Trans-Catheter Double-Frequency Ultrasound Ablator for The Treatment of Aortic Valve Leaflets Calcification

Calcifications of the aortic valve leaflets, due to inflammatory conditions and specific biological processes [1], are quite common after 60 years of age. In particular, aortic valve degenerative stenosis, typical of elderly, has a prevalence of 3% in the Mediterranean area alone (4.6% for age> 74 years) and reaches 10% in America and North America [1]. The prevalence of a clinically significant stenotic valve condition is about 20% in patients in the 65-75 age group, about 35% in the 75-85 age group and 48% in the 85-age group [2]. The aortic valve is constantly subjected to cyclical mechanical stresses, due to the systo-diastolic motion, with consequent leaflets’ stress and to the transvalvular pressure gradient during diastole for each cardiac cycle (about 80mmHg). Valve calcification is a gradual accumulation of calcium and phosphate minerals: deposits that can thicken the leaflets creating mineralized structures ARTICLE INFO ABSTRACT

. Dystrophic calcification is concentrated in the areas most subjected to mechanical stress, causing a progressive deterioration of cardiac hemodynamics. The pathogenesis of dystrophic valve calcifications is the result of a chronic inflammatory process and pro-atherosclerotic conditions [3]. To prevent irreversible damage to the heart muscle, calcified aortic valves are replaced by artificial valves, mechanical or bioprosthetic, that can be implanted by conventional open-heart surgery or minimally invasive procedure [2,4].
However, in some cases, surgery may be particularly risky, due to the patient's particular condition (for example, advanced age and other co-morbidities). In case of very old patients with heavy comorbidities, also TAVI is not recommended and the only remaining option is to administer a medical therapy with a low probability of survival at 2 years. It is estimated that more than 200,000 patients, every year, are not treated [4] due to these shortcomings. Patients who are not candidates for surgical or transcatheter prostheses implantation could benefit from a "calcium debridement procedure" of their diseased aortic valve in order to restore, at least in part, the valve functionality. Ultrasonic waves, in particular shockwaves, are daily used to treat calcifications in urological [5,6], or orthopedic fields, and this technique has begun being used for vascular lithotripsy, e.g., at the level of the coronary arteries (10.1016/j.jacc.2020.09.603; 10.2217/fca-2020-0034). To efficiently treat the aortic valve, shockwave generators, external to the body, cannot be sufficiently selective. In fact, in order to achieve an effective lithotripsy, it is necessary to act directly in the site of calcium deposits, without increasing the local temperature that would damage the tissues. It is therefore necessary to use low power energies, while maintaining the calcium breaking capacity.
The spaces that can be used to reach the area to be treated are also very limited: for example, if the cardiac valve is to be treated, only arterial catheters, with a diameter of a few millimeters, can be used.
In the present report, we describe the first transcatheter debridement device (TDD) that has been specifically designed to deliver shockwaves to the aortic valve tissue and fracture the accumulated calcium deposits with ultrasound fields generated by opposed piezoelectric transducers at different frequencies [7][8][9]. The TDD is an ablation device based on physical energies, in particular ultrasound fields that act directly on the calcium deposits present outside and inside the valve leaflets, with the aim at increasing their pliability and restoring sufficient transvalvular flow. Other than for restoring, at least in part, the function of the valve, the TDD device could be also employed to remove calcium deposits from the aortic ring in preparation of conventional or minimally invasive prosthetic valve implantation.

Biophysical Analysis of Ultrasonic Fields Combination
The technique implemented in the TDD device is the ultrasound histotripsy. Ultrasounds suitably modulated in intensity, frequency, and waveform, can be used to produce fractures and structural changes in calcified deposits. Ultrasonic shockwaves can be generated with piezoelectric generators. The acoustic pressure "bumps" on the calcification, with a force of mechanical stress and, indirectly, by the collapse of cavitation bubbles formed inside the biological tissue [10]. The shockwave is a short duration  ns) positive pressure pulse, followed by a negative pressure pulse.
As described in the literature, these impulse waves, consisting of a positive peak followed by a negative peak, act on calcium deposits with combined effects of forces called spallation at the interface of tissue concretion, shear stress and superfocusing. A particular disruptive action on the deposited calcium, created by ultrasound, is given by the cavitation bubbles [10][11][12][13]. The phenomenon of cavitation can be seen as the breaking of a liquid and the consequent formation of bubbles, containing dissolved gas. The action of the ultrasonic field can create acoustic cavitation, distinct in inertial (transient) and non-inertial (stable). When a bubble is exposed to an ultrasonic field, the acoustic pressure acts as an external force that changes its radius. The bubble behaves like an oscillating system with an elasticity given by the gas contained inside it, and an inertia given by the liquid that surrounds the bubble and which oscillates with the wall of the bubble itself.
The bubble therefore has its own frequency which is inversely proportional to its radius (in equilibrium conditions). The relationship between frequency of acoustic field and radius of the bubble can be simplified like this: When the frequency of the acoustic field approaches the proper frequency of the bubble, resonant phenomena occur: the bubble expands during the negative phase of the pressure wave and collapses very quickly and violently, at the arrival of the positive pressure [14]. During the collapse, the bubble can fragment and break or repeat the expansion and collapse cycles. The implosion of the bubbles causes mechanical erosion, due to the release of concentrated energy. The frequencies normally used to induce cavitation, range from tens of kHz to few MHz. Higher frequencies induce thermal increase, controllable by reducing the amplitude of the wave [14]. For the design of the TDD, we considered low intensity so that the thermal increase is negligible. Cavitation bubbles are inherently unstable, but repetition of impulses creates a "bubble cloud" confined to the focal volume of the ultrasound field [7][8][9]. The combination of frequencies has shown greater therapeutic efficacy. The hypothesis at the base of a higher efficacy is that the combination of multiple frequencies of the ultrasonic field directly affects cavitation. By combining two frequencies, one about 30 times greater than the other, results an increase in cavitation effect [7]. In particular, low-frequency stimulation contributes by amplifying the effects of cavitation created by the higher frequency, also extending the cavitation volume [7].
The number of bubbles is 5 times higher by combining the two frequencies, compared to a frequency only, at equal power supply, while a larger number of bubbles is obtained at lower power [8]. Further enhancement is achieved by increasing the higher frequency, in the range of MHz [9]. The combination of two sources at two different frequencies increases both the intensity and the bandwidth, "enhancing" the effect of the ultrasound field.
Furthermore, non-linear effects combined with the combination of the two frequencies reduce the threshold for generating cavitation effects. The use of frequencies in the 3-4MHz range allows to act on small diameter bubbles and to exert the action at a lower distance.
The higher the frequency, the quicker the signal attenuates, "discharging" the energy at a short distance.
The cavitation enhancement is due to the contribution of some according to the equations of Minaret [15] and the approximation of Leighton [16]. Since the duration of these bubbles is greater than the time necessary to vary the frequency of the ultrasounds generated, it is possible to obtain the effect of cavitation that is the expansion and collapse of the bubbles, at different times. The lower frequency field nucleates larger bubbles while the higher frequency field, subsequently applied, creates the cavitation effect with the consequent growth and implosion of the bubbles obtaining a greater efficacy in the treatment of calcium deposits than the use of a single frequency [17][18][19][20]. The PZT, are introduced by a catheter, with an outer diameter of 6 mm, designed for access via a femoral artery, reaching the leaflets of the aortic valve.

Conclusion
The use of a combination of ultrasound frequencies helps to maximizes the disruptive effects on dystrophic calcifications of human aortic valve leaflets. For this reason, TDD seems to be a More tests are needed to better understand the interaction between the ultrasound field and the calcified tissue to demonstrate the clinical efficacy of the treatment.