Cardiopulmonary Bypass: Principles and Practice

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P. H. Hornick Department of Cardiac Surgery, National Heart and Lung Institute, Imperial College School of Medicine at Hammersmith Hospital, London, W12 ONN United Kingdom.

K. M. Taylor: Department of Cardiac Surgery, National Heart and Lung Institute, Imperial College School of Medicine at Hammersmith Hospital, London, W12 ONN United Kingdom.

Immune responses offer protection to the organism from a variety of pathologic insults. The immune system comprises two fundamental features that act to effect such protection by generating both innate and acquired responses to the insult. Innate immunity depends on a variety of immunologic effector mechanisms that are neither specific for a particular infectious agent nor improved by repeated encounters with the same agent. In contrast to the innate system with its phagocytic and natural killer cells and soluble factors such as complement, lysozyme, and acute phase proteins, the adaptive immune system concerns T- and B-cell function and soluble factors such as antibody. Adaptive (or acquired) immunity is specific for the inducing agent and is marked by an enhanced response upon repeated encounters with that agent. The key features of the adaptive immune response are therefore memory and specificity. In practice, there is considerable overlap between these two types of immunity, because the adaptive immune system can direct elements of the innate system, such as phagocytes or complement factors.

Cardiopulmonary bypass (CPB) impinges on these two elements of the immune response. CPB generates an unphysiologic innate immune response manifested as a whole body systemic inflammatory response. At the same time, CPB also induces the cellular and humoral constituents of the adaptive immune system to undergo quantitative and qualitative changes, leading to a temporary immunodeficiency.

Because CPB induces a nonphysiologic (detrimental) inflammatory response, additional specific immunologic deficit has the potential to further increase the adverse effects of CPB by reducing susceptibility to infection.


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At the outset, it is important to define what is meant by the systemic inflammatory response in the context of CPB. A variety of terms are used to describe this pathologic condition: "sepsislike" syndrome; hyperdynamic circulation; postperfusion syndrome; disseminated intravascular post-pump syndrome; and pathologic similarities to the "systemic inflammatory response syndrome" (SIRS); adult respiratory distress syndrome; and multiple system organ failure.

It is generally accepted that CPB produces a "whole body" inflammatory response. In its most severe form, a spectrum of injury may be observed that includes one or more of the following clinical manifestations: pulmonary, renal, gut, central nervous system, and myocardial dysfunction; coagulopathy; vasoconstriction; capillary permeability; vasodilatation; accumulation of increased interstitial fluid; hemolysis; pyrexia; and increased susceptibility to infections and leukocytosis (1–6). Certainly this "post-bypass" syndrome would come within the rather loose and unsatisfactory catch-all term of SIRS (7). However, the breadth of the diagnostic criteria that permit inclusion and the absence of CPB in the terminology leads to the suggestion of yet another acronym to indicate the systemic inflammatory response after bypass, SIRAB. The potential diagnostic criteria for SIRAB would at least combine the noninfectious clinical picture(s) mentioned above with certain hemodynamic and hematologic parameters together with specific biochemical markers of inflammation. An obvious pitfall here is distinguishing end-organ damage due to inflammatory phenomena per se (microemboli, hyperdynamic circulation, etc.) versus that due to suboptimal perfusion during CPB of an organ system that already has a compromised vascular supply and little endogenous reserve.

Until we define specific criteria for the inflammatory response after CPB, it will remain difficult to accurately define its incidence and prevalence and to properly apportion its pathologic effects in terms of patient morbidity and mortality. Moreover, any strategies aimed at either therapy or prevention can only be properly assessed in the light of any established criteria.

Spectrum of Response

Most patients who undergo operations requiring CPB experience few clinically identifiable adverse sequelae and convalesce normally (8,9). One could almost be forgiven for viewing CPB as near perfect. That significant clinical morbidity is relatively unusual probably is due to the patient's innate ability to compensate for the damaging effects of CPB than to extracorporeal circulatory perfection or to any specific therapy aimed at abrogating the inflammatory response.

There appears to be an inevitability about the occurrence of the inflammatory response, and once again confusion abounds as to what exactly is meant until the diagnostic criteria are properly defined. It is clear that if the definition for SIRS is used, most if not all post-bypass patients would be included. Indeed, Cremer et al. (10) estimated its occurrence in approximately 10% of all their open heart operations. However, we previously emphasized that the inflammatory response demonstrates a spectrum of severity (11). The severity of this response is not predictable nor are its clinical sequelae. The length of CPB is frequently but not necessarily (10) regarded as a risk factor. The adverse sequelae of SIRAB do in fact account for substantial morbidity in pediatric surgery, the aged or infirm, and in patients undergoing long complex surgical procedures (3,12,13).

Initiation of systemic inflammatory response after bypass

Normal blood flow is altered by nonpulsatile perfusion. This is most commonly used in most cardiac surgical units worldwide. The relative benefits of pulsatile and nonpulsatile perfusion systems are an issue beset by controversy (14). The initial unsuccessful attempts at the inception of cardiac surgery to develop pulsatile systems that would reproduce normal physiologic pulsatile blood flow led to the adoption of nonpulsatile CPB. These systems were compatible with patient survival (15), and as confidence increased, nonpulsatile perfusion has become routinely established. As William Harvey might have predicted, it appears that there are distinct benefits to pulsatile flow that may prove advantageous to particular high-risk patient subgroups (14).

The damaging effects of CPB are most reasonably attributed to altered arterial blood flow patterns and to the exposure of blood to abnormal surfaces and substances during the period of bypass. The pathophysiologic responses that are responsible for SIRAB may continue long after the discontinuation of CPB.

SIRAB is initiated by a number of injurious processes that impinge on both cellular and noncellular (humoral) elements of blood. These processes generate microemboli, disrupt hemostasis, and lead to a generalized whole body inflammatory response. Most importantly, they set in motion a sequence of cytokine-mediated events that activate vascular endothelium, allowing further neutrophil-mediated inflammatory injury. It is important to appreciate that none of these events occur in isolation but that they are simultaneous and frequently stimulate or catalyze other reactions in the pathologic cycle of SIRAB. The scenario may be further compounded by cardiogenic shock and also endotoxemia.

Damaging process

Shear stress forces are generated by blood pumps, cardiotomy suction devices (including intracardiac venting), and cavitation around the tip of the arterial cannula. The greatest hematologic damage and activation of formed and unformed blood components and vascular endothelium during CPB, however, derives from the repeated passage of the patients blood volume through the extracorporeal circuit. Contact activation is a series of host-defense mechanisms designed to isolate and destroy the foreign surface and gaseous interface that the various blood components recognize. Synthetic materials that comprise the membrane in membrane oxygenators and abnormal blood–gas interfaces in bubble oxygenators are far removed from the normal, physiologic, endothelial cell contact.

Cellular components of blood

Red blood cell

Erythrocytes are mainly damaged during CPB by sheer stresses (16). Red cell deformability is reduced by CPB, possibly due to mechanical damage to red cells. This has the effect of inducing changes to ionic pumps at the cell surface, leading to abnormal accumulations of intracellular cations (17). Another damaging factor is produced by the membrane and is attacked by the membrane attack complex (MAC) generated from the activation of complement (1). Red cell lifespan is reduced, and this may be one reason for the anemia frequently seen in the postoperative period (18). Red cell injury is deleterious through a number of mechanisms. Free hemoglobin in plasma may be damaging to tissue function by increasing plasma oncotic pressure and viscosity. Cytotoxic oxygen free radicals are also released after autooxidation of hemoglobin. ADP released from red cell lysis may alter platelet function (19). In addition, arrhythmias may also develop as a result of potassium released from the red blood cell. Lipid red cell membrane ghosts may occlude the microcirculation and lead to organ dysfunction. Decreased red cell deformability can also lead to reduced tissue flow by alteration of the rheologic properties of the blood. This leads to reductions in tissue metabolism and oxygenation.

Neutrophil and vascular endothelium

Leukocytes are particularly sensitive to shear stresses that cause destruction or functional impairments such as decreased aggregation, decreased chemotactic migration, and impaired phagocytosis (20).

The neutrophil is a central cell type in the mediation of the inflammatory response, and its recruitment, activation, and cytotoxic capability are an essential aspect of the body's ability to ward off infection. Neutrophil activation through interaction with activated vascular endothelium may be responsible for much of the clinical sequelae of SIRAB. The inextricable physiologic association between the endothelium and neutrophils is important. CPB initiates a humoral cascade that results in activation of vascular endothelium. This process results in the expression of adhesion molecules that promote adhesion of leukocytes to the vascular endothelium. Once adherent to the endothelium, neutrophils release cytotoxic proteases and oxygen-derived free radicals that produce some of the end-organ damage after CPB. Neutrophil-mediated injury can occur from both activation of complement and endothelial adhesion. Neutrophil-derived proteases have been demonstrated in the circulation after CPB. These proteases break down extracellular structure and matrix, which contributes to the capillary leakage that results in extracellular volume overload and imbalance of electrolytes postoperatively (21).

Under resting conditions, the vascular endothelium offers a relatively inert surface that regulates the passage of intravascular substrates to the extravascular space and ensures the unhindered flow of cellular and serum components through the capillary network. Inflammatory signals, which include complement activation products, hypoxia, cytokines, oxygen-derived free radicals, and lipopolysaccharide (endotoxin), result in changes in gene expression, leading to activation of the endothelium with ensuing cytokine release and protein expression. These cytokines and proteins promote inflammatory reactions and thrombosis (22,23). Under normal physiologic circumstances, endothelial activation is crucial in neutrophil recruitment and promoting coagulation to limit the spread of local infection. CPB, however, induces this response at a systemic level with the release of cytokines; endothelial activation over large areas; and the recruitment, margination, and degranulation of neutrophils on a larger scale. Attenuation of this systemic response by feedback mechanisms appears to function less well when compared with the local level. The result of endothelial activation is widespread leukocyte adhesion molecule and tissue factor expression, resulting in end-organ damage, microemboli, and consumption of clotting factors, which contributes to the coagulopathy that frequently complicates cardiac operations (24,27).

The recruitment of leukocytes from the circulation to an inflammatory site is a three-stage process (Fig. 15.1, A and B) involving members of several adhesion receptor families (28,29). The first stage is the activation of the vascular endothelium that leads to the rapid expression of members of the selectin family (30). First is P-selectin, which is stored, preformed, in Weibel-Palade bodies within the endothelial cell and is released by exocytosis within minutes of activation by agents such as histamine, thrombin, H2O2, and C5a. This is followed by the expression of the related E-selectin. Both molecules interact with Sialyl LewisX carbohydrate groups expressed by leukocytes and lymphocytes, causing the free flow of the cells to be slowed down, initiating a rolling of the cells along the endothelial surface. In addition to E- and P-selectin interactions, L-selectin expressed on the cells can interact with charged fucosylated carbohydrate groups on the endothelium.

FIG 15.1. A: Adherence of circulating neutrophils (A) is triggered by activation of the endothelial cells, which causes the neutrophils to roll along the endothelium (B), a process mediated by selectin interaction with carbohydrate ligands. Activation of the neutrophils (e.g., by interleukin-8, C5a, or PAF) results in activation of integrin molecules (e.g., by LFA-1) on the neutrophil surface. This results in firm adhesion of the neutrophils to activated endothelium (C) through receptors belonging to the Ig family (e.g., ICAMs). Activated adherent neutrophils extravasate through the endothelium into the tissue interstitial space (D). B: Activation of the endothelium is associated with expression of a range of adhesion molecules, including selectins (P and E) and the Ig superfamily (ICAM-1, -2, -3, and VCAM-2). Each molecule has its own kinetics of expression. The rapid response of P-selectin is achieved through release of preformed molecules. Ig superfamily molecules are synthesized with endothelial activation. Many are either not expressed at all or only at low levels before endothelial activation.

The second stage of the recruitment process is activation of the leukocytes. This typically occurs through interaction of the cells with C5a, members of the chemokine family of cytokines [such as interleukin (IL)-8 and MIP-1b], which bind to the proteoglycan groups expressed on the surface of the endothelium (31,33), or with molecules such as platelet-activating factor produced by activated endothelial cells. All these molecules interact with a family of receptors on the leukocytes, the seven-transmembrane domain rhodopsin-like G protein-coupled receptors (34). These molecules also act as chemoattractant agents, in some cases by creating a gradient of the agent immobilized to the surface of the vascular endothelium. Interaction of receptors with the chemokines leads to activation of the integrin family of proteins expressed on the cells (e.g., LFA-1 and Mac-1), causing them to interact with their ligand (ICAM-1, -2, and -3).

Activation of neutrophils during CPB is shown by loss of L-selectin and upregulation of CD11b/CD18 (Mac-1). Increased production of reactive oxygen intermediates (35) and neutrophil-derived elastase, and similar molecules, have also been reported (35,38). Studies in complement-deficient dogs suggest that at least some of the neutrophil activation seen in CPB is due to complement activation (24). In addition to the presence of activated complement products during CPB, increased expression of neutrophil activating cytokines, such as IL-8, is also observed (39).

This process of neutrophil activation, firm adhesion, and sequestration can lead to obstruction of capillaries and local ischemia (40). In addition, the release by activated cells of cytotoxic products can cause direct cellular damage. These cytotoxins include both preformed agents that are present in the granules of neutrophils and newly synthesized molecules. There are two forms of neutrophil granules. The primary, or azurphil, granules have a predominantly intracellular role. These granules contain various proteases and myeloperoxidase that convert hydrogen peroxide to hypochlorous acid. Neutrophil secondary granules are released from the neutrophil and function primarily extracellularly. The secondary granules contain a number of cell surface molecules, such as the receptor for breakdown products of C3b and for chemoattractants. They also contain soluble mediators of inflammation, including activators of the C5 complement component and macrophage chemotactic agents.

In addition to causing obstruction in small vessels and the release of preformed molecules, neutrophils (and mononuclear phagocytes) synthesize novel substances including leukotrienes (38) and reactive oxygen intermediates (35). This synthesis can be readily detected by a dramatic increase in the oxygen consumption of the cells. These species are largely responsible for the so-called ischemia-reperfusion injury. Ischemic injury results when the blood supply to a tissue is impaired or suboptimal, which may occur with CPB. The paradox is that a more severe tissue injury occurs when blood flow is restored on reperfusion. The onset of ischemia is accompanied by the depletion of cellular adenosine triphosphate as a result of its degradation by hypoxanthine. Normally, hypoxanthine is oxidized by the enzyme xanthine dehydrogenase to xanthine using NAD in a reaction converting NAD to NADH. During ischemia however, xanthine dehydrogenase, which is usually present in large quantities, is converted to xanthine oxidase.

Also, anaerobic metabolism results in the production of lactic acid and altered cellular homeostasis with the loss of ion gradients across cell membranes. Reperfusion injury is initiated by a series of biochemical events that result in the generation of reactive oxygen metabolites. Reduction of oxygen leads to the production of the superoxide anion (Fig. 15.2, reaction 1), which is able to penetrate through cell membranes where it is converted into other more toxic oxygen species. Thus, the dismutase reaction (catalyzed by superoxide dismutase) leads to the conversion of the superoxide anion into hydrogen peroxide (Fig. 15.2, reaction 2). This can lead to the production of hypochlorous acid (the major bactericidal component of domestic bleach) by action of myeloperoxidase (Fig. 15.2, reaction 3) or, via interaction with iron salts, in the Haber-Weiss reaction to generate the highly toxic hydroxyl radical (Fig. 15.2, reaction 4). The toxicity of the hydroxyl radical results from its ability to take electrons from a wide range of molecules, leading to the formation of a new radical that can continue the reaction (Fig. 15.2, reaction 5).

FIG 15.2. The five principle reactions leading to the production of the superoxide anion (1), hydrogen peroxide (2), hypochlorous acid (catalyzed by myeloperoxidase, MPO) (3), the hydroxyl radical (4), and oxidation of other cellular molecules (R), leading to new free radical species (5).

This family of reactive oxygen intermediates exert their toxic effect through being highly reactive agents that oxidize or chlorinate a wide range of molecules, including proteins and membrane lipids. This leads to disruption of cellular function and, eventually, cell death. In addition, low nontoxic concentrations of some reactive oxygen intermediates participate in cellular activation pathways, such as the NF-B system (41), thereby potentially activating a wide range of cell types.


CPB is associated with a transient deficit in platelet function and number. These effects are manifest as a derangement in postoperative hemostasis. The normal platelet is able to adhere to a damaged endothelial cell or the subendothelial layer. Adherence is achieved by a bridge of a molecule of the multimeric form of the von Willebrand factor from the endothelium to the platelet at the glycoprotein (GpIb) receptor site. The platelet then undergoes a conformational change with exposure of different glycoproteins that include the complex GpIIb/IIIa, which can bind to fibrinogen. Fibrinogen is an important cofactor in platelet adhesion and is essential for platelet to platelet binding that occurs during irreversible aggregation. The aggregate is stabilized by the protein complex thrombospondin. Thromboxane A2 is also released, which produces vasoconstriction and platelet aggregation. Platelet response to a foreign surface is different in that platelet reactivity is related to fibrinogen. Mechanisms responsible are direct activation of the platelet, ADP release from granules of activated and damaged platelets, and stimulation of the platelet receptor to thrombin. Binding sites are subsequently exposed and platelets subsequently attach to the fibrinogen on the surface of the bypass circuit. Much evidence shows abnormalities of platelet numbers and function associated with CPB, including a rapid consumption of platelets during bypass (42), a decreased reactivity to known agonists (43), an increase in the concentration of the -granule compounds in plasma (43), and an increase in the stable metabolite of thromboxane A2 (thromboxane B) released from aggregating platelets (44). The observation that the bleeding time is prolonged after a period of extracorporeal circulation has been directly related to the time on bypass; however, the time course of these observations and the precise mechanism is less clear (45). It has been observed that platelets of patients deficient in the GpIIb/IIIa complex did not bind to a foreign surface and that the proteins adherent to the extracorporeal system were primarily made up of fragments of the GpIIb receptor (46). This suggests that the GpIIb/IIIa complex is the adhesive glycoprotein most affected by extracorporeal circulation and therefore most likely to be associated with platelet aggregation and activation. Other authors have suggested that any platelet stimulation associated with the initial phase of bypass is of little consequence and that only a small proportion of the platelets proceed beyond the initial contact phase. The argument is based on a number of observations. Zilla et al. (47) found that platelet release reaction products, such as -granule constituents and thromboxane, increase progressively during the course of bypass, suggesting that platelets are undergoing a lytic process throughout the period of perfusion (47). Whatever the case, all studies do agree that platelet functions are abnormal during the period of extracorporeal circulation.

Humoral components of blood

Kirklin (3) initially hypothesized that the deleterious effects of CPB were secondary to the exposure of blood to nonendothelial surfaces in the bypass circuit, initiating a "whole body inflammatory response." He noted that this response was characterized by activation of coagulation, the kallikrein system, fibrinolysis, and complement (3,13,48). We now recognize the importance of cytokines (49) and the combined effects of this humoral cascading in the activation of endothelial cells and neutrophil adhesion.

Inflammatory cascades

The principal event is the activation of factor XII (Hageman factor) to factor XIIa, which stimulates a number of inflammatory systems (Fig. 15.3). After surface contact, factor XII undergoes a conformational change and becomes attached to a high-molecular-weight kininogen. This complex attaches itself to the foreign surface and, after limited proteolysis, releases kallikrein and also bradykinin. In addition, limited proteolysis of factor XII releases factor XIIa. This active proteolytic factor can initiate the intrinsic coagulation cascade by direct effects on factor XI, which binds to the foreign surface and can also induce activation of factor VII, thereby further augmenting the intrinsic cascade of coagulation. Factor XIIa is also involved in a positive feedback system involving kallikrein. Factor XIIa releases kallikrein from prekallikrein, and the former is in turn able to act on factor XII to produce factor XIIa (50). Kallikrein can further activate neutrophils and in so doing can produce further activation of inflammatory cascades by producing oxygen free radicals and proteolytic enzymes. Furthermore, kallikrein and bradykinin can stimulate the fibrinolytic system. Kallikrein stimulates plasmin production by its action on pro-urokinase and bradykinin by releasing tissue-type plasminogen activator from the endothelium.

FIG 15.3. Contact activation cascade systems that result in the production of the systemic inflammatory response after bypass (SIRAB).

Complement system

One of the most important immunologic mechanisms involved in the inflammatory process is the complement system. This consists of more than 30 proteins that serve both as an effector arm of the immune response and as a primitive recognition system capable of self-/non–self-discrimination (51). Its functions include mediating inflammation, opsonization of antigenic particles, and causing membrane damage to pathogens. The complement components interact with each other so that the products of one reaction form the enzyme for the next. Thus, a small initial stimulus can trigger a cascade of activity with consequent biologic activation. The importance of the complement system is seen in the fact that 5% to 10% of serum proteins are complement components.

There are three major pathways to the complement system consisting of a series of preformed, inactive, proteins that, upon activation, activate the next protein in the pathway. This results in massive amplification of the initial activating event and to a cascade of different members being activated, such that the primary trigger leads to the production of a large number of different effector functions. Such an amplification system needs tight control, and so it is no surprise that nearly half the proteins in the complement system are regulatory molecules.

There are two activation pathways in the system, the classic and the alternative pathway. Both pathways culminate in the production of C3 convertase molecules, which cleave the central component of the complement system, C3, into the active C3a and C3b fragments. The classic pathway, which is probably the least important to the present discussion, is initiated by the binding of antibodies to target antigens on an appropriate surface. The clustered Fc regions of the antibodies are recognized by the first component of the classic pathway, C1, thus initiating the cascade of activation leading to the production of the classic pathway C3 convertase, C4b2b (Fig. 15.4). The alternative pathway, which is the most primitive part of the system, operates by a feedback loop in which the C3b component interacts with factor B (and subsequent activation of factor B by factor D) to generate the alternative pathway C3 convertase, C3bBb (Fig. 15.4). Thus, C3 acts as both a substrate and a component of the alternative pathway. The original C3b that initiates the alternative pathway can be generated either as a product of the classic pathway (in which case the alternative pathway acts as an amplification loop for complement activation) or through the spontaneous activation of C3 that occurs at a low level on a continuous basis. This "tickover" of C3 activation means that the complement system is continually being triggered, and to prevent the consequent activation of the whole cascade, it is necessary for cells to express regulatory proteins that inactivate the alternative pathway C3 convertase. This system provides the complement cascade with its primitive self-/non–self-recognition system in which surfaces (e.g., those of viruses and bacteria) that lack the necessary regulatory components are "recognized" by the alternative pathway, leading to the activation of the complement cascade and generation of inflammatory effector mechanisms, as discussed below.

FIG 15.4. Three pathways of the complement system. Conversion or interaction of serum components is indicated by solid lines; enzymatic reactions are shown as broken lines. The classic pathway leads to the conversion of C3 to C3b, which triggers the lytic pathway. The alternate pathway can be initiated as an amplification loop for the classic pathway by means of C3b or by C3b, resulting from the spontaneous hydrolysis of C3, in combination with factors B and D, to produce C3bBb, which is stabilized by properdin (P) to yield C3bBbP, which is capable of hydrolyzing C3 to produce more C3b.

The final pathway of the complement system is the lytic or terminal pathway. This pathway is initiated by C3b, produced by either the classic or alternative pathway C3 convertase. C3b activates C5 to generate C5a, a soluble molecule, and C5b, which is bound to the cell surface. This leads to the subsequent binding to the surface of C6, C7, and C8 and the polymerization of C9 to form a pore through the membrane that can result in target cell lysis (MAC). The MAC thus forms a transmembrane channel that allows the influx of ions and water into the cell, which is unable to maintain osmotic and chemical equilibrium (52).

There are a number of effects of activation of the complement system. First, a number of proteins, most notably the anaphylatoxins C3a, C4a, and C5a, are produced. These molecules act through receptors on mast cells and basophils, leading to their degranulation and release of a wide range of inflammatory mediators (including histamine). They also act directly on smooth muscle and endothelium, leading to muscle contraction and an increase in vascular permeability. In addition, C5a acts as a chemotactic and activating agent for neutrophils and other myeloid cells, leading both to their recruitment and release of lysosomal enzymes, reactive oxygen species, and other inflammatory mediators (53).

In addition to the production of soluble molecules, a number of components bind to surfaces after activation, including the MAC, whose role has already been mentioned. In addition, there are receptors for C4b and, more importantly, C3b and its breakdown products. These are involved in the opsonization of complement coated particles and the clearance of immune complexes.

During CPB, activation of the complement system is observed, as shown both by consumption of complement components (54) and the appearance in the circulation of C3a and C5a and the MAC. This activation of the cascade is probably due to a number of causes, including the bioincompatible nature of the surfaces encountered in the CPB apparatus and the release of endotoxin into the systemic circulation (55,56). Furthermore, physical effects of CPB may be responsible, for example, interaction with bubbles of oxygen have been shown to cause the breakdown of C3 (57).

Further contributions to systemic inflammatory response after bypass

In addition to the inflammatory interactions that result from the bypass circuit, cardiogenic shock and endotoxemia may further compound the response (58). Cardiogenic shock can further contribute to SIRAB, and prolonged nonpulsatile perfusion or periods of circulatory arrest can lead to diffuse end-organ ischemia as well (59). The end-organ hypoxic insult likely causes endothelial cells, circulating monocytes, and tissue-fixed macrophages to release cytokines and oxygen-derived free radicals that drive this response. Once the patient is resuscitated from shock and after hypoxic end organs are reperfused, a form of systemic ischemic-reperfusion injury results (58).

Another form of inflammatory activation that results from extracorporeal circulation and episodes of systemic ischemia-reperfusion is endotoxemia. Endotoxin is frequently detected in high concentration in the systemic circulation after CPB (60). Endotoxin is a potent stimulant of both complement and of endothelial cell activation, resulting in the surface upregulation of adhesion molecules and tissue factor (23). Endotoxin is a potent agonist of macrophage tumor necrosis factor release, which may explain why the level of this cytokine is elevated in some patients after CPB. Although the precise mechanism of endotoxemia after CPB is unclear, this may derive from a translocation of bacteria from the gut, resulting from the systemic stress of CPB and splanchnic ischemia (see below) coupled with impaired Kupfer cell function (56). The result is a transient endotoxemia that contributes to the overall state of systemic inflammation after CPB.

Endotoxin is also relevant in considering the "hyperdynamic circulation" and the associated profound fall in systemic vascular resistance, which is sometimes seen after CPB (10). This condition is very similar to the early phases of sepsis or endotoxemia (11). Until recently it was thought that endotoxin exerted its myocardial depressant effects by reducing the response to adrenergic stimulation by desensitization of the B-1 receptor. Recent data indicate that G proteins (guanine nucleotide binding proteins) may in fact be the mediators of endotoxin-induced defects in inotropic regulation (61).

Organ dysfunction and cardiopulmonary bypass

Lung and pulmonary circulation

The pulmonary circulation takes the whole of the cardiac output as the patient is weaned from CPB. This results in a high load of activated cells passing through this region. In addition, evidence suggests that activated neutrophils will preferentially lodge in the pulmonary circulation (62,63) and that free radicals derived from activated neutrophils are responsible for lung injury. Royston et al. (64) showed evidence for increased oxygen-derived free radical activity. This study demonstrated an increase in the peroxides of the polyunsaturated lipids of the cell membrane as a result of their reaction with free radicals. The rise in peroxidized products also showed a very close temporal relationship with the sequestration of neutrophils in the lung. However, other authors have provided evidence that there is in fact a reduction in the neutrophil-derived free radical output during bypass and that release of the aortic clamp and reperfusion of the lungs is associated with release of the superoxide anion by neutrophils (65).

Intravenous infusions of complement activated plasma result in an increase in pulmonary vascular resistance, a fall in arterial oxygen tension, and sequestration of neutrophils within the pulmonary vascular bed (66,67). Further experimental studies have shown that intravascular complement activation can result in pulmonary endothelial injury, leading to an increase in vascular permeability and interstitial pulmonary edema (66). The central role of neutrophils in complement-induced lung injury can be demonstrated by depletion experiments (68). It has been demonstrated that complement-induced neutrophil-dependent lung injury could be attenuated with scavengers of oxygen radicals.

Renal function and cardiopulmonary bypass

Renal function is influenced by hemodilution, hypothermia, and endocrine effects during nonpulsatile perfusion. Changes in the rennin–angiotensin–aldosterone system promote an increase in renal vascular resistance and a tendency to sodium and water retention. The vasoconstrictive effects of hypothermia reduce renal cortical blood flow, glomerular filtration rate, tubular function, and free water and osmolar clearance. Hemodilution protects against renal damage during CPB by increasing outer cortical renal plasma flow and increasing sodium, potassium, osmolar, and free water clearance (5). In general, urine output will be diminished during hypothermia and improved at normothermia and after the resumption of pulsatile perfusion. Renal dysfunction may also result from microemboli (including lipid membrane ghosts) that lodge within the microcirculation and also from hemoglobinuria.

Brain and cardiopulmonary bypass

A range of neurologic and neuropsychological abnormalities are seen after coronary artery bypass graft. The incidence of these complications has been recorded by Shaw et al. (69,73). As a result of autoregulation of the cerebral circulation, cerebral perfusion is assumed to be adequate when the mean arterial pressure is above 50 to 60 mm Hg. This is assuming that there is no flow limiting carotid stenosis or that the patient is not chronically hypertensive. In these situations, the minimally acceptable cerebral perfusion pressure is higher than normal, because the cerebral blood flow autoregulation curve shifts to the right and cerebral blood flow (CBF) therefore becomes pressure dependent at a higher than normal perfusion pressure. Postoperative cerebral dysfunction in the absence of the aforementioned conditions is commonly attributed to macro- and microembolization during surgery. Studies using intraoperative transcranial Doppler and fluorescein retinal angiography have documented a high incidence of cerebral and retinal microemboli of blood cell aggregates during CPB (74,76). Magnetic resonance imaging identifies brain swelling after coronary artery bypass graft (Fig. 15.5) (77). In this pilot series of six patients, no major neurologic deficits were seen. The mechanism of the swelling is uncertain. Hypotheses include stasis of interstitial fluid caused by an absence in arterial pulsations but not of flow during aortic cross clamping (78) and increased cerebral blood flow due to post-bypass hyperemia (79). Brain swelling may provide further insight into the cause of neuropsychological deficits seen after coronary artery bypass surgery.

FIG 15.5. Periventricular and deep white matter lesions consistent with ischemic disease were seen in all patients on the preoperative scans. All patients showed brain swelling with reduction in size or even obliteration of cerebral sulci fissures and cisterns on the early postoperative scans.

Gut and cardiopulmonary bypass

Gut mucosal ischemia is common during CPB but rarely produces clinical evidence of gastrointestinal disease in the absence of postoperative low cardiac output syndrome (80). Factors that contribute include splanchnic vasoconstriction induced by elevated angiotensin II levels during nonpulsatile perfusion, splanchnic shunting during rewarming from systemic hypothermia, microemboli comprising platelet or leukocyte aggregates with the release of vasoactive substances, and preexisting atherosclerotic disease in the splanchnic bed (81).

Therapeutic avenues

Effective amelioration of SIRAB has yet to become a reality. Certain proactive strategies may be instituted when there is difficulty in maintaining an adequate perfusion pressure before the discontinuation of bypass, which may indicate a profoundly low systemic vascular resistance. The only current therapy in this situation is vasoconstrictor and inotropic drugs, recognizing that such treatment does not address the pathologic basis of the hypotension.

The recognition that the deleterious effects of SIRAB are of lesser magnitude and shorter duration in the presence of robust postoperative cardiac performance suggests that at least an optimal cardiac output increases the clearance of inflammatory proteins and mediators. Accordingly, improved methods of myocardial preservation and organ perfusion offer the possibility of ameliorating, at least to some extent, the deleterious effects of humoral activation.

To prevent or ameliorate SIRAB, the optimal strategy would be to develop a CPB system that does not produce contact activation of blood components rather than to systemically administer anti-inflammatory therapeutic agents because of their potential to render the patient more susceptible to postoperative infection.

In terms of technical advances in relation to the CPB circuit, pulsatile perfusion may have beneficial effects compared with nonpulsatile flow (14). There is increasing clinical and experimental evidence for the benefit of membrane oxygenators over bubble oxygenators (48,82). The membrane oxygenator system provides superior oxygenation compared with a bubble oxygenator and produces less trauma to blood elements during prolonged use. In addition, membrane oxygenators are associated with a reduction in microembolism when compared with bubble oxygenators (83). By reducing complement activation (84), the membrane oxygenator may reduce the incidence of pulmonary dysfunction after CPB (85). It seems unlikely that modifications of bypass equipment alone will provide a unique solution to the problem of contact activation in the absence of any other manipulations.

In contrast to the aforementioned, Videm et al. (86) showed that the MAC (terminal effector unit, C5–C9) is increased in the plasma to a similar degree no matter what type of oxygenator was used. Other interventions include the coating of membranes and CPB apparatus with heparin. This coating appears to provide a more biocompatible surface and to decrease complement activation (87,89). Another strategy makes use of hemofiltration. This allows the removal of neutrophils from the circulation. In so doing, their biologic availability to promote inflammatory injury is decreased (90). The problem appears to revolve around a return of leukocytes to the circulation when the filter is discontinued. Use of heparin-bonded circuits and hemofiltration is still far from routine in clinical practice.

In terms of pharmacologic manipulation of SIRAB, many strategies have been proposed to inhibit complement activation, but such is the complexity of the complement pathways that an agent has yet to be found that can inhibit effectively all aspects of the system. Many agents have a limited inhibitory action in vitro but are often restricted to one or other of the complement pathways. Corticosteroids inhibit the formation of C3 and C5 convertases in vitro, but there have been conflicting reports as to their efficacy at inhibiting complement activation during CPB (91). A number of protease inhibitors, for example FUT-175, have been proposed as effective inhibitors of the complement pathways (92). It is nonspecific in that it also inhibits factor XIIa, kallikrein, and thrombin. It is, however, a much more potent inhibitor of the classic pathway than the alternative pathway and is therefore of limited interest in regard to the prevention of the complement activation that accompanies CPB. Pharmacologic agents, such as corticosteroids, may be useful in preventing activation, though the results are conflicting. At the present time, their function as routine prophylaxis has not found clinical use because there is no evidence of effective in vivo complement inhibition, cellular host defense mechanisms are impaired, and elevated endotoxin levels have been reported in treated patients (93).

Aprotinin (Trasylol) is a serine protease inhibitor isolated from bovine lung. It inhibits several proteinases, including trypsin, plasmin, and kallikrein. In view of its antiplasmin effect, it is also considered to be an antifibrinolytic agent. The precise mechanism of action of aprotinin is still unclear. The broad range of antiprotease activity might indicate several potentially interlinked mechanisms (94,97). Aprotinin has been used in the past in a variety of clinical situations, including acute pancreatitis, adult respiratory distress syndrome, septic shock, and hemorrhagic shock. In 1963, Tice et al. (98) reported its use in cardiac surgical patients. Despite subsequent use by others indicating that it might be effective in reducing bleeding after cardiac surgery (99), aprotinin's potential role in cardiac surgery was largely ignored for many years. The Hammersmith group found that high-dose aprotinin therapy, which they used in an attempt to block complement and kallikrein-induced lung damage, showed remarkable efficacy in reducing blood loss and blood transfusion requirements in cardiac surgical patients (100,102). The experience with aprotinin has recently been extended and confirmed in a larger study in the United Kingdom (103). Aprotinin has been suggested to have a dose-responsive inhibitory effect on complement activation in vivo (104). However, a detailed study by Blauhut et al. (105) demonstrated no inhibitory effect on complement cascades during CPB. Aprotinin has, however, been shown to reduce plasma concentrations of neutrophil elastase after aortic cross-clamp removal (94). The fact that aprotinin acts as a nonspecific antiprotease and blocks the action of kallikrein and the fibrinolytic pathway does nonetheless offer considerable potential for the role of this drug as an inhibitor of SIRAB and as a hemostatic agent.

The prevention of neutrophil activation would be of use in preventing many problems associated with CPB. A number of pharmacologic agents, such as glucocorticoids and other inflammatory agents, have been used that reduce neutrophil activation (25,106,107). These reports suggest that some specific antineutrophil therapy may be possible to prevent tissue injury in the future.

Lowering the temperature of the patient has also been shown to reduce adherence of neutrophils to the endothelium (108); however, although hypothermia is beneficial when the patient is cold, the benefits are rapidly lost when the patient rewarms (109,110).

Techniques designed to impair neutrophil-endothelial cell interaction have the potential to make a substantial difference if not actually attenuate SIRAB. There is much interest currently in the use of monoclonal antibodies as therapeutic agents to block specific adhesion molecules expressed by activated endothelium. Such an approach, or similar strategies using soluble ligands of the adhesion molecules, has been shown to block neutrophil recruitment in a number of experimental systems, including CPB (25,59). A further therapeutic avenue is the use of monoclonal antibodies directed against cytokine receptors. Such antibodies directed against tumor necrosis factor and IL-1 receptors in animal models of sepsis appear to prevent endotoxin-induced shock (111,112). Such interventions may result in inhibition of T- and B-cell proliferation and lymphokine generation. This will likely result in a diminution of host defense mechanisms and hence make the patient more vulnerable to severe infection. Similarly, adhesion molecule blockade has been shown to increase the body's susceptibility to infection, an effect that severely limits this approach as a therapeutic strategy (113). At this time a multiplicity of monoclonal antibodies exists with the capability to block adhesion molecules. Boyle et al. (58) emphasized the importance of addressing the time courses of expression of specific endothelial adherence molecules so that interventions can be directed to each specific adhesion molecule at the time when it can be expected to be maximally expressed. Other approaches that appear attractive include the inhibition of adhesion molecule expression. Efforts to characterize the molecular events that result in endothelial activation genes could conceivably result in utilization of gene-based therapies to modulate or attenuate SIRAB.

The ability to produce novel molecules using recombinant DNA technology has also shown some promise for the future. In an animal model, a soluble version of the complement receptor 1 (sCR1, normally a cell surface inhibitor of complement activation) has been shown to decrease complement activation during CPB (114). It is likely that an increasing number of similar molecules will be produced and tested in both preclinical and clinical settings over the next few years. Furthermore, it is possible to envisage incorporation of such molecules onto the surfaces of membranes and tubes to prevent local activation.


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The cellular and humoral constituents of the adaptive immune system undergo changes in both function and number after CPB. The concern is that postoperative morbidity and mortality related to infection is partially due to a loss of T and B cells and immunoglobulin consumption resulting from CPB. This section examines the effect of CPB on adaptive immunity and, specifically, T-cell function.

Immunodeficiency and cardiopulmonary bypass

Numerous studies have demonstrated that a depression of the acquired immune response occurs after surgical procedures (115,124). The degree of immunosuppression is directly related to the magnitude and duration of the surgical procedure and to the volume of transfused blood (123). Therefore, one would expect that cardiac operations would be highly immunosuppressive. The clinical prevalence of infections after cardiac surgery indicates that prolonged operating time and duration of CPB are significantly correlated to the incidence of infections (125,126). The risk factors for the development of infections identified in general surgical patients are applicable to cardiac surgery (127,128). However, it has been shown that anesthesia in open heart surgery has only minor immunologic effects (129,130).

The result of CPB is a depression of immunologic reactivity, which makes patients susceptible to perioperative infections with the development of septic shock as a possible, and frequently fatal, result (131). In one cardiac institution, 55% of perioperative mortality was due to multisystem organ failure associated with sepsis (132). Whether sepsis was the primary factor initiating the multisystem organ failure or was the result of endotoxin and bacterial translocation produced by a primary circulatory abnormality could not be determined. With the advent of interventional catheterization laboratory techniques and better medical management of patients with coronary artery disease, older and sicker patients are being referred for operation. These patients comprise a group that is at higher risk for the development of infections in the postoperative period. Of additional concern is that any depression of the acquired immune response may be detrimental because it could increase the susceptibility of cardiac surgical patients (many of whom have prosthetic material in the vascular system) to endocarditis.

Under normal conditions, relatively low levels of immunoglobulins and complement are sufficient for adequate opsonization of bacteria (133). However, if the load of infectious microorganisms increases, as may occur in patients undergoing open heart surgical procedures (134), then an imbalance may develop in the homeostasis of the host defense. The quantitative and qualitative exhaustion of humoral and cell-mediated immune mechanisms may have an adverse effect on clinical outcome when an additional injury occurs to a patient with a downregulated immune system in the early postoperative period. It is therefore not surprising that opportunistic pathogens are frequently found in cardiac surgical patients experiencing one or more perioperative complications, for example renal failure or low cardiac output. This patient group is likely to develop sepsis-related multisystem organ failure that can account for a high proportion of perioperative fatalities (135). At the present time, the lack of correlation between laboratory data and clinical findings in surgical patients with or without CPB has been noted by several groups of investigators (124,130,136,137). It may be possible to relate clinical outcome and laboratory results if more discriminative tests are applied and by using large patient numbers.

Humoral immunity

In patients undergoing CPB, serum levels of immunoglobulins and complement are markedly reduced (3,4,48,136,138–140). As a consequence, the contribution of these proteins to host defense is quantitatively affected, resulting, for example, in reduced opsonization of bacteria in vitro (139). Bubble oxygenators produce a greater deleterious effect in this regard than membrane oxygenators (141,142). Leukocyte counts fall with the onset of CPB. The sequestration of leukocytes in tissue is increased after their activation by anaphylatoxins C3a and C5a. After CPB, the chemotactic ability of granulocytes is impaired (141,144), which may also contribute to a higher susceptibility to bacterial infections. Other studies show that not only the phagocytic function but also the metabolic function of leukocytes is impaired after CPB (145).

It is unclear what happens to B-cell numbers after CPB. Some investigators report no change (6) and others a decrease (146). Roth et al. (137) reported that relative levels (percentage) of B cells increase after CPB with no significant increase in absolute numbers. The secretion of IgG, IgM, and IgA by B cells in response to pokeweed mitogen is diminished after CPB (137). The bactericidal activity of serum is depressed after CPB. Complement is consumed, and all components of humoral immunity are decreased as a consequence of the hemodilution caused by bypass (147). The denaturation, flocculation, and clearing of these microparticles by the reticuloendothelial system is responsible for the continued fall in concentration of the immune proteins during bypass (148). Exposure of immune proteins to contact with gaseous and foreign surfaces results in denaturation. Surface depolarizing forces result in the disruption of the sulphydryl and hydrogen bonds that stabilize the secondary and tertiary structure of the protein molecules. The resulting unfolding of globular protein molecules may expose otherwise masked chemical groups and create a randomly coiled molecule. The macromolecules thus formed then tend to flocculate. These changes in plasma proteins may also lead to coalescence of serum lipids and an increase in plasma viscosity because of protein denaturation (6,149).

Natural killer cells

Natural killer cells are a heterogeneous subpopulation of lymphoid cells that are not T or B lymphocytes. Natural killer cells have been shown to produce cytotoxic responses in virus-infected cells and transformed target cells (e.g., tumor cells). Decreases in both number and function of natural killer cells have been shown after CPB (139,150,151).

Reticuloendothelial system

The reticuloendothelial system is made up of tissue macrophages in the spleen, lymph nodes, lung, and liver. These cells are initially derived from bloodborne monocytes. The normal function of the reticuloendothelial system includes clearing of bacteria, endotoxin, platelets, denatured proteins, chylomicrons, plasma hemoglobin, thrombin, fibrin, fibrin degradation products, thromboplastin, and plasminogen activator from the circulating blood. The function of the reticuloendothelial system has been shown to be depressed after CPB. The microparticles generated by CPB are a major factor in the depression of this system (152).

T cells

T cells are lymphocytes that develop in the thymus. This organ is seeded by lymphocytic stem cells from the bone marrow during embryonic development. These cells then develop their T-cell antigen receptors and differentiate into the two major peripheral T-cell subsets, one of which expresses the CD4+ marker (helper cells) and the other CD8+ (cytotoxic cells). T helper cells play a central role in the initiation and regulation of the acquired immune response. T helper cells recognize antigen presented on the surface of antigen presenting cells in association with class II molecules encoded by the major histocompatibility complex (MHC). T-cell activation requires other specific costimulatory signals generated by the antigen presenting cell. Cytotoxic T cells recognize antigen presented on the surface of antigen presenting cells in association with class I molecules encoded by the MHC. T helper cells provide "help" in the form of lymphokine secretion. Such lymphokines help B cells to divide, differentiate, and produce antibody. The lymphokines also are required for the development of leukocyte lines from hematopoietic stem cells and development of cytotoxic T cells. They also cause activation of macrophages, allowing them to destroy the pathogens they have taken up. Cytotoxic T cells are capable of destroying virus-infected target cells or allogeneic (transplanted) cells.

Quality and quantity

Phenotypic changes induced by CPB have been investigated using standard commercially available leukocyte labeling monoclonal antibodies together with flow cytometric analysis. There is a decrease in CD3+, CD4+, and CD8+ cells with a reversal of the normal CD4+/CD8+ ratio. Such changes are maximal on postoperative day 1 and remain low for approximately 1 week (151,153–155). T lymphocytes are significantly reduced in numbers [relative and absolute (137)].

The capacity to counteract microbial infection by the innate immune system is reduced in patients after cardiac operation due to a waste of complement factors (3,48,140) and a decrease in the cellular elements of the innate immune response, for example neutrophils and natural killer cells (137,150,156). Hisatomi et al. (157) showed in patients having cardiac operations that lymphocyte response to the mitogen phytohemagglutinin (PHA) was low on the first and seventh days after operation and that IL-2 production was greater than 90% depressed on the first postoperative day. There was no significant change in a control group of patients who underwent cholecystectomy. Furthermore, improvement in IL-2 production occurred immediately in patients without blood transfusion from random donors and reached normal levels by postoperative day 3. However, IL-2 production remained depressed on day 3 in all patients with transfusion from random donors and remained significantly diminished on day 7 in patients in NYHA classes III and IV.

Markewitz et al. (135) showed a similar depression of the response to PHA and a specific antigen cocktail (purified protein derivative, tetanus toxoid, streptolysin, mumps, and vaccinia antigen). Using in vitro cytolytic assays, Nguyen et al. (151) showed a decrease in cytotoxic T cell activity that was maximal on postoperative day 1 with a return to baseline values on postoperative day 3.

Morphologic correlation

Scanning electron microscopy of T cells after CPB shows profound alterations to the plasma membrane of the T cells. There is a decreased number of microvilli and a decrease in the folded aspect of the lymphocyte surface. Furthermore, after CPB, the membrane does not accommodate monoclonal microbeads.


The mechanism responsible for the decrease in T lymphocytes in circulating blood after CPB is not defined. A high level of cortisol probably plays an important role in postoperative immunosuppression, yet it is unlikely to be the only factor in this complex phenomenon (158). Elevation of serum corticosteroids may cause a decrease in circulating T-cell levels by causing a redistribution to lymphoid tissues, although this is usually mild and transient (153,159). Lymphocyte transformation by PHA is retarded by corticosteroids. Although serum cortisol levels are elevated after operation, the lack of correlation between changes in serum cortisol and changes in lymphocyte number and function suggests that serum cortisol is not an etiologic factor in postoperative immunosuppression (137). Blood dilution as a consequence of CPB, fluid shifts between extravascular and intravascular compartments, and mechanical destruction and consumption are likely causes of quantitative and qualitative change (160). Ide et al. (155) suggested T-cell redistribution between bone marrow, lymphoid tissue, and peripheral blood may occur.


T cells are quantitatively reduced and qualitatively affected by CPB. The literature does not, however, address two fundamental issues. First, one of the main factors that determines the magnitude of an immune response is the number of antigen or allo-specific lymphocytes available to respond at the time of antigenic or allogeneic challenge. The experiments described thus far give no more than an overall impression of the diminution in quality of the immune response and have inherently poor quantitation. The qualitative defect has, therefore, not been quantified at the cellular level. Such quantification is essential if one is to compare the immunomodulatory effect of certain agents or to make appropriate judgments regarding the effects of CPB on the T-cell response. Furthermore, it is not possible to make any judgments as to the effect of CPB on the T-cell response to nominal as opposed to allo-antigen, the latter being of importance in the context of cardiac transplantation. The second issue is the assessment of whether CPB affects the ability of antigen presenting cells (macrophages, B cells, and dendritic cells) to present antigen to T cells. This would also affect the quality of the T-cell response. Because all other aspects of the immune response appear to be affected by CPB, it would not be surprising to find out that such a defect was produced by CPB. This could also be quantified and the impact of putative immunomodulatory agents properly assessed.

The most sensitive and quantitative technique for measuring reactive T cells at a population level is to determine their frequencies by limiting dilution. Limiting dilution analysis defines a previously unknown frequency in a population of cells and is the only way to quantitate the immune response in humans at the cellular level. It is used both as a research tool and clinically in the context of bone marrow transplantation to predict the risk of developing graft-versus-host disease. Methods are established for assaying the frequencies of reactive cytotoxic T lymphocytes and IL-2 secreting T helper cells (161,162). These assays, however, are able to estimate the frequency of T helper cells or cytotoxic T cells to a specific nominal or allo-antigen. This technique alone could provide powerful information as to the impact of CPB on the ability of patient T cells to respond to antigen and also antigen presenting cells to present antigen, or both. In a less quantitative manner, antigen presenting function of patient cells could be assessed by performing bulk culture experiments. In these, a T-cell clone, restricted for the patient's class II MHC and specific for a peptide (e.g., hemagglutinin), is used. The patients antigen presenting cells present peptide to this clone, and proliferation of the T-cell clone is measured by tritiated thymidine incorporation into replicating DNA. The assumption is that if the antigen presenting cells are affected by CPB, the clone will proliferate to a different extent compared with pre-bypass proliferation. In this manner, more accurate quantitative comparisons may be made.


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It is clear that the inflammatory and immunologic sequelae of CPB are not responsible for a large morbidity or mortality per se. However, they assume more importance in longer more complex surgery performed on patients who are at the extremes of age and who have significant comorbid conditions. Furthermore, the contribution that the inflammatory and immunologic sequelae to CPB make to postoperative infection remains unknown. At this time, CPB is far from perfect. Despite some technologic improvement in CPB equipment, no single anti-inflammatory or immunologic therapy has yet found routine use in clinical practice. Until more is known about the specific time course of SIRAB and which molecules assume central importance, pharmacologic therapy (including the future promise offered by monoclonal antibodies and gene-based strategies) should be treated with caution due to the risk of compounding postoperative immunodeficiency and thus increasing postoperative sepsis.


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  • Systemic Inflammatory Response After Bypass (SIRAB)

  • CPB induces a whole body inflammatory response involving cellular and noncellular elements of blood.

    • Red cell damage is due to shear stress and produces cell lysis, release of hemoglobin, and production of membrane "ghosts."

    • Both neutrophils and vascular endothelial cells are activated by CPB.

      • Neutrophils adhere to endothelium and degranulate, releasing cytotoxic substances and causing small vessel obstruction.

    • Platelets activate, degranulate, and adhere to CPB components.

    • Humoral inflammatory cascade begins with activation of Hageman factor (factor XII).

      • Factor XII activates the intrinsic coagulation cascade, kallikrein, bradykinin, and plasmin (through kallikrein).

      • Complement is activated, leading to formation of the MAC.

      • Endotoxin circulates in high concentrations after CPB.

    • SIRAB is associated with significant pulmonary, renal, and central nervous system pathology.

    • Heparin coating of CPB circuitry and use of protease inhibitors (e.g., aprotinin) may ameliorate SIRAB.

  • Cellular and humoral immune function is depressed after CPB.

  • Numbers and function of T and B lymphocytes, killer T cells, RES cells decrease after CPB.

    • Altered T-cell plasma membrane after CPB.

    • Decreased lymphocyte response to PHA and decreased IL-2 production by lymphocytes after CPB.


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