Matas Lecture. Heparin--controversies and misconceptions

The previous dogmas regarding heparin therapy are currently being challenged. It is apparent that the experimental work on which guidelines for heparin therapy are based do not necessarily have any relevance clinically. Once clotting has been initiated, there are multiple factors that result in a relative refractory response to heparin anticoagulation. In addition, it is now apparent that heparin's effect cannot be accurately monitored with current tests of anticoagulation. Most importantly, the risk of bleeding does not correlate with heparin levels but with clinical risk factors and to the presence or absence of functioning platelets. For this reason, sufficient heparin should be given initially to ensure that clotting is under control. If this is not done, all of the risk of heparin anticoagulation is assumed with none of the benefit. Life-threatening clotting conditions require high doses of heparin, equivalent to those required for cardiopulmonary bypass. Even though there is no good laboratory test available to ascertain the adequacy of anticoagulation, assessment of the clinical response is sufficient. When heparin levels are adequate, clinical improvement is evident as manifest by decreased pain and improvement in well-being, cardiac function, and/or collateral flow.