The power of ultrasound: treating secondary MR with sound waves.

Innovations in physics and its applications often enable medical advances. Collaboration between physicists who developed a high-frame-rate ultrasound system and cardiovascular investigators has allowed non-invasive measurement of vascular and myocardial stiffness. In this current issue of the European Heart Journal— Cardiovascular Imaging, members of these groups, using a related technology, have employed ultrasound to treat as well as to image the heart in mitral regurgitation (MR) secondary to myocardial infarction (MI) that causes mitral valve (MV) leaflet tethering mediated by chordae to the displaced papillary muscles (PMs). This is a condition for which a strong need is perceived for improved approaches. Although there is evidence for adaptive MV growth in secondary MR, this extra leaflet tissue is often insufficient to compensate adequately for mitral annular (MA) enlargement and chordal tethering. The MV leaflets thus become increasingly taut, being pulled by the annulus towards the base, and by chordae towards the left ventricular (LV) apex. The normally convex MV closing configuration towards left atrium becomes concave, and the anterior MV leaflet appears like a ‘hockey stick’ on Echo. Once the necessary leaflet tissue redundancy for MV coaptation is exhausted, MR will develop, which will over time further advance annular remodelling and tethering by the collagen-based chordae and therefore MR (Figure 1A). Current medical and surgical and transcatheter repair therapies for secondary MR thus aim to reduce overall leaflet tethering to restore leaflet coaptation by addressing the impaired LV (optimal medical heart failure therapy, revascularization, resynchronization, potential myocardial regeneration) and the mitral annulus (restoration of normal sinus rhythm, annuloplasty). Not as clinically established, but shown to be safe and effective is to directly target the chordal apparatus by cutting secondary chordae attaching to the leaflet bodies to relieve their tethering and to restore a greater surface for leaflet coaptation (Figure 1B). Such chordal cutting reduces both MR and its associated LV remodelling and has been successfully applied by several surgical groups. The pathological changes observed in chordae post-infarction and the recent reported relation between secondary MR and chordal shortening, imposing additional restriction on the tethered leaflets, lend further support to the potential benefits of chordal cutting to relieve such MR. The current paper reports a proof of concept for using pulsed cavitational focused ultrasound (histotripsy) to cut chordae non-invasively guided by real-time 3D echocardiography. The authors show that secondary anterior chordae can be selectively cut in vitro and in vivo in the beating sheep heart with a high-energy