Identification of Patients at Risk
The coagulopathy associated with CPB is related primarily to the interaction of blood components with the artificial surfaces of the CPB circuit, which results in derangements in platelet function, abnormal functioning of the coagulation cascades, and excessive fibrinolysis. The administration of high-dose heparin to prevent coagulation within the CPB circuit and hypothermia achieved during bypass further contribute to hemostatic derangements. Finally, although the use of asanguineous crystalloid prime rather than the whole-blood pump prime used historically has reduced the amount of blood transfused during CPB dramatically, the resulting hemodilution contributes to the risk of low intraoperative and postoperative hematocrit, which can be independent risk factors for transfusion in the postoperative period. Other risk factors can be assessed in the preoperative state that can identify those patients who may be at high risk of bleeding or who have a low red cell mass, both of which may require autologous blood transfusions.
One of the most important predictors of postoperative bleeding in the surgical patient is a personal or family history of any excessive bleeding or documented bleeding disorders. Many disorders can be confirmed with simple laboratory tests demonstrating some level of coagulation derangement. However, in the cardiothoracic patient population, medications and acquired medical diseases and their associated hemostatic defects are likely to be the most common risk for bleeding. Notably, the use of aspirin alone or included in other medications intended for pain relief or treatment of other ailments is very common. The prevalence among patients undergoing unplanned surgery may be as high as 50%, and this may be even higher among patients with previously diagnosed coronary artery disease.17–19 The currently published data suggest that this does not represent a significant bleeding risk, and there is little evidence to suggest that bleeding times correlate with operative blood loss in these patients.20 Heparin will inhibit factors II and X, and may lead to immune-mediated thrombocytopenia. Coumadin can block gamma-carboxylation and lead to multiple factor deficiency.
Herbal Extracts for Cardiovascular Health
The use of herbal extracts and complementary medicine has become popular in the prevention of arterial thrombotic disease. In 1997, an estimated $21 billion was spent in the United States on complementary and alternative therapies.21,22 Herbs such as thyme and rosemary have been shown to have a direct inhibitory effect on platelets.23
Despite the increased potassium contained in fruits and nuts, menu planning for patients on warfarin can include a healthy diet of these foods without compromising the stability of their oral anticoagulation therapy because most fruits are not important sources of vitamin K, with the exception of some berries, green fruits, and prunes.24 In a case report regarding fish oil supplementation, it was demonstrated that additional anticoagulation could have resulted from an interaction with warfarin therapy. This case reveals that a significant rise in the international normalization ratio (INR) occurred after the dose of concomitant fish oil was doubled. Fish oil, an omega-3 polyunsaturated fatty acid, consists of eicosapentaenoic acid and docosahexaenoic acid. This fatty acid may affect platelet aggregation and/or vitamin K–dependent coagulation factors. Omega-3 fatty acids may lower thromboxane A2 supplies within the platelet, as well as decrease factor VII levels.25
The popularity of herbal additives can be evidenced by the more than $700 million spent annually and the continued expected spending on such products.26 Despite the potential benefits of many herbs with regard to improved well-being, many adverse risks exist. Primarily there is an increased cardiovascular risk among the more commonly used supplements, herbs such as garlic, ginger, and gingko are associated with platelet dysfunction–derived bleeding, whereas supplements such as ginseng and licorice may lead to elevated blood pressure.27
Health-care workers can play a crucial role in identifying possible drug interactions by asking patients taking warfarin about herbal and other alternative medicine product use. Furthermore, the clinical importance of herb–drug interactions depends on many factors associated with the particular herb, drug, and patient. Herbs should be labeled appropriately to alert consumers to potential interactions when used concomitantly with drugs and should recommend a consultation with one's general practitioner.
Autologous Blood Donation
One of the primary concerns in blood conservation is the patient's size and preoperative red blood cell volume. Two early reports by Cosgrove and Utley demonstrated these factors in addition to preoperative anemia as independent risk factors for blood transfusion.28,29 As discussed later in this chapter, there are several relatively simple manipulations to the CPB circuit that can be made to reduce the amount of hemodilution that the patient experiences while on bypass to reduce the overall risk of transfusions.
Preoperative autologous donation (PAD) is a recognized strategy to reduce the risk of homologous blood transfusion in the perioperative period. Although this technique has been in practice since the 1960s, its use in cardiac surgery did not achieve widespread acceptance until the 1980s, with the advent of HIV, as an effort to reduce homologous blood exposures. Unfortunately, in cardiac surgery, the acuteness of the operations and dealing with an older and sicker patient population often preclude PAD because there must be enough preoperative time for autologous collection as well as red blood cell mass regeneration before arriving in the operating room.
Several preoperative characteristics can identify the cardiac patient who is eligible for PAD. The first criterion is that the patient be able to wait the required time for donation and red blood cell regeneration. This length of time typically varies depending on the type of surgical procedure planned (larger operative procedures likely requiring a larger amount of blood) and patient characteristics (eg, body size, blood volume, and hematocrit). In general, this time is a minimum of 2 weeks per unit of blood donated to allow for red blood cell regeneration. The second criterion is that the patient be healthy enough to undergo donation. This criterion would preclude patients with severe left main stem stenosis, critical aortic stenosis, congestive heart failure, and idiopathic hypertrophic subaortic stenosis, as well as patients with severe coronary artery disease and ongoing ischemia, given that many of these patients would have been screening failures for the first criterion. The third criterion is that the patients not have active endocarditis. The time between donation and receiving the PAD unit is ample time for bacteria to replicate in the donated unit with resulting bacteremia, which is potentially life threatening. The fourth criterion is that the patient has an adequate hematocrit and red blood cell mass. A preoperative hematocrit of less than 33% regardless of red blood cell mass is a contraindication to PAD according to the American Association of Blood Banks (AABB) guidelines. A patient with a hematocrit of greater than 33% may be eligible provided that other criteria are met.
Several options exist for patients who historically were not eligible for PAD. Recombinant erythropoietin can be used to accelerate red blood cell production in anemic patients; this strategy is used commonly in Jehovah's Witness patients to increase their red blood cell mass before surgery.30 However, this can be quite costly; as a result, it is usually reserved for patients who are unable to tolerate homologous blood transfusions whether for religious reasons or because they have a rare blood type. An alternative strategy is to stimulate the body to increase the release of endogenous erythropoietin by allowing patients with hematocrits below the traditional cutoff to undergo PAD. The resulting anemia experienced by the patient in the postdonation period is a strong stimulant for endogenous erythropoietin production, ultimately leading to an increase in red blood cell mass.30
Red blood cell mass is related to patient body size.31 A traditional cutoff of 110 lb had been used for PAD. However, the AABB does make specific allowances for PAD in smaller patients. The current recommendation is that no more than 15% of the patient's effective blood volume should be removed at any given time, which takes into account smaller patient body size. PAD should be pursued aggressively for these small patients with low red blood cell mass and hematocrit because they are at highest risk of receiving a blood transfusion at some time during their hospital stay. The mean rate of red blood cell generation of the studies that provided adequate data is 0.46 units per week, or slightly less than 1 unit every 2 weeks.32 In conjunction with PAD, oral iron therapy should be initiated at the time of first donation to ensure adequate iron stores for red blood cell regeneration.
Owing to the relatively acute illness of the population, rarely is there sufficient time for PAD in the cardiothoracic surgical patient.33 In addition, PAD has been supplanted largely by intraoperative blood salvage techniques for reasons of cost-effectiveness (ie, blood withdrawal, preparation, storage, and potential erythropoietin therapy add to costs) and advances in intraoperative blood salvage techniques such as intraoperative autologous donation (IAD; discussed later in this chapter), retrograde autologous prime of the CPB circuit, use of cell salvage, and regular use of cardiotomy suction.