Therapeutic ultrasound to noninvasively create intracardiac communications in an intact animal model

Abstract

Objective: To determine if pulsed cavitational ultrasound therapy (histotripsy) can accurately and safely generate ventricular septal defects (VSDs) through the intact chest of a neonatal animal, with the eventual goal of developing a noninvasive technique of creating intra-cardiac communications in patients with congenital heart disease.

Background: Histotripsy is an innovative ultrasonic technique that generates demarcated, mechanical tissue fractionation utilizing high intensity ultrasound pulses. Previous work has shown that histotripsy can create atrial septal defects in a beating heart in an open-chest canine model.

Methods: Nine neonatal pigs were treated with transcutaneous histotripsy targeting the ventricular septum. Ultrasound pulses of 5-μsec duration at a peak negative pressure of 13 MPa and a pulse repetition frequency of 1 kHz were generated by a 1 MHz focused transducer. The procedure was guided by real-time ultrasound imaging.

Results: VSDs were created in all pigs with diameters ranging from 2 to 6.5 mm. Six pigs were euthanized within 2 hrs of treatment, while three were recovered and maintained for 2-3 days to evaluate lesion maturation and clinical side effects. There were only transient clinical effects and pathology revealed mild collateral damage around the VSD with no significant damage to other cardiac or extra-cardiac structures.

Conclusions: Histotripsy can accurately and safely generate VSDs through the intact chest in a neonatal animal model. These results suggest that with further advances, histotripsy can be a useful, noninvasive technique to create intracardiac communications, which currently require invasive catheter-based or surgical procedures, to clinically stabilize newborn infants with complex congenital heart disease.

Figure 1

Histotripsy setup. a) The neonatal pig is placed in a water bath. The therapy and imaging transducers are placed into a cylinder filled with degassed water and coupled to the pig's chest by a plastic membrane filled with water. b) Close-up view of 1MHz therapy transducer with an imaging transducer protruding through center opening.

Figure 2

Generation of a cavitation bubble cloud. a) Prior to treatment, a cavitation bubble cloud is formed in degassed water to mark the focal point, seen by inserted arrow. b) The marked point is then positioned over the interventricular septum (IVS). c) Histotripsy is started and a hyperechoic bubble cloud is formed on the right ventricular (RV) side of the ventricular septum. LV (left ventricle)

Figure 3

Creation of VSDs by histotripsy. a) Image of the ventricular septum in long axis prior to treatment. b) After treatment a VSD is seen on 2 dimensional ultrasound imaging, and (c) shunting is seen across the ventricular septum with color flow Doppler. Another example of a created VSD is seen in short axis with images prior to treatment (a), post treatment (b) and with color flow Doppler (c).

Figure 4

VSD two days after histotripsy therapy. a–b) Parasternal long axis view of VSD two days after creation in 2D and with color flow Doppler. c) 3-dimensional representation of same VSD shown in a modified apical four chamber view showing persistence of the septal defect. Note the hyperechoic (bright) rim surrounding the defect marked by white arrows.

Figure 5

Cardiac pathology after histotripsy treatment. Gross pathology of the heart revealed no damage to the epicardial surface on the anterior (a), basilar (b), or posterior regions (c). Dissection revealed demarcated damage across with ventricular septum with flanking zones of hemorrhage in immediately euthanized animals (d). e) Similar lesions were observed in animals with delayed euthanasia but with smoother borders and less apparent flanking hemorrhage and/or damage.

Figure 6

Lung pathology. a) Gross evaluation of the lungs revealed mostly healthy appearing tissue with only focal areas of hemorrhage or congestion (arrow). On histological analysis (b) no microscopic evidence of thrombo-embolic events were visualized.

Figure 7

Histology of ventricular septum. a) Histological analysis revealed a central area of acellular debris and necrosis and flanking areas of contraction necrosis with hemorrhage in immediately sacrificed animals. (b) High power imaging of this border reveals tight demarcation of necrosis and myocyte injury (solid arrow) with normal appearing myocytes in the top left of image (dashed arrow). c) In animals with delayed euthanasia, the VSD is more clearly demarcated without acellular debris and d) there is evidence of cellular remodeling with presence of activated fibroblasts (arrows).