Presentazioni e Articoli Tecnici

Minimally invasive surgery (MIS) for cardiac procedures is a valid alternative to open-heart surgeries, allowing for a faster post-operative recovery [1]. Nowadays, during MIS interventions, fluoroscopic (i.e., x-rays-based technique) and ultrasound (US) images, in particular using a Trans-Esophageal Echocardiographic (TEE) probe, are relied on simultaneously [2]. Both in vitro and in vivo studies observed that cell death is triggered by a specific range of thermal doses, which is why TEE commercial devices automatically switch off at 42.5°C [3], for avoiding thermal damage to the internal walls of the human esophagus. Based on numerical simulations and experimental results, a significantly stable miniaturized cooling device for TEE probes is proposed, thus enhancing the reliability of US images in the intraoperative scenario, as they are completely harmless to both patients and doctors. In the presented context, heat exchange phenomena appear as a key stakeholder – being the esophagus an endoluminal setting - and are studied by means of thermo-fluid dynamic simulations conducted with COMSOL Multiphysics: their aim is to validate the mechanical design and serve as a basis for the temperature control part.

The presented cooling system has the shape of a silicone cap – Figure 1- characterized by an inner tube in which water - the cooling fluid - flows at a constant temperature of 25°C. The starting point is a 3D geometry, imported in COMSOL from SolidWorks. Physiological conditions are also considered by inserting the probe into a hollow cylinder – Figure 2 - simulating the esophagus, to which the properties of the musculature are attributed. The simulation involves a dual heat transfer module – Heat Transfer in Solids and Fluids: convection between the cooling fluid and the silicone cap (ht), and conduction between the cap and the probe (ht2). The simulation is based on the combination of these two physical phenomena – Multiphysics - and represents a case of Time-Dependent study. Analyzing ht, although the pipe diameter is 2 mm only, turbulent flow is verified through Reynolds Number, and this is attributable to the complexity of the geometry. The applied turbulence model is the k-ε model, including (i) Turbulent intensity: Medium (0.05); (ii) Turbulence length scale: geometry-based. A weakly compressible flow is considered and the wall condition was set as no slip, so to consider boundary conditions. Furthermore, the open boundary condition describes boundaries in contact with a large volume of fluid. The flow parameters are set according to the hydraulic circuit, being the flow governed by a pumping device. As far as ht2 is concerned instead, a laminar flow is evaluated. Figure 3 shows how the probe starting from 38°C reaches a safe temperature (≈37°C, physiological body temperature) in 0.5 minutes, which is an absolutely acceptable time frame, especially when compared with the duration of the surgical procedure (approximately 2h) [2]. Figure 4 reports the silicone cap temperature map.

To conclude, these results enabled a proper evaluation of the cooling system functioning, which was then confirmed by experimental results, paving the way to the integration of the device into clinical practice.