Intra-operative antithrombotic therapy with low molecular weight heparin in aortic surgery. How should heparin be administered?

Interventions for preventing venous thromboembolism following abdominal aortic surgery

BACKGROUND

Deep vein thrombosis (DVT) is one of the most common, preventable complications of surgery. Although the relationship between surgery and DVT is well established in general surgical operations and most other subspecialties, the same cannot be said about arterial surgery. Deep vein thrombosis is believed to be less common in aortic surgery where its management is rather controversial with a reported incidence of DVT from 2% to 18%.Intra-operative heparin is believed to provide protection during the period when DVT is most likely to develop. However, the practice of using intra-operative heparin could increase the risk of haemorrhagic complications if further heparin is used during the recovery period. This can significantly limit the use of such prophylactic measures especially with the low perceived risk of venous thromboembolism (DVT or pulmonary embolism (PE)) following abdominal aortic surgery. However, vascular patients are usually older, with more co-morbidity and are subject to prolonged immobility, all of which increase the likelihood of developing venous thromboembolism.

OBJECTIVES

To determine the efficacy of anticoagulant prophylaxis (with or without mechanical devices) in patients undergoing surgery for abdominal aortic aneurysm.

SEARCH STRATEGY

The Cochrane Peripheral Vascular Diseases Group searched their trials register (last searched 8 August 2007) and the Cochrane Central Register of Controlled Trials (CENTRAL) in The Cochrane Library, (last searched Issue 3, 2007). The authors searched for additional trials through reference lists of retrieved studies and conference proceedings.

SELECTION CRITERIA

Randomised controlled trials comparing the use of anticoagulants (with or without mechanical devices) with control or no intervention in preventing DVT or PE after abdominal aortic operations.

DATA COLLECTION AND ANALYSIS

Three authors independently selected potential trials and assessed trial quality.

MAIN RESULTS

Two studies (n=147) were included. Both studies had methodological limitations. There were no data to indicate that post operative anticoagulation, with or without the use of mechanical devices, can safely reduce the incidence of DVT after aortic surgery. Neither study reported a significant effect of anticoagulants on the incidence of PE or related mortality. One study was terminated before recruiting sufficient participants due to a higher incidence of bleeding with the use of anticoagulants. The incidence of minor bleeding events was also slightly higher with anticoagulants.

AUTHORS' CONCLUSIONS

There is not enough evidence to make a definitive conclusion about the use of anticoagulant drugs (with or without mechanical devices) for DVT prophylaxis in patients undergoing abdominal aortic surgery.

[Low molecular weight heparin in patients undergoing vascular surgery]

OBJECTIVES

Despite the fact that there are no approved indications by the French regulatory agency and despite the absence of recommendations, low molecular weight heparins (LMWH) are frequently used in vascular surgery, an area at both high risk of perioperative arterial and venous thrombosis as well as of bleeding. The aim of this study was to investigate medical practice in vascular surgery, using a survey of prescribers of antithrombotic agents.

STUDY DESIGN

Survey of physicians from different specialities involved in patient care before, during, and after vascular surgery.

PATIENTS AND METHODS

Between March and June 2003, 301 physicians filled a questionnaire providing information about their antithrombotic regimens depending on the type of vascular surgery. These physicians are involved in an important part of vascular surgery activity in France.

RESULTS

The survey confirmed the use of LMWH in near 80% of patients scheduled for vascular surgery. The prescribed LMWH is frequently associated with antiplatelets agents. During patient's hospitalization, LMWH prescription is guided by numerous factors, making the prescription protocol rather complex. Major risk factors favoring continued prophylaxis after patient's hospital discharge include surgery with a high risk of thromboembolic events and limited ambulation. Analysis of the data also showed that the surgeon and the referring physician are both implicated in the majority of the postoperative care following patient's discharge, and therefore, in prescribing the antithrombotic drug.

CONCLUSION

This survey gives information on practical use of LMWH in the vascular surgery setting. This information could lead to prospective studies of LMWH in situations other than venous thromboembolic disease.

Low molecular mass heparin instead of unfractionated heparin during infrainguinal bypass surgery

OBJECTIVES

To test whether low molecular mass heparin (LMMH) is comparable to unfractionated heparin (UFH) as an anticoagulant during infrainguinal bypass surgery and to investigate laboratory evidence of hypercoagulation in patients undergoing infrainguinal bypass surgery.

MATERIALS AND METHODS

Eighteen patients were randomised to receive either UFH or LMMH (70 anti-Xa units/kg b.w.). Soluble fibrin, measured as fibrin monomers (FM) and fibrinopeptide A (FPA), were measured in blood from the femoral vein before, during and after release of the occluding clamps during surgery. In addition, fibrinogen prothrombin complex, thrombin-antithrombin complex, platelets and antithrombin were measured before surgery. Heparin levels (Anti Xa) were measured during surgery.

RESULTS

Increased levels of fibrinogen, FPA, thrombin antithrombin complex and FM were recorded prior to surgery. During surgery no further increase was noted. The anti Xa levels were slightly higher in patients with LMMH than in patients receiving UFH. Levels of FM were significantly lower in patients receiving LMMH. No difference in FPA was noted. A positive correlation between fibrinogen and FPA and FM respectively was recorded. Four patients, two in each group, were reoperated for graft occlusion. One patient in the UFH group required reoperation because of bleeding.

CONCLUSIONS

LMMH is comparable to UFH as an anticoagulant during infrainguinal bypass surgery. Variables reflecting hypercoagulability are elevated in this group of patients and are positively correlated to the fibrinogen level. High fibrinogen levels could thus be a risk for perioperative thrombosis.

Effective and less toxic reversal of low-molecular weight heparin anticoagulation by a designer variant of protamine

PURPOSE

This investigation assessed protamine reversal of heparin anticoagulation by formation of a protamine-heparin alpha-helix by use of a new designer-variant protamine [+18BE] that was made from an existing protamine variant [+18B] whose non-alpha-helix-forming amino acid proline (P) was replaced by an alpha-helix-forming glutamic acid (E). The rate of administration of the new [+18BE] variant protamine on efficacy and toxicity in comparison to that of [+21] standard protamine and [+18B] was also studied.

METHODS

Acetyl-EAA(K2A2K2A)4K2-Amide [+18BE] was administered intravenously in a 1:1 dose to low-molecular-weight heparin (LMWH)-anticoagulated (intravenous 150 IU antifactor Xa/kg) dogs over 10 seconds or 3 minutes (n = 7, each group). Reversal efficacy was documented by measuring activated clotting time, thrombin clotting time, antifactor Xa, and antifactor IIa. Toxicity was defined by measuring systemic blood pressure, heart rate, cardiac output, pulmonary artery pressure, and oxygen consumption. Measurements were made at baseline, after administration of LMWH, before its reversal, and for 30 minutes thereafter. Results were compared with those after LMWH reversal with [+21] standard protamine and the [+18B] variant. A total toxicity score (TTS) was calculated for each compound from maximal declines in blood pressure, heart rate, cardiac output, and oxygen consumption.

RESULTS

LMWH anticoagulation reversal was significantly (p < 0.01) less toxic over 10 seconds and 3 minutes with the [+18BE] designer variant (TTS -2.3, -2.2) compared with the [+21] standard protamine (TTS -6.4, -7.2). Percent LMWH reversal at 3 minutes revealed [+18BE] to have antifactor Xa activity as high as 91%, compared with 68% for protamine [+21], when given over 3 minutes (p < 0.05).

CONCLUSIONS

This investigation documents that a new designer variant of protamine [+18BE] has superior efficacy compared with [+21] standard protamine for reversal of LMWH anticoagulation and that this occurs with a highly favorable toxicity profile.

Effect of dextran and enoxaparin on early ePTFE graft thrombogenicity in sheep

OBJECTIVES

To evaluate the effect of low molecular weight heparin (LMWH), dextran 70 and their combination on platelet adhesion and fibrinogen uptake in ePTFE grafts in an experimental sheep model.

DESIGN

Prospective open study.

SETTING

Animal Laboratory of a University Hospital.

MATERIALS

Early thrombogenicity of ePTFE grafts was studied after interposition in the two common carotid arteries of 40 adult sheep. The animals received one of four different treatment regimens in a double blind randomised way: enoxaparin and polygeline, saline and dextran 70, enoxaparin and dextran 70 or saline and polygeline (control). The substances were administered i.v. with a total dose of 73 antifactor-Xa U/kg for enoxaparin and 1.0 g/kg for dextran 70. Polygeline and saline were used as placebo substances in equivalent volumes. On one side (random allocation) the carotid blood flow was restricted to 25 ml/min, on the other side it was left unrestricted.

CHIEF OUTCOME MEASURES

The following variables were studied: 1) fibrinogen uptake; 2) platelet uptake; 3) early graft patency; 4) blood flow in patient grafts; 5) visible presence of graft thrombus; 6) thrombus weight.

MAIN RESULTS

The results verified the importance of adequate blood flow as only 30% of grafts with restricted blood flow in the control group were patent compared with 80% of those with unrestricted blood flow (p = 0.038). Dextran 70, enoxaparin and the combination of the two increased early graft patency (p < 0.05) and reduced thrombus weights (p < 0.05) in grafts with restricted blood flow. The relative number of grafts with thrombus free surface was increased in the unrestricted blood flow situation.

CONCLUSIONS

Dextran 70 and enoxaparin appeared to be equally effective in decreasing fibrinogen and platelet uptake in the grafts. Their combination was not significantly more effective although there was a favourable trend.

Low-molecular-weight heparin vs. unfractionated heparin in femorodistal reconstructive surgery: a multicenter open randomized study. Enoxart Study Group

Several clinical trials have been conducted to study the role of low-molecular-weight heparin (LMWH) in the prevention and treatment of venous thrombosis. In contrast, there have been few studies investigating LMWH in the prophylaxis in arterial thrombosis. After informed consent and institutional approval were obtained, 201 consecutive patients scheduled for femorodistal reconstructive surgery under general anesthesia were enrolled in an open randomized multicenter (n = 14) study (from November 1990 to November 1992). Immediately before arterial cross-clamping, patients were given an intravenous bolus of either enoxaparin (ENX), 75 anti-Xa IU/kg (n = 100), or unfractionated heparin (UFH), 50 IU kg (n = 101). Meanwhile the saphenous vein or a prosthetic graft was flushed with ENX (25,000 anti-Xa IU) or UFH (25,000 IU) in 250 ml of saline solution. Subsequent treatment consisted of subcutaneous administration of ENX, 75 anti-Xa IU/kg, or UFH, 150 IU kg, beginning 8 hours after the intravenous injection and then every 12 hours thereafter for 10 days. The primary end point was graft patency on day 10 +/- 2 after surgery as assessed clinically and/or by arteriography on day 10 +/- 2 and/or during reintervention or autopsy. Analysis of patients on an intention-to-treat basis (patients who received at least on injection of ENX or UFH and who had at least one end-point evaluation) showed that graft thrombosis occurred in 30 of 199 cases: eight (8%) in the ENX group and 22 (22%) in the UFH group (p = 0.009). Among the 131 patients who were evaluated by arteriography before day 12, twelve (9.1%) had graft thrombosis: four (6%) in the ENX group and eight (12.5%) in the UFH group (NS). There were no significant differences between the two groups in terms of safety--that is, there were 12 major hemorrhages in each group, and during the follow-up period five patients in the ENX group died compared to nine in the UFH group (NS). These results indicate that ENX is as safe as but more effective than UFH when used for the prevention of early graft thrombosis in patients undergoing femorodistal reconstructive surgery.

Advantageous effect of low-molecular-weight heparin administration on hepatic mitochondrial redox state

The effect of low-molecular-weight heparin (LMWH) on hepatic mithocondrial metabolism was compared with that of unfractionated heparin (UH) after the intravenous administration of these two kinds of heparin to normal rabbits. The magnitude of decrease in blood triglyceride levels 5 min after administration of UH (200 U/kg) was significantly greater than after LMWH (200 U/kg). Free fatty acid levels in the blood were significantly higher after this dose of UH than after LMWH. Blood total ketone body levels (acetoacetate + 3-hydroxybutyrate) 15 min after injection of 50 U/kg of UH were significantly higher than those after a dose of 50 U/kg of LMWH, and levels after 200 U/kg of UH were significantly higher than those after 200 U/kg of LMWH at 15, 30, 45 and 60 min. Enhanced ketogenesis was not noted after LMWH at any of the doses, or after UH at 3 U/kg. Arterial ketone body ratio (AKBR; acetoacetate/3-hydroxybutyrate), which reflects the hepatic mitochondrial oxidation-reduction state (NAD+/NADH), was maintained above 1.0 in all groups except in the U-200 group, while AKBR in that group was significantly decreased to 0.99 +/- 0.14 at 30 min, and further decreased to 0.80 +/- 0.08 at 60 min. These results indicate that LMWH has less effect on lipolysis than UH and does not enhance ketogenesis, resulting in less deterioration of mitochondrial redox state.