Bradykinin Receptor Antagonism During Cardiopulmonary Bypass

Morbidity of cardiopulmonary bypass. Each year more than a million patients worldwide undergo cardiac surgery. Nearly all cardiac surgeries are performed on unbeating hearts supported by CPB. Although the use of off-pump coronary artery bypass surgery procedures are increasing, concerns regarding incomplete revascularization and reduced venous graft patency limit the use of this technique to specific patients. CPB activates various humoral cascades including the coagulation cascade, the KKS, the fibrinolytic cascade, and causes a systemic inflammatory response syndrome. Activation of these systems can lead to hypotension, fever, disseminated intravascular coagulation, diffuse tissue edema, or, in extreme cases, to multiple organ failure. Activation of the KKS contributes to the hemodynamic perturbations, fibrinolysis and inflammatory response observed in patients undergoing CPB. Aprotinin, a non-specific serine protease inhibitor, that works in part by decreasing bradykinin generation, decreases fibrinolysis, hypotension and the systemic inflammatory response associated with CPB. Aprotinin decreases blood loss and transfusion requirements, however, its use is mainly limited to redo-cardiac surgery because of cost. Other factors that may limit the widespread use of aprotinin include an increased risk for renal dysfunction, allergic reaction and non-specificity of the drug. Bradykinin mediates most of the effects of the KKS. Thus, bradykinin receptor antagonism has the potential to modulate the effects of KKS activation during CPB. The purpose of this proposal is to test the hypothesis that endogenous bradykinin contributes to the hemodynamic, fibrinolytic and inflammatory response to CPB and that bradykinin receptor antagonism will reduce hypotension, inflammation and transfusion requirements. The proposed studies promise to lead to novel therapies to reduce morbidity associated with CPB.

Cardiopulmonary bypass activates the kallikrein-kinin system (KKS). Several groups, including ours, have reported that bradykinin concentrations increase during CPB. For example, Campbell et al demonstrated that bradykinin levels increase 10 to 20-fold during the first 10 minutes of CPB, returned to basal levels by 70 minutes of CPB and remained 1.7 to 5.2-fold elevated after CPB. Plasma and tissue kallikrein were reduced by 80 and 60% respectively, during the first minute of CPB. Similarly, we have demonstrated that bradykinin increases significantly during CPB and that ACE inhibition and smoking potentiate the kinin response during CPB.

Fibrinolytic response to cardiopulmonary bypass. CPB increases t-PA antigen and activity in a time-dependent manner. The fibrinolytic response during CPB is heterogeneous, with t-PA levels varying as much as 250-fold. The mechanism of t-PA release during CPB is likely multifactorial. As outlined above, we and others have shown that CPB increases bradykinin, a potent stimulus to t-PA release. In addition, thrombin or complement generated during CPB may stimulate the release of t-PA from endothelium. In addition to the changes in t-PA concentrations during CPB, PAI-1 activity falls because of hemodilution and the rise in t-PA release which consumes active PAI-1. Plasmin generation increases over 100-fold while D-dimer generation increases 200-fold within 5 minutes of CPB initiation. For the remainder of the CPB, average plasmin and D-dimer levels remain 20-fold to 30-fold above baseline levels. The postoperative period is marked by a systemic inflammatory response caused by a combination of CPB and surgery producing an acute -phase response that results in increased PAI-1 production. PAI-1 levels begin to rise about 2 hours after surgery. Once CPB is over, PAI-1 levels continue to rise and peak during the first 12-36 hours postoperatively and return to normal by the second postoperative day. Thus, the fibrinolytic response to CPB is characterized by an initial hyperfibrinolytic phase that begins with a rapid rise in t-PA, plasmin, and D-dimer concentrations followed by a postoperative hypofibrinolytic phase associated with a rise in PAI-1 secretion and a fall in t-PA concentrations.

Interaction between the renin-angiotensin system (RAS), the KKS and fibrinolytic system. There is evidence that fibrinolytic balance is regulated by the RAS and the KKS. ACE is strategically poised to control fibrinolytic balance by promoting the breakdown of bradykinin and the conversion of Ang I to Ang II. Ang II causes the release of PAI-1 thus inhibiting fibrinolysis. Bradykinin stimulates t-PA release through its B2 receptor. ACE inhibition decreases PAI-1 antigen levels and increases endothelial t-PA release through endogenous bradykinin. In addition, ACE inhibition enhances exogenous bradykinin-mediated vasodilation and t-PA release. The augmentation of bradykinin-induced vasodilation, the increase in t-PA and the decrease in PAI-1 described with ACE inhibition in patients with ischemic heart disease may contribute to the primary mechanism of the anti-ischemic effects associated with chronic ACE inhibitor therapy. We have demonstrated that inpatients undergoing coronary artery bypass grafting (CABG) requiring CPB, not only did ACE inhibition increase fibrinolytic activity by decreasing PAI-1 antigen and increasing t-PA activity, but also enhanced the kinin response. Increased PAI-1 concentrations in the perioperative period are associated with acute vein graft thrombosis. Thus, ACE inhibitors have a potential to reduce the risk of acute graft thrombosis through their effects on Ang II generation by attenuating the PAI-1 response after CABG. As opposed to the beneficial effects of ACE inhibition on PAI-1, the augmentation of the kinin response during CPB may have detrimental effects including increased fibrinolysis with consequent bleeding and hypotension. The effect of angiotensin II type 1 (AT1) receptor antagonist on the fibrinolytic response to CPB is not known. Inpatients with essential hypertension AT1 receptor antagonist decreases PAI-1 antigen in some but not other studies. In Specific Aim 1 we will test the hypothesis that angiotensin-converting enzyme inhibitors and AT1 receptor antagonist modulate the fibrinolytic and inflammatory response to CPB differently.

Bradykinin receptor antagonism could reduce the hypotensive response to CPB. Low systemic vascular resistance (SVR) commonly occurs during and early after CPB. It is usually transient and easy to treat. Occasionally, patients have a more severe and persistent fall in SVR, referred to postoperative vasodilatory shock. Risk factors for vasodilatory shock includes the preoperative use of ACE inhibitors, low left ventricular ejection fraction and heart failure syndrome. Treatment is frequently required to maintain adequate perfusion pressure during CPB and to establish satisfactory hemodynamics when ready to separate the patient from bypass. This usually entails counteracting the effect of the vasodilatory mediators by administration of drugs such as norepinephrine or phenylephrine. Although usually effective and safe, these drugs can redistribute blood flow in such a way as to compromise the splanchnic and renal circulation. Several mediators are thought to be responsible for producing postoperative shock, including bradykinin. For example, there is an inverse correlation between bradykinin concentrations and mean arterial pressure during CPB, suggesting that bradykinin is an important mediator in the decrease in SVR. We and others have shown that bradykinin induces vasodilation through its B2 receptor. In contract, B1 receptor stimulation does not cause vasodilation. As outlined under PRELIMINARY STUDIES, we have demonstrated that endogenous bradykinin contributes to protamine-related hypotension following CPB and that bradykinin receptor antagonism administered just prior to protamine attenuates this hypotensive response. In Specific Aim 2 we will test the hypothesis that bradykinin receptor antagonism modulate the hemodynamic changes observed during CPB.