The STOP CLOT Pilot Study: Study of Low Molecular Weight Heparin in High Risk Cesarean Section

BACKGROUND: Venous thromboembolism (VTE) remains the most common cause of maternal mortality in the developed world. VTE is up to 10 times more common in pregnant women than non-pregnant women of comparable age. More than a third of pregnancy related VTE occur during the brief postpartum period (6 weeks). When compared with vaginal delivery, cesarean delivery further increases the risk of pregnancy associated VTE by three-fold. Pregnancy associated VTE is unique in that isolated iliac vein thrombosis is more likely and long term morbidity (post-phlebitic syndrome) is common.

There is an absence of randomized controlled trials (RCTs) of thromboprophylaxis after C-section to guide practice. Many national societies have guidelines on thromboprophylaxis however these are not evidence based and compliance with these guidelines is poor. Thromboprophylaxis may be associated with adverse effects (bleeding, heparin induced thrombocytopenia, skin reactions), inconvenience and cost. It is critical that the efficacy and safety of thromboprophylaxis following cesarean delivery be assessed in well designed and conducted randomized trials.

OBJECTIVES: We are conducting a randomized double-blind placebo-controlled pilot study. The pilot study seeks to answer the question: Is a randomized double-blind placebo-controlled multicentre trial of low molecular weight heparin thromboprophylaxis feasible in moderate to high risk women post cesarean section with DVT detected by pelvic vein (MRV) or leg vein (leg compression ultrasound) imaging? Our ultimate goal is to determine:

1. Is LMWH effective in preventing postpartum DVT following cesarean section in women at moderate to high risk of VTE?
2. Is LMWH safe in preventing postpartum DVT following cesarean section in women at moderate to high risk of VTE?
3. Is LMWH cost effective in preventing postpartum DVT following cesarean section in women at moderate to high risk of VTE?

STUDY DESIGN: We propose a randomized double-blind placebo-controlled pilot study of prophylactic LMWH in women at moderate to high risk for VTE following cesarean section. Eligible, consenting and randomized participants will receive once-daily injections of study drug (4,500 IU tinzaparin sodium (Innohep®;) or placebo) within 6 to 24 hours postpartum and continue until 3 to 7 days postpartum. On the day of hospital discharge, bilateral leg imaging with compression leg ultrasounds and pelvic vein imaging with MRV will be completed. The primary outcome will be adjudicated DVT documented on ultrasounds or MRV on the day of hospital discharge. Secondary outcomes will include symptomatic DVT and PE, death from PE, major and minor bleeding, HIT during the six week postpartum period. All outcomes will be adjudicated by an independent committee of experts blinded to study drug allocation.

With a sample size of 134 patients we will have over 80% power to detect a 50% relative risk reduction in the primary outcome event rate and a large enough sample to determine the feasibility objectives of the pilot study (i.e. obtain a precise estimate of the primary outcome event rate [DVT], a precise estimate of the multicentre recruitment rate, feasibility and acceptability of blinded study drug and placebo administration, feasibility of obtaining local study centre MRV and central interpretation of MRV and a preliminary relative risk reduction estimate with study drug compared to placebo to inform final study sample size determination).

RELEVANCE: Maternal mortality is a devastating outcome with far reaching emotional and societal implications. Evidence to guide thromboprophylaxis in women at risk for the number 1 cause of maternal mortality is required.