SNOWMASS, COLO. — When it comes to managing anticoagulation in the pregnant patient with a mechanical heart valve, there is simply no ideal solution, Dr. Carole A. Warnes stressed at a conference sponsored by the Society for Cardiovascular Angiography and Interventions.
“This is not the same as getting your patient through noncardiac surgery. It's very different. The blood is stickier than at any other time you'll have to manage a mechanical valve,” cautioned Dr. Warnes, professor of medicine at the Mayo Clinic, Rochester, Minn.
Other normal physiologic changes in pregnancy that increase the risk of thromboembolic events in patients with mitral or aortic valve prostheses include a nearly 50% increase in circulating blood volume, accompanied by a 30% rise in cardiac output and a 10–20 beat-per-minute increase in resting heart rate. Also, uterine contractions can trigger sudden jumps in systolic and diastolic blood pressure.
What makes managing thromboembolic risk in pregnant patients with a mechanical heart valve so challenging is the need to trade off maternal versus fetal risk.
Unfractionated heparin doesn't cross the placenta. It is often considered safer for the fetus than warfarin in pregnancy. But unfractionated heparin is a poor anticoagulant in pregnancy. The response to the standard dosage varies widely because of the background increases in factor VIII and fibrinogen. As a result, the risk of a thrombosed valve or other thromboembolic event with prolonged heparin is about 10%. The maternal hemorrhage risk is increased as well.
Warfarin is far more effective than unfractionated heparin at preventing valve thrombosis in pregnancy. However, it crosses the placenta, and fetal exposure during gestational weeks 6–9 can result in warfarin embryopathy. The risk is approximately 6% but might be dose dependent. In one older Italian study involving 58 pregnancies, no cases of embryopathy occurred at warfarin doses of 5 mg/day or less, compared with a 9% rate at doses above 5 mg/day (J. Am. Coll. Cardiol. 1999;33:1637–41).
“The fetal risk is probably not as high with warfarin as you might think, but for medicolegal reasons you probably want to avoid it in most circumstances,” Dr. Warnes observed at the conference, which was cosponsored by the American College of Cardiology.
Some advocate low-molecular-weight heparin throughout pregnancy as the best approach, but Dr. Warnes is leery. The supporting data are extremely limited. Moreover, she has seen thromboembolic complications occur even when LMWH dosing was guided by monitoring of factor Xa levels rather than relying on fixed-dose therapy.
The most popular management strategy in the United States entails a switch from warfarin to unfractionated heparin as soon as pregnancy is diagnosed, with a switch back to warfarin at 13 weeks' gestation, after the risk of embryopathy is over.
This is followed by yet another switch back to heparin at about 35 weeks in anticipation of delivery, because the fetus can't safely pass through the birth canal while anticoagulated.
The heparin is stopped for as short a time as possible around delivery. Heparin is resumed 6–12 hours post partum, because that's still a high-risk period for valve thrombosis.
If this strategy is employed, it's important to give heparin at an adequate intensity. This means maintaining the activated partial thromboplastin time at greater than twice control. If factor Xa monitoring is used, aim for 0.35–0.7 U/mL of anti-factor Xa, Dr. Warnes urged.
The highest-risk situation in pregnancy in terms of thromboembolism involves a tilting disc prosthesis in the mitral position. This is a situation in which continued use of warfarin throughout pregnancy is a reasonable strategy up until the switch to intravenous heparin at week 35, even though the Physicians Desk Reference lists warfarin as contraindicated in pregnancy, the cardiologist said.
Warfarin throughout pregnancy is a particularly attractive strategy in a high-risk woman who was well controlled on the anticoagulant at 5 mg/day or less prior to pregnancy, which might lessen the risk of warfarin embryopathy, she continued.
Whatever anticoagulation strategy is used in pregnancy, a daily baby aspirin during the second and third trimesters is safe and probably beneficial. It should be used routinely, according to Dr. Warnes.
'This is not the same as getting your patient through noncardiac surgery.' DR. WARNES