BIO11: Ambulatory Rabbit ECLS Model for Long-term Biocompatibility Testing

Abstract

Background: Thrombosis causes rapid failure of oxygenators within extracorporeal life support (ECLS) circuits. To combat this problem, various groups are developing new surface coatings and anticoagulants, but their testing typically relies on either (a) inexpensive, but overly-brief (< 8 hour) in vitro or small animal in vivo testing or (b) expensive, long-term (days to weeks) testing in large animals. A more inexpensive model is thus needed to assess the long-term biocompatibility of new anticoagulant technologies. In this study, the feasibility of a long-term, ambulatory rabbit ECLS model was assessed. Methods: A miniaturized ECLS circuit was attached to rabbits (2.5-4 kg) under anesthesia using a pumpless arteriovenous (AV) configuration. The circuit contained a miniature artificial lung (surface area of 400 cm2) using the same materials as full-scale artificial lungs. The circuit was tunneled behind the animal’s back and secured just below the scruff of the neck. IV heparin was provided via a battery-powered infusion pump (SAI 3D Mini Infusion Pump) within a small jacket. Twelve non-recovery experiments were used to optimize the model, and two subsequent recovery experiments were run until oxygenator failure (blood flow rate below 5 mL/min for over 2 hours and/or resistance > 5x baseline). Non-recovery experiments were used to optimize the cannulation strategy. This included cannula type and circuit placement on the rabbit. Once optimized, two survival studies were performed. The average circuit blood flow during the first experiment was 50-60 mL/min. The circuit blood flow was then limited to 35-45 mL/min with a Hoffman clamp for the second experiment. Blood flow resistance was measured to assess anticoagulation effectiveness, and physiology was monitored through hepatic and renal panels. Results: Bilateral cannulation was ideal for circuit placement. A flexible 16G single-lumen central line was chosen because of its low blood flow resistance and resistance to kinking. The circuit was tunneled 40 cm, emerging just below the scruff. This enabled safe handling of the animal without impacting its mobility and ability to eat. The recovery experiments lasted 47 and 21 hours prior to oxygenator failure. Animals were completely ambulatory with free access to food and water. Heparin delivery could be controlled simply using the infusion pump, and coagulation could be assessed by drawing blood samples and measuring oxygenator resistance. The first survival rabbit developed hyperlipidemia on Day 1. The cause is unclear at this time but could be attributed to high blood flow through AV circuit. Conclusion: This study demonstrates the feasibility of using a lower-cost rabbit model for long-term anticoagulation testing that requires less staffing. Future studies will use this model to compare the anticoagulation effectiveness of heparin, a selective Factor XIIa inhibitor, and surface coatings for a period of five days.