1. Epicardial Atrial Ablation with High Intensity Focused Ultrasound on the Beating Heart. Mathew Williams, Mauricio Garrido, Susan Kourpanidis, Jennifer Casher, Alessandro Barbone, Paul DiGiorgi, David D’Alessandro, Michael Argenziano, Yoshifumi Naka Division of Cardiothoracic Surgery College of Physicians and Surgeons of Columbia University Background The Cox-Maze III remains the most successful curative treatment of Atrial Fibrillation (AF). We and others have demonstrated a 70-80% success rate in curing AF during concurrent surgery utilizing various energy sources to isolate only the pulmonary veins in lieu of the standard ‘cut & sew’ Maze incisions. The ability to create lesions on the beating heart will be paramount to the evolution of the surgical treatment of atrial fibrillation. High intensity focused ultrasound (HIFU) is a method for creating transmural epicardial lesions on the beating heart. HIFU has several potential advantages including rapid lesion creation and easy adjustment of ablation depth. Additionally, imaging (to assess lesion creation) is readily facilitated by this mode. Purpose To determine if transmural lesions can be created on the beating heart using HIFU. To determine if HIFU lesions can be successfully applied through epicardial fat. To determine if HIFU lesions can be visualized with ultrasound imaging. Methods 4 dogs with open chests. 59 spot lesions created on both right and left atria. Ablation created using HIFU at 7.3 MHz (nominal) with a focus at 5 mm. Power varied between 10 and 100 Watts (into transducer) and ablation time from 0.5 - 4 seconds. Animals allowed to survive for 2 hours after ablation Tissue stained with vital dye- triphenyl-tetrazolium chloride (TTC). Tissue depth, fat depth and lesion depth were characterized. Imaging Results A separate study was performed to determine if lesions could be visualized after creation. Lesions were created in vitro on the ventricle of a pig. Representative results shown below. Conclusions HIFU can create transmural lesions on the beating heart in a short amount of time. HIFU reliably ablates at a defined depth. The ability to electronically alter this depth is required and should result in reliably transmural lesions. Early attempts at visualizing lesions are promising. B-mode images obtained using an ATL HDI Ultramark 9 Imaging System and L10-5 Linear Probe 42 (71%) of the 59 lesions were transmural. The average lesion depth was 2.5± 1.7 mm. In comparing non-transmural v. transmural specimens, the total wall thickness (myocardium + fat) was thicker in non-transmural specimens (6.4 mm ±3.9 mm v. 2.7 mm ± 2.1 mm, p < 0.05). However, the ablation depths were similar (2.8 mm ± 1.8 mm v. 2.3 mm ±1.3 mm, p=ns), indicating HIFU ablates at a depth based on its focal length irrespective of epicardial fat, endocardial blood or location. For additional questions please contact: Mathew Williams, MD via email- mw365@columbia.edu Examples of the appearance of HIFU lesions created in pig ventricle. HIFU Physics Spectrasonics F/1.5 23 mm diameter spherically focused transducer was used to create lesions. Water path standoff was used to adjust lesion depth within myocardium. The acoustic focus was located just anterior to the endocardial surface to obtain the most reliable transmurality. Shown below is a laser image of a representative acoustic field.