Extracardiac application of high intensity focused ultrasound for ablation of ventricular myocardium

Introduction: High intensity (2-3 kW/cm²) focused ultrasound (HIFU) has been applied to internal organs from outside the body to ablate tissue. No prior studies have assessed the feasibility of ablating cardiac tissue with HIFU. The purpose of this study was to determine, using an in vitro model, if HIFU can be accurately focused and controlled to create lesions of various sizes within the heart. Methods and Results: Six canine hearts were suspended in a degassed water bath (28°C) 28 mm from a single element (1.4 MHz) spherically focused ultrasound transducer. Three lesions (2 endocardial, 1 mid-wall) were made in the left ventricular free wall. Each lesion was made with 18 applications of HIFU (two 3 s pulses per grid location, 2.8 kW/cm²), in a 3 x 3 grid (9 grid locations), with 2 mm between grid locations. Lesions were well-demarcated. Endocardial lesions were accurately focused at the endocardial surface with normal overlying tissue. Mid-wall lesions were enveloped by normal epicardial and endocardial tissue. Mean lesion volume was 1.8 ± 1.1 cm³ (range 0.2 to 3.4 cm³), with a mean of 2.1 ± 1.1 cm³ for endocardial lesions and 1.36 ± 0.86 cm³ for mid-wall lesions. Larger and smaller lesions were also created and accurately positioned within the left ventricular wall. Conclusions: Extracardiac HIFU appears to be a feasible energy source to create large and small lesions accurately positioned within the heart. Lesions of different sizes and shapes can be created by altering the number of HIFU applications in the grid, or the shape of the grid.