reasonable practice to ensure that the blood bank has an adequate platelet supply available for the operating room but to withhold transfusion until the child demonstrates pathologic bleeding.

Several additional points should be considered⁶¹:

Special Processing of Cellular Blood Components

Leukocytes collected with whole blood donations partition into platelet and PRBC components, and few intact leukocytes are present in FFP. Passenger leukocytes are responsible for most febrile, nonhemolytic transfusion reactions, HLA alloimmunization, and transmission of cytomegalovirus (CMV). To prevent the complications from these leukocytes, they can be very effectively removed (2 to 3 log reduction) by passage through leukocyte-reduction filters that may be done shortly after collection (prestorage leukoreduction) or at the bedside. Leukocyte reduction of RBCs by filtration is a superior technique to washing or freezing-deglycerolizing, which was used in the past. Table 10-8 provides indications for who may benefit from receiving leukocyte-reduced cellular components (i.e., RBCs or platelets).

TABLE 10-8 Indications for Leukocyte-Reduced Cellular Blood Components

CMV transmission can also be reduced by screening donors for CMV exposure (testing for antibody to CMV), although leukocyte reduction is the more widely used approach. Even though primary CMV infection is benign in children with intact immune systems, some pediatric populations are at risk for developing systemic disease and should be protected from CMV transmission by blood. Only patients who have not previously been infected with CMV (i.e., CMV seronegative) are at risk. Children particularly susceptible to systemic CMV infections are listed in E-Table 10-1.

E-TABLE 10-1 Children at Risk for Cytomegalovirus Disease*

AIDS, Acquired immunodeficiency syndrome; CMV, cytomegalovirus; HIV, human immunodeficiency virus.

^*Applies to CMV-seronegative patients only.

Transfused lymphocytes may mediate a graft-versus-host process in some recipients with impaired cellular immunity. Because this process involves the bone marrow and the usual targets (i.e., skin and gastrointestinal tract), the fatality rate is substantial. Transfusion-associated graft-versus-host disease (GVHD) can be prevented by exposing cellular blood components to gamma irradiation that disables the donor lymphocytes. Children who are considered to be at risk for transfusion-associated GVHD and who should receive irradiated cellular components are listed in E-Table 10-2. This complication can also occur in children with intact immune systems in the unusual circumstance when the transfusion donor is homozygous for an HLA haplotype that is shared with the recipient. In this case, the recipient’s immune system, although fully functional, cannot recognize the donor lymphocytes as foreign. The donor lymphocytes mount a GVHD attack on the recipient’s tissues, recognizing the mismatched haplotype. This situation is more likely to occur when the donor is a blood relative of the recipient. It is for this reason that blood and HLA-matched platelets donated by family members are routinely irradiated.

E-TABLE 10-2 Children at Risk for Transfusion-Associated Graft-versus-Host Disease

HLA, Human leukocyte antigen system.

Fresh Frozen Plasma

Fresh frozen plasma (FFP) represents the fluid portion of whole blood that is separated and frozen within 8 hours of collection. After thawing at 37° C, which usually requires 30 minutes, it may be administered within 24 hours if stored at 1° to 6° C. The volume of 1 unit varies from 180 to 300 mL and represents 7% to 10% of the coagulation factor activity in a 70-kg patient. It contains all of the clotting factors and regulatory proteins at approximately the native concentration, but after 6 hours at 1° to 6° C, the levels of the labile factors V and VIII begin to diminish.⁶² FFP does not provide functional platelets, nor does it contain leukocytes or RBCs.

FFP should be ABO-compatible with recipient red cells because it contains the anti-A and anti-B isoagglutinins. If the recipient’s blood type is not known, plasma from a donor with blood type AB, which contains neither anti-A nor anti-B, may be administered. Because the citrate anticoagulant is present in the plasma, rapid administration of FFP is more likely to be associated with citrate toxicity than the transfusion of components with smaller volumes of plasma (e.g., PRBCs).

FFP is frequently administered without justification by evidence-based medicine.⁶³ One major surgical indication for FFP is to correct the coagulopathy associated with massive blood transfusion (see Table 10-7). Other indications include a prolongation in the prothrombin time (PT) before surgery or in the setting of bleeding, the emergency reversal of warfarin, or the presence of a specific congenital or acquired coagulation protein deficiency for which a factor concentrate or a recombinant factor is not available (e.g., for factor XI deficiency).⁶⁴ Administration of vitamin K should not be overlooked in children who are taking warfarin, have hepatic insufficiency, have been exclusively breastfed,^65–67 who have been on broad-spectrum antibiotics (which often eliminate normal vitamin K–producing gastrointestinal flora), who have been on total parenteral nutrition for inadequate oral caloric intake, or who may have prolonged hospitalizations. Correction of a mild increase in the PT (e.g., international normalized ratio [INR] <1.5) is rarely necessary. Figure 10-1 shows the relationship between the level of coagulation factors and the in vitro clotting times, in this case the PT. Relatively modest levels of coagulation factors can support normal hemostasis, even though the PT is prolonged. When the PT is very prolonged (see Fig. 10-1, point A), the transfusion of 1 unit of FFP, which increases the coagulation factor levels by 7% to 10% in an adult, has a dramatic effect on shortening the PT. When the PT is only mildly prolonged, as at point C, where factor levels are already adequate for hemostasis, the increase in the factor levels by an additional 7% to 10% with 1 unit of FFP (in an adult) has a much smaller effect on the PT. This small effect does not achieve any improvement in hemostasis.

FIGURE 10-1 Nonlinear relationship between levels of coagulation factor and clotting test results. Decreases in clotting factor levels to about 30% of normal prolong the clotting test times but still support normal hemostasis. Treating an adult or child at point A with fresh frozen plasma to raise the level of coagulation factors to point B has a marked effect on the prothrombin time and the international normalized ratio (INR). The same amount of fresh frozen plasma administered to an adult or child at point C, however, has only a minor effect on the prothrombin time (PT).

(Modified from Dzik WH, Stowell CP. Transfusion and coagulation issues in trauma. In: Sheridan RL, editor. The trauma handbook of Massachusetts General Hospital. Philadelphia: Lippincott Williams & Wilkins; 2004. p. 139.)

Cryoprecipitate

Cryoprecipitate is prepared by thawing FFP at 4° to 10° C and expressing most of the plasma, leaving behind precipitated protein that is then resuspended in a small volume of residual plasma (15 to 25 mL) and refrozen. This component contains 20% to 50% of the factor VIII from the original unit of plasma. It also contains von Willebrand factor (vWF), fibrinogen (approximately 250 mg), and factor XIII. It is indicated for the treatment of factor XIII deficiency, dysfibrinogenemia, and hypofibrinogenemia (see Table 10-7).^68–77 It is no longer used for the treatment of von Willebrand disease or hemophilia A. Plasma concentrates of factor XIII and fibrinogen have been available in Europe for several years and recently have been licensed in the United States for use in patients with these congenital deficiencies.

Plasma-Derived and Recombinant Factor Concentrates

The most commonly administered factor concentrate is factor VIII, used in the treatment of hemophilia A. Children with hemophilia can have many problems related to their disease, including splenomegaly, abnormal liver function, and joint disease related to hemarthrosis. In the past, the use of pooled plasma products was associated with very high rates of transmission of viral hepatitis (especially hepatitis C virus) and HIV.^78–80 The use of more rigorous viral removal and inactivation processes and the introduction of recombinant factor VIII and IX products^81–85 have greatly reduced these problems.^68–7586 Initial concerns that there may be an increased incidence of inhibitors in children who receive recombinant therapy compared with plasma-derived factor therapy have not been borne out.⁸⁷ Mild hemophilia usually responds well to desmopressin (1-deamino-8-d-arginine vasopressin [DDAVP]) therapy.^88,89

Children with hemophilia B (i.e., Christmas disease or factor IX deficiency) are managed with recombinant human factor IX and highly purified factor IX (preparations with various amounts of factors VII, X, and prothrombin) that are treated to inactivate or remove viruses.^{70,71,73,90–103} Careful planning of any surgical procedure for these children includes close communication with the child’s hematologist to ensure optimal therapy while reducing unnecessary transfusions (see Chapter 9).

von Willebrand disease is routinely treated with DDAVP or plasma-derived factor VIII concentrates that are also rich in vWF, such as Humate-P, Alphanate, and Koate DVI. In children who have von Willebrand disease and are resistant to DDAVP or for whom DDAVP is contraindicated (e.g., central nervous system [CNS] bleeding, allergic reaction), it is reasonable to withhold treatment with blood-derived products until surgery has begun unless surgery is performed in an area where bleeding is potentially life-threatening, to reduce unnecessary transfusions. These children often do not demonstrate pathologic bleeding. Adjunctive therapies that can further minimize the use of blood products include use of the antifibrinolytic agent ε-aminocaproic acid (Amicar), which can be administered orally or intravenously, and topical hemostatic agents, including topical collagen and fibrin glues.

Desmopressin

DDAVP, a synthetic analogue of vasopressin, can increase the levels of factor VIII : C (i.e., coagulant activity) and factor VIII : vWF in children with mild hemophilia A or von Willebrand disease.^89,104–108 An IV dose of 0.3 µg/kg (maximum 20 µg; a subcutaneous preparation is available in Europe) increases the levels of both factors twofold to threefold within 30 to 60 minutes, with a half-life of 3 to 6 hours.¹⁰⁵ Intranasal DDAVP is also effective, but onset is less rapid. Between 80% and 90% of children with von Willebrand disease are responders,^109,110 and affected children should be tested for their responsiveness to IV DDAVP. This treatment is best suited to bleeding from surgical procedures, which ceases within 2 to 3 days. When bleeding continues beyond this period, as can occur with some orthopedic procedures, daily IV Humate P (or Alphanate or Koate DVI) can obviate possible tachyphylaxis with DDAVP. Products rich in the vWF allow better control over peak levels of factor VIII. When in excess of 200%, factor VIII predisposes to postoperative deep venous thrombosis and pulmonary embolism.

DDAVP has been used to treat the coagulopathy associated with uremia and cirrhosis.^111,112 It may reduce elective surgical bleeding when the potential for blood loss is substantial, such as in cardiac surgery and spinal fusion.^89,113–118 Although initial reports apparently demonstrated a benefit in patients who did not have a preexisting coagulopathy, other controlled studies failed to show an effect despite increases in factor VIII:C and vWF, and its use for these indications has largely been abandoned.^119–121 Because of the potential for hyponatremia from water retention, use of DDAVP is avoided in children younger than 2 years, in children with CNS lesions, including a brain tumor, history of CNS irradiation, or recent neurosurgery or CNS trauma, and in elderly adults.

Albumin, Dextrans, Starches, and Gelatins

Solutions of several high-molecular-weight molecules (i.e., colloids) have been used for volume replacement, although there is no clear advantage to their use over crystalloid solutions. These colloids include albumin, dextrans, starches, and gelatins.

Albumin has the longest track record and the fewest adverse side effects.^122,123 In the past, dextrans (i.e., high- and low-molecular-weight glucose polymers) were administered for volume expansion and hemodilution in children,^124,125 but currently, their primary use is for antithrombosis, although their value for even this indication is questionable.¹²⁶

Starches are branched polysaccharide polymers available in high-, medium-, and low-molecular-weight ranges (480,000 to 70,000 daltons). However, these compounds alter hemostasis by diluting clotting factors and impairing platelet function and the coagulation cascade.^127,128 An additional concern, especially in children, is their accumulation in the reticuloendothelial system and the potential for unknown long-term adverse effects.¹²⁹ Several studies have been carried out in children with minor changes in coagulation parameters that occurred when transfusions exceed 20 mL/kg.^130–134 A 6% hydroxyethyl starch (HES 130/0.4) yielded clinical and physiologic profiles similar to those for 5% albumin in volumes up to 16 mL/kg in noncardiac surgery and in volumes up to 50 mL/kg in cardiac surgery, although at smaller cost.^135,136 Several major reviews regarding the use of starches and gels in adult patients who are critically ill have raised significant concerns regarding adverse effects on coagulation^137,138 and renal function,^139,140 and they found inadequate overall safety data, even for the third-generation products.^18,141 If these concerns have been raised in adult populations, we should have even greater concern about their use in children.

Gelatins are polypeptides derived from bovine collagen that seem to have a minimal effect on coagulation and provide reasonable plasma volume expansion. However, a significant number of life-threatening anaphylactic or anaphylactoid reactions have been reported, and their use in children remains somewhat limited.^{122,142–146}

Red Blood Cell Substitutes

Blood substitutes offer the promise of agents with universal compatibility, minimal infectious risks, and prolonged shelf life (years rather than days) to carry oxygen to vital organs.¹⁴⁷ Early efforts to develop these products involved a variety of human, bovine, and genetically engineered hemoglobin polymer solutions, perfluorocarbons, and lipid-encapsulated hemoglobin. Most failed in clinical trials because of severe complications such as renal failure, stroke, and vasoconstriction.^148,149 One formulation, Hemospan (Sangart, Inc., San Diego), is still in clinical trials.^150–152 It is designed to maximize oxygen delivery at the tissue level (i.e., capillary beds with impaired perfusion) rather than maximize oxygen-carrying capacity and have equivalency to hemoglobin.^{148,153–157} Further research is needed before these blood substitutes become available for use in children.

Coagulopathy

The coagulation system in children and adults involves platelets, coagulation proteins, and localized tissue factor, which initiate all steps of hemostasis. Figure 10-2 shows that an initial step is platelet adhesion to a wound or site of vessel wall injury, with adhesion being mediated by the vWF through its receptor on the platelet, the glycoprotein Ib–glycoprotein IX complex (GPIb-IX), and fibrinogen through the fibrinogen receptor on the platelet, the glycoprotein IIb–glycoprotein IIIa complex (GPIIb-IIIA). In flowing blood, initial platelet attachment is facilitated by vWF, whereas platelet spreading and more secure (shear stress–resistant) platelet-platelet aggregation is driven by fibrinogen and by the GPIIa-GPIIIa complex. However, there is also evidence that platelets attach even to intact endothelium, which has an activated phenotype, as after inflammatory cytokine exposure or sepsis. Platelets attach to the endothelium through high-molecular-weight von Willebrand multimers. Fibrinogen is attached to endothelium through upregulated integrins and selectins.

FIGURE 10-2 A blood clot forms when platelets adhere to one another through the GPIIb-IIIa complex (purple), which serves as a receptor for the adhesive protein fibrinogen (orange). Platelet interaction with an injured vessel wall requires synergy between the GPIb-IX (blue) and GPIIb-IIIa complexes and the adhesive proteins of von Willebrand factor (green) and fibrinogen, respectively. In high-velocity gradients, the efficacy of GPIb-IX interaction with the von Willebrand factor is significantly impaired.

Initial platelet hemostasis (i.e., platelet plug formation) is accompanied by local generation of fibrin as the end product of the action of at least three surface-active enzyme complexes. Clotting is initiated by the tissue factor/factor VIIa surface-active enzyme complex and amplified by the factor VIIIa/IXa/X and factor II/Va/Xa complexes. A mural platelet thrombus, which includes platelets and fibrin, then forms a scaffold on which healing of the vessel wall can take place. The scaffold is removed when no longer needed by means of thrombolysis and the effects of macrophages.

The surface-active enzyme complexes (Fig. 10-3) are active on the phospholipid surfaces provided by platelets, leukocytes, and endothelial cells but not in the bulk of the blood. In this regard, the conventional coagulation cascade shown in Figure 10-4 is oversimplified, although it is a convenient approach to understanding the PT and PTT. All of the steps must be considered in the milieu of flowing blood, such that the high-velocity gradients of arterioles (i.e., mucous membranes of the uterus, gastrointestinal tract, upper respiratory tract, oral cavity, and gums) and arteries favor thrombi with a greater proportion of platelets (i.e., white thrombi), and low-velocity gradient states such as those found in stasis or blood accumulation within a body cavity favor a greater proportion of red cells (i.e., red thrombi). This explains why a patient with von Willebrand disease, characterized by a defect in the protein that allows blood platelets to adhere to a wound, tends to bleed from mucous membranes, sites of high-velocity (arteriolar) gradients, whereas hemophiliacs tend to bleed into joint spaces and muscle planes, sites of low- or near-zero–velocity gradients.

FIGURE 10-3 The key clotting factor enzyme complexes in coagulation. Tissue factor (A) (shown on the surface of a fibroblast) initiates coagulation, leading to the activation of clotting factors IX and X (shown on the surface of a platelet) and the processing of prothrombin (factor II) to form thrombin (factor IIa). Factor XIa has a contributory role in factor IX activation. Key to this cascade are three surface-active enzyme complexes: (A) the complex of tissue factor and factor VIIa; (B) the complex of factor IXa, factor VIIIa, and factor X; and (C ) the complex of factor Xa, factor Va, and factor II (where the letter “a” denotes the activated form of a factor). Prothrombin fragment 1 + 2 and thrombin-antithrombin complexes are markers of the generation of thrombin. Fragment 1 + 2 is an inactive fragment formed during the processing of prothrombin; thrombin-antithrombin complex is formed when antithrombin III binds to thrombin, resulting in the inactivation of thrombin. Not shown in this diagram is the important influence of blood flow; for example, arteriolar and arterial velocity gradients, along with the presence of red cells, promote collisions between platelets and, consequently, platelet aggregation.

(Modified with permission from Grabowski EF. The hemolytic-uremic syndrome toxin, thrombin, and thrombosis. N Engl J Med 2002;346:58-64.)

FIGURE 10-4 The conventional activated partial thromboplastin time (aPTT) is considered to be a measure of the intactness of the intrinsic coagulation system, which includes clotting factors XII, XI, IX, VIII, V, X, II, and I (fibrinogen). The prothrombin time (PT) is a measure of the intactness of the extrinsic coagulation system and encompasses tissue factor–bearing membrane surfaces and microparticles and factors VII, V, X, II, and I. In the PTT test, the blood clots without the need for an exogenous agent and comprises an intrinsic or complete clotting system. In practice, an agent such as diatomaceous earth is added to speed the reaction in the laboratory, and the term activated is added to the designation (aPTT). In the PT test, the blood clots by virtue of an extrinsic activator (i.e., tissue factor). This view in the diagram obscures the central role of tissue factor in clot initiation. Tissue factor circulates in an inactive form in the blood and is no longer considered only an extrinsic factor to the blood itself.

The coagulopathy associated with massive blood transfusions is usually attributable to the dilution of clotting factors or platelets, or both. The point at which the deficiency in clotting factors is sufficient to produce a coagulopathy depends on the volume of blood lost and the type of blood component transfused (i.e., PRBCs or whole blood). Dilutional thrombocytopenia sufficient to cause clinical bleeding depends on the starting platelet count and the volume of blood replaced (Fig. 10-5). In some cases, the cause of bleeding is a consumptive coagulopathy such as fibrinolysis or disseminated intravascular coagulation (DIC).^51,159–179 In other scenarios, bleeding is caused by hypothermia, severe metabolic acidosis, poor tissue perfusion, and the release of tissue factors. Body temperature should be maintained by using efficient blood-warming devices, acidosis should be treated, and normovolemia and cardiac output should be restored to prevent a coagulopathy from developing.^{166,180–183}

FIGURE 10-5 A study that considered the use of citrated whole blood (blue line) in adults found that bleeding in most cases resulted from dilutional thrombocytopenia. The magenta line represents estimated points of dilutional clotting factor deficiency if solely citrated packed cells are transfused.

(Modified from Miller RD. Transfusion therapy and associated problems. ASA Refresher Courses in Anesthesiology 1973;1:107.)

Dilutional Thrombocytopenia

Because of the dearth of published data on the effects of massive blood transfusion in children, we must rely on our clinical experience and data extrapolated from adults to anticipate the effects in children. A study of trauma patients during the Vietnam War reported that the onset of clinical bleeding occurred after about 15 units of whole blood had been transfused. The incidence of coagulopathy was unrelated to an abnormal PT or partial thromboplastin time (PTT) but correlated closely with a platelet count of less than 65,000/mm³.⁵¹ In a 70-kg adult, the estimated blood volume is approximately 5 L (70 mL/kg × 70 kg = 4900 mL), or approximately 10 units of whole blood. To relate the data in Figure 10-6 to children, assume the 70-kg adult has 5 L of blood, or 10 units of whole blood. If each 10 units of whole blood is one blood volume, some children are expected to develop a coagulopathy after losing about 1.5 blood volumes, but most develop clinical bleeding after losing 2.0 to 2.5 blood volumes. Studies of massive blood loss with whole blood replacement appear to support the conclusion that the coagulopathy at these levels of blood loss results from thrombocytopenia rather than a clotting factor deficiency.^51,166–175 Consequently, children should be monitored for thrombocytopenia and possible transfusion of platelets or clotting factor deficiency (see further) after the loss of the first 1.0 to 1.5 blood volumes. After the platelet count has decreased to 50,000/mm³, it is likely that approximately one platelet dose (i.e., 6 units for an adult or 10 to 15 mL/kg for a child) will be required for each blood volume replaced. If a coagulopathy develops earlier than expected (i.e., before a 1.0-blood volume loss), a search should be initiated for other causes of bleeding, such as increased arterial or venous pressure in the surgical field or DIC.

FIGURE 10-6 Percent of change in platelet count versus blood volumes transfused in adults and children.^51,52 The magenta line represents observed values, whereas the blue line represents calculated values. This difference suggests increased bone marrow production and/or splenic recruitment of platelets during massive transfusion.

(From Miller RD, Robbins TO, Tong MJ, Barton SL. Coagulation defects associated with massive blood transfusions. Ann Surg 1971;174:794-801; Coté CJ, Liu LM, Szyfelbein SK, Goudsouzian NG, Daniels AL. Changes in serial platelet counts following massive blood transfusions in pediatric patients. Anesthesiology 1985;62:197-201.)

Studies of adult and pediatric patients with acute dilutional thrombocytopenia found that clinical bleeding correlated closely with a platelet count of 65,000/mm³ or less.^51,166–175 Figure 10-6 compares the calculated reduction in platelet count with the observed decline in platelet count in adults and children. The observed decrement differs from the calculated reduction because platelets are mobilized from the bone marrow, lungs, and lymphatic tissues. The platelet count usually does not decrease to concentrations that cause bleeding until 2.0 to 2.5 blood volumes are lost in children, which is consistent with our observations, or until 20 to 25 units of whole blood are transfused in adults.^52,173,184 Clinical bleeding does not usually occur in children whose platelet counts remain above 50,000/mm³, despite blood losses as great as 5.0 blood volumes (Fig. 10-7, A).⁵²

FIGURE 10-7 A, Serial changes in platelet counts are plotted for 26 pediatric patients who lost one to five blood volumes. Most of the children suffered from severe thermal injuries, and many had relatively high platelet counts at baseline. Clinically evident signs of coagulopathy appeared when the platelet count decreased below 50,000/mm³. B, Platelet counts of five children abstracted from A. The baseline platelet count is invaluable in estimating potential platelet needs in relation to blood volumes transfused. A low initial count suggests the need for early exogenous platelet transfusion, whereas a high initial platelet count indicates that exogenous platelets may not be required until several blood volumes have been lost. The three children who developed a coagulopathy began surgery with a relatively low platelet count, whereas the two children with a very high platelet count did not require platelet transfusion despite the loss of four and five blood volumes. (It should be noted that these children received sufficient FFP so as to maintain the PT and PTT within a normal range.)

(Reproduced with permission from Coté CJ, Liu LMP, Szyfelbein SK, Goudsouzian NG, Daniels AL. Changes in serial platelet counts following massive blood transfusions in pediatric patients. Anesthesiology 1985;62:197-201.)

The starting platelet count is important for estimating how much blood loss can be tolerated before critical thrombocytopenia occurs. For example, with a starting platelet count of 600,000/mm³, dilutional thrombocytopenia is unlikely to occur until 4.0 or more blood volumes are lost, whereas with a starting count of 100,000/mm³, dilutional thrombocytopenia may be expected after 1.0 blood volume has been lost (see Fig. 10-7, B). Prophylactic transfusion of platelets typically is not indicated without documented evidence of dilutional thrombocytopenia, visible microvascular bleeding, and ongoing blood loss, although it should be anticipated based on the starting platelet count and the volume of blood lost.^{52,56,162,166,185}

Although the primary platelet defect in massive transfusion is thrombocytopenia, some data suggest that platelets may not function normally (i.e., thrombocytopathy) after massive trauma or in the presence of hypothermia.^182,186,187 This has not been our experience in the children we studied whose temperature remained within the normal range.^52,167 The only simple test to assess platelet function is the bleeding time. However, this test is also sensitive to thrombocytopenia and its predictive value is of equivocal utility.^162,186,188 The PFA-100 test shows less potential as a rapid screening tool than it once did, because the device uses citrated blood warmed to 37° C and is relatively insensitive to milder defects in platelet-vessel wall interaction, such as that in mild von Willebrand disease. There remains a critical need for a point-of-care device or simple test to assess platelet function. Currently, the platelet count is our best indication for the need for platelet transfusions in situations involving rapid blood loss.¹⁸⁹ Other devices to measure whole blood clotting have been used to guide transfusion therapy, but their efficacy in improving outcomes has not been established.¹⁹⁰

In several in vitro and animal model systems, recombinant factor VII (rFVIIa) activates factors IX and X on the surface of activated platelets, probably through the binding of rFVIIa to the platelet membrane (from which rFVIIa can also be taken up into storage sites within the platelet) and subsequent recruitment of circulating tissue factor.^191–196 Although this has significantly improved hemostasis for hemophiliacs with inhibitors to factor VIII, there is limited evidence that rFVIIa reduces mortality for off-label use, as in cardiovascular surgery, trauma, and intracerebral hemorrhage. This agent increases the risk of thromboembolism.¹⁹⁷

Basic clotting studies (e.g., PT, PTT, fibrinogen, platelet count) must be performed before elective surgery when major blood loss can be anticipated to determine the cause of underlying coagulopathies and provide adequate quantities of blood components.

Factor Deficiency

Laboratory assessment of developing deficiencies in clotting factors plays an important part in transfusion decisions in managing massive transfusion. The PT (for the extrinsic system) measures the adequacy of factors VII, X, and V; prothrombin; and fibrinogen,⁵¹ whereas the PTT (for the intrinsic system) measures the adequacy of factors XII, XI, IX, VIII, X, and V; prothrombin; and fibrinogen (see Fig. 10-4). Banked whole blood contains normal plasma concentrations of all of the clotting factors and regulatory proteins, but it has a reduced concentration of factors V and VIII (20% to 50% of normal at the time of outdate). For coagulopathy due to a clotting factor deficiency to develop, factor VIII must be less than 30% of the normal concentration and factor V less than 20% of normal.¹⁶⁷ For these to occur, at least 3.0 blood volumes must be exchanged with whole blood. In this scenario, the first coagulation test that is abnormal is the PTT because factor VIII is diluted to less than 30%.¹⁶³

If blood loss is replaced with PRBCs, as is the current practice with blood banking techniques, the amount of plasma that is transfused is minimal because most of it was sequestered in the FFP fraction when it was separated. Massive replacement of blood loss with PRBCs and no other blood products quickly dilutes all of the clotting factors, including fibrinogen (see Fig. 10-4).* Data have confirmed that the PT and PTT are prolonged in children with multiple clotting factor deficiencies (e.g., during massive transfusion) at concentrations of clotting factors that are greater than in children with single clotting factor deficiencies (e.g., congenital coagulopathies).^174,175 This was documented in adult patients who were transfused exclusively with PRBCs and crystalloid. The dilution of multiple clotting factors correlated with the volume of blood and crystalloid transfused.¹⁴⁸ Diluting clotting factors to approximately 30% of normal may be expected replacing 1.0 to 1.5 blood volumes with PRBCs and crystalloid exclusively. Because moderate prolongations of the PT and PTT exist without overt signs of clinical bleeding,¹⁷⁴ the indication for FFP should be onset of clinical coagulopathy. However, to avoid falling behind, the anesthesiologist should anticipate that the clotting factors will be diluted and that FFP should be infused after 1.0 blood volume of blood loss has been replaced with a combination of PRBCs and crystalloid. Documented deficiency of fibrinogen (<80 mg/dL) may also be corrected by transfusing FFP, but marked deficiency, particularly in the presence of a consumptive coagulopathy (e.g., DIC, fibrinolysis), may require the addition of cryoprecipitate (0.2 to 0.4 unit/kg).^{3,199,201–203} No published studies have examined massive blood replacement in infants and children using exclusively component therapy.

Our experience with 26 children (12 ± 4 years old, weight of 41.9 ± 15.8 kg; 22 Harrington rod procedures, three tumor excisions, and one Whipple procedure) who did not receive FFP or whole blood during surgery that involved blood losses between 0.5 and 1.0 blood volumes showed that none of the children demonstrated clinical signs of coagulopathy. Slight prolongations of the PT or PTT occurred when blood loss was 1.0 blood volume or less (Table 10-10). Two children who lost 1.5 blood volumes, and one who lost 2.0 blood volumes had prolonged PT and PTT values. The only child who had clinical coagulopathy was the one who lost 2.0 blood volumes, and the coagulopathy could have been ascribed to simultaneous dilutional thrombocytopenia.^204,205

The magnitude of the change in PT or PTT that predicts a clinical coagulopathy is not well defined. However, the consensus panel of the National Institutes of Health and others suggest that greater than 1.5 times normal (or INR >2.0) should be considered pathologic.^{56,169,173,206–209} Our studies suggest that the PT and PTT are prolonged to more than 1.5 times normal when the blood loss is 1.5 or more blood volumes and when the blood loss has been replaced with only PRBCs and crystalloid or PRBCs and 5% albumin.^204,205 Our clinical practice is to initiate FFP after loss of more than 1.0 blood volume has been replaced with PRBCs (Table 10-11). At that point, FFP is administered in a ratio of 1 unit for every 2 units of PRBCs transfused. The indications for and timing of FFP depend on which blood product has been transfused, the volume of that transfusion as it relates to the child’s blood volume, and whether the blood loss will continue perioperatively. The PT, PTT, fibrinogen concentration, and platelet count should be determined after each blood volume has been replaced and used to guide the need for additional FFP and platelets.

TABLE 10-11 Minimal Fresh Frozen Plasma Recommendations according to the Type of Blood Product Transfused and the Volume of Blood Lost

FFP, Fresh frozen plasma; PRBCs, packed red blood cells; U, unit.

Recombinant factor VIIa (rFVIIa; NOVO Seven) is approved for use in the United States for hemophiliacs with high-titer inhibitors and for congenital factor VII deficiency. Several anecdotal reports describe its efficacy in controlling hemorrhage in a variety of other settings. However, in randomized clinical trials enrolling patients with partial hepatectomy, liver transplantation, prostate surgery, pelvic (orthopedic) surgery, trauma, or upper gastrointestinal tract bleeding, it conferred little or no benefit.^210–215 In a large randomized clinical trial of intracranial hemorrhage, the patients in the largest-dose group showed a small improvement in hematoma expansion, but 10% also experienced thromboembolic complications. In adults receiving rFVIIa, there is a 1.4% to 10% incidence of major thromboembolic complications, including acute myocardial infarction and stroke.²¹⁶ There are anecdotal reports of its successful use in children, but they provide little guidance because of the limited data.^{1,2,217–219,219a} Until the results of controlled trials demonstrate a clear benefit for its use, rFVIIa should be used with great caution for off-label indications.^{210–215,218,220,221}

Several studies^215,222 have compared different transfusion strategies for treating trauma incurred during combat in the Middle East A consensus conference on massive transfusion concluded that the evidence did not support the up-front use of a 1 : 1 : 1 ratio of units of PRBC, FFP, and platelets, but it did support the early use of an antifibrinolytic medication (i.e., tranexamic acid).²²² The consensus conference also recommended an integrated approach to managing massive transfusion that included rapid provision of PRBCs, the use of antifibrinolytics, and a foundation ratio of blood components directed by the results of standard coagulation testing (e.g., PT, PTT, platelet count, fibrinogen) or clot viscoelasticity, or both.²²²

The dilutional coagulopathy associated with massive blood transfusion is reasonably predictable. When using whole blood, dilutional thrombocytopenia usually develops first and may occur as early as after the first blood volume (if the initial platelet count is low) has been replaced. In most cases, clotting factors (particularly factors V and VIII) are not diluted until the blood loss exceeds three blood volumes. On the other hand, when PRBCs are used to replace blood loss, the clotting factors and platelets may be diluted after as little as 1.0 blood volume is lost. However, the predictable coagulopathy of dilution is only an approximate guide. We should monitor the PT, PTT, fibrinogen, and platelet count during massive transfusions and use this information to guide replacement therapy.

Disseminated Intravascular Coagulation and Fibrinolysis

DIC and fibrinolysis are frequently associated with shock, trauma, and other forms of tissue damage, with release of procoagulants (e.g., tissue factor) and fibrinolytics (e.g., tissue plasminogen activator). In the presence of massive blood loss, these processes must be differentiated from dilutional coagulopathy. Differentiation may be difficult, because both are associated with pathologic oozing of blood in the surgical field and each may result in prolongation of the PT and PTT, as well as thrombocytopenia.^223,224 With massive replacement using whole blood or PRBCs and adequate FFP, the fibrinogen level should remain normal; with uncompensated (acute) DIC, it may be decreased. However, replacing the blood loss with PRBCs, albumin, and crystalloid also leads to a reduction in fibrinogen.

The most helpful test for DIC and fibrinolysis is documentation of a significant increase in the level of D-dimer, a small peptide fragment generated during the digestion of fibrin by ongoing thrombolysis (i.e., through plasmin), along with evidence on the peripheral blood smear of schistocytes and helmet cells (i.e., microangiopathic hemolytic anemia).^195,225,226 Abnormal RBCs and RBC fragments are thought to arise from the slicing action of immobilized fibrin strands in the microcirculation, although the precise mechanism remains unknown. A scoring system used for screening for potential DIC has been devised but not evaluated in the operating room setting.²²⁷ If pathologic oozing in the surgical field is observed and 1.0 blood volume or less has been lost in a child who had a normal platelet count and normal PT and PTT values preoperatively, it should be suspected that the child has developed a consumptive coagulopathy.

The most effective treatment for DIC is to eliminate the cause, such as correcting shock, acidosis, or sepsis. Heparin therapy remains controversial even in children with thrombotic manifestations of DIC and is not advisable in children with active bleeding, especially in the operative setting.^224–226228

Hyperkalemia

RBCs leak potassium into the extracellular fluid during storage, particularly as the units age. The concentration of adenosine triphosphate (ATP) decreases, and the ATPase-driven Na⁺/K⁺ pump activity decreases. When the blood is outdated, 1 unit of whole blood or PRBCs has approximately 6 mEq of K⁺ in the plasma at a concentration that varies considerably (see Table 10-4) because this 6 mEq of K⁺ is distributed in the plasma and the anticoagulant solution of the stored whole blood and PRBC units. The K⁺ leak is more rapid from RBCs that have been irradiated.^229–231 To avoid the accumulation of large amounts of extracellular K⁺, irradiated RBCs are stored for a maximum of 28 days (versus 35 or 42 days for whole blood or PRBCs).

Although extracellular K⁺ is present in banked RBCs, clinically important hyperkalemia after routine transfusion has not been reported.^232–238 A study of serum potassium in neonates after exchange transfusion with PRBCs documented a decrease in serum potassium concentrations.²³⁹ A retrospective study of children undergoing massive intraoperative transfusion with PRBCs documented transient, but not life-threatening, hyperkalemia.²⁴⁰ It appears that clinically important hyperkalemia does not usually occur when PRBCs are administered at normal, slow infusion rates through peripheral IV access.²⁴¹ This may be explained by of the combination of the small absolute amount of K⁺ (about 6 mEq per unit); its rapid reabsorption into the potassium-depleted, transfused RBCs; the large volume of distribution; and dilution with crystalloid or albumin during administration. Hyperkalemia in the setting of massive transfusion is usually the consequence of extensive tissue injury or acidemia resulting from inadequate tissue perfusion.

The need to relate the size of the patient to the rate of blood replacement is infrequently a problem in adults, but it is vitally important to consider in an infant or small child. An alert from the Society for Pediatric Anesthesia²⁴² described 4 deaths among 11 children who developed hyperkalemia during transfusion; 8 were younger than 1 year, and 6 were younger than 6 months old. The Perioperative Cardiac Arrest Registry reported 8 hyperkalemic cardiac arrests related to blood transfusion.^242,243 Hyperkalemia may become a problem when large volumes of whole blood or PRBCs are administered very rapidly in adults (rates ≥120 mL/min) and in infants and children undergoing rapid blood transfusion, particularly through a central venous catheter.^240,244,245 A rapid transfusion rate of 120 mL/min in a 70-kg adult is equivalent to 1.5 to 2 mL/kg/min of blood, which is relatively easy to infuse in an infant or small child using a pressure bag or a rapid-transfusion device. An adult sustained a cardiac arrest and died after receiving Adsol-preserved PRBCs with a supernatant potassium concentration of 24 to 34 mEq/L at a rate of 6.4 mL/kg/min through a rapid infusion device (see Chapter 51).²⁴⁶ Similar transfusion rates and greater are possible in infants and children without such devices.^240,247,248

The principle is to avoid falling behind in replacing the blood loss and to avoid a situation in which a rapid and massive infusion of blood is required. Warming the blood and administering it through a peripheral IV line (rather than a central venous catheter) reduce the severe effects of hyperkalemia on the cardiac conduction system. When the rate of infusion of whole blood or PRBCs exceeds 1.5 to 2.0 mL/kg/min, the electrocardiogram must be closely monitored. If ventricular arrhythmias occur with peaked T waves in the setting of hyperkalemia (see Fig. 8-7), appropriate treatment should be instituted (e.g., calcium chloride or calcium gluconate, hyperventilation, sodium bicarbonate, albuterol, glucose and insulin); Kayexalate is the slowest and least effective in this situation and not recommended as an acute intervention.

Rapid, massive transfusion of whole blood or PRBCs to a neonate, particularly blood that has been stored for several weeks, can cause hyperkalemic cardiac arrest. It is common practice to administer RBC-containing components that are relatively young to avoid hyperkalemia in neonates requiring massive blood transfusion. If relatively young units are not available and time permits, it may be possible to wash the PRBCs to reduce the potassium concentration. However, RBC transfusion to an actively bleeding infant should not be delayed to obtain less than 7-day-old PRBCs or to wash the units. Similarly, blood for intrauterine transfusion, exchange transfusion, or neonates is chosen to be relatively young (usually less than 7 days).

These practices are consistent with the recommendations from the Society for Pediatric Anesthesia Wake Up Safe quality improvement initiative, which directs the physician to anticipate the blood loss and transfuse early and cautions that transfusing the hypovolemic child should be done slowly and through a peripheral IV catheter rather than rapidly through a central venous catheter.²⁴² If the infant requires irradiated RBC blood components, it is preferable to transfuse them soon after irradiating the blood.

Hypocalcemia and Citrate Toxicity

Citrate works as an anticoagulant for stored blood components by chelating ionized calcium (iCa²⁺). As citrate in the blood component is transfused, it is rapidly taken up and metabolized by every nucleated cell in the body, although the primary site of clearance is the liver. During massive transfusion, particularly of whole blood or FFP, the influx of citrate may temporarily overwhelm the capacity to clear it, resulting in its accumulation, which causes the plasma concentration of iCa²⁺ to decrease.^249–253 The plasma fraction in a unit of PRBCs, which contains the citrate, is a much smaller volume than in a unit of whole blood or FFP. Clinically, it is rare for the iCa²⁺ level to decrease unless the transfusion rate is very rapid; in adults, this rate must be 1.0 or more units of whole blood or FFP in 3 to 4 minutes.²⁴⁹ This effect on the iCa²⁺ concentration has been reported in neonates undergoing exchange transfusion and is more likely to occur in more-premature and lower-weight infants.²⁵³ In adult cardiac surgical patients who received whole blood at 1.5 mL/kg/min, the ventricular function curve did not improve (i.e., cardiac output did not increase although the iCa²⁺ decreased). When a similar volume of heparinized blood was administered at the same rate, the Frank-Starling response to volume loading was normal (i.e., cardiac output increased, but iCa²⁺ level did not change).²⁵⁴ These findings correlate well with earlier studies in that a decrease in iCa²⁺ concentration of clinical importance (i.e., decreased cardiac contractility) is expected to occur when the rate of infusion of citrated whole blood exceeds 1.5 to 2.0 mL/kg/min.^249,254

The change in the iCa²⁺ level after transfusion of large volumes of PRBCs is less than that observed with whole blood or FFP. Although measurable decreases in the iCa²⁺ concentration have been observed during rapid transfusion, they have only rarely been associated with cardiac toxicity in adults. The rates of infusion of citrated whole blood that produce hypocalcemia and hyperkalemia are almost identical, whereas the cardiac electrophysiologic effects produced by hypocalcemia and hyperkalemia are opposite. It is important to observe the electrocardiogram for abnormalities, especially widening of the QRS complex, prolonged QT interval, and peaking of the T wave.^167,255

Treatment for hypocalcemia and hyperkalemia is administration of exogenous calcium. Evidence from normal animals and children with extensive thermal injuries demonstrated that calcium chloride and calcium gluconate dissociate at a similar rate; hepatic metabolism of the gluconate moiety is not necessary (Fig. 10-8).²⁵⁶ Other studies during the anhepatic phase of liver transplantation also found equal ionization of calcium chloride and calcium gluconate, establishing that hepatic metabolism is not required to release calcium from gluconate.²⁵⁷ We recommend using calcium chloride or calcium gluconate to treat acute ionized hypocalcemia, with the caveat that calcium gluconate, which contains one third of the ionizable calcium of calcium chloride (by weight), be administered at a threefold greater dose (milligrams per kilogram) than the latter. Frequent, small boluses are as effective as single large boluses and result in smaller fluctuations in plasma iCa²⁺ values.²⁵⁶ Ideally, both forms of calcium should be slowly administered through a large peripheral or central vein, because both are sclerosing medications.

FIGURE 10-8 A, Changes in arterial iCa²⁺ levels in dogs after three equal elemental calcium doses of calcium chloride (4, 8, and 12 mg/kg) or calcium gluconate (14, 28, and 42 mg/kg). The rate of change in the iCa²⁺ concentration was identical for each form of calcium at each dose. There was no significant difference between the highest and lowest doses after 2 minutes, suggesting that frequent low doses are equally effective and perhaps safer than large boluses of exogenous calcium. B, Changes in arterial iCa²⁺ levels in children who received equal elemental doses of calcium chloride and calcium gluconate. At 30 seconds, both forms of calcium dissociated equally, and these data indicate that hepatic metabolism of the gluconate moiety is not required to liberate ionized calcium from calcium gluconate.

(From Coté CJ, Drop LJ, Daniels AL, Hoaglin DC. Calcium chloride versus calcium gluconate: comparison of ionization and cardiovascular effects in children and dogs. Anesthesiology 1987;66:465-470.)

The volume of plasma and therefore the absolute amount of citrate in a unit of FFP are slightly less than the amounts in a unit of whole blood. However, the citrate load can be more rapidly delivered in FFP because it can be infused more rapidly owing to its low viscosity. It is easy to give a large citrate load in a brief period. Caution is urged when FFP or whole blood is rapidly infused, especially if the child already has a low iCa²⁺ concentration or impaired hepatic function (e.g., neonates and children undergoing liver transplantation). Figure 10-9, A, shows the changes in iCa²⁺ concentrations that occurred in children who had extensive thermal injuries and who received rapid FFP infusions at a rate between 1.0 to 2.5 mL/kg/min for 5 minutes. The maximum decrease in iCa²⁺ levels occurred between the fourth and fifth minute. There was no difference in the nadir in the iCa²⁺ concentration among the three largest rates of FFP infusion.²⁵⁸

FIGURE 10-9 A, Changes in arterial iCa²⁺ levels in children with severe thermal injuries during infusions of fresh frozen plasma at a rate of 1.0 to 2.5 mL/kg/min for 5 minutes through an infusion pump. Notice the dangerous although transient decrease in the iCa²⁺ concentration, with the nadir occurring between the fourth and fifth minute. Ionized hypocalcemia occurs when the infusion rate equals or exceeds 1 mL/kg/min. B, Changes in iCa²⁺ levels occurred in four thermally injured children who received calcium chloride (arrow) after 2 minutes of fresh frozen plasma infusion. There were no sharp increases or decreases in iCa²⁺ levels.

(From Coté CJ, Drop LJ, Hoaglin DC, Daniels AL, Young ET. Ionized hypocalcemia after fresh frozen plasma administration to thermally injured children: effects of infusion rate, duration, and treatment with calcium chloride. Anesth Analg 1988;67:152-60.)

If exogenous calcium is administered during rapid FFP transfusion, large decreases in the iCa²⁺ level can be avoided (see Fig. 10-9, B). We transfused FFP at a rate of 2 mL/kg/min for 10 minutes (equivalent to an average adult receiving 1400 mL FFP over 10 minutes) in six children with extensive burn injuries and measured very significant decreases in iCa²⁺ levels but observed no consistent adverse cardiovascular events. However, because these children had extensive burn injuries, we presumed they were hypermetabolic and therefore able to rapidly metabolize the excess citrate, which limits our ability to extrapolate these data to children without burn injuries (see Chapter 34).²⁵⁸

In dogs anesthetized with halothane, we determined that citrate-induced ionized hypocalcemia caused significantly greater cardiovascular depression as the concentration of expired halothane increased.²⁵⁹ These findings are consistent with the combined myocardial depression caused by ionized hypocalcemia and the myocardial depressant effects of calcium channel blockade caused by the halothane.^260,261 Although halothane seems to exert the greatest calcium channel blocking activity of the inhalational anesthetics, all inhalational anesthetics depress the myocardium through this mechanism and therefore should augment the myocardial dysfunction associated with citrate-induced hypocalcemia, although similar data with sevoflurane and desflurane have not been forthcoming.^262,263

The adverse cardiac effects of citrate-induced hypocalcemia may be increased if FFP is rapidly administered through a central venous catheter because there is less time to dilute the FFP and metabolize the citrate before it enters the heart and coronary vessels. FFP may be more safely administered through a peripheral IV site. Calcium should be administered during rapid transfusion of FFP (>1 mL/kg/min) to attenuate this transient but dangerous citrate toxicity, especially in the presence of potent inhalational anesthetics.^259–263 Our clinical impression is that neonates and small infants are particularly vulnerable to the developing citrate toxicity because it is easier to administer a relatively large volume of FFP over a brief period to them and because citrate may not be eliminated as rapidly (i.e., first-pass effect through the liver) in infants. In addition to thermally injured patients, children undergoing liver transplantation and cardiac surgery are likely to require FFP and may develop hypocalcemia.^264,265 Liver transplantation recipients are particularly susceptible to decreased iCa²⁺ levels during the anhepatic phase and during the pre-anhepatic phase of surgery because of impaired hepatic metabolism, impaired hepatic blood flow, and the reduced ability to metabolize citrate.^266–270 An IV preparation of calcium should always be available when a major transfusion with FFP is anticipated.

Acid–base Balance

Massive transfusions usually occur in one of two situations: severe trauma with shock or major surgery with massive blood loss. In the first situation, severe metabolic acidosis may occur because of low cardiac output and diminished oxygen delivery. Correction of the acidosis with sodium bicarbonate may be a necessary part of the resuscitation, along with blood volume replacement. In this situation, impaired coagulation may occur because of the acidosis.^{164,180,271–273} In the operating room, intravascular volume is usually maintained, and because most instances of massive blood loss are anticipated, replacement of acute blood loss is more controlled. Even with repeated massive blood loss, metabolic acidosis is not usually a problem provided severe hypovolemia is avoided.^274–276 Sodium bicarbonate therapy must be governed by the child’s acid–base status because metabolic acidosis does not usually occur with massive transfusion unless accompanied by severe hypovolemia, low cardiac output, or hypoxemia.

With massive blood transfusion, the child may develop a moderate to severe metabolic alkalosis due to the large volume of transfused citrate and its conversion to bicarbonate, which causes a metabolic alkalosis within hours of a massive transfusion.^{250,265,277–279} If exogenous bicarbonate therapy is superimposed on this endogenous metabolic alkalosis, a leftward shift of the oxyhemoglobin dissociation curve may result. It is important to determine the acid–base status before administering sodium bicarbonate to avoid overcorrecting the pH and shifting the oxyhemoglobin dissociation curve further.

Hypothermia

Hypothermia may be a significant problem associated with major blood loss and its replacement. Hypothermia decreases oxygen consumption and reduces oxygen demand. It may also increase oxygen consumption if the child shivers and decrease tissue delivery of oxygen by a leftward shift of the oxygen-hemoglobin dissociation curve; severe hypothermia (about 32° C) may induce a refractory ventricular tachycardia.^167,280,281 Hypothermia may also profoundly compromise platelet function and impair the coagulation cascade.^{164,180–182,271,272,282,283}

Prevention of hypothermia by all available means is considered an essential part of damage-control resuscitation of trauma patients.^284–287 Banked blood products are stored between room temperature and 4° C, depending on the blood component. In the setting of high-volume transfusions, all blood products should be infused through a blood warmer. No other method should be considered (e.g., storing blood in a warming cupboard, immersing in hot water) because RBCs hemolyze readily with prolonged warming or overheating (>42° C). Warming blood and all other IV infusions with a high-capacity blood warmer, using hot air warming blankets and radiant warmers, placing plastic wrap around extremities, inserting a heated humidifier in the anesthesia circuit, covering the child’s head, and maintaining a warm to hot operating room contribute to maintaining thermal neutrality. Rapid-transfusion devices markedly improve the rapidity of transfusion and the thermokinetics involved.^288–294 In one case, one author (CJC) and two nurses transfused more than 50,000 mL of blood products and crystalloid in less than 1 hour, and the child’s temperature remained at or exceeded 34.5° C.²⁸⁸

Monitoring during Massive Blood Transfusion

If massive blood loss can be anticipated, adequate monitoring should be instituted before surgery begins so that baseline information can be recorded. Large-bore peripheral IV access is preferable (E-Fig. 10-1) because these catheters have reduced resistance and they avoid giving blood products and drugs directly into the heart, as with a CVP line. If a child arrives in the operating room in shock (e.g., trauma patient), the physician must be careful to differentiate hypovolemia from other causes of shock (e.g., tension pneumothorax, cardiac tamponade) (see Chapters 38 and 39). Invasive monitoring inserted during resuscitation helps with the differentiation. Our philosophy is one of aggressive invasive monitoring to provide maximum data for evaluation and management of a critically hypovolemic child.

E-FIGURE 10-1 Rapid infusion intravenous catheters are available. A 20-gauge catheter is inserted into a peripheral vein, and a guidewire is advanced. A moderate skin nick is made with the provided scalpel blade, and the dilator (blue catheter) is advanced into the vein. The Rapid Infusion Catheter (white catheter; Rapid Infusion Catheter Exchange Set, Arrow International, Erding Germany) is then advanced into the vein, and the dilator and guidewire are removed. The Rapid Infusion Catheter is sutured in place (side hole on white catheter). In this case, an 8.5-Fr catheter that is only 6.4 cm long is in place, providing a large-bore, low-resistance catheter that is able to adequately function with rapid transfusion devices (see Figs. 51-1 and 51-2). These catheters are available in smaller sizes and from various manufacturers, and the devices are best combined with large-bore extension catheters.

FIGURE 10-10 Changes in the contour of an arterial tracing with hypovolemia. A, The normal tracing shows a sharp upswing of the arterial pulse wave and position of the dicrotic notch. B, There is movement of the dicrotic notch and widening of the pulse wave. C, The pulse wave widens further. D, There is further widening of the pulse wave and loss of the dicrotic notch. An exaggerated (“picket fence”) respiratory variation of pulse wave is shown in the right tracing compared with the left. Factors other than hypovolemia, such as hypothermia, deep anesthesia, vasodilator therapy, or damped tracing (e.g., clot, air bubble), may produce artifactual changes in the shape of the arterial waveform.

Monitors and the data they generate are helpful, but anesthesiologists must rely on more than numbers. It serves no purpose to have sophisticated monitoring if the data provided cannot be interpreted and related to clinical events. The final monitor is ultimately the anesthesiologist’s attention and judgment.

Thromboelastography provides a standardized means of quantitating the rapidity and quality of clot formation and a means for identifying fibrinolysis.^303,304 This device was first used primarily during massive blood transfusion situations related to liver transplantation and now is used in several areas, particularly cardiac surgery.^303–312 Some studies have found that thromboelastographic screening was not useful in predicting bleeding after cardiac surgery and was associated with a large percentage of false-positive results.^313,314 Use of heparinase helps to improve the accuracy of the results by eliminating the effects of heparin, but this requires two machines to simultaneously sample blood (with and without heparinase) to have results within a useful period.³¹⁵ This monitor may also be used to guide the effectiveness of antifibrinolytic therapy.^310,311,316 The exact role of thromboelastography in the routine care of pediatric cardiac surgical patients, liver transplant recipients, and children who have had massive blood loss with ongoing coagulopathy has not been established. Tables 10-10 and 10-11 summarize the expected changes in various blood components when administered on a per kilogram basis and the estimated FFP requirement.

Infectious Disease Considerations

It is important for anesthesiologists to use basic precautions to minimize their risk when administering blood products and contacting body fluids. Blood and body fluids, even in infants, are capable of transmitting hepatitis B, hepatitis C, and HIV through parenteral exposure (e.g., cuts, needlestick), mucous membrane contact, or exposure to nonintact skin.^4,317–326 Accidental needlesticks were the most common means of exposure in the operating room for anesthesiologists. The introduction of safe IV needles, a needleless IV system, the use of stopcocks, and never recapping used needles has reduced the incidence of this problem.^191–193327 The incidence of HIV seroconversion after needle puncture is estimated to be 0.2% to 0.5% (∼1 in 300), although the conversion rate is much greater after a needlestick injury from individuals with hepatitis.^{4,194,317,323–325,328}

Anesthesiologists must practice universal blood and body fluid precautions (e.g., gloves, goggles) and should minimize the use of needles and especially the practice of recapping needles. The management of infants may be less than optimal with the use of three-way stopcocks because of the fluid required to flush the system and the ease of introducing air into the IV line. In these infants, single-use needles without recapping or needleless systems are recommended. If an exposure occurs from a known HIV-positive patient or puncture from a needle of unknown origin, consult a specialist immediately to determine the need for immediate institution of prophylactic medical therapy as soon as possible.^327,328 Early institution of drug therapy is recommended to reduce the potential for seroconversion (see Tables 49-6 and 49-7).

Erythropoietin

Use of recombinant erythropoietin to promote endogenous RBC production can reduce the need for allogeneic RBC transfusions. This therapy has proved useful in many populations, including preterm infants, children on chemotherapy, children with renal failure, children of Jehovah’s Witnesses, and children undergoing elective major reconstructive surgery, spinal surgery, liver transplantation, or cardiac surgery. Coordination with the hematology department, blood banking, and the primary patient care team is required to take full advantage of this form of therapy.^329–347 Although usually well tolerated, erythropoietin should be used with careful monitoring in patients with hypertension. Erythropoietin is available in a lyophilized (freeze-dried form for reconstitution) or in a dilute albumin solution. Some members of the Jehovah’s Witness faith who refuse albumin should receive the lyophilized formulation, which is albumin free.

Preoperative Autologous Blood Donation

Donation and storage of blood before elective surgery have reduced the use of allogeneic RBCs.^{230,330,343,348–361} Banked units of PRBCs may be stored for 35 to 42 days in the liquid state,^362,363 permitting the donation of several units and the time required for the patient (usually teenagers) to regenerate the RBC mass before surgery. Children unable to mount an erythropoietic response to phlebotomy may only succeed in making themselves anemic, so administration of iron, vitamin C, and folate is important, as is monitoring for reticulocytosis to ensure the bone marrow is replenishing the donated RBCs. Autologous donation should not be attempted in children with significant cardiac ischemic disease or those with an active infection because bacteria can seed the collected unit and overgrow during storage. Autologous donation should be discouraged before procedures for which RBC transfusion is unlikely. Donated blood that is not used by the donor must be discarded rather than enter the general blood bank pool.

Patients or family of pediatric patients may wish to obtain blood from family members or friends (i.e., directed donation). Despite the perception that this donor pool may be safer than the pool of volunteer, allogeneic donors, there is no evidence that this is true. Directed donors are considered to be allogeneic donors and are screened and tested in the same manner as any volunteer donor. Because there is a greater risk of transfusion-associated GVHD with cellular components from a donor who is a blood relative, these units are irradiated to eliminate the possibility of this usually fatal complication of transfusion.^364–366

Intraoperative Blood Recovery and Reinfusion: Autotransfusion

Recovery of blood from an operative site and reinfusion after some form of processing has been applied to major vascular, cardiac, and multiple trauma situations for many years.^{353,358–360,367–377} The common techniques used wash the recovered blood in a centrifuge so the product consists of the child’s RBCs suspended in saline at a hematocrit of 50% to 60%.^368,375 Cellular debris, excess citrate or heparin, free hemoglobin, activated clotting factors, and clotted blood are almost completely removed. Autotransfusion avoids the infectious and immunologic risks of allogeneic transfusion and, if reinfusion is carried out in the operating room, minimizes the opportunity for mistransfusion.^{367,375,376,378}

Intraoperative blood recovery is not widely used in infants and children.* The equipment is designed for adults, although some manufacturers have adapted standard devices for pediatric use. We have found blood recovery to be a useful adjunct to minimize allogeneic blood transfusion during scoliosis surgery. This technique may also be used in conjunction with preoperative autologous blood donation, further reducing the need for allogeneic RBC transfusions.^{358–360,371,372} The capital investment for the devices and the costs for the disposables and a trained operator are significant. However, these expenses can be offset if three fewer units of allogeneic RBCs are transfused. Development of pediatric-sized equipment should make this technique more widely used and more cost-effective even in smaller children.³⁸³

Controlled Hypotension

Controlled hypotension, the intentional reduction of systemic perfusion pressure, has been used to reduce intraoperative blood loss or to provide a relatively bloodless operating field.^{329,387–398} Hypotensive anesthesia may be accomplished with several techniques, including continuous infusion of vasodilators, β-adrenergic blockade, deep inhalational anesthesia, and large-dose opioid infusions (e.g., remifentanil).^{388,399–401}

Controlled hypotension is reserved for older children and teenagers undergoing major reconstructive or orthopedic surgery. The choice of technique and the degree of induced hypotension depend on the surgical procedure. For procedures in which a dry surgical field is the end point with little potential for rapid blood loss, a technique that may take some time for recovery is acceptable (e.g., deep inhalational agent with or without β-blockade). If surgery carries the possibility of rapid or massive blood loss, a technique that is rapidly reversed (e.g., nitroprusside, nitroglycerin, remifentanil) is probably safer. Controlled hypotension with mean arterial pressure (MAP) of 55 to 60 mm Hg is used less in pediatric anesthesia in recent years, although there are insufficient data to categorically state that the risks of controlled hypotension outweigh the purported benefits. Moderate hypotensive techniques with a MAP of 65 to 70 mm Hg are a more common practice and are likely associated with less risk, although no studies have been published in this regard.³⁴⁷

Normovolemic Hemodilution

Intentional isovolemic hemodilution is a useful adjunct to an anesthesiologist’s strategy for reducing allogeneic blood transfusions.^{329,330,353,460,473–480} Two basic methods can be applied:

The latter technique is preferable because it reserves a quantity of the child’s own blood, which can be returned at the end of the surgical procedure. For a Jehovah’s Witness, this technique often conforms to religious guidelines if direct continuity is maintained with the child’s circulation.^{460,481–488}

During acute normovolemic hemodilution under anesthesia, the distribution of blood flow improves with the reduced hematocrit. Improved blood rheology is the major compensatory mechanism for maintaining oxygen delivery despite a reduced hematocrit. Oxygen extraction increases in the presence of an inadequate circulating blood volume or when the hematocrit decreases to less than 20%. If the hematocrit decreases to less than 15%, subendocardial myocardial ischemia may develop.^489–492 At this extreme level of anemia, dissolved oxygen begins to assume a more important role in oxygen delivery.⁴⁹³ In several reports, extreme acute normovolemic hemodilution (hemoglobin as low as 2 g/dL) were well-tolerated,^{489,494–497} although we cannot endorse the use of this technique in children because it is impossible to assess the effects of such an extreme hemoglobin concentration on the long-term cognitive ability. Nonetheless, these reports^489,497 indicate that healthy children can tolerate these extreme hematocrit concentrations provided they are anesthetized, normovolemic, slightly hypothermic, and ventilated with 100% oxygen. We limit the absolute minimum hematocrit to 15%, but we prefer to maintain the hematocrit closer to 20% at all times.