# Cardiac Radiofrequency Ablation Simulation Using a 3D-Printed Bi-Atrial Thermochromic Model
## 4 Discussion  
The aim of this study was to produce a novel 3D-printed, irreversible thermochromic bi-atrial model, meeting as many of the requirements for a high-fidelity physical simulator for cardiac RFA as possible. We combined Layfomm-40, a new tissue-mimicking 3D-printable polymer, with an irreversible thermochromic pigment and barium sulphate using acrylic paint as a carrier. We measured the electrical and thermal conductivities of Layfomm-40 soaked in saline solution. The thermal properties were found to be similar to those of the myocardium. Although the electrical conductivity was substantially lower it was sufficient to allow use of the material with RFA and EAM systems. We developed an irreversible thermochromic paint and demonstrated that Layfomm-40 coated with this paint was sensitive to RFA and produced visible lesions whose size was influenced by the power and temperature settings of the ablation system. Lesions with diameters similar to those produced in isolated muscle and in vivo could be generated. We overcame issues with the X-ray visibility of Layfomm-40 by formulating a barium-sulphate-doped acrylic paint that produced a similar tissue-to-background image contrast to that seen in clinical cardiac X-ray fluoroscopy images. The thermochromic paint and the barium-doped paint were combined into a four-layer coating for applying to Layfomm-40, both internally and externally. A bi-atrial model was constructed using these strategies, starting with a patient CT scan. A custom base and enclosure were designed and constructed that allowed insertion of catheters, stabilized the model during catheter manipulation and allowed compatibility with standard RF generators and the CARTO3 EAMS. Experimental validation of the simulator by a cardiologist demonstrated the successful ability to perform X-ray fluoroscopy and cone beam CT imaging, to insert and exchange interventional devices without leaks, to manipulate devices within the bi-atrial model without causing damage, to perform mapping using CARTO3, and to perform ablations. Inspection of the cut away model after the experiment clearly showed the ablation lesions. The striking novel feature of our simulator is the ability to visualize delivered radiofrequency ablation lesions. To the best of our knowledge a common limitation of all prior physical simulators is the inability to visualize the delivered therapy. The state-of-the-art simulators developed by [PERSON] et al. [3], Heartroid, and Pangolin are limited by not being RFA-sensitive. Since the goal of electroanatomic mapping systems is to guide the delivery of radiofrequency ablation therapy we argue that visualization of this therapy is an essential requirement to support effective simulator-based training of electrophysiologists. In this work we have developed a simple but effective methodology supporting this visualization which can be easily applied to any 3D-printed cardiac chamber model which potentially could be of value during training or device evaluation scenarios. As mentioned previously, there are many factors that affect RFA lesion formation [4]. For lesions 1-5 shown in Figure 10B, we see that increasing power increased the lesion diameter while keeping all other parameters constant, which was as expected. One parameter which is difficult to keep constant is the contact force and this ranged from an average (over the time of each ablation) of 11.1 g to 21.7 g during these five ablations. [PERSON] and [PERSON] conducted experiments to investigate the effect of varying contact force on RFA lesion formation in an in vitro set up [16]. They measured the surface lesion diameter, the intramural width, and the intramural depth. Although they used different power and temperature settings (30 W, 30 \({}^{\circ}\)C) for their irrigated ablation catheter compared to our experiments, the average surface lesion diameters that they measured were 4.1 mm (for 2 g force) to 6.9 mm (40 g), with our measurement being 3.4 mm (19.7 g average, 14.4-23.4 g range for lesions 6-10 (Figure 10B)).