Background: High-intensity focused ultrasound (HIFU) produces immediate focal lesions without direct tissue contact. Previously, we reported the HIFU potential for cardiac ablation. The purpose of this study was to evaluate the possibility of myocardial ablation in the left ventricle of beating dog hearts with monitoring by 2-dimensional echocardiography.
Methods: The operating frequency and the acoustic intensity were 5.25 MHz and 23 kW/cm(2), and the focal length and diameter were 3.3 mm axial and 0.37 mm wide at a distance of 35 mm from the transducer. Three dogs underwent a left-sided thoracotomy. The right ventricular surface was coupled with the transducer. The timing of the HIFU exposure was set during the early systolic phase using an electrocardiographic triggering system. The focal point was set in the left ventricular septum using 2-dimensional echocardiography mounted in the HIFU transducer. Ultrasound energy was delivered for 0.2 seconds. For each dog, we created 18 lesions. Exposures were performed 20, 30, or 40 times. Lesion size was assessed by manually measuring its length and width.
Results: All lesions except one were clearly visible. The histologic lesion area was 18.7 +/- 8.3, 26.3 +/- 8.7, and 35.5 +/- 15.7 mm(2) (20, 30, and 40 times, respectively). The intraclass correlation coefficients were found to be 0.72, 0.63, 0.75, and 0.73 for lesion length, width, area, and depth, respectively.
Conclusion: HIFU can be used to create targeted, well-demarcated thermal lesions in the ventricular septum myocardium during cardiac contraction.