Prevention of blood clots in patients undergoing cardiac or thoracic surgery

Background

Patients undergoing surgery have an increased probability of developing blood clots in their veins (venous thromboembolism). These clots may be in the deep veins (deep vein thrombosis) or travel to the lungs (pulmonary embolism). As in other types of surgery, effective prevention of blood clots (thromboprophylaxis) after cardiac or thoracic surgery may reduce the risk of postoperative vein clots. These potential benefits, however, have to be balanced against the associated risks of bleeding. This systematic review looked at the effectiveness and safety of anticoagulants (medicines that reduce the ability of the blood to clot), mechanical interventions (such as pneumatic pumps on the legs to promote blood flow), and caval filters (a type of vascular filter, implanted into the main abdominal vein to prevent movement of clots from the legs to the lungs) in patients undergoing cardiac or thoracic surgery.

Study characteristics and key results

We identified 13 randomised controlled trials (6923 participants), six for cardiac surgery (3359 participants) and seven for thoracic surgery (3564 participants). The evidence is current to May 2014. No study evaluated fondaparinux, the new oral direct thrombin or direct factor Xa inhibitors, or caval filters. Data could not be combined because of the different comparisons and the lack of data. Data for clinically relevant outcomes such as pulmonary embolism (blockage of one or more arteries of the lung) or major bleeding were often lacking. In cardiac surgery, symptomatic venous thromboembolism occurred in 71 out of 3040 participants from three studies. In a study of 2551 participants, representing 85% of the review population in cardiac surgery, the combination of unfractionated heparin with intermittent pneumatic compression was associated with an important reduction of symptomatic venous thromboembolism compared to unfractionated heparin alone. Major (important) bleeding was reported in one study only, and the best estimate was that bleedings occurred seven times more often in participants on vitamin K antagonists compared to participants on platelet inhibitors, but the true estimate may lay between one and a half to 30. In thoracic surgery, symptomatic venous thromboembolism occurred in 15 out of 2890 participants from six studies. Combined analysis could not be performed, but the largest study evaluating unfractionated heparin versus an inactive control did not show a benefit in terms of reduced occurrence of symptomatic venous thromboembolism. Major bleeding was reported in two studies that did not find significantly different rates between fixed-dose and weight-adjusted low molecular weight heparin (2.7% versus 8.1%) and between unfractionated heparin and low molecular weight heparin (6% and 4%).

Quality of the evidence

Overall, the evidence on the use of thromboprophylaxis in cardiac and thoracic surgery appeared to be scarce, so we are very uncertain about the benefit-to-risk balance. All studies had major study design flaws and most lacked a placebo or no treatment control group. We typically graded the quality of the overall body of evidence for the various outcomes and comparisons as low, due to imprecise estimates of effect and risk of bias. Our data suggest that thromboprophylaxis cannot be suggested for all patients undergoing these types of surgery, but should rather be considered case-by-case based on the individual risk of venous thromboembolism and bleeding.

Authors' conclusions: 

The evidence regarding the efficacy and safety of thromboprophylaxis in cardiac and thoracic surgery is limited. Data for important outcomes such as pulmonary embolism or major bleeding were often lacking. Given the uncertainties around the benefit-to-risk balance, no conclusions can be drawn and a case-by-case risk evaluation of VTE and bleeding remains preferable.

Read the full abstract...
Background: 

Cardiac and thoracic surgery are associated with an increased risk of venous thromboembolism (VTE). The safety and efficacy of primary thromboprophylaxis in patients undergoing these types of surgery is uncertain.

Objectives: 

To assess the effects of primary thromboprophylaxis on the incidence of symptomatic VTE and major bleeding in patients undergoing cardiac or thoracic surgery.

Search strategy: 

The Cochrane Peripheral Vascular Diseases Group Trials Search Co-ordinator searched the Specialised Register (last searched May 2014) and CENTRAL (2014, Issue 4). The authors searched the reference lists of relevant studies, conference proceedings, and clinical trial registries.

Selection criteria: 

Randomised controlled trials (RCTs) and quasi-RCTs comparing any oral or parenteral anticoagulant or mechanical intervention to no intervention or placebo, or comparing two different anticoagulants.

Data collection and analysis: 

We extracted data on methodological quality, participant characteristics, interventions, and outcomes including symptomatic VTE and major bleeding as the primary effectiveness and safety outcomes, respectively.

Main results: 

We identified 12 RCTs and one quasi-RCT (6923 participants), six for cardiac surgery (3359 participants) and seven for thoracic surgery (3564 participants). No study evaluated fondaparinux, the new oral direct thrombin, direct factor Xa inhibitors, or caval filters. All studies had major study design flaws and most lacked a placebo or no treatment control group. We typically graded the quality of the overall body of evidence for the various outcomes and comparisons as low, due to imprecise estimates of effect and risk of bias. We could not pool data because of the different comparisons and the lack of data. In cardiac surgery, 71 symptomatic VTEs occurred in 3040 participants from four studies. In a study of 2551 participants, representing 85% of the review population in cardiac surgery, the combination of unfractionated heparin with pneumatic compression stockings was associated with a 61% reduction of symptomatic VTE compared to unfractionated heparin alone (1.5% versus 4.0%; risk ratio (RR) 0.39; 95% confidence interval (CI) 0.23 to 0.64). Major bleeding was only reported in one study, which found a higher incidence with vitamin K antagonists compared to platelet inhibitors (11.3% versus 1.6%, RR 7.06; 95% CI 1.64 to 30.40). In thoracic surgery, 15 symptomatic VTEs occurred in 2890 participants from six studies. In the largest study evaluating unfractionated heparin versus an inactive control the rates of symptomatic VTE were 0.7% versus 0%, respectively, giving a RR of 6.71 (95% CI 0.40 to 112.65). There was insufficient evidence to determine if there was a difference in the risk of major bleeding from two studies evaluating fixed-dose versus weight-adjusted low molecular weight heparin (2.7% versus 8.1%, RR 0.33; 95% CI 0.07 to 1.60) and unfractionated heparin versus low molecular weight heparin (6% and 4%, RR 1.50; 95% CI 0.26 to 8.60).

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