Hemostatic Mechanism In Extracorporeal Circulation

Natural and synthetic antifibrinolytics in cardiac surgery

In an effort to reduce morbidity associated with transfusion of blood products, the use of antifibrinolytics to decrease bleeding and transfusions after cardiopulmonary bypass (CPB) is receiving widespread attention. The predominant haemostatic defect induced by CPB and, therefore, the mechanisms by which natural (aprotinin) or synthetic antifibrinolytics (sigma-amino-caproic acid, tranexamic acid) exert their effects have been difficult to define. Nonetheless, all three substances appear to be effective in the treatment or in the prevention of excessive bleeding associated with cardiac surgery. However, the administration of these drugs should not attempt to replace meticulous surgical and anaesthetic care. In particular, the importance of an appropriate transfusion practice cannot be overemphasized. The efficient use of these, sometimes expensive, drugs must take into account not only the initial cost, but also the short- and long-term economic consequences for the health care provider of using, or not using, a given medication. Unfortunately, the comprehensive data on which authoritative conclusions may be reached are not yet available. Pending availability of these data, the present use of antifibrinolytics at the Montreal Heart Institute is the following: (1) patients undergoing elective primary myocardial revascularization or valve surgery do not receive prophylactic antifibrinolytics; (2) patients undergoing repeat myocardial revascularization, repeat valve surgery, or primary or repeat combined procedures, receive prophylactic sigma-aminocaproic acid; (3) sigma-aminocaproic acid may be used to treat excessive chest drainage in the postoperative period; (4) the prophylactic and the therapeutic uses of low doses of aprotinin are currently under investigation.

To pulse or not to pulse

Pulsatile and nonpulsatile blood flow have been intensely studied for cardiopulmonary bypass (CPB), isolated organ perfusion, and myocardial preservation. Although early studies differed, later ones have shown the benefits of pulsatile flow. Kidney function, lymph flow, and oxygen consumption are increased during pulsatile perfusion. Also, nonpulsatile CPB increases total peripheral resistance and mean arterial pressure, which are related to time of perfusion. Theories to account for the superiority of pulsatile flow include: (1) "vascular shocks" causing physical displacement of tissues, which changes the boundary layer of interstitial fluid around cell membranes and enhances diffusion ;(2) increased lymph movement during pulsatile flow; and (3) pulsatile energy ensuring the patency of the vascular beds and preventing shunting. New methods to create pulsatile flow and their adaptation to the standard roller pump are discussed.