Mechanism of Tissue Heating During High Intensity Focused Ultrasound Pulmonary Vein Isolation: Implications for Atrial Fibrillation Ablation Efficacy and Phrenic Nerve Protection

Introduction: The purpose of this study was to examine tissue temperatures around pulmonary veins (PVs) during high intensity focused ultrasound (HIFU) balloon ablation for atrial fibrillation. The thermodynamics and extent of PV and phrenic nerve (PN) heating during HIFU ablation have not been established. Methods and Results: Tissue temperatures were recorded from epicardial thermocouples at right superior (RS) PV orifice and PN in eight dogs undergoing 51 RSPV HIFU ablations (40 seconds, 40 W). Maximum tissue temperatures, reflected by 288 recording profiles, were negatively correlated with distance from balloon surface (r =–0.79, P < 0.001). Average maximum RSPV temperature was 56.8 ± 16.8 °C (distance: 6.6 ± 4.1 mm), resulting in full-thickness, circumferential PV isolation in 7 of 8 animals. Direct local mechanical heating restricted to the focused ultrasound zone showed temperature rise to ≥50°C within 10 seconds to a maximum temperature of 82.6 ± 8.9°C (n = 31). Apparent conductive heating showed gradual heating patterns (<50°C within 10 seconds) at greater distance. PN temperature at all recording sites was 45.0 ± 11.3°C (distance: 9.2 ± 5.0 mm). Intentional PN injury (10 of 51 [19.6%] ablations), occurred at 63.7 ± 16.0°C with closest distance of 4.4 ± 2.5 mm, which was closer than in PNs without injury (7.3 ± 4.3 mm, P = 0.005). Conclusions: HIFU ablation is highly effective in generating temperatures needed for full-thickness, circumferential ablation through rapid direct mechanical heating. Gradual heating of surrounding tissue supports the presence of conductive heating. This study also discloses direct HIFU effects as the mechanism of PN injury occurring within 4–7 mm from balloon surface.