Modified Folts models in the rabbit: variations on a well worn theme

Editor, We read with interest the paper by Dellamonica et al.[1] regarding an arterial cyclic thrombosis model in rabbit. As we work on the same model, we would like to share some comments regarding the procedure. In 1976, John Folts developed a canine model of coronary stenosis with intimal injury in order to reproduce experimentally the clinical situation of unstable angina [2]. In this dynamic model, a vascular wall injury associated with a stenosis of the artery (both induced experimentally) produces cyclic flow reductions (CFRs) characterized by a gradual decrease in blood flow followed by a sudden restoration of flow, either spontaneous or facilitated. The endothelial damage induces the formation of a platelet plug, which embolizes and forms again in a cyclic fashion. This model was extended by Folts [3] and others [4,5] to different species and vascular territories. We, as others, have adapted the Folts model to the rabbit's carotid in our respective laboratories. We all monitored carotid thrombosis formation by measuring electromagnetic flow while controlling haemodynamic and ventilation variables. Although all three groups worked on the potential prothrombotic effect of recombinant activated factor VII, our observations were not similar [6,7]. Fortunately, we were able to discuss the differences in our results, particularly those concerning CFRs and their significance. This letter, as a link to the work by Dellamonica et al.[1], gives us an opportunity to share our conclusions. First, there is concern about the ventilation parameters, as pO2 affects the occurrence of CFR. The frequency and the tidal volume are established for rabbits as follows: frequency = 40–45 cycles min−1 and tidal volume = 5 ml kg−1. Therefore, the only variable parameter is FiO2. Dellamonica et al.[1] show that an abrupt decrease of pO2 decreases the number of CFR. This result is very interesting and similar to the finding by Godier et al.[6] (same low number of CFR in 21% FiO2) and Charbonneau et al.[7] (CFR trend to vanish in room air ventilation). Therefore, FiO2 should be adapted depending on the prothrombotic or antithrombotic trend of the experiment. Second, anaesthetic procedures are achieved by three different products: isoflurane gas and intramuscular curare injection, intravenous mixture of ketamine and xylazine or intravenous penthotal infusion. These therapeutics lead to variations in blood pressure, heart rate and platelet aggregation. Isoflurane provides the best haemodynamic stability and no platelet interaction, but its use is much more expensive. Furthermore, it does not dispense with the use of analgesia, which will interact with platelets and the haemodynamics. The effective dose of pentobarbital is close to the lethal dose in rabbit and makes its use tricky and is not recommended by veterinary surgeons. Therefore, the combination of ketamine and xylazine could be the best solution, even if ketamine interacts with platelets, regarding its efficacy on anaesthesia–analgesia without vasoplegia. Third, although we all define CFRs as a progressive reduction in blood flow followed by a sudden return to the initial level (in the absence of haemodynamic variations), the characterization of CFRs varies. The Nancy group shakes the clamp to induce CFRs, the Paris group considers only spontaneous emboli CFRs and does not mechanically dislodge the thrombus by shaking, and the Montreal group accepts both induced and spontaneous CFRs (and identifies them as such). Thus, stenosis and carotid injury should be standardized. The stenosis can be achieved by a silicone occluder or a metal vascular clamp to obtain a consistent 10% decrease in mean carotid flow. Arterial deendothelialization is produced by external compression of the artery. However, even if nontraumatic forceps are always used, forceps differ from one laboratory to another and can lead to unavoidable differences in the degree of arterial trauma. We investigated whether the use of a specific forceps induces more or less severe lesions. In the same rabbit, the injury was made with the Nancy group's forceps on one carotid artery and with the Paris group's forceps on the other. We then compared histological injuries. The Nancy group's forceps resulted in a higher degree of trauma; although the length of injury is similar, depth is obviously different. We postulate that more traumatic forceps will result in more thrombotic lesions. Logically, in the Nancy group's technique, as the injury is more traumatic, the model is more thrombotic, and the clamp must be shaken to obtain CFRs rather than irreversible thrombosis. In the Paris group's technique, the lesion is less pronounced, and spontaneous emboli may occur. In conclusion, it is important to consider variations in experimental design. We suggest that the degree of trauma should be chosen with respect to the aim of the study and the drug under study; the more traumatic forceps and elevated FiO2 should be used to study antithrombotic drugs, whereas the less traumatic forceps and room air ventilation should be used to test prothrombotic drugs.

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