Thromboprophylaxis for the ICU patient
Critically ill patients frequently have several coexistent risk factors for thromboembolic disease, in particular prolonged immobility, surgery, and indwelling vascular access catheters. Therefore they may benefit from thromboprophylaxis. However, they may also be at increased risk of hemorrhage due to recent surgery, hemostatic dysfunction secondary to renal failure (which also reduces heparin clearance), severe hepatic disease, or thrombocytopenia. These factors are relative contraindications to anticoagulation. As these patients form a highly heterogeneous group, the risk-benefit ratio will vary from patient to patient. There is a paucity of data on the risks and benefits of thromboprophylaxis or anticoagulant regimens in this group of patients.
Subcutaneous heparin, either unfractionated or low molecular weight, is the most common thromboprophylaxis used. Compared with unfractionated heparin at standard dosage, low-molecular-weight heparin has been shown to have increased efficacy in the high-risk situation of orthopedic surgery. Its disadvantages in critically ill patients are the need for subcutaneous absorption, which may be impaired due to poor peripheral perfusion, and the lack of easy reversibility due to the long plasma half-lives of these drugs. Low-molecular-weight heparin can be monitored to allow more precise control using an anti-activated factor X (anti-factor Xa) plasma level. However, this would not be feasible on a frequent basis in most institutions. A logical approach may be the administration of continuous low-dose intravenous unfractionated heparin at an initial dose of 500 IU/h. This has several advantages in the ICU setting; it is easily monitored using routine laboratory tests and, as it has a short plasma half-life, it can be stopped shortly before surgical procedures or the insertion of indwelling catheters. This regimen should be monitored, aiming to ensure that the activated partial thromboplastin time ratio remains less than 1.5.