Blood Transfusion in Surgery II Cardiac and Vascular Surgery

Blood transfusion in cardiac surgery accounts for 10-14% of all red cells transfused in the United States. Mean usage/patient is about 5 units, although there is a huge variation between different institutions. This results in 1.2 million units per year transfused in the United States. Transfusion practices in cardiac surgery are, therefore, of great importance to hospital blood banks.

The cause(s) for this variation in practice is not entirely clear, but current evidence indicates that certain kinds of patients have an increased likelihood of blood transfusion. Female gender, increased age (over 70), low preoperative hematocrit and extensive procedures such as combined bypass and valve procedures with long pump runs are predictive of increased blood usage. Other determinants of blood use appear to be choice of the vascularization vessel, either saphenous graft or internal mammary graft. Even allowing for these known determinants, there is evidence of a strong influence of transfusion styles (Chapter 9).

The causes for blood transfusion in cardiac surgery are shown in Table 10.1. An important reason for red blood cell transfusion in cardiac surgery is extracorporeal circulation since this causes a dilution of the red cell mass of the patient. For patients with high hematocrits and a large intravascular volume, this dilution rarely precipitates a need for red cell transfusion. In some patients, however, preoperative hematocrits or intravascular volume or both may be low, (such as low weight females). Under these circumstances, the extracorporeal circuit will cause a significant dilution of the red cell mass, often to a hematocrit of less than 16. Extensive resections and lack of attention to good local hemostasis will also result in excessive bleeding which may also require red cell replacement. A third reason is extracorporeal damage occurring to platelets and activation of soluble systems such as the inflammatory and fibrinolytic systems. When the patient comes off the pump and has been neutralized with protamine, this may manifest as excessive oozing. Furthermore, the use of fluids to expand the intravascular volume, such as crystalloids and/or colloids, may further dilute blood cells and coagulation factors, with a resulting dilutional coagulopathy. Attachment of platelets to a large aortic graft may result in thrombocytopenia and also contribute to a bleeding disorder, which may require treatment with blood components, either platelets, possibly plasma, or both.

Intraoperative platelet transfusion in cardiac surgery in very controversial. Prophylactic transfusions have not been shown to be effective. The rationale for the use of therapeutic platelets is the presence of unexpected, excessive bleeding (wet field) as observed by the anesthesiologist or surgeon. Since the duration and threshold for this observation prior to ordering platelets may vary from surgeon to

Table 10.1. Reasons for blood transfusion in cardiac surgery

1. Extracorporeal circuit dilutes the red cell mass, causing anemia.

2. Excessive bleeding with dissection of the chest or graft source.

3. Long pump runs can cause platelet dysfunction, and activate the inflammatory and fibrinolytic system causing an acquired bleeding disorder.

4. Intravenous fluids and the transfusion of salvage red cells in saline will cause a dilutional coagulopathy.

5. Large aortic arch grafts will consume platelets, causing thrombocytopenia.

6. Excessive bleeding due to #3, #4, or #5 will increase the need for red cell replacement.

surgeon, this likely explains much of the variation in platelet use. The empiric use of plasma or even cryoprecipitate may also occur in this context, often at ineffective doses. Although use of tests of hemostasis may be helpful in guiding the transfusion of these components, in practice, the turnabout time is often too long to be of practical use. Studies using intraoperative coagulation devices with a short turnabout time have been able to reduce plasma and cryoprecipitate transfusion by measuring clotting times or fibrinogen levels. Clotting times such as the prothrombin time (PT) or activated partial thromboplastin time (aPTT) are frequently prolonged. However, a PT or aPTT ratio of 1.5 times mean in the presence of excessive bleeding is sometimes used as an indication for plasma transfusion (10-15 ml/Kg). Although hematologists often regard a fibrinogen of less than 100 mg/dl as an indication for cryoprecipitate transfusion (fibrinogen replacement), surgical services may use higher thresholds, e.g., 150 mg/dl or 200 mg/dl. Lack of agreement on the above accounts for the substantial intraoperative use, and variation in use, of blood components in cardiac surgery.

Postoperatively, excessive bleeding is manifested by an increase in the volume of chest tube drainage (> 400 ml in first two hours). This is often treated (appropriately) with red cell replacement therapy. Empirical treatment with platelets, plasma, and/or cryoprecipitate can also occur. Separating this bleeding from surgical site bleeding can be difficult with potential for over transfusion of blood components, especially platelets. Overall, institutions vary in the percentage of patients who receive platelet transfusions, from less than 5% to greater than 80%. It is likely that some patients may benefit from these platelet transfusions. However, it is also likely that a substantial number do not benefit, resulting in blood component wastage.

Modest postoperative normovolemic anemia (Hct 24-30; Hb 8-10 g/dl) is common and usually well tolerated, and the practice of routinely transfusing red cells to maintain the hematocrit greater than 30 (Hb > 10 g/dL) likely reflects a transfusion style.

The role of plasma and cryoprecipitate in ameliorating postoperative clinical bleeding in cardiac surgery is controversial. Mild prolongations of clotting times and modest reduction in fibrinogen are very common in postoperative cardiac

patients. Administration of these products in the presence of significant clotting time prolongation time (greater than 1.5 times control) or severe reduction in fibrinogen (less than 100 mg%), is reasonable, but treatment of bleeding in the presence of borderline abnormalities may simply delay the need for surgical reexploration.

There have been numerous approaches to reduce allogeneic blood transfusion in cardiac surgery. These are listed in Table 10.2. Predeposit autologous donation (Chapter 3) may be useful in reducing the transfusion of allogeneic red blood cells under certain circumstances. This is particularly the case if preoperative erythropoietin is used to increase the number of collections. However, there is potential danger from acute hypotension occurring during the predeposit donation in this high-risk population. In addition, this approach is cumbersome for the patient preoperatively and involves an additional expense. As such, given the low cost benefit, it is unlikely to become wide spread practice in an era of cost containment.

Desmopressin (DDAVP) was initially described in the mid-1980s as being of benefit in reducing bleeding and transfusions in patients undergoing cardiac surgery. Subsequent studies have failed to reproduce the original data with regard to the beneficial effect, and interest in the use of this drug in cardiac surgery has decreased. An agent of accepted benefit, however, is the anti-protease, aprotinin. Aprotinin is a 65 kD protein derived from bovine lung. This anti-protease has been shown in numerous studies to reduce the transfusion of red cells and other blood components. Aprotinin is known to inhibit kallikrein and, therefore, reduces the inflammatory response. Dosages are expressed in kallikrein inhibitory units (KIU). In addition, it inhibits plasmin and, therefore, reduces fibrinolytic activity. Aprotinin commonly is administered in one of two dosage regimens: 2 million KIU pre-pump; 2 million in the pump and 500,000 KIU/h as a continuous infusion post pump. Half-dose regimens have also been used and shown to be equally efficacious in reducing allogeneic transfusion. Aprotinin is a very expensive agent, and the half dose regimen is, therefore, more attractive. There has been concern in the United States with regard to postoperative graft thrombotic events,

Table 10.2. Approaches to reduce allogenic blood transfusion in cardiac surgery

1. Preoperative erythropoietin with, or without, predeposit autologous donation.

2. Intraoperative blood salvage.

3. Preoperative hemodilution or platelet sequestration.

4. Pharmacologic agents:

(b) Amino caproic acid or tranexamic acid.

(c) Aprotinin.

although studies in Europe have failed to show such an adverse effect. Other problems associated with aprotinin are the possibility of hypersensitivity and for this reason a test dose is administered initially. Aprotinin is likely to be most useful in patients undergoing extensive procedures with long pump runs or re-do procedures. Aminocaproic acid has not been as extensively formally studied as aprotinin in this patient population. Aminocaproic acid is an inhibitor of plasmin and a lower cost pharmaceutical. Empiric use has been more widespread for this reason. Topical thrombin or fibrin glue are agents which may be useful when excessive microvascular oozing occurs with difficult dissections such as re-do procedures.

Preoperative hemodilution (Chapter 3) is attractive since it supplies an autologous product with fresh platelets and blood coagulation factors to the patient. In prospective studies, however, preoperative hemodilution has been disappointing in demonstrating any decrease in the need for red cell transfusion. Platelet sequestration is a modification of preoperative hemodilution in which platelets are collected using an apheresis device, but its role in decreasing the need for blood transfusion is controversial. Lastly, intraoperative salvage of blood is common in cardiac surgery. Autologous red cells shed from the dissection fields may be aspirated into the reservoir of a salvage device, subsequently washed and reinfused. Blood from the cardiac bypass pump may be given directly intravenously. However, it is a more common practice in the United States to process this blood through the salvage machine with the red cells being returned suspended in saline. Alternatively the contents of the pump and reservoir may be ultrafiltrated; this produces a product rich in colloids with a lower total volume.

An important consideration for overall transfusion in cardiac surgery is agreement regarding thresholds at which decisions are made with regard to transfusion. These are: (1) acceptable hematocrit tolerated on the pump, (2) intraoperative platelet transfusions in suspected excessive bleeding after protamine neutralization and (3) transfusing red cells postoperatively in normovolemic patients.

Vascular surgical procedures vary greatly in potential to require the transfusion of allogeneic blood. The most important vascular surgical procedure in this regard is aortic abdominal aneurysectomy (Triple A). The procedure is typically associated with the need for a large volume transfusion of red blood cells and occasionally plasma and platelets due to the development of a dilutional coagulopathy (see Chapter 14). One of the more important aspects of managing AAA resections is the use of intraoperative blood salvage, and this can result in a dramatic reduction in allogeneic blood transfusion in these patients. The role of predeposit autologous blood and/or preoperative hemodilution in elective cases is unsettled. These patients may have compromised cardiac function and depositing blood preoperatively may potentially expose the patient to donation risk without achieving any substantial reduction in the transfusion of allogeneic blood. Other types of revascularization procedures, such as femoro-popliteal bypass or endarterectomies, are not, in general, associated with large volume transfusions. The use of intraoperative salvage has sometimes been advocated in some of these procedures, although the volume of salvaged blood tends to be minimal.

Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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