Low-molecular-weight heparins and heparinoids

To the Editor: In a valuable review of low-molecular-weight heparins (LMWH), Eikelboom and Hankey1 stray off the beaten path into the unwelcoming area of obstetric therapeutics — a notoriously hostile environment replete with traps and hazards. Their statement that "low-molecular-weight heparins are being used increasingly in pregnant women with prosthetic heart valves and for the prevention and treatment of venous thromboembolism" is contentious and requires considerable qualification.

The Journal has already published a position statement concerning the use of these heparins in pregnancy.2 It clearly stated that the initial treatment for pulmonary embolism in pregnancy remains intravenous unfractionated heparin, because so far there are no trials of LMWH in pulmonary embolism in pregnancy. The guidelines of the American College of Chest Physicians do endorse the use of LMWH for this indication,3 but base that view on data in non-pregnant patients. We believe that, as yet, there is insufficient evidence to recommend LMWH for the initial management of pulmonary embolism in pregnancy, although, on theoretical grounds, the treatment seems attractive.

In anticoagulation therapy for artificial heart valves in pregnancy, there are serious problems. Unfortunately, the conscientious adviser must be very circumspect in counselling women with these prostheses. Pregnancy for these women presents significant risks. None of the heparins, unfractionated or low molecular weight, has been shown to protect reliably against embolism from, or thrombosis of, these valves in pregnancy.

Whether LMWH is better than unfractionated heparin has not been established and awaits appropriate trials. Warfarin, which crosses the placenta, remains a valid, but worrying, choice in pregnancy for antico-agulation in patients with prosthetic heart valves. This drug provides optimal protection from valve thrombosis, but with the potential for teratogenicity in the first trimester and fetal (and maternal) haemorrhage later in pregnancy. Many experts use heparin and warfarin sequentially in this situation.3

Thus, anticoagulation therapy for pregnant women with serious medical problems remains, as always, perplexing, difficult and dangerous. While LMWH offer considerable promise and have undoubted utility in several areas, there are very compelling caveats about their current use for pulmonary embolism and prosthetic heart valves in pregnant women.

For these reasons and others, women with prosthetic heart valves planning pregnancy, as well as those already pregnant, should be counselled about these problems by a physician experienced in managing medical problems in pregnancy.