Dependence of the Rate and Completeness of Fibrin Clot Destruction on the Acoustic Dose and Ultrasound Intensity.

The kinetics of fibrin clot destruction under catheter-delivered 32- to 45-kHz ultrasound (US) has been studied at 36°C-38°C in isotonic saline solution. A pseudo-first-order rate constant increased linearly from 0.06/min to 0.57/min with increasing US intensity I0 from 21.6 to 51.2 W/cm2. At I0 = 4.4 and 11.4 W/cm2, the degree of clot destruction did not exceed 11%-15% regardless of the time of US exposure. Starting from I0 = 21.6 W/cm2, the maximum achievable level of clot destruction increased linearly with US intensity, reaching 68% at I0 = 51.2 W/cm2 after 3 min of US exposure. Thus, US intensity is a key parameter determining the maximum achievable level of clot destruction. However, an increase in US intensity above 30 W/cm2 is limited by the intensified negative sonochemical effect on the enzymatic system of hemostasis caused by an increase in inertial cavitation. The best effect can be achieved with ultrasound of a sufficiently high intensity that ensures a large contribution of stable cavitation, generating microstreaming flows, and a minimum contribution of inertial cavitation, generating microjets and shock waves.

Adaptation of cardiovascular system stent implants.

Time-consuming design and manufacturing processes are a serious disadvantage when adapting human cardiovascular implants as they cause unacceptable delays after the decision to intervene surgically has been made. An ideal cardiovascular implant should have a broad range of characteristics such as strength, viscoelasticity and blood compatibility. The present research proposes the sequence of the geometrical adaptation procedures and presents their results. The adaptation starts from the identification of a person's current health status while performing abdominal aortic aneurysm (AAA) imaging, which is a point of departure for the mathematical model of a cardiovascular implant. The computerized tomography scan shows the patient-specific geometry parameters of AAA and helps to create a model using COMSOL Multiphysics software. The initial parameters for flow simulation are taken from the results of a patient survey. The simulation results allow choosing the available shape of an implant which ensures a non-turbulent flow. These parameters are essential for the design and manufacturing of an implant prototype which should be tested experimentally for the assurance that the mathematical model is adequate to a physical one. The article gives a focused description of competences and means that are necessary to achieve the shortest possible preparation of the adapted cardiovascular implant for the surgery.

Ultrasound-assisted thrombolysis with streptokinase improves thrombus resolution with minimal distal embolisation.

Catheter-directed ultrasound (US) has a synergistic effect on thrombus with streptokinase (SK). We aimed to assess whether a new method of arterial thrombolysis based on a combination of short-term US and intravenous SK administration can improve efficacy and minimize distal embolisation as compared to these two interventions applied separately. Experiments have been done on 23 mongrel dogs with ligature-induced femoral thrombosis divided into groups treated with (i) enzymatic thrombolysis (intravenous SK, n = 6), (ii) 36 kHz US-assisted thrombolysis (n = 6), (iii) US+SK applied together (n = 6), and (iv) control group with no treatment (n = 5). US intensity at the distal end of the waveguide was 10-15 W/cm(2). Selective angiography, plethysmography and sphygmography have been used to assess thrombus resolution and distal embolisation. US-assisted thrombolysis alone was associated with good thrombus resolution, but substantial distal embolisation. SK-induced fibrinolysis alone did not provoke distal embolization but showed delayed thrombus resolution compared to US-treated group. Dual US+SK therapy resulted in high rate of US destruction without significant Under the combined US+SK action, nearly additive summation of US cavitation and SK effects as well as synergistic effects of both these factors on hemostasis parameters (activated partial thromboplastin, prothrombin, and thrombin time; fibrinogen, FDP, D-dimers, antithrombin III, plasminogen, and α2-antiplasmin) have been observed. Combination of CK and US-induced thrombolysis shows high efficacy with minimal distal embolisation. Arterial thrombus destruction by the combination of gradual (40 min) SK intravenous administration followed by short-time (1.5 min) intense US exposure improves shows positive effect of parameters of haemostasis. The magnitude and clinical significance of possible adverse effects of the dual fibrinolytic intervention related to endothelial injury and risk of bleeding needs to be further assessed in longer-term experiments and appropriately designed clinical trials.

Development and study of novel non-contact ultrasonic motor based on principle of structural asymmetry.

The article presents novel design of non-contact rotary ultrasonic motor consisting of ring-shaped stator vibrating in in-plane flexural mode and rotor provided with blades. In contrast to other motors with similar design proposed motor relies on the use of standing ultrasonic waves. This simplifies design and electronic control of motor and becomes possible due to introduction of artificial asymmetry, for example by tilting one or several blades of the rotor relative to the surface normal. Operating principle of the proposed motor is based on acoustic radiation torque exerted on rotor by ultrasonic waves propagating in air or fluid gap between rotor and stator. This torque is calculated using finite element method by means of COMSOL Multiphysics software. Dynamics of rotor is studied using MathCad software and general theory of nonlinear conservative oscillators. Role of asymmetry is explained on the basis of comparative analysis of potential functions and phase trajectories for symmetric and asymmetric cases. It is shown that direction of rotation is determined by structural parameters of motor, particularly tilting direction (clockwise or counter-clockwise) of the blades. Conceptual design of motor with bidirectional rotation is described. Direction and velocity of rotation in the proposed conceptual design can be potentially controlled by changing excitation frequency of stator.

Combined low-frequency ultrasound and streptokinase intravascular destruction of arterial thrombi in vivo.

To prevent a distal embolization in the course of ultrasound (US) angioplasty, we combined US thrombus disruption in peripheral artery in vivo with simultaneous administration of streptokinase (SK). Acute thrombosis was induced in the femoral arteries of 23 dogs. Two hours after thrombus formation, thrombus destruction was performed using US (36 kHz) and by a combined US+SK (75,000 U/kg) administration. The results showed that thrombi were disrupted completely by 1.5 ± 0.5 min US. A combined US+SK action resulted in activation of fibrinolysis, as indicated by the increase in the content of fibrinogen and fibrin degradation products and D-dimers by a factor of 1.5-2.0 after 120 min from start of treatment compared with the SK lysis. The duration of clot destruction did not change; the distal embolization was not indicated; platelet aggregation activity dropped after thrombus destruction. In summary, intravascular thrombus destruction by a combined US and SK action in vivo is accompanied by enhancing the enzymatic fibrinolysis and lowering the platelet aggregation activity that assists in preventing the distal embolization of the resulting clot debris.

Modeling of flexible waveguides for ultrasonic vibrations transmission: longitudinal and flexural vibrations of non-deformed waveguide.

The article presents the mathematical model allowing to investigate longitudinal and flexural vibrations of stepped flexible waveguides with transitional section without regard to various vibration modes interaction. The model uses original numerical-analytic calculations based on analytical solutions of the equation of waveguide steps vibrations and their continuous matching with numerical solution of the equation of transitional section vibrations. The proposed model can be considered as an initial approximation to the solution of the problem of flexible waveguides design, which makes it possible to determine and validate effective methods of its addressing. Resonant curves of longitudinal and flexural vibrations of two-step waveguide are traced for the given vibration frequency. Step lengths values providing simultaneous resonance of longitudinal and flexural vibrations for the given frequency are determined. Validity of the proposed model is proved by the results of finite elements method (FEM) modeling using ANSYS software. Application of Timoshenko's model instead of Euler-Bernoulli's model for description of flexural vibrations enabled reduction of relative deviation of resonant frequencies calculated using ANSYS from the value specified during resonant curves tracing down to negligible value (0.17%).