Background: Ovine models are widely used in cardiovascular research because of their comparability to the human cardiovascular system. Sheep are well available, easy to handle, mostly indulgent and anesthesia is similar in comparison with humans. Nevertheless, differences are known concerning the composition of blood constituents and rheological properties as well as oral administration and resorption of some drugs. Amongst those, platelet inhibitors, vitamin K antagonists and low molecular weight heparin (LMWH) exhibit different pharmacodynamics. During implantation of heart valves it is essential to reduce the coagulation ability with the aim of preventing thromboembolic events. Concurrently, it is essential to avoid any bleeding complications by accumulation of anticoagulants. In most sheep experiments for cardiovascular research the anticoagulant regime follows the guidelines for humans in the lack of knowledge about specific ovine requirements. To date, no standardized anticoagulation strategy for the ovine model using LMWH is established. This study aimed to establish a reliable regime for LMWH administration in sheep at high risk of thromboembolic events. Target was an anti-factor-Xa (AFXa) level of 0.6 to 0.8 units known from human studies to be preventive and therapeutic at moderate to high risk of thromboembolism. The primary aim of the study was to improve the antithrombotic management after pulmonary valve implantation in three sheep. These results were then applied and re-evaluated during minimally-invasive heart valve implantation within four sheep.
Materials and Methods: Seven adult grey horned heath ewes (mean body weight 43.4 kg) were included in this study prior to heart valve implantation. The study design regarded interventional heart valve implantation as an intermediaterisk situation for thromboembolic events. During a period of two weeks a weekly incremental dose of dalteparin was administered once a day at 7 AM subcutaneously. Venous blood samples were collected three times per day to monitor the response over 24 hours. The first measurement of anti-factor-Xa was conducted immediately before administration of heparin (baseline). The second measurement was conducted four hours and the last measurement 12 hours after administration. The starting dose in week 1 was 250 IU (international units). The following dose in week 2 comprised 350 IU. The dose with an anti-factor-Xa level closest to 0.6 to 0.8 units was tested in four sheep in which a pulmonary valve was implanted transvenously.
Results: Data showed a peak concentration of anti-factor-Xa four hours after administration. 12 hours after injection the level had decreased to 50% of the peak level and 24 hours after administration there was no measurable level of anti-factor-Xa left. Sheep needed 350 units dalteparin once daily to achieve an anti-factor-Xa level of 0.6 to 0.8 units. No thromboembolic event occurred in those four sheep which received the weight- adapted dose of dalteparin (350 IU/ day) according to the results of the dose-finding study after the implantation of a transcatheter pulmonary valve. The macroscopic and histologic results showed no signs of any thrombotic affection of the graft.
Conclusions: Sheep require much greater quantities of dalteparin (LMWH) than humans to achieve comparable blood levels of anti-factor-Xa.
Kerstin Brakmann, Hendrik Spriestersbach, Felix Berger and Boris Schmitt