Mrs. ACS is a 74-year-old female who is admitted to the intensive care unit (ICU) for an acute coronary syndrome. She presents to the emergency department with substernal chest pain with radiation to the neck, diaphoresis, and dizziness. Her electrocardiogram (ECG) shows T-wave flattening in the inferior leads (II, III, and aVF). No ST-segment elevations are noted. The patient is started on aspirin, nitroglycerin, and metoprolol. As you write for a heparin drip, the nurse asks, “Why don’t we do Lovenox [enoxaparin], that way I don’t have to start a drip.”

Acute coronary syndrome is a relatively new classification term for a set of acute coronary diseases. Causes of acute coronary syndrome include ST-elevation myocardial infarction (STEMI), non-ST-elevation myocardial infarction (NSTEMI), and unstable angina. STEMI is defined as myocardial infarction involving ST-segment elevation of 0.1 mV in two or more consecutive leads or a new left bundle branch block. NSTEMI refers to acute myocardial damage from atherosclerotic disease that results in release of cardiac biomarkers without causing ST-segment elevation. Unstable angina refers to myocardial damage that does not result in release of cardiac biomarkers or ST-segment elevation. These definitions are important because they relate to the pathogenesis, severity of disease, and management strategy of the affected patients.

In the treatment of acute coronary syndrome, anticoagulant therapy is indicated to minimize further thrombus formation. Heparin is the recommended parenteral anticoagulant in treatment of acute coronary syndrome. Heparin accelerates the action of antithrombin. This leads to inactivation of factors IIa (thrombin), IXa, and Xa. This leads to decrease thrombus formation. Unfractionated heparin is a mixture
of different weights of heparin (5 to 30k) that have varying effects on factors IIa, IXa, and Xa. Low-molecular-weight heparin (LMWH) is a subset of heparin (weight ∼5k) that has activity on factor Xa, but not on IIa. LMWH has the advantage of less binding to protein and dose-independent clearance. This leads to more predictable dosing and a longer half-life (3 to 6 hours). Because of the longer half-life, LMWH can be administered subcutaneously twice a day as opposed to a continuous infusion for unfractionated heparin (half-life 1 to 2 hours).