Reproducibility of left atrial ablation with high-intensity focused ultrasound energy in a calf model

Electrophysiological changes correlated with temperature increases induced by high-intensity focused ultrasound ablation

To gain better understanding of the detailed mechanisms of high-intensity focused ultrasound (HIFU) ablation for cardiac arrhythmias, we investigated how the cellular electrophysiological (EP) changes were correlated with temperature increases and thermal dose (cumulative equivalent minutes [CEM43]) during HIFU application using Langendorff-perfused rabbit hearts. Employing voltage-sensitive dye di-4-ANEPPS, we measured the EP and temperature during HIFU using simultaneous optical mapping and infrared imaging. Both action potential amplitude (APA) and action potential duration at 50% repolarization (APD50) decreased with temperature increases, and APD50 was more thermally sensitive than APA. EP and tissue changes were irreversible when HIFU-induced temperature increased above 52.3 ± 1.4°C and log10(CEM43) above 2.16 ± 0.51 (n = 5), but were reversible when temperature was below 50.1 ± 0.8°C and log10(CEM43) below -0.9 ± 0.3 (n = 9). EP and temperature/thermal dose changes were spatially correlated with HIFU-induced tissue necrosis surrounded by a transition zone.

Evaluation of an integrated bipolar and unipolar ablation device

OBJECTIVE

Bipolar lesion creation is accepted as the most effective method to create transmural atrial ablations. However, the creation of an ablation pattern on the left and the right atrium with current bipolar devices is difficult on the beating heart. We have evaluated a novel integrated ablation device that creates both bipolar and unipolar lesions in a linear pattern on the beating porcine heart.

METHODS

Using six porcine models, we evaluated the effectiveness of creating beating heart ablation lesions by transmurality and conduction block. Isolating lesions were created on the left and right atria of the beating porcine heart. After we created sequential bipolar and unipolar lesions, we confirmed conduction block and examined the lesion depth histologically.

RESULTS

Linear lesions were created successfully on the surface of the beating porcine heart. Conduction block at 20 mV was confirmed at all isolated areas, and 96.4% of the lesion sections had full thickness and were transmural at histology.

CONCLUSIONS

This device enables unipolar and bipolar lesion creation in a linear and low-profile manner, enabling effective creation of a biatrial lesion pattern on the epicardial surface of the beating heart.

Human comparative experimental study of surgical treatment of atrial fibrillation by epicardial techniques

Background

To set up an experimental model of cadaveric heart in order to evaluate and compare histologic transmurality of lesions immediately caused by different energy sources of anti-arrhythmic epicardial devices.

Methods

Procedures were performed on a cadaveric human heart in orthotopic position with an ischemic time of 48 h at 37° and supported through the use of cardiopulmonary bypass. Three anti-arrhythmic epicardial devices were studied: the bipolar forceps Cardioblate BP (Medtronic) for the radiofrequency, the Epicor Ultracinch LP Ablation device (St. Jude) for ultrasound and the Cardioblate CryoFlex (Medtronic) device for cryoablation. Histological features of lesions made at the pulmonary venous confluence assessed the effectiveness of different energy sources.

Results

Over 45 experimentations performed, only 28 were considered correct and retained for histological analysis. Three distinct groups were studied according to the type of procedure performed: group 1 (Radiofrequency, n = 12), group 2 (ultrasound, n = 4), group 3 (cryoablation, n = 10) and controls (n = 2). All analysed samples showed histological changes with a success rates of transmurality of 33% for radiofrequency, 25% for ultrasound and 90% for cryotherapy (p <0.001). The average length of transmurality, when it was reached and the proportion of transmurality over the total length of the lesion were respectively 12 ± 6 mm and 37 ± 18% for group 1, 10 mm and 33% for group 2 and 11.1 ± 1.1 mm and 35 ± 5% for group 3.

Conclusion

Immediate detectable histological transmural lesions after epicardial procedure are discontinuous whatever the kind of energy source tested in this work and it strongly encourages the repetition of radiofrequency procedures. Nevertheless, our experimental model seems inadequate to assess ultrasound energy efficacy.

Heart ablation using a planar rectangular high intensity ultrasound transducer and MRI guidance

The aim of this study was to evaluate a flat rectangular (3×10mm(2)) MRI compatible transducer operating at 5MHz. The main task was to explore the feasibility of creating deep lesions in heart at a depth of at least 15mm. The size of thermal necrosis in heart tissue was estimated as a function of power and time using a simulation model. The system was then tested in an excised lamb heart. In this study, we were able to create lesions of 15mm deep with acoustic power of 6W for an exposure of approximately 1min. The contrast to noise ratio (CNR) between lesion and heart tissue was evaluated using fast spin echo (FSE). The CNR value was approximately 22 using T1W FSE. Maximum CNR was achieved with repetition time (TR) between 300 and 800ms. Using T2W FSE, the corresponding CNR was approximately 13 for the 14 in vivo experiments. The average lesion depth was 11.93mm with a standard deviation of 0.62mm. In vivo irradiation conditions were 6W for 60s. The size of the lesion in the other two dimensions was close to 3×10mm(2) (size of the transducer element).

Chronic histological transmurality of high-intensity focused ultrasound ablation

We evaluated histologic results for surgical left maze with an high-intensity focused ultrasound (HIFU) energy source. Two patients came to our attention 6 and 48 months, respectively, after ablation concomitant to a valve procedure. Tissue specimens, obtained from the lesion site on the mitral isthmus and from the "box lesion" around the pulmonary veins were analyzed histologically. A complete transmural lesion was found in all specimens. Chronic lesions exhibited replacement of the muscular band with connective tissue. The atrial wall maintained normal thickness and vascularization. HIFU ablation represents an acceptable energy source to create transmural lesions on the beating human left atrium.