The Inflammatory Response to Cardiopulmonary Bypass: Part 1—Mechanisms of Pathogenesis

REVIEW ARTICLE William C. Oliver, Jr, MD Gregory A. Nuttall, MD Paul G. Barash, MD Section Editors

The Inflammatory Response to Cardiopulmonary Bypass: Part 1—Mechanisms of Pathogenesis Oliver J. Warren, MRCS (Eng),* Andrew J. Smith, MRCS (Eng),* Christos Alexiou, PhD,† Paula L.B. Rogers, MBBS,* Noorulhuda Jawad, BSc (Hons),* Charles Vincent, PhD,* Ara W. Darzi, FMedSci KBE,* and Thanos Athanasiou, PhD*

C

ARDIOPULMONARY BYPASS (CPB) temporarily replaces the functions of the heart and lungs during cardiac surgery, allowing the heart to be opened and operated on. To achieve this requires 2 key postulates to hold true: the circulation of blood around a patient’s vasculature can be maintained by mechanical pumps while the heart is arrested, and venous blood can be artificially “arterialized” in an extracorporeal gaseous exchange device.1 Although physiologists such as Brown-Sequard and Starling attempted to create artificial forms of both organs as far back as the mid-19th century, the genesis of the machines that are now regarded as CPB circuits started in October 1930, when John Gibbon2 pondered how to save a patient dying from a blood clot in her pulmonary artery. Two decades later, on May 6, 1953, Gibbon performed the first successful human intracardiac operation (an atrial septal defect repair in a young woman) using a mechanical extracorporeal pump oxygenator.3 Since 1953, what was once a perilous, experimental procedure has been transformed into a relatively standard medical act, performed on more than half a million patients worldwide per annum. This transformation has occurred because of a number of factors. The expertise of surgeons, anesthesiologists, and perfusionists has increased rapidly, pharmacologic research has resulted in an increased armamentarium being available to clinicians and a number of key biomaterial technologies have developed. The latter has created much smaller volume circuits with predominantly disposable components, more biocompatible surfaces, and gas-permeable microporous membranes. As a result, the blood trauma previously witnessed has reduced and with it thrombogeneity and embolic particle creation.4,5 However, despite these major improvements, CPB continues to be associated with an undesirable inflammatory reaction.6 Inflammation is the initial, nonspecific response of vascularized tissue to a variety of injuries and represents the body’s attempt to protect itself from an injuring agent. Irrespective of the cause, the inflammatory response follows qualitatively similar patterns of activation, involving both humoral and cellular inflammatory pathways. While aiming to be protective, the inflammatory response can on occasions become exaggerated, damaging the very host it aims to protect. The inflammatory response witnessed in CPB-assisted cardiac surgery is no dif-

ferent. Major surgery and its associated prolonged anesthesia cause the body to undergo a major inflammatory response. CPB, an inherently unnatural process, magnifies this reaction. The pump and oxygenator both function in a nonphysiologic manner (without any feedback from normal homeostatic mechanisms); thus, intravascular pressures and blood gas composition stray outside normal ranges.6 Significant hemodilution occurs, causing intercompartmental fluid shifts, significant fluid retention, and dilution and denaturing of important plasma proteins7-9; blood is exposed to nonendothelial surfaces and abnormal shear stresses, activating blood elements to produce a number of vasoactive mediators, altering capillary permeability, and causing hemolysis.10 The coagulation system is simultaneously activated and impaired. In summary, the body’s homeostatic mechanisms are thrown into disarray, and, as a result, a systemic inflammatory response syndrome (SIRS) occurs. In most circumstances, the resulting organ dysfunction is transient and self-terminating because the homeostatic defense mechanisms are able to compensate. However, on occasion, the patient may experience major complications, increased intensive care and hospital length of stay, and even death. The inflammatory response to CPB can be divided into 2 key phases: “early” and “late.” The early phase occurs as a result of blood coming into contact with nonendothelial surfaces, and the late phase is driven by ischemia-reperfusion injury (I/R injury) and endotoxemia. These 2 phases and the varying

From the *Department of BioSurgery and Surgical Technology, Imperial College London, London, United Kingdom; and †Department of Cardiothoracic Surgery, Barts and the London NHS Trust, London, United Kingdom. Address reprint requests to Oliver J. Warren, MRCS (Eng), Department of BioSurgery and Surgical Technology, Imperial College London, 10th Floor QEQM Building, St Mary’s Hospital, London W2 1NY, United Kingdom. E-mail: [email protected] © 2009 Elsevier Inc. All rights reserved. 1053-0770/09/2302-0020$36.00/0 doi:10.1053/j.jvca.2008.08.007 Key words: cardiopulmonary bypass, inflammation, cardiac, surgery, cells

Journal of Cardiothoracic and Vascular Anesthesia, Vol 23, No 2 (April), 2009: pp 223-231

223

224

WARREN ET AL

Fig 1. A summary of the inflammatory response to CPB.

components involved are the focus for this article and are summarized in Figure 1. THE EARLY PHASE: CONTACT ACTIVATION

The early phase of the inflammatory response occurs on initiation of extracorporeal circulation and is thought to be caused by the contact of blood components (both cellular and humoral) with the synthetic material of the extracorporeal circuit. In normal circumstances, blood only makes contact with the smooth endothelial cell lining of the vasculature, a surface with an important role in maintaining the equilibrium of the circulation. By producing balanced amounts of procoagulant and anticoagulant substances, endothelial cells ensure that blood is maintained in its fluid form, until such time that vessel injury occurs and blood clot formation becomes favorable. The nonendothelial surfaces of the CPB machine shift this equilibrium in the direction of thrombosis, and, in order to avoid this, it is essential that adequate doses of heparin are administered before CPB initiation. When heparinized blood comes into contact with the tubing of the CPB circuit, plasma proteins are instantly adsorbed onto the circuit, forming a monolayer.6 Some of these proteins undergo conformational changes exposing receptors to circulating blood proteins and cells. This leads to the activation of 5 plasma protein systems (contact, intrinsic coagulation, extrinsic coagulation, fibrinolytic, and complement) and 5 cell groups (endothelial cells, lymphocytes, monocytes, neutrophils, and platelets).11 The roles of these 5 protein systems and 5 cell groups are interlinked, complex, and still not fully understood; however, the vasoactive substances, enzymes, and microemboli produced by these activated mediators initiate the “whole-body inflammatory response”7 and are responsible for the major complications associated with CPB, namely coagulopathy, tissue edema, and temporary organ dysfunction.12 PROTEIN COMPONENTS

Contact System The contact system consists of 4 primary plasma proteins: factors XII (Hageman factor) and XI, prekallikrein, and high– molecular-weight kininogen (HK). When blood comes into contact with a negatively charged, nonendothelial cell surface, factor XII, in the presence of prekallikrein and HK, is autoactivated and cleaves into 2 serine proteases, factor XIIa and

factor XIIf (a collective term for the remaining fragments). Factor XIIa activates factor XIa, thus initiating the intrinsic coagulation pathway. Factor XIIa also activates HK to form bradykinin,13 a potent vasoactive peptide that causes vasodilatation and nonvascular smooth muscle contraction. Finally, factor XIIa cleaves prekallikrein to produce kallikrein. Kallikrein is a major mediator that directly activates neutrophils, drives fibrinolysis, accelerates further cleavage of factor XII in a positive feedback loop, and thus amplifies the inflammatory response.14 These relatively complex interactions are shown in Figure 2. Activation of the contact system has been shown, and attempts made to attenuate it, in both simulated and clinical CPB.15-17 It is clear that during CPB the contact system directly activates the intrinsic coagulation pathway and neutrophils but beyond this appears to indirectly activate platelets, the fibrinolytic system, complement, and endothelial cells.6 Intrinsic and Extrinsic Coagulation Systems The activation of the intrinsic coagulation pathway begins on activation of the contact system when blood is exposed to an artificial surface or to collagen within a damaged vascular wall. In the presence of HK, factor XIIa activates factor XI to factor XIa. This is the initial step of the intrinsic coagulation pathway, which proceeds through factor IX, in the presence of phospholipids, Ca2⫹, and factor VIII to activate factor X. Factor Xa is the key point at which the intrinsic and extrinsic (see later) pathways meet. Factor Xa, if in the presence of factor V, Ca2⫹, and phospholipids, acts as a protease to convert the inactive precursor molecule prothrombin to thrombin. Thrombin has a wealth of hemostatic actions including the activation of factors V, VIII, and XI; the activation of woundbased factor VII; and the stimulation of subendothelial smooth muscle cells to constrict, hence staunching blood loss from an injured vessel.18 However, the principal actions of thrombin during extracorporeal circulation are to cleave fibrinogen to fibrin, activate factor XIII to crosslink fibrin, activate platelets via thrombin-specific receptors, and stimulate endothelial cells to produce von Willebrand factor (thus aiding platelet aggregation).19 This creates a surface that supports the binding of coagulation factors and thereby facilitates the full thrombin burst necessary for hemostasis. This pathway is the predominant route for coagulation during all applications of extracor-

INFLAMMATORY RESPONSE TO CPB: PART 1

Fig 2. CPB.

225

Activation of the contact system by

poreal circulation. Beyond the coagulation system, thrombin has a wide range of actions and is one of the key inflammatory mediators stimulating the production of a variety of growth factors and, at sites of injury, inducing production of chemoattractants and vasoactive substances, which promote neutrophil adhesion, attract macrophages, and increase vascular permeability.18 The extrinsic pathway is initiated by trauma to a vessel wall and thus is the predominant pathway involved in hemostasis in wounds. Although the 2 pathways do overlap, the intrinsic pathway plays less of a role in this setting. Vessel injury exposes nonvascular tissue cells to the blood. Tissue factor (TF), an integral membrane glycoprotein on these cells, then binds to circulating factor VII, forming a TF-FVIIa complex, which in the presence of Ca2⫹ and phospholipids catalyzes the conversion of factor X into its active form (Xa). However, recent work has led to the realization that what is known of factor VIIa-TF interaction in physiologic conditions may not be true during the systemic inflammatory response witnessed in major cardiac surgery with extracorporeal circulatory support. Under these conditions, TF expression is highly unlikely to be restricted to the subendothelium. Several groups of investigators have reported the presence of physiologically active “blood-borne TF” in proinflammatory conditions, including cardiac surgery.20-22 What form this takes remains unclear; blood-borne TF has been reported as being located on blood cells, being an undefined mixture of procoagulant microparticles (0.1-1 ␮m), or being soluble procoagulant TF fragments.23-25 Proinflammatory cytokines released during CPB can stimulate neutrophils and monocytes to produce and present TF on their surface,26,27 and blood-borne TF in combination with activated monocytes may activate FVII in cardiac surgical patients more than when combined with activated platelets.20,28 Regardless of the source of TF, once factor Xa is generated, thrombin is generated and the 2 coagulation pathways follow a common course. Surgery using CPB results in extensive activation of both arms of the coagulation cascade,29 necessitating systemic heparinization to prevent clot formation within the

bypass circuit. Heparin potentiates the activity of antithrombin III by causing a conformational change that exposes its active site, thus indirectly inhibiting thrombin formation.30 Systemic heparinization, however, is not risk free; platelet activation leading to heparin-induced thrombocytopenia31 and possible (but rare) hyperkalemia secondary to aldosterone inhibition are both well-documented side effects.32 Heparinization may inhibit clot formation, but it acts in the final common stage of both the intrinsic and extrinsic pathway. As a result, the coagulation cascade itself is therefore fully activated and thrombin generation unchecked even in full systemic heparinization, as manifested by elevated levels of thrombin-antithrombin throughout CPB. This progressive thrombin generation, located predominantly within the pericardial wound,33 produces a consumptive coagulopathy that is responsible for many of the thromboembolic and nonsurgical bleeding complications associated with these operations.34 Eventually, direct thrombin inhibitors, such as bivalirudin, may offer the possibility of completely suppressing thrombin generation, thus displacing heparin from its half-century dominance in cardiac surgery, but currently they are in restricted clinical use.35,36 Complement The fourth plasma protein system activated during extracorporeal circulation is the complement system: an innate, cytotoxic host immune defense system composed of approximately 35 interacting plasma and membrane-associated proteins. This system initiates and amplifies the inflammatory response and acts as a “complement” to antibody-mediated immunity against microbial infection. Contained within this system are several soluble factors that prevent spontaneous complement activation from occurring and several regulatory proteins that protect the host from unintended complement-mediated attack.37,38 Complement activation occurs via 3 major pathways: the classic pathway, which is immune complex (antibodies bound to antigens)– dependent; the alternative pathway, which can be activated solely by microbial cell or nonhost surfaces; and the

226

mannose-binding lectin (MBL) pathway, which is activated by a plasma lectin that binds to mannose residues found on microbes.39 When the classic pathway requires antibodies to activate it, both the alternative and MBL pathways are nonantibody-dependent and therefore nonspecific. All 3 pathways eventually generate variants of a protease called C3 convertase, after which point the complement system follows the same path regardless of the means of activation (alternative, classic, or MBL). The C3-convertase role is to cleave and activate C3, creating C3a and C3b and causing a cascade of cleavage and activation events. The alternative pathway is thought to be the principal pathway via which the complement cascade is activated during CPB (Fig 3). Activation occurs immediately on blood coming into contact with the foreign synthetic materials of the bypass circuit40 and occurs by spontaneous hydrolysis of C3 to form C3a and C3b. C3b allows plasma protein factor B to bind to it, where it is cleaved into factor Ba and Bb. Factor Bb remains attached to C3b to form C3bBb, whereas Ba is released into the surrounding medium. C3bBb is the alternative pathway’s C3 convertase, and, although only produced in small amounts, it cleaves multiple further C3 proteins into C3a and C3b. Binding of another C3b fragment to the C3 convertase of the alternative pathway creates C3bBbC3b, a C5 convertase analogous to that created by the other pathways. This activates C5 to C5a and C5b. C5a directly activates neutrophils, and C5b initiates formation of the membrane attack complex (MAC), consisting of C5b, C6, C7, C8, and polymeric C9. The MAC is the endproduct of the complement cascade and is a transmembrane channel capable of producing osmotic cell lysis and death. CPB also activates complement via the classical pathway. There are 3 presumed triggers for this: the formation of protamine-heparin complexes after protamine reversal of heparin at the end of CPB,41 the release of endotoxin by intestinal flora during I/R,40 and the activation of C1 by factor XIIa produced by the contact system. The classic pathway requires 3 proteins, C1, C2, and C4. C1, when bound to an antibody-antigen

WARREN ET AL

complex, is able to bind to C2 and C4 and split them to form C2a and C4b. These 2 bind to form the C3 convertase of the classic pathway whose action is described earlier. The classic pathway proceeds in sequential steps, but the alternative pathway contains a feedback loop (C3bBb) that serves to amplify complement activation and thus the alternative system predominates in CPB-induced complement activation.11 Complement activation during CPB-assisted cardiac surgery plays an important role in the development of perioperative tissue injury. C3a, C4a, and C5a are anaphylatoxins (C5a a particularly potent one) that increase capillary permeability and alter vasomotor tone, resulting in airway smooth muscle contraction and hypotension.42 C5a rapidly binds to neutrophils43 and in combination with the MAC stimulates neutrophil activation and some aspects of platelet activation. Furthermore, the membrane attack complex causes cell lysis and can damage platelets, endothelial cells, and cardiac myocytes.44 The inhibition of the MAC prevents upregulation of neutrophil CD11b receptors, expression of platelet GMP-140 receptors, and formation of platelet-neutrophil and platelet-monocyte conjugates.6,45 Complement activation during CPB is associated with free oxygen radical generation and significant morbidity.6,7 Fibrinolysis The fifth plasma protein system activated during extracorporeal circulation is the fibrinolytic system, the counterbalance to the coagulation system. If coagulation were to be activated without regulatory mechanisms, the resulting continuous production of cross-linked fibrin would cause widespread thrombosis. The fibrinolytic system limits this process, localizing clot formation to the site of tissue or vessel injury and thus preventing widespread thrombotic vessel occlusion and secondary tissue ischemia. Plasminogen is an inactive protein synthesized in both the liver and endothelial cells. Although it is incapable of cleaving fibrin, it has an affinity for it and is incorporated into clots when

Fig 3. Activation of the complement system, particularly the alternative pathway, by CPB.

INFLAMMATORY RESPONSE TO CPB: PART 1

they form. Plasminogen is converted into its active form, plasmin, predominantly by tissue plasminogen activator (t-PA). Plasmin is a serine protease, which cuts through fibrin strands, producing several degradation products (the smallest of which is D-dimer) and eventually leading to clot solubilization.46 In normal circumstances, t-PA is slowly released by the endothelium of damaged blood vessels such that clot dissolution occurs over several days, without disruption of local vessel repair. Plasmin activation and inhibition are further regulated by a variety of proteins including ␣2-antiplasmin, ␣2-macroglobulin, and thrombin-activatable fibrinolysis inhibitor. Fibrinolysis occurs continuously throughout cardiac surgery, particularly within the pericardial wound.47,48 This is compounded by CPB in which fibrinolysis occurs within the circuit,49 as shown by progressively increasing t-PA and D-dimer levels throughout its duration.29 This activation is caused by thrombin stimulation of endothelial cells to produce t-PA50 as well as elevated factor XIIa and kallikrein levels.39 The extent to which fibrinolysis occurs, as indicated by D-dimer levels, has been positively correlated with increased perioperative hemorrhage.51 Finally, the activation of this system is likely to affect other aspects of hemostasis, such as reduced platelet adhesion and aggregation capabilities because of redistribution of glycoprotein Ib and IIb/IIIa receptors. CELLULAR COMPONENTS

Endothelial Cells Although endothelial cells do not come into direct contact with the extracorporeal circuit, they are in constant contact with blood. By responding to a variety of blood-borne agonists (principally thrombin, C5a, and the cytokines, interleukin-1␤ [IL-1␤], and tumor necrosis factor-␣ [TNF-␣]) and producing or inactivating other substances, they play a number of roles in the inflammatory response to bypass; they maintain the fluidity of blood, influence vascular tone, maintain the integrity of the vascular system, and are involved in the adhesion and transmigration of leukocytes into tissues.52 IL-1␤ and TNF-␣ stimulate endothelial cells to produce and express the cell adhesion molecules E-selectin and P-selectin. These bind with high affinity to ligands on activated leukocytes and mediate “rolling” of leukocytes.53 They also induce expression of intracellular adhesion molecule (ICAM) 1 and vascular cell adhesion molecule 1, which firmly bind neutrophils and monocytes to the endothelium and facilitate leukocyte migration through the endothelium into the extravascular space to mediate many of the inflammatory manifestations of CPB. During CPB, endothelial cells produce a variety of anticoagulants and hemostatic agents54 including heparin sulphate, antithrombin, thrombomodulin and protease nexin 1 (both of which remove thrombin), protein S (which accelerates the natural anticoagulant protein C), t-PA, and finally tissue factor pathway inhibitor, a single-chain polypeptide that reversibly inhibits factor Xa and indirectly inhibits the factor VIIa-TF complex that drives the extrinsic coagulation pathway.55 They also influence vasomotor tone through the production or inactivation of a variety of chemicals including nitric oxide (endothelium-derived relaxing factor), endothelin-1, histamine, noradrenaline, and bradykinin.

227

Leukocytes Initially, leukocyte counts are reduced by CPB because of the hemodilutional effect of the fluid used to prime the extracorporeal circuit. However, they subsequently increase in both number and activity during and after CPB.11 CPB activates 3 types of leukocyte: neutrophils, monocytes, and lymphocytes. Neutrophils Neutrophils are the key effector cell of the defense reaction and when activated release a number of cytotoxic enzymes from intracellular granules, including neutrophil elastase, myeloperoxidase, and several lysozymes. Through a “respiratory burst,” they are also capable of producing oxygen-free radicals, hydrogen peroxide, acid groups, and other highly cytotoxic substances.43 The release of these various noxious substances may occur within the vasculature or directly within tissues. The range of substances within these granules and their effects are summarized in Table 1. CPB activates neutrophils, as shown by elevated levels of neutrophil elastase, neutrophil-produced proinflammatory cytokines, and the formation of platelet-leukocyte conjugates.56 A number of mechanisms are responsible for this. Predominantly, neutrophils are activated by the components of both the complement and contact systems, namely C3a and C5a, and kallikrein and factor XIIf. The effect of activated complement in particular is marked, with neutrophil activation occurring within seconds of exposure.57 However, other mediators play a role; thrombin, tumor necrosis factor-␣, heparin, endotoxin,

Table 1. Granular Contents of Activated Neutrophils Released Into the Circulation, Intracellular, and Extracellular Spaces Product Category

ROS

Product Type

Effect

Superoxide anion Hydrogen peroxide Hydroxyl radical Myeloperoxidase

Dismutation 1 Capillary permeability Endothelial cell damage Granular Drives respiratory burst, enzymes cytotoxic Elastase Elastin degradation, platelet activation Collagenase Collagen breakdown Cathepsin-G Collagen depolymerization Neutral protease Damage of basement membranes ␤-glucuronidase Digest extracellular matrix, antimicrobial activity Gelatinase Collagen and extracellular matrix degradation Lactoferrin Antimicrobial activity Vasodilatation, pyrogenesis Arachidonic acid Prostaglandin E2 Thromboxane Vasoconstrictor, platelet metabolites aggregation Leukotriene B4 Leukocyte adhesion and activation, chemoattractant, induction of ROS formation Leukotriene C4, D4 Vasoconstriction NOTE. These products influence the inflammatory response and directly destroy tissue.

228

histamine, and neutrophil-activating peptide 2 released from platelets all have been shown to activate neutrophils during CPB.39,40,58 Finally, neutrophils are activated through direct interaction with endothelial cells, as described earlier, and this neutrophil-endothelial cell interaction plays a key role in the late phase of the inflammatory response. The steps involved in neutrophil chemotaxis and activation at the site of tissue injury are well described. Endothelial cells are activated by exposure to physiologic stressors such as proinflammatory cytokines, trauma, I/R injury, and bacteria. Subsequent to this activation, they start to recruit neutrophils from the blood. Initially, neutrophils “roll” along the endothelial layer, as mediated by the increased expression of L-selectin on the neutrophil surface and E- and P-selectin on endothelial cells. These create low-affinity bonds, but for full adhesion and transmigration to occur, require the creation of high-affinity bonds mediated by cell adhesion molecules. Neutrophils express an integrin, the MAC-1 receptor (consisting of 2 subunits, CD18 and CD11b), which is upregulated by CPB, and endothelial cells express ICAM-1 and ICAM-2 and platelet endothelial cell adhesion molecule 1. The interaction between these adhesion molecules ensures neutrophils bind firmly to the endothelial surface. Here they may clump, resulting in microvascular occlusion and end-organ ischemia. Once bound, neutrophils undergo shape change to facilitate transmigration into the interstitial compartment through the endothelial monolayer. This is facilitated by adhesion molecule expression at endothelial cell junctions and the release of chemotactic agents into the subendothelial space, which creates a concentration gradient down which migration can occur. Once within the extracellular matrix, neutrophils release the cytotoxic contents of their intracellular granules, resulting in increased microvascular permeability, interstitial edema, thrombosis, and parenchymal cell death.59,60 This has been shown to occur within the lungs, heart, and other tissues and organs during and after CPB.61-63 Most investigators consider neutrophils to play the central role in the tissue and organ injury that result from extracorporeal circulation, and, thus, they remain the target of many different therapeutic efforts, both mechanical64 and pharmaceutical.65,66 Monocytes Monocytes are also activated during CPB, but more slowly than other systems such as complement and neutrophils, with peak levels of activity likely to occur a few hours after CPB commences.67 How monocytes are activated is still not entirely clear; C3b generation during complement activation may be one mechanism, but it is likely that other factors such as contact with the extracorporeal circuit, C5b-9, interactions with soluble TF, and endotoxin may contribute to this process.11,28,58 When stimulated, monocytes produce a host of pro- and anti-inflammatory cytokines, including IL-1, IL-2, IL-3, IL-6, and IL-8. The role of TNF-␣, another monocyte-produced cytokine, within CPB is contentious; although some studies have shown an increase, others have not. Plasma levels of these significant mediators of the inflammatory response peak some hours after CPB, suggesting their impact to be largely in the early postoperative period. Monocytes play an increasingly appreciated role in hemostasis during cardiac surgery, both independently and in con-

WARREN ET AL

junction with platelets, with whom they form platelet-monocyte conjugates68 via the granule membrane protein-140 receptor. When stimulated by proinflammatory cytokines, monocytes produce and present tissue factor on their surface,26,27 both in the pericardial wound and in the circuit. Blood-borne TF in combination with activated monocytes may activate factor VII in cardiac surgical patients more than when combined with activated platelets.20,28 The expression of TF initiates the extrinsic coagulation pathway (see earlier). Lymphocytes Total lymphocyte concentration falls over the duration of CPB, an effect mirrored in nearly all specific subsets, including B-lymphocytes, natural killer cells, T-helper cells, and T-suppressor lymphocytes. This decrease remains for around 3 to 7 days after surgery.69 The reduction in number is combined with an impairing of white-cell phagocytosis and an inhibition of monocyte’s ability to present antigen and synthesize IL-1 and results in a weakening of the cellular immune response and an increase in the susceptibility of postbypass patients to acquire infection.70 Platelets CPB activates platelets, resulting in a decrease in their numbers of around 30% to 50% and impairment of their function, both of which contribute to postoperative coagulopathy. Platelet numbers decrease almost instantly because of dilution by circuit priming fluids, but this mechanism is not sufficient alone to account for the thrombocytopenia witnessed in CPB.71 Other factors likely to contribute to platelet consumption include mechanical destruction, adhesion to the circuit surface, sequestration in certain organs, and consumptive coagulopathy.39 Platelet activation is multifactorial, with mediators including the creation of the surgical wound, heparin dosing, hypothermia, and direct contact with the extracorporeal circuit.72 Once on-pump surgery starts, low concentrations of thrombin are generated in both the pericardial wound and within the circuit.73 Thrombin is a potent platelet agonist and likely initiates platelet activation. However, as surgery continues, activated complement (C5b-9), leukotrienes, plasmin, platelet-activating factor, surface contact, and collagenases activate platelets.74,75 Once activated, platelets adhere to surface-adsorbed fibrinogen, von Willebrand factor, and fibronectin on circuit surfaces. They express glycoprotein IIb/IIIa receptor complexes, which allow them to adhere to each other by way of fibrinogen bridges and also express P-selectin, which contributes to the formation of monocyte and (less so) neutrophil conjugates, (as described earlier), by binding P-selectin glycoprotein 1.76 Pselectin expression also stimulates monocytes to express tissue factor, thus contributing to the evolution of thrombus formation.77 Some platelets, both attached and circulating, release some or all of their granules, which contain chemoattractants, coagulation proteins (eg, HK and VWF), and vasoactive substances (eg, thromboxane A2). Platelets also contribute to the formation of microemboli, as disrupted fragments that have sheared off surface-adsorbed platelet aggregates, as part of platelet-leukocyte conjugates or as platelet-fibrin emboli.78 As platelet func-

INFLAMMATORY RESPONSE TO CPB: PART 1

229

tion and levels drop, a few new platelets enter the circulation from the bone marrow. CPB thus has a profound effect on the platelet population, and, as a result, by the end of CPB the platelet population has become highly heterogenous. This effect varies among patients and perfusion systems,79 but all will exhibit thrombocytopenia and increased bleeding times to some extent. Bleeding times usually return to the normal range within 4 to 12 hours.80 LATE PHASE

As the duration of CPB increases, the activation of the humoral and cellular components described earlier diminishes. This is thought to be the result of a “passivation” process of binding sites on the biomaterial surfaces of the circuit, which occurs as a result of protein adsorption onto the inner surface of the extracorporeal circuit, rendering the surface more biocompatible. However, a second, later phase of the inflammatory response has been shown and is thought to be related to I/R injury during and after CPB and endotoxemia, likely after the release of endotoxins from intestinal microflora.81 I/R INJURY

During cardiac surgery, aortic cross-clamping removes the entire blood supply to the heart and the majority of it to the lungs (which receive some oxygenated blood via the undisturbed bronchial arteries). Consequently, the heart, and less so the lungs, become ischemic and on release of the clamp are both fully reperfused. This is associated with a further inflammatory reaction resulting in increased capillary permeability, accumulation of interstitial fluid, leukocytosis, coagulopathy, and end-organ dysfunction. I/R injury is predominantly mediated through neutrophilendothelial interactions, as described earlier. The ischemic phase causes high levels of endothelial injury, resulting in neutrophil activation and sequestration on reperfusion. However, one component is independent of leukocytes and involves the production of highly toxic reactive oxygen species (ROS) (eg, superoxide anions and hydrogen peroxide), the release of soluble arachidonic acid metabolites (eg, prostacyclin), the release of proinflammatory cytokines by ischemic cells (eg, TNF-␣ and interleukins), and activation of the humoral protein systems (eg, complement and coagulation) as described in detail earlier.82 The reintroduction of oxygen during reperfusion creates a high concentration of injurious ROS within previously ischemic cells, a situation compounded by the depletion of energy stores (eg, adenosine triphosphate) and anti-

oxidant defenses during the ischemic period. ROS particularly affects endothelial cells in the microcirculation but within any cell may directly damage cell membranes, denature proteins, and leach from cells to disrupt local structures or enter the circulation to act as second messengers to stimulate the acute inflammatory response.40 ENDOTOXIN

Endotoxin is a lipopolysaccharide from the cell wall of gram-negative bacteria and is recognized to be a major stimulus for the development of SIRS.83 Endotoxemia in patients undergoing surgery involving CPB has been widely recognized,84 but the magnitude of the elevation in endotoxin varies widely among studies, because in part of the heterogeneity of the literature.40 There are many possible sources of endotoxin release during bypass, and the pathogenesis involved in this phenomenon is not fully elucidated. However, gut translocation traditionally has been perceived as the primary source.85 Splanchnic vasoconstriction occurs during CPB, resulting in an enteric mucosal ischemia that is thought to lead to changes in microbial viability and intestinal permeability. However, these postulates remain very difficult to show in vivo, and many investigators have struggled to show any clear relationship or causality among key variables, such as the duration of CPB, levels of intestinal permeability, and endotoxin levels at the end of CPB.86-88 The increased levels of endotoxin resulting from CPB are known to activate complement via the alternative pathway, stimulate the release of proinflammatory cytokines (eg, TNF-␣) and nitric oxide, and increase levels of postoperative oxygen consumption.89 CONCLUSION

The use of CPB in major cardiothoracic surgery initiates an acute inflammatory response that is complex, unpredictable and can cause significant morbidity and mortality. The pathways involved in this process are diverse and complex but can be summarized as being contact activation of the blood by the nonendothelial surfaces of the bypass circuit, ischemia-reperfusion injury, endotoxemia, and operative trauma. The complexity of this pathophysiology has both benefits and detriments. Although multiple targets for intervention are available, it is unlikely that any single solution will safely or effectively attenuate the inflammatory response associated with CPB. Pharmacologic and technical efforts to do so have been developed and investigated, and these will be reviewed in the second article of this pair.

REFERENCES 1. Galletti PM: Cardiopulmonary bypass: a historical perspective. Artif Organs 17:675-686, 1993 2. Gibbon JH Jr: The development of the heart-lung apparatus. Am J Surg 135:608-619, 1978 3. Gibbon JH Jr: Application of a mechanical heart and lung apparatus to cardiac surgery. Minn Med 37:171-185, 1954 4. Edmunds LH Jr: Advances in the heart-lung machine after John and Mary Gibbon. Ann Thorac Surg 76:S2220-2223, 2003 5. Wahba A: Centrifugal blood pump use in routine cardiac surgery. Interact Cardiovasc Thorac Surg 5:299-300, 2006

6. Edmunds LH: Why cardiopulmonary bypass makes patients sick: strategies to control the blood-synthetic surface interface, in Karp RB, Laks H and Wechsler AS (eds): Advances in Cardiac Surgery (vol 6). St Louis, MO, Mosby-Yearbook, 1995, pp 131-167 7. Kirklin JK, Westaby S, Blackstone EH, et al: Complement and the damaging effects of cardiopulmonary bypass. J Thorac Cardiovasc Surg 86:845-857, 1983 8. Cleland J, Pluth JR, Tauxe WN, et al: Blood volume and body fluid compartment changes soon after closed and open intracardiac surgery. J Thorac Cardiovasc Surg 52:698-705, 1966

230

9. Pacifico AD, Digerness S, Kirklin JW: Acute alterations of body composition after open intracardiac operations. Circulation 41:331341, 1970 10. Smith EE, Naftel DC, Blackstone EH, et al: Microvascular permeability after cardiopulmonary bypass. An experimental study. J Thorac Cardiovasc Surg 94:225-233, 1987 11. Edmunds LH Jr: Inflammatory response to cardiopulmonary bypass. Ann Thorac Surg 66:S12-S16, 1998 12. Edmunds LH: Extracorporeal perfusion, in Edmunds LH (ed): Cardiac Surgery in the Adult. New York, NY, McGraw-Hill, 1997, pp 255-294 13. Colman RW: Surface-mediated defense reactions. The plasma contact activation system. J Clin Invest 73:1249-1253, 1984 14. Sainz IM, Pixley RA, Colman RW: Fifty years of research on the plasma kallikrein-kinin system: From protein structure and function to cell biology and in-vivo pathophysiology. Thromb Haemost 98:7783, 2007 15. Wachtfogel YT, Kettner C, Hack CE, et al: Thrombin and human plasma kallikrein inhibition during simulated extracorporeal circulation block platelet and neutrophil activation. Thromb Haemost 80:686-691, 1998 16. te Velthuis H, Baufreton C, Jansen PG, et al: Heparin coating of extracorporeal circuits inhibits contact activation during cardiac operations. J Thorac Cardiovasc Surg 114:117-122, 1997 17. Saatvedt K, Lindberg H, Michelsen S, et al: Activation of the fibrinolytic, coagulation and plasma kallikrein-kinin systems during and after open-heart surgery in children. Scand J Clin Lab Invest 55:359-367, 1995 18. Edmunds LH Jr, Colman RW: Thrombin during cardiopulmonary bypass. Ann Thorac Surg 82:2315-2322, 2006 19. Valen G, Blomback M, Sellei P, et al: Release of von Willebrand factor by cardiopulmonary bypass, but not by cardioplegia in open heart surgery. Thromb Res 73:21-29, 1994 20. Khan MM, Hattori T, Niewiarowski S, et al: Truncated and microparticle-free soluble tissue factor bound to peripheral monocytes preferentially activate factor VII. Thromb Haemost 95:462-468, 2006 21. Diamant M, Nieuwland R, Pablo RF, et al: Elevated numbers of tissue-factor exposing microparticles correlate with components of the metabolic syndrome in uncomplicated type 2 diabetes mellitus. Circulation 106:2442-2447, 2002 22. Giesen PL, Rauch U, Bohrmann B, et al: Blood-borne tissue factor: Another view of thrombosis. Proc Natl Acad Sci USA 96:23112315, 1999 23. Engelmann B: Initiation of coagulation by tissue factor carriers in blood. Blood Cells Mol Dis 36:188-190, 2006 24. Rauch U, Nemerson Y: Circulating tissue factor and thrombosis. Curr Opin Hematol 7:273-277, 2000 25. Nieuwland R, Berckmans RJ, Rotteveel-Eijkman RC, et al: Cell-derived microparticles generated in patients during cardiopulmonary bypass are highly procoagulant. Circulation 96:3534-3541, 1997 26. Nijziel M, van Oerle R, van’t Veer C, et al: Tissue factor activity in human monocytes is regulated by plasma: Implications for the high and low responder phenomenon. Br J Haematol 112:98-104, 2001 27. Maugeri N, Brambilla M, Camera M, et al: Human polymorphonuclear leukocytes produce and express functional tissue factor upon stimulation. J Thromb Haemost 4:1323-1330, 2006 28. Hattori T, Khan MM, Colman RW, et al: Plasma tissue factor plus activated peripheral mononuclear cells activate factors VII and X in cardiac surgical wounds. J Am Coll Cardiol 46:707-713, 2005 29. Hunt BJ, Parratt RN, Segal HC, et al: Activation of coagulation and fibrinolysis during cardiothoracic operations. Ann Thorac Surg 65:712-718, 1998

WARREN ET AL

30. Pixley RA, Schapira M, Colman RW: Effect of heparin on the inactivation rate of human activated factor XII by antithrombin III. Blood 66:198-203, 1985 31. Arnoletti JP, Whitman GJ: Heparin-induced thrombocytopenia in coronary bypass surgery. Ann Thorac Surg 68:576-578, 1999 32. Edes TE, Sunderrajan EV: Heparin-induced hyperkalemia. Arch Intern Med 145:1070-1072, 1985 33. Philippou H, Adami A, Davidson SJ, et al: Tissue factor is rapidly elevated in plasma collected from the pericardial cavity during cardiopulmonary bypass. Thromb Haemost 84:124-128, 2000 34. Philippou H, Davidson SJ, Mole MT, et al: Two-chain factor VIIa generated in the pericardium during surgery with cardiopulmonary bypass: Relationship to increased thrombin generation and heparin concentration. Arterioscler Thromb Vasc Biol 19:248-254, 1999 35. Warkentin TE, Greinacher A, Koster A: Bivalirudin. Thromb Haemost 99:830-839, 2008 36. Warkentin TE: Anticoagulation for cardiopulmonary bypass: Is a replacement for heparin on the horizon? J Thorac Cardiovasc Surg 131:515-516, 2006 37. Walport MJ: Complement. Second of two parts. N Engl J Med 344:1140-1144, 2001 38. Walport MJ: Complement. First of two parts. N Engl J Med 344:1058-1066, 2001 39. Day JR, Taylor KM: The systemic inflammatory response syndrome and cardiopulmonary bypass. Int J Surg 3:129-140, 2005 40. Wan S, LeClerc JL, Vincent JL: Inflammatory response to cardiopulmonary bypass: Mechanisms involved and possible therapeutic strategies. Chest 112:676-692, 1997 41. Carr JA, Silverman N: The heparin-protamine interaction. A review. J Cardiovasc Surg (Torino) 40:659-666, 1999 42. Downing SW, Edmunds LH Jr: Release of vasoactive substances during cardiopulmonary bypass. Ann Thorac Surg 54:12361243, 1992 43. Chenoweth DE, Hugli TE: Demonstration of specific C5a receptor on intact human polymorphonuclear leukocytes. Proc Natl Acad Sci USA 75:3943-3947, 1978 44. Moat NE, Shore DF, Evans TW: Organ dysfunction and cardiopulmonary bypass: The role of complement and complement regulatory proteins. Eur J Cardiothorac Surg 7:563-573, 1993 45. Hamilton KK, Hattori R, Esmon CT, et al: Complement proteins C5b-9 induce vesiculation of the endothelial plasma membrane and expose catalytic surface for assembly of the prothrombinase enzyme complex. J Biol Chem 265:3809-3814, 1990 46. Walker JB, Nesheim ME: The molecular weights, mass distribution, chain composition, and structure of soluble fibrin degradation products released from a fibrin clot perfused with plasmin. J Biol Chem 274:5201-5212, 1999 47. Khalil PN, Ismail M, Kalmar P, et al: Activation of fibrinolysis in the pericardial cavity after cardiopulmonary bypass. Thromb Haemost 92:568-574, 2004 48. Tabuchi N, de Haan J, Boonstra PW, et al: Activation of fibrinolysis in the pericardial cavity during cardiopulmonary bypass. J Thorac Cardiovasc Surg 106:828-833, 1993 49. Stibbe J, Kluft C, Brommer EJ, et al: Enhanced fibrinolytic activity during cardiopulmonary bypass in open-heart surgery in man is caused by extrinsic (tissue-type) plasminogen activator. Eur J Clin Invest 14:375-382, 1984 50. Levin EG, Santell L: Stimulation and desensitization of tissue plasminogen activator release from human endothelial cells. J Biol Chem 263: 9360-9365, 1988 51. Gram J, Janetzko T, Jespersen J, et al: Enhanced effective fibrinolysis following the neutralization of heparin in open-heart surgery increases the risk of postsurgical bleeding. Thromb Haemost 63:241-245, 1990

INFLAMMATORY RESPONSE TO CPB: PART 1

52. Boyle EM Jr, Pohlman TH, Johnson MC, et al: Endothelial cell injury in cardiovascular surgery: The systemic inflammatory response. Ann Thorac Surg 63:277-284, 1997 53. Springer TA: Traffic signals for lymphocyte recirculation and leukocyte emigration: The multistep paradigm. Cell 76:301-314, 1994 54. Cardigan RA, Mackie IJ, Machin SJ: Hemostatic-endothelial interactions: A potential anticoagulant role of the endothelium in the pulmonary circulation during cardiac surgery. J Cardiothorac Vasc Anesth 11:329-336, 1997 55. Ranucci M: The endothelial function in cardiac surgery. Minerva Anestesiol 72:503-506, 2006 56. Butler J, Parker D, Pillai R, et al: Effect of cardiopulmonary bypass on systemic release of neutrophil elastase and tumor necrosis factor. J Thorac Cardiovasc Surg 105:25-30, 1993 57. Craddock PR, Fehr J, Brigham KL, et al: Complement and leukocyte-mediated pulmonary dysfunction in hemodialysis. N Engl J Med 296:769-774, 1977 58. Asimakopoulos G: Mechanisms of the systemic inflammatory response. Perfusion 14:269-277, 1999 59. Collard CD, Gelman S: Pathophysiology, clinical manifestations, and prevention of ischemia-reperfusion injury. Anesthesiology 94:1133-1138, 2001 60. Ratliff NB, Young WG Jr, Hackel DB, et al: Pulmonary injury secondary to extracorporeal circulation. An ultrastructural study. J Thorac Cardiovasc Surg 65:425-432, 1973 61. Ng CS, Wan S, Arifi AA, et al: Inflammatory response to pulmonary ischemia-reperfusion injury. Surg Today 36:205-214, 2006 62. Clark SC: Lung injury after cardiopulmonary bypass. Perfusion 21:225-228, 2006 63. Vinten-Johansen J: Involvement of neutrophils in the pathogenesis of lethal myocardial reperfusion injury. Cardiovasc Res 61:481497, 2004 64. Warren O, Alexiou C, Massey R, et al: The effects of various leukocyte filtration strategies in cardiac surgery. Eur J Cardiothorac Surg 31:665-676, 2007 65. Wakayama F, Fukuda I, Suzuki Y, et al: Neutrophil elastase inhibitor, sivelestat, attenuates acute lung injury after cardiopulmonary bypass in the rabbit endotoxemia model. Ann Thorac Surg 83:153-160, 2007 66. Ando M, Murai T, Takahashi Y: The effect of sivelestat sodium on postcardiopulmonary bypass acute lung injury in a neonatal piglet model. Interact Cardiovasc Thorac Surg 2008 Jul 2 [Epub ahead of print] 67. Chung JH, Gikakis N, Rao AK, et al: Pericardial blood activates the extrinsic coagulation pathway during clinical cardiopulmonary bypass. Circulation 93:2014-2018, 1996 68. Weerasinghe A, Athanasiou T, Philippidis P, et al: Plateletmonocyte procoagulant interactions in on-pump coronary surgery. Eur J Cardiothorac Surg 29:312-318, 2006 69. DePalma L, Yu M, McIntosh CL, et al: Changes in lymphocyte subpopulations as a result of cardiopulmonary bypass. The effect of blood transfusion. J Thorac Cardiovasc Surg 101:240-244, 1991 70. Roth JA, Golub SH, Cukingnan RA, et al: Cell-mediated immunity is depressed following cardiopulmonary bypass. Ann Thorac Surg 31:350-356, 1981 71. Holloway DS, Summaria L, Sandesara J, et al: Decreased platelet number and function and increased fibrinolysis contribute to postoperative bleeding in cardiopulmonary bypass patients. Thromb Haemost 59:62-67, 1988

231

72. Paparella D, Yau TM, Young E: Cardiopulmonary bypass induced inflammation: Pathophysiology and treatment. An update. Eur J Cardiothorac Surg 21:232-244, 2002 73. Gikakis N, Khan MM, Hiramatsu Y, et al: Effect of factor Xa inhibitors on thrombin formation and complement and neutrophil activation during in vitro extracorporeal circulation. Circulation 94:II341II346, 1996 74. Sims PJ, Faioni EM, Wiedmer T, et al: Complement proteins C5b-9 cause release of membrane vesicles from the platelet surface that are enriched in the membrane receptor for coagulation factor Va and express prothrombinase activity. J Biol Chem 263:18205-18212, 1988 75. Smith BR, Rinder HM, Rinder CS: Cardiopulmonary bypass, in Michelson AD (ed): Platelets. Amsterdam, Netherlands, Academic Press, 2002, pp 727-741 76. Larsen E, Celi A, Gilbert GE, et al: PADGEM protein: A receptor that mediates the interaction of activated platelets with neutrophils and monocytes. Cell 59:305-312, 1989 77. Celi A, Pellegrini G, Lorenzet R, et al: P-selectin induces the expression of tissue factor on monocytes. Proc Natl Acad Sci USA 91:8767-8771, 1994 78. Weerasinghe A, Taylor KM: The platelet in cardiopulmonary bypass. Ann Thorac Surg 66:2145-2152, 1998 79. Edmunds LH Jr, Ellison N, Colman RW, et al: Platelet function during cardiac operation: Comparison of membrane and bubble oxygenators. J Thorac Cardiovasc Surg 83:805-812, 1982 80. Despotis GJ, Goodnough LT: Management approaches to platelet-related microvascular bleeding in cardiothoracic surgery. Ann Thorac Surg 70:S20-S32, 2000 81. Rossi M, Sganga G, Mazzone M, et al: Cardiopulmonary bypass in man: Role of the intestine in a self-limiting inflammatory response with demonstrable bacterial translocation. Ann Thorac Surg 77:612618, 2004 82. Krishnadasam B, Griscavage-Ennis J, Aldea GS, et al: Reperfusion injury during cardiopulmonary bypass, in Matheis G, Moritz A, Scholz M (eds): Leukocyte Depletion in Cardiac Surgery and Cardiology. Basel, Switzerland, Karger, 2002, p 54 83. Opal SM: The host response to endotoxin, antilipopolysaccharide strategies, and the management of severe sepsis. Int J Med Microbiol 297:365-377, 2007 84. Rocke DA, Gaffin SL, Wells MT, et al: Endotoxemia associated with cardiopulmonary bypass. J Thorac Cardiovasc Surg 93:832-837, 1987 85. Aydin NB, Gercekoglu H, Aksu B, et al: Endotoxemia in coronary artery bypass surgery: A comparison of the off-pump technique and conventional cardiopulmonary bypass. J Thorac Cardiovasc Surg 125:843-848, 2003 86. Ohri SK, Bjarnason I, Pathi V, et al: Cardiopulmonary bypass impairs small intestinal transport and increases gut permeability. Ann Thorac Surg 55:1080-1086, 1993 87. Andersen LW, Landow L, Baek L, et al: Association between gastric intramucosal pH and splanchnic endotoxin, antibody to endotoxin, and tumor necrosis factor-alpha concentrations in patients undergoing cardiopulmonary bypass. Crit Care Med 21:210-217, 1993 88. Riddington DW, Venkatesh B, Boivin CM, et al: Intestinal permeability, gastric intramucosal pH, and systemic endotoxemia in patients undergoing cardiopulmonary bypass. JAMA 275:1007-1012, 1996 89. Oudemans-van Straaten HM, Jansen PG, Hoek FJ, et al: Intestinal permeability, circulating endotoxin, and postoperative systemic responses in cardiac surgery patients. J Cardiothorac Vasc Anesth 10:187-194, 1996