Ultrasonic clot disruption: an in vitro study

Branched silver-iron oxide nanoparticles enabling highly effective targeted and localised drug-free thrombolysis

Ultrasound has been widely used as an external stimulus to trigger drug release from nanomaterials in thrombosis treatment. Here, we introduce a novel strategy leveraging nanomaterials not for drug delivery, but for enhancing US-induced thrombolysis. This innovative strategy is particularly significant, as thrombolytic drugs inherently pose a risk of systemic bleeding. We combined branched silver-iron oxide nanoparticles (AgIONPs) with low-intensity focused ultrasound to evaluate their thrombolytic potential. Binding assays in in vitro human blood clots and in a thrombosis mouse model confirmed that the targeted AgIONPs specifically bound to thrombi. Upon ultrasound activation, AgIONPs facilitated thrombolysis via two key mechanisms: hyperthermia driven by the nanoparticle-mediated thermal conversion, and mechanical shear forces induced by ultrasound. The combination of AgIONPs and US generated a synergistic thrombolytic effect, demonstrating significant efficacy in both in vitro and in vivo.

Kinetics of fibrin clots destruction under ultrasonic cavitation

We studied the kinetic features of fibrin clot destruction in vitro under the action of ultrasonic cavitation generated by low-frequency (36 kHz) ultrasound (US) with the intensity I0  of 4.4–51.2 W/cm2, using a flexible waveguide concentrator. It was established that the rate of US destruction of clots immersed in saline at the initial stage of the process is proportional to I0 in the range of 12–51 W/cm2, corresponds to first-order kinetics, and is determined by the erosive processes without the formation of D-dimers and other fibrinolysis products at a minimum contribution of sonochemical reactions. The clot destruction rate is maximum at the initial time moment and decreases with increasing the US exposure duration (by 35 % in 1 min and by 72 % by the end of the second minute at I0 = 51.2 W/cm2). It was shown that in order to increase the completeness of clot destruction at a minimum administered US dose, it is advisable to minimize the US exposure time when using the highest values of the US intensity limited by the level of safe cavitation exposure to the vascular wall, hemostasis, and blood cells.

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.

Preclinical modeling of mechanical thrombectomy

Mechanical thrombectomy to treat large vessel occlusions (LVO) causing a stroke is one of the most effective treatments in medicine, with a number needed to treat to improve clinical outcomes as low as 2.6. As the name implies, it is a mechanical solution to a blocked artery and modeling these mechanics preclinically for device design, regulatory clearance and high-fidelity physician training made clinical applications possible. In vitro simulation of LVO is extensively used to characterize device performance in representative vascular anatomies with physiologically accurate hemodynamics. Embolus analogues, validated against clots extracted from patients, provide a realistic simulated use experience. In vitro experimentation produces quantitative results such as particle analysis of distal emboli generated during the procedure, as well as pressure and flow throughout the experiment. Animal modeling, used mostly for regulatory review, allows estimation of device safety. Other than one recent development, nearly all animal modeling does not incorporate the desired target organ, the brain, but rather is performed in the extracranial circulation. Computational modeling of the procedure remains at the earliest stages but represents an enormous opportunity to rapidly characterize and iterate new thrombectomy concepts as well as optimize procedure workflow. No preclinical model is a perfect surrogate; however, models available can answer important questions during device development and have to date been successful in delivering efficacious and safe devices producing excellent clinical outcomes. This review reflects on the developments of preclinical modeling of mechanical thrombectomy with particular focus on clinical translation, as well as articulate existing gaps requiring additional research.

Bioinspired In Vitro Brain Vasculature Model for Nanomedicine Testing Based on Decellularized Spinach Leaves

Animal testing is often criticized due to ethical issues and complicated translation of the results obtained to the clinical stage of drug development. Existing alternative models for nanopharmaceutical testing still have many limitations and do not significantly decrease the number of animals used. We propose a simple, bioinspired in vitro model for nanopharmaceutical drug testing based on the decellularized spinach leaf's vasculature. This system is similar to human arterioles and capillaries in terms of diameter (300-10 μm) and branching. The model has proven its suitability to access the maneuverability of magnetic nanoparticles, particularly those composed of Fe₃O₄. Moreover, the thrombosis has been recreated in the model's vasculature. We have tested and compared the effects of both a single-chain urokinase plasminogen activator (scuPA) and a magnetically controlled nanocomposite prepared by heparin-mediated cross-linking of scuPA with Fe₃O₄ nanoparticles. Compositions were tested both in static and flow conditions.

Quantification of microbubble-induced sonothrombolysis in an ex vivo non-human primate model

BACKGROUND

In vitro studies with ultrasound (US) and microbubbles (MB) have reported that sono-thrombolysis can be achieved at high peak rarefactional acoustic pressure amplitudes (PRAPAs) using 0.25 and 1.05 MHz US frequencies.

OBJECTIVE

The aim of the current study was to determine if these parameters work on an ex vivo physiological model of thrombosis.

METHODS

A thrombogenic device was placed in an ex vivo chronic arteriovenous shunt in juvenile baboons. Platelet accumulation was measured by dynamic imaging of the device and the 10 cm thrombus tail with 111 In-labeled platelets. After 15 minutes of thrombus formation, treatment with either low-dose recombinant tissue plasminogen activator (rtPA) or low-dose rtPA + MB+US was performed for 20 minutes. Four US settings at 0.25% duty cycle were used: 0.25 MHz at PRAPAs of 1.20 and 2.20 MPa, and 1.05 MHz at 1.75 and 4.75 MPa.

RESULTS

Platelet accumulation was not inhibited by low-dose rtPA or MB with US alone. Platelet accumulation was significantly reduced with 0.25 MHz US at 2.20 PRAPA (P < .001) and with 1.05 MHz at 1.75 MPa and 4.75 MPa (P < .05) when used with MB and low-dose rtPA. Although this approach prevented platelet accumulation it did not cause thrombolysis on the device.

CONCLUSIONS

rtPA + MB + US (0.25 and 1.05 MHz) resulted in inhibition of platelet accumulation on the thrombogenic device when moderately high PRAPAs (≥1.75 MPa) were used. These results taken in context with lytic effects of US on myocardial microthrombi and direct effect on myocardial blood flow and function provide direction for the use of therapeutic US in acute coronary syndromes.

MRI-Guided High-Intensity Focused Ultrasound as an Emerging Therapy for Stroke: A Review

Stroke, either ischemic or hemorrhagic, accounts for significantly high morbidity and mortality rates around the globe effecting millions of lives annually. For the past few decades, ultrasound has been extensively investigated to promote clot lysis for the treatment of stroke, myocardial infarction, and acute peripheral arterial occlusions, with or without the use of tPA or contrast agents. In the age of modern minimal invasive techniques, magnetic resonance imaging-guided high-intensity focused ultrasound is a new emerging modality that seems to promise therapeutic utilities for both ischemic and hemorrhagic stroke. High-intensity focused ultrasound causes thermal heating as the tissue absorbs the mechanical energy transmitted by the ultrasonic waves leading to tissue denaturation and coagulation. Several in-vitro and in-vivo studies have demonstrated the viability of this technology for sonothrombolysis in both types of stroke and have warranted clinical trials. Apart from safety and efficacy, initiation of trials would further enable answers regarding its practical application in a clinical setup. Though this technology has been under study for treatment of various brain diseases for some decades now, relatively very few neurologists and even neurosurgeons seem to be acquainted with it. The aim of this review is to provide basic understanding of this powerful technology and discuss its clinical application and potential role as an emerging viable therapeutic option for the future management of stroke.

Therapeutic application of contrast ultrasound in ST elevation myocardial infarction: Role in coronary thrombosis and microvascular obstruction

In the past few decades, cardiac ultrasound has become a widely available, easy-to-use diagnostic tool in many scenarios in acute cardiac care. The introduction of microbubbles extended its diagnostic value. Not long thereafter, several investigators explored the therapeutic potential of contrast ultrasound on thrombus dissolution. Despite large improvements in therapeutic options, acute ST elevation myocardial infarction remains one of the main causes of mortality and morbidity in the western world. The therapeutic effect of contrast ultrasound on thrombus dissolution might prove to be a new, effective treatment strategy in this group of patients. With the recent publication of human studies scrutinising the therapeutic options of ultrasound and microbubbles in ST elevation myocardial infarction, we have entered a new stage in this area of research. This therapeutic effect is based on biochemical effects both at macrovascular and microvascular levels, of which the exact working mechanisms remain to be elucidated in full. This review will give an up-to-date summary of our current knowledge of the therapeutic effects of contrast ultrasound and its potential application in the field of ST elevation myocardial infarction, along with its future developments.

Effectiveness of mechanical endovascular thrombectomy in a model system of cerebrovascular occlusion

BACKGROUND AND PURPOSE

A number of thrombectomy devices are currently undergoing clinical evaluation; meanwhile, various novel devices are under investigation. The aims of this study were to quantify flow restoration and the particle size distribution of the effluent pursuant to MET in an in vitro occlusion model.

MATERIALS AND METHODS

The model system was composed of 3 elements: an ICA/MCA replica, a clot model with mechanical properties similar to those of thrombi found in patients at risk of stroke, and a pulsatile flow loop. Different thrombectomy mechanisms including mechanical retrieval, aspiration, and waveguide induced cavitation were used. The efficacy end points were recanalization rate and amount of flow restoration. The risk of the embolic shower was assessed to evaluate device safety.

RESULTS

The recanalization rates were the following: Merci, 67%; Solitaire, 100%; Penumbra, 83%; Enterprise, 17%; and the waveguide, 0%. In experiments in which recanalization was achieved, the amount of flow restoration for the Merci, Solitaire, and Enterprise devices was 100%, 92%, and 86%, respectively. The mean sizes of generated small and large clot fragments were between 23 and 37 and 215 and 285 μm, respectively, depending on the device used. The Merci device generated the fewest number of large fragments compared with the Penumbra system (P < .05) and Solitaire (not significant).

CONCLUSIONS

The risk of embolic shower was influenced by the mechanism of action for the thrombectomy device. Clinically reported recanalization rates for the Solitaire, Penumbra, and Merci devices were nearly identical in this model system, suggesting that this model may provide a predictive tool for preclinical evaluation of MET.

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.

Mechanical characterization of thromboemboli in acute ischemic stroke and laboratory embolus analogs

BACKGROUND AND PURPOSE

Mechanical behavior of the thromboembolus is one of the key factors that determine the efficacy of thrombectomy devices for revascularization in AIS. We characterized the mechanical properties and composition of thromboemboli from clinical cases and compared them with commonly used EAs.

MATERIALS AND METHODS

Thromboemboli were obtained from patients with AIS by using aspiration devices and from carotid atherosclerotic plaques harvested during endarterectomy. In the laboratory, common EAs were created by varying blood donor species (human, porcine, and bovine), thrombin concentration, and presence of barium sulfate. Stiffness and elasticity of the specimens were measured with DMA. Scanning electron microscopy and histology were used to investigate the ultrastructure and composition of all specimens.

RESULTS

Red thromboemboli from patients composed mainly of fibrin and erythrocytes were much softer than the calcified and cholesterol-rich material. Of the EAs created in the laboratory, those made from bovine blood presented the highest stiffness that was independent of thrombin concentration. Addition of thrombin increased the stiffness and elasticity of human and porcine EAs (P < .05). The presence of barium sulfate significantly reduced the elasticity of all EAs (P < .05).

CONCLUSIONS

Endovascular device testing and development requires realistic EAs. The stiffness and elasticity of the cerebral thromboemboli analyzed in this study were closely matched by recalcified porcine EAs and thrombin-induced human EAs. Stiffness of the thrombus extracted from carotid endarterectomy specimens was similar with that of the thrombin-induced bovine and porcine EAs.

Sonothrombolysis for intraocular fibrin formation in an animal model

Vascular diseases such as diabetic retinopathy or retinal arterial occlusion are always associated with retinal and/or choroidal vasculopathy and intravascular thrombosis is commonly found. The ultrasound (US) therapy is a recently developed technique to accelerate fibrinolysis and it is being applied to some clinical fields. The present study was to observe the effects of extraocular US exposure on intraocular fibrin, which is a deteriorating factor in various ocular diseases. Tubes containing human blood (2 mL) in the following groups were irradiated with US; US alone, US with tissue plasminogen activator (tPA), tPA alone, and saline (control). Fibrinolysis was quantified by measuring D-dimer after 2h. In rat eyes, intracameral fibrin (fibrin formation in the anterior chamber of the eye) was induced by YAG-laser-induced iris bleeding. Then, eyes in the following groups were irradiated with US; US alone, subconjunctival tPA alone, US and subconjunctival tPA, control. Intracameral fibrin was scored on day 3 (3+ maximum to 0). The temperatures of rat eyes were measured by infrared thermography. Histologic evaluation was also performed. D-dimer was increased by US with statistical significance (p <0.05) or tPA (p <0.01). D-dimer in US with tPA group was significantly higher than either US alone or tPA alone group (p <0.01). In rat eyes, the average intracameral fibrin score on day 3 was 1.4 in control group and 1.2 in subconjunctival tPA alone group; however, it decreased significantly in the US alone group (0.75; p <0.05, vs. control), US and subconjunctival tPA group (0.71; p <0.01, vs. control). The temperature was less than 34 degrees C after US exposure. No histologic damage was observed. US irradiation from outside accelerated intracameral fibrinolysis without causing apparent tissue damage. This noninvasive method might have therapeutic value for intraocular fibrin.

Blood clot disruption in vitro using shockwaves delivered by an extracorporeal generator after pre-exposure to lytic agent

The standard methods for recanalyzing thrombosed vessels are vascular stenting or administration of thrombolytic drugs. However, these methods suffer from uncertain success rate and side-effects. Therefore, minimally-invasive ultrasound methods have been investigated. In this article, we propose to use shockwaves after pre-exposure to fibrinolytic agent for disrupting thrombus. Shockwaves were delivered by an extracorporeal piezocomposite generator (120 mm in diameter, focused at 97 mm, pulse length = 1.4 micros). In vitro blood clots, made from human blood, were placed at the focal point of the generator. The clots were exposed to shockwaves either with or without prior immersion in a solution of streptokinase. The percentage of lysed clot was determined by weighing the clot before and after treatment. The proportion of lysed clot increased with the pressure at the focus and with the number of shocks. A mean clot reduction of 91% was obtained for 42 MPa in 4-min treatment duration only, without using streptokinase. For a treatment of 2 min at 29 MPa, the clot reduction increased significantly (p < 0.01) from 47% without streptokinase to 82% when streptokinase was used prior to shockwaves. These results also showed no significant damage to streptokinase due to exposure to shockwaves. This study suggests that extracorporeal shockwaves combined with streptokinase is a promising pharmaco-mechanical method for treating occlusive thrombus, and should be confirmed by in vivo trials. Additional studies must also be conducted with other fibrinolytic agents, whose abilities to penetrate clots are different.

Ultrasound thrombolysis

Ultrasound energy for thrombolysis dates back to 1976. Trubestein et al. demonstrated first in vitro that a rigid wire delivery low frequency ultrasound energy could disrupt clot. These investigators also showed that this system had potential for peripheral arterial clot dissolution in vivo in animal studies [G. Trubestein, C. Engel, F. Etzel, Clinical Science 51 (1976) 697s-698s]. Subsequently, four basic approaches to ultrasonic thrombolysis have been pursued--two without pharmacological agents: (1) catheter-delivered external transducer ultrasound, (2) transcutaneous-delivered HIFU external ultrasound without drug delivery and ultrasound in conjunction with thrombolytic drugs and/or microbubbles or other agents, (3) Catheter-delivered transducer-tipped ultrasound with local drug delivery, and (4) transcutaneous-delivered low frequency ultrasound with concomitant systemic (intravenous) drug delivery for site specific ultrasound augmentation. This article reviews recent data on therapeutic ultrasound for thrombolysis in vitro, in vivo, in animal studies, as well as in human clinical trials.

Pulsed-high intensity focused ultrasound enhanced tPA mediated thrombolysis in a novel in vivo clot model, a pilot study

INTRODUCTION

Thrombotic disease continues to account for significant morbidity and mortality. Ultrasound energy has been investigated as a potential primary and adjunctive treatment for thrombotic disease. We have previously shown that pulsed-high intensity focused ultrasound (HIFU) enhances thrombolysis induced by tissue plasminogen activator (tPA) in vitro, including describing the non-destructive mechanism by which tPA availability and consequent activity are increased. In this study we aimed to determine if the same effects could be achieved in vivo.

MATERIALS AND METHODS

In this study, pulsed-HIFU exposures combined with tPA boluses were compared to treatment with tPA alone, HIFU alone and control in a novel in vivo clot model. Clots were formed in the rabbit marginal ear vein and verified using venography and infrared imaging. The efficacy of thrombolytic treatment was monitored via high resolution ultrasonography for 5 h post-treatment. The cross-sectional area of clots at 4 points along the vein was measured and normalized to the pre-treatment size.

RESULTS

At 5 h the complete recanalization of clots treated with pulsed-HIFU and tPA was significantly different from the partial recanalization seen with tPA treatment alone. tPA treatment alone showed a significant decrease in clot versus control, where HIFU was not significantly different than control. Histological analysis of the vessel walls in the treated veins showed no apparent irreversible damage to endothelial cells or extravascular tissue.

CONCLUSIONS

This study demonstrates that tPA mediated thrombolysis can be significantly enhanced when combined with non-invasive pulsed-HIFU exposures.

Treatment of Acute Iliofemoral Deep Venous Thrombosis: A Strategy of Thrombus Removal

Patients with acute iliofemoral deep vein thrombosis (DVT) suffer the most severe postthrombotic sequelae. The majority of physicians treat all patients with acute DVT with anticoagulation alone, despite evidence that postthrombotic chronic venous insufficiency, leg ulceration, and venous claudication are common in patients treated only with anticoagulation. The body of evidence to date in patients with iliofemoral DVT suggests that a strategy of thrombus removal offers these patients the best long-term outcome. Unfortunately, currently published guidelines use outdated experiences to recommend against the use of techniques designed to remove thrombus, ignoring recent clinical studies showing significant benefit in patients who have thrombus eliminated. Contemporary venous thrombectomy, intrathrombus catheter-directed thrombolysis, and pharmacomechanical thrombolysis are all options that can be offered to successfully remove venous thrombus with increasing safety. The authors review evidence supporting the rationale for thrombus removal and discuss the most effective approaches for treating patients with acute iliofemoral DVT.

Pulsed high-intensity focused ultrasound enhances thrombolysis in an in vitro model

PURPOSE

To evaluate the use of pulsed high-intensity focused ultrasound exposures to improve tissue plasminogen activator (tPA)-mediated thrombolysis in an in vitro model.

MATERIALS AND METHODS

All experimental work was compliant with institutional guidelines and HIPAA. Clots were formed by placing 1 mL of human blood in closed-off sections of pediatric Penrose tubes. Four experimental groups were evaluated: control (nontreated) clots, clots treated with pulsed high-intensity focused ultrasound only, clots treated with tPA only, and clots treated with pulsed high-intensity focused ultrasound plus tPA. The focused ultrasound exposures (real or sham) were followed by incubations of the clots in tPA with saline or in saline only. Thrombolysis was measured as the relative reduction in the mass of the clot. D-Dimer assays also were performed. Two additional experiments were performed and yielded dose-response curves for two exposure parameters: number of pulses per raster point and total acoustic power. Radiation force-induced displacements caused by focused ultrasound exposures were simulated in the clots. A Tukey-Kramer honestly significant difference test was performed for comparisons between all pairs of experimental groups.

RESULTS

The clots treated with focused ultrasound alone did not show significant increases in thrombolysis compared with the control clots. The clots treated with focused ultrasound plus tPA showed a 50% ([30.2/20.1]/20.1) increase in the degree of thrombolysis compared with the clots treated with tPA only (P < .001), further corroborating the d-dimer assay results (P < .001). Additional experiments revealed how increasing both the number of pulses per raster point and the total acoustic power yielded corresponding increases in the thrombolysis rate. In the latter experiment, simulations performed at a range of power settings revealed a direct correlation between increased displacement and observed thrombolysis rate.

CONCLUSION

The rate of tPA-mediated thrombolysis can be enhanced by using pulsed high-intensity focused ultrasound exposure in vitro.

Development of new screening system for Alzheimer disease, in vitro Abeta sink assay, to identify the dissociation of soluble Abeta from fibrils

Abeta is one of the primary therapeutic targets for Alzheimer disease (AD). Abeta vaccination induces the disappearance of Abeta deposits. Since few reports have focused on the reverse phase of Abeta aggregation, we established a new screening system, the in vitro Abeta sink assay, to clarify the process of dissociation of soluble forms from fibrils. Abeta42 was more resistant to dissociation from fibrils to monomers and/or low molecular weight (LMW) soluble oligomers than Abeta40. We applied this system to find a potential therapy for AD. Ultrasound irradiation significantly enhanced the dissociation of soluble Abeta from fibrils, while ultrasound experiments also confirmed the difference between Abeta40 and Abeta42. We found that some compounds enhanced the dissociation of Abeta from fibrils. Here, we proposed that Abeta42 was more resistant to dissociation from fibrils to monomers and/or LMW soluble oligomers than Abeta40, and this system might be useful to identify dissociation of soluble Abeta from fibrils.

Ultrasound angioplasty: an update review

The use of therapeutic ultrasound to treat atherosclerosis and thrombosis has been appreciated for decades. However, it was only the explosive growth of angioplasty in the 1980s that brought real momentum to the development of therapeutic catheter ultrasound. The idea behind this technique was that ultrasound, by its bioselectivity, might provide a solution to some of the shortcomings of balloon angioplasty. In the late 1980s, two groups, headed by Rosenschein and Siegel, began serious work to address the technical challenge of developing a catheter that would provide efficient external ultrasound energy to the lesion. Current catheters from both groups consist of a solid metal probe which is connected to a piezoelectric transducer. In the distal segment, the wire is specially designed to increase energy delivery. Initial in vitro studies concentrated on understanding the mechanisms of ablation and the effects of mechanical vibration, thermal phenomena and cavitation. Clinical studies of ultrasound ablation were initially performed in peripheral vessels. Later, after safety had been assured, clinical studies involving the coronary arteries began to take place. In this article we aim to update the reader about the experimental and limited clinical experience in this novel technique for treating different kinds of arterial obstruction.

Ultrasound and thrombolysis

Thrombolytic therapy and mechanical interventions are frequently used in the treatment of both arterial and venous thrombotic disease. Limitations to these approaches include failure to achieve reperfusion and complications including bleeding and vessel wall damage. Increasing evidence indicates that the use of ultrasound offers potential therapeutic advantages. This review considers two distinct approaches which include the use of high intensity ultrasound to mechanically fragment clots and also the use of low intensity ultrasound to augment enzymatic fibrinolysis. High intensity ultrasound can be delivered via catheter or transcutaneously to disrupt clots in vitro or in animal models into small fragments. Initial clinical studies demonstrate potential clinical value in peripheral and coronary arterial thrombosis and occluded saphenous vein bypass grafts treated with the catheter approach. Studies in vitro indicate that low intensity ultrasound accelerates enzymatic thrombolysis through non-thermal mechanisms involving improvement in drug transport. The effect is larger at low frequencies, which also offer better tissue penetration and less heating. The ability to accelerate thrombolysis has been confirmed in animal models demonstrating markedly increased reperfusion and minimal toxicity. The use of ultrasound to mechanically disrupt occlusive thrombi or to accelerate enzymatic thrombolysis offers a new approach to treating occlusive thrombotic disease.

Ultrasound, microbubbles, and thrombolysis

Although dissolution of thrombus using ultrasound has been attempted for over 25 years, the clinical use of this technique remains limited. The ability of microbubbles to potentiate ultrasound-induced thrombolysis has renewed interest in this technique, which recanalizes occluded vessels without the need for fibrinolytic therapy. In this article, the potential mechanisms by which ultrasound and microbubbles produce thrombus dissolution are explored. In vitro and in vivo studies using ultrasound alone and ultrasound in combination with microbubbles to cause thrombolysis are reviewed. Potential clinical implications of more recent findings are explored.

Ultrasound fibrin clot destruction in vitro in the presence of fibrinolytic agent

Ultrasound fibrin clot destruction was investigated in vitro using electron microscopy and by monitoring the changes in the light transmission of clot debris suspension. It has been established that in the course of a combined action of ultrasound and fibrinolytic agent at high ultrasound intensities and short sonification periods, fibrin clot is disrupted mainly due to sonomechanical treatment, while fermentative lysis takes place in parallel and at a significantly lower rate. However, the streptokinase action prevails after ultrasound switching off and results in the prevention of clot debris conglomeration.

Arterial thrombus dissolution in vivo using a transducer-tipped, high-frequency ultrasound catheter and local low-dose urokinase delivery

PURPOSE

To examine the hypothesis that a transducer-tipped high-frequency ultrasound drug-delivery catheter may augment the thrombolytic effects of locally delivered low-dose urokinase and result in improved recanalization rates and reduced residual thrombotic burden.

METHODS

Thrombi were induced in situ bilaterally in 5- to 6-cm-long segments of the superficial femoral arteries in 9 dogs by intraluminal thermal damage and injection of thrombin. A transducer-tipped high-frequency local drug-delivery catheter was applied at 1.1 MHz and 0.6 W for 60 minutes to one superficial femoral artery segment, and an identical catheter with an inactivated ultrasound transducer was used to treat the contralateral control segment. Urokinase (5000 IU/kg) was delivered bilaterally into the thrombi during the treatment interval.

RESULTS

Angiography documented TIMI grade 2 or 3 flow in 9 (100%) segments in the ultrasound-treated group versus 6 (67%) of the controls (no ultrasound) (p = 0.058). Angiographically detected distal embolization was found in 2 ultrasound-treated segments compared with 5 controls (p = 0.02). Protruding or occlusive thrombi were seen angioscopically in 8 (89%) control segments but in only 1 (11%) of the ultrasound-treated arteries (p < 0.001). By histopathology, 7 (78%) segments in the control group had occlusive thrombi, whereas only 3 nonocclusive thrombi were found in the ultrasound-treatment group (p < 0.001).

CONCLUSIONS

Catheter-delivered high-frequency ultrasound and local low-dose urokinase infusion is efficacious for the treatment of acute thrombotic occlusions as evaluated by angiography, angioscopy, and histopathology.

Cavitational mechanisms in ultrasound-accelerated thrombolysis at 1 MHz

Inertial cavitation is hypothesized to be a mechanism by which ultrasound (US) accelerates the dissolution of human blood clots when the clot is exposed to a thrombolytic agent such as tissue plasminogen activator (t-PA). To test this hypothesis, radiolabeled fibrin clots were exposed or sham-exposed in vitro to 1 MHz c.w. US in a rotating sample holder immersed in a water-filled tank at 37 degrees C. Percent clot dissolution after 60 min of US exposure was assessed by removing the samples, centrifuging, and measuring the radioactivity of the supernatant fluid relative to the pelletized material. To suppress acoustic cavitation, the exposure tank was contained within a hyperbaric chamber capable of pneumatic pressurization to 10 atmospheres (gauge). Various combinations of static pressure (0, 2, 5, and 7.5 atm gauge), US (0 or 4 W/cm(2) SATA), and t-PA (0 or 10 microg/mL) were employed, showing statistically significant reductions in thrombolytic activity as static pressure increased. To gain further insight, an active cavitation detection scheme was employed in which 1-micros duration tonebursts of 20-MHz US (< 1 kPa peak negative pressure, 1 Hz PRF) were used to interrogate clots subjected to US and static pressure. Results of this cavitation detection scheme showed that scattering from within the clot and broadband acoustic emissions that were both present during insonification were significantly reduced with application of static pressure. However, only about half of the acceleration of thrombolysis due to US could be removed by static pressure, suggesting the possibility of other mechanisms in addition to inertial cavitation.

Ultrasound enhancement of thrombolytic therapy: observations and mechanisms

Fibrinolytic therapy is a proven approach for achieving reperfusion of occluded coronary arteries during myocardial infarction, resulting in reduced mortality and preservation of ventricular function. The amount of myocardial muscle loss is proportional to the duration of ischemia. Bleeding complications are not infrequent. Adjuvant therapy by ultrasound might enhance the rate of fibrinolysis and reduce the concentrations of lytic agents required to achieve an equivalent degree of clot lysis. Noninvasive ultrasound at low intensities and high frequencies, parameters that potentially could be applied and tolerated in vivo, have been proven to significantly accelerate the rate of fibrinolysis in both in vitro and in vivo models, in pure fibrin as well as whole blood clots. Such enhancement is not drug-specific. These effects were achieved by nonthermal mechanism. Ultrasound exposure did not cause mechanical fragmentation of the clot, did not alter the size of plasmatic derivates and degradation products. Ultrasound caused increased flow rate through thrombi, probably by cavitation-induced changes in fibrin ultrastructure; disaggregation of uncrosslinked fibrin fibers into smaller fibers has been shown. This resulted in increased transport of the lytic agent into the clot, alteration of binding affinity and increased maximum binding. Presence of echo-contrast agent induced further acceleration of thrombolysis by ultrasound.

Effect of External Ultrasound Frequency on Thrombus Disruption In Vitro

Objectives: This in vitro study assesses the effect of different external ultrasound frequencies on the disruption of human thrombi. Background: Ultrasound energy has been shown to disrupt human thrombi in vitro. However, there have been no previous studies to assess the effect of a range of different ultrasound frequencies on the rate and extent of thrombus disruption. Methods: In vitro, we exposed 56, 1- to 3-hour-old human blood thrombi to continuous wave ultrasound (2.9 W/cm2) for 3 minutes. Seven different frequencies, ranging from 243 kHz to 25 kHz, were used. Results: There was a gradual increase in the total reduction of thrombus weight as well as the percent thrombus disruption with the use of lower ultrasound frequencies, reaching 99% at 25 kHz (p < 0.001) and 86% (p < 0.001) at 39 kHz, compared with 25% at 243 kHz. The average particle size of the disrupted thrombi was 3.26 µm (range 2.8-3.8). Conclusions: Our in vitro data with external ultrasound show that for a given power intensity of ultrasound, the extent and magnitude of thrombus disruption is progressively increased as frequencies decrease from 243 to 25 kHz. This might be related to the fact that larger acoustic bubbles are induced by lower frequency ultrasound, which gives rise to greater mechanical energy for thrombus disruption during bubble vibration and their collapse.

Histological observations and the process of ultrasound contrast agent enhancement of tissue plasminogen activator thrombolysis with ultrasound exposure

Although the enhancement of tissue plasminogen activator (tPA) induced thrombolysis by ultrasound has been reported to be augmented by ultrasound contrast agents (UCA), few data exist regarding its process. The present study evaluated the effect of a galactose based UCA on the efficacy of ultrasonic enhancement of tPA thrombolysis and observed the serial changes in the acoustic property and histopathology. A catheter-type transducer capable of ultrasound emission in both continuous (CW) and pulsed wave (PW) was used. The tPA thrombolysis was studied in 30 artificial white thrombi, which were assigned to 4 study groups based on insonation modes and with and without UCA. Each sample was suspended in 100ml saline in a beaker. Five minutes after tPA (8000U) administration, ultrasound was applied for 10min. For the UCA-treated groups, UCA (0.25g) was added 5 min after the start of ultrasound exposure. The alteration of the thrombus was monitored with echography. Weight reduction of the thrombus was -25+/-6% in PW and -30+/-7% in CW, which was significantly enhanced by UCA treatment, 40+/-3% (p<0.005) in PW+UCA and -43+/-7% (p<0.005) in CW+UCA. The area of thrombus echo image minimally decreased with ultrasound alone (-12+/-6%: PW, -23+/-11%: CW). In the UCA groups, UCA induced a remarkable reduction of size (-36+/-3%: PW+UCA, -43+/-7%: CW+UCA) with a high-echo intensity in the superficial layer of the thrombus, where multiple cavity formation was observed by light microscope. UCA markedly enhanced the effect of ultrasound on tPA thrombolysis. The altered acoustic property and corresponding histological microcavity formation in the shallow layer within the thrombus suggests that UCA augmented infiltration of tPA into the thrombus.

Ultrasonic thrombolysis: catheter-delivered and transcutaneous applications

Ultrasonic thrombolysis has been proved to be an efficient and safe modality for the treatment of acute arterial occlusions in vitro and in vivo in animal studies. There have been and are ongoing parallel improvements in ultrasound technology and adjuvant pharmacological treatments for therapeutic applications. Thus therapeutic ultrasound for thrombolysis holds great promise in overcoming the limitations of current available therapies.

Enhancement of fibrinolysis with 40-kHz ultrasound

BACKGROUND

Ultrasound at frequencies of 0.5 to 1 MHz and intensities of > or =0.5 W/cm2 accelerates enzymatic fibrinolysis in vitro and in some animal models, but unacceptable tissue heating can occur, and limited penetration would restrict application to superficial vessels. Tissue heating is less and penetration better at lower frequencies, but little information is available regarding the effect of lower-frequency ultrasound on enzymatic fibrinolysis. We therefore examined the effect of 40-kHz ultrasound on fibrinolysis, tissue penetration, and heating.

METHODS AND RESULTS

125I-fibrin-radiolabeled plasma clots in thin-walled tubes were overlaid with plasma containing tissue plasminogen activator (tPA) and exposed to ultrasound. Enzymatic fibrinolysis was measured as solubilization of radiolabel. Tissue attenuation and heating were examined in samples of porcine rib cage. Fibrinolysis was increased significantly in the presence of 40-kHz ultrasound at 0.25 W/cm2, reaching 39+/-7% and 93+/-11% at 60 minutes and 120 minutes, compared with 13+/-8% and 37+/-4% in the absence of ultrasound (P<0.0001). The acceleration of fibrinolysis increased at higher intensities. Attenuation of the ultrasound field was only 1.7+/-0.5 dB/cm through the intercostal space and 3.4+/-0.9 dB/cm through rib. Temperature increments in rib were <1 C/(W/cm2).

CONCLUSIONS

These findings indicate that 40-kHz ultrasound significantly accelerates enzymatic fibrinolysis at intensities of > or =0.25 W/cm2 with excellent tissue penetration and minimal heating. Externally applied 40-kHz ultrasound at low intensities is a potentially useful therapeutic adjunct to enzymatic fibrinolysis with sufficient tissue penetration for both peripheral vascular and coronary applications.

Thrombolysis of mural thrombus by ultrasound: an experimental in vitro study

RATIONALE AND OBJECTIVES

The authors perform an in vitro evaluation of the thrombus fragmentation to determine the efficacy and degree of downstream clot fragment embolization that occurs during transcatheter ultrasound treatment of fibrin-rich mural thrombus in a peripheral venous flow model with variable diameter tubing.

METHODS

The authors used a 22.5-kHz prototype intravascular ultrasound device with a flexible 0.8-mm (.032-inch) titanium wire probe encased in a 7-French teflon guide catheter, at the tip of which is a 2-mm ball. In 50 silicone tube segments (inner diameter 3, 5, 7, 9, and 11 mm; n = 10 each), firmly adherent mural thrombus was produced using bovine blood in a modification of the Chandler's loop technique. Ultrasound energy (30-36 watts/cm), maximal longitudinal catheter tip amplitude 70 m) was applied to the thrombus while a continuous flow of water was maintained in the closed loop system. Clot fragment emboli were trapped in "downstream" polyethylene filters.

RESULTS

The mean rate of thrombus removal ranged from 99% +/- 0.3% in the 3-mm segments to 76% +/- 6% in the 11-mm segments. The average weight of the fragments that embolized "downstream" and were trapped in the filters, expressed as a percentage of the initial clot weight, was 11% in the 3-mm segment, 14% in the 5-mm segment, 30% in the 7-mm segment, 29% in the 9-mm segment, and 28% in the 11-mm segments. The majority of the embolized fragments appear to be larger than 1 mm.

CONCLUSIONS

In this in vitro venous flow model a lack of catheter steerability was the major obstacle to complete thrombus fragmentation in vessel calibers larger than two times the tip diameter. The rate of embolism and the amount of remaining thrombus that could not be removed from the vessel were higher in the larger vessels.

Declotting of embolized temporary vena cava filter by ultrasound and the Angiojet: comparative experimental in vitro studies

RATIONALE AND OBJECTIVES

The authors perform an in vitro evaluation of the thrombolytic efficacy and the amount of "downstream" embolization induced by two new mechanical thrombectomy devices when applied to clots trapped in a temporary vena cava filter.

METHODS

The first device used was a 22.5-kHz prototype intravascular ultrasound device with a flexible 0.8-mm (.032-inch) titanium 2-mm ball-tipped wire probe ensheathed in a 7-French teflon guide catheter. The device was inserted through a 10-French steering catheter. Under fluoroscopic control, ultrasound energy (26 +/- 4 watts/cm2, maximal longitudinal catheter tip amplitude 54 microm) was applied to 10 Ultravist-filled porcine thrombi (mean, 3500 mg). The second device, the Angiojet catheter, was applied to five Ultravist-filled porcine thrombi (mean, 3640 mg). The thrombi were treated while trapped in a temporary Günther vena cava filter (Cook Europe, Bjaverskov, Denmark) mounted in a vena cava flow model. The resultant "downstream" emboli were trapped in two tandem filters of decreasing pore size and weighed.

RESULTS

Mean thrombus dissolution rate was 53% +/- 22% standard deviation (SD) for the ultrasound device (n = 10) and 63 % +/- 8% SD for the Angiojet (n = 5) (difference statistically significant at P = 0.03). For the ultrasound device, the mean embolic particle weight caught by the filters with mesh widths of 1 mm and 0.1 mm was 42% +/- 14% SD and 4% +/- 2% SD, respectively, of the initial thrombus weight. For the Angiojet, the respective numbers were 35% +/- 16% SD and 3% +/- 1% SD. Mean treatment time was 216 +/- 45 seconds SD for the ultrasound device and 153 +/- 21 seconds SD for the Angiojet.

CONCLUSIONS

The thrombolytic efficacy of the Angiojet was significantly greater and the treatment time was significantly shorter than that of the ultrasound device. Both systems had a high embolization rate.

Dissolution of thrombotic arterial occlusion by high intensity, low frequency ultrasound and dodecafluoropentane emulsion: an in vitro and in vivo study

OBJECTIVES

We examined the effectiveness of the microbubbles of an echo contrast agent, dodecafluoropentane (DDFP) emulsion, to enhance low frequency ultrasound clot disruption in vitro and in vivo.

BACKGROUND

Ultrasound is reported to facilitate clot dissolution, and microbubbles could theoretically enhance ultrasound clot dissolution by augmenting cavitational effects.

METHODS

IN VITRO STUDIES

The disruption rate of fresh human clots by ultrasound (24 kHz, 2.9 W/cm2) was examined in saline and DDFP emulsion. In vivo studies: Using a rabbit iliofemoral thrombotic occlusion model, recanalization rate and histopathologic findings were compared among groups treated with DDFP emulsion alone, transcutaneous ultrasound (20 kHz, 1.5 W/cm2) alone and with DDFP emulsion and ultrasound combined.

RESULTS

The ultrasound clot disruption rate was significantly (p < 0.01) increased, from 72 +/- 18% (mean +/- SD) in saline to 98 +/- 4% in DDFP emulsion in 3 min in vitro. No vessel was recanalized by DDFP emulsion alone (0%), and only a single artery was patent after ultrasound treatment alone (9%). In contrast, 82% of iliofemoral arteries were angiographically recanalized after ultrasound treatment with DDFP emulsion. Histologically, the patent arteries had only minimal focal mural thrombus, with no evidence of vessel wall damage. However, substantial damage was observed in rabbit dermis and subcutaneous tissue.

CONCLUSIONS

1) DDFP emulsion, an echo contrast agent, significantly enhances the clot-disrupting effect of low frequency ultrasound in vitro and in an in vivo rabbit iliofemoral occlusion model. 2) This simple combination therapy has potential for clinical application in patients with thrombotic arterial occlusions.

Intravascular therapeutic ultrasound thrombolysis in acute myocardial infarctions

Catheter-delivered, therapeutic ultrasound was shown to effectively dissolve thrombus in vitro and in vivo. This first study in 14 patients with acute myocardial infarctions demonstrates that it is a safe and effective treatment alternative that deserves further clinical evaluation.

Physics of ultrasonic surgery using tissue fragmentation

The ultrasonic surgical aspirator employs a vibrating metal tip to fragment tissue and then aspirates the debris through the hollow center of the tip. The mechanism of interaction has been stated to be poorly understood, most likely related to cavitation, possibly acting in concert with other mechanical actions. The role of stroke, suction, frequency, tissue type, and tip area have been examined with regard to tissue fragmentation rate. Suction is shown to make a significant contribution to the interaction. Photographic and acoustic data from experiments in water and on a range of fresh pig tissues are used to investigate the fragmentation effect. A model for the primary mechanism for tissue fragmentation is presented. This involves the horn-tip impact and other mechanical forces, operating in combination with hydrodynamic forces applied to the tissue on the forward stroke in each cycle. No evidence of cavitation in tissue was observed.

Catheter-delivered ultrasound potentiates in vitro thrombolysis

PURPOSE

To develop a catheter-directed method to enhance urokinase- mediated thrombolysis with use of ultrasound.

MATERIALS AND METHODS

A prototype catheter was constructed by using a 9-F piezoelectric crystal capable of producing 640-kHz pulsed ultrasound energy. Clots formed in vitro from whole blood were trace-labeled with iodine-125 fibrinogen, and the release of radiolabeled fibrin degradation products was measured in the presence of urokinase, ultrasound, or a combination of urokinase and ultrasound.

RESULTS

By 30 minutes, clot lysis was more complete with urokinase plus ultrasound (78.7% +/- 5.3 [mean +/- SD]) than with ultrasound alone (19.3% +/- 10.0) or urokinase alone (47.9% +/- 10.0) (P < .001 for ultrasound and urokinase vs either alone). The time to 50% clot lysis was shortened by 46% on average with the application of urokinase and ultrasound compared with urokinase alone (P < .03).

CONCLUSIONS

Catheter-based ultrasound enhances enzymatic thrombolysis in vitro and may be a practical means to reduce the dose of enzyme and the time needed to achieve clot lysis in vivo.

Physics of ultrasonic surgery using tissue fragmentation: Part I

The ultrasonic surgical aspirator employs a vibrating metal tip to fragment tissue and then aspirates the debris through the hollow center of the tip. The mechanism of interaction has been stated to be poorly understood, most likely related to cavitation, possibly in concert with other mechanical actions. In Part I (of two parts), the role of stroke, suction, frequency, tissue type and tip area are examined with regard to tissue fragmentation rate. A tissue quantifier which can be used to relate the performance of the ultrasonic aspirator and a selected tissue is described. Suction is shown to make a significant contribution to the interaction. Thermal and tip load experiments are used to estimate the acoustic pressures and powers at the tip. In Part II, photographic and acoustic data from experiments in water and on a range of fresh pig tissues are used to further investigate the fragmentation effect.

Ultrasound accelerates transport of recombinant tissue plasminogen activator into clots

Fibrinolysis is accelerated in vitro in an ultrasound field, and externally applied high frequency ultrasound also accelerates thrombolysis in animal models. Although the mechanism of this effect is not known, ultrasound does not cause mechanical disruption of clots but rather accelerates enzymatic fibrinolysis. To determine if accelerated fibrinolysis could be related to increased transport of enzyme into clot, we have examined the effect of insonification on the distribution of plasminogen activator between clot and surrounding fluid in vitro. Plasma clots were overlayed with plasma containing 125I-radiolabeled, active-site-blocked recombinant tissue plasminogen activator (rt-PA) and incubated in the presence of 1-MHz ultrasound at 4 W/cm2 or in the absence of ultrasound. The rate of uptake of rt-PA was significantly faster in the presence of ultrasound, reaching 15.5 +/- 1.4% at 4 h compared to 8.2 +/- 1.0% in the absence of ultrasound (p < 0.0001). Similarly, ultrasound increased transport of enzyme from the clot into the surrounding fluid. To determine the effect of ultrasound on the spatial distribution of enzyme, plasma clots were overlayed with plasma containing radiolabeled rt-PA and incubated in the presence or absence of ultrasound. The clots were then snap-frozen, and the radioactivity in serial cryotome sections was determined. Exposure to ultrasound altered the rt-PA distribution, resulting in significantly deeper penetration of rt-PA into the clots. We conclude that exposure to ultrasound increases uptake of rt-PA into clots and also results in deeper penetration. These effects of ultrasound on enzyme transport may contribute to the accelerated fibrinolysis observed in an ultrasound field.

Acceleration of fibrinolysis by ultrasound in a rabbit ear model of small vessel injury

High frequency ultrasound has been previously shown to accelerate fibrinolysis in vitro at intensities that are potentially applicable for noninvasive administration clinically. To extend these findings in vivo, we have investigated the effects of ultrasound on fibrinolysis induced by streptokinase in a rabbit model of small vessel injury. Full thickness puncture wounds were made in rabbit ears with a scalpel blade. The rabbits were rested for 2-3 hours after cessation of bleeding to allow maturation of hemostatic plugs. Saline or streptokinase was then infused intravenously, and ultrasound was applied to some lesions at 1 MHz with a 50% duty cycle at 1 W/cm2 net intensity. Ear lesions in rabbits treated with saline showed no bleeding after 30 minutes whether they were exposed to ultrasound or not. Streptokinase alone induced bleeding after 19.7 +/- 5.5 minutes. Application of ultrasound significantly reduced the time to bleeding in streptokinase treated rabbits to 7.5 +/- 3.9 minutes (p < .002). The times to bleeding with "sham" ultrasound (18.8 +/- 5.6 minutes) and heating of the ear (18.0 +/- 5.6 minutes) during streptokinase administration were not significantly different compared to streptokinase alone. Histologic examination revealed that application of ultrasound resulted in a mild increase in interstitial edema and accumulation of polymorphonuclear leukocytes but did not cause vascular or other tissue damage. We conclude that the noninvasive, percutaneous application of ultrasound significantly accelerated streptokinase-induced fibrinolysis in this rabbit model of small vessel injury.

High intensity, low frequency catheter-delivered ultrasound dissolution of occlusive coronary artery thrombi: an in vitro and in vivo study

OBJECTIVES

This study assessed the efficacy of a new high intensity, low frequency therapeutic coronary ultrasound catheter for thrombus dissolution in vitro and in vivo in canine coronary arteries.

BACKGROUND

Therapeutic ultrasound has been shown to dissolve thrombi in vitro and in peripheral arteries in vivo. There have been no previous studies on in vivo coronary thrombus dissolution by ultrasound.

METHODS

In vitro, we exposed 1- to 4-h old human blood clots for 3 min to pulsed-wave ultrasound. Clot dissolution under various conditions was evaluated. In vivo occlusive coronary thrombi were induced in 18 dogs.

RESULTS

In vitro irrigation alone (10 ml/min of normal saline solution) and ultrasound alone each contributed to a reduction of clot weight by 47.1 +/- 11.4 mg and 84.6 +/- 25.6 mg, respectively, after 3 min (p < 0.001). Ultrasound plus irrigation resulted in a reduction of clot weight by 216.5 +/- 31.5 mg after 3 min (p < 0.001). The magnitude of clot dissolution was considerably amplified when ultrasound energy was combined with irrigation, probably because of cavitational effects. In vivo, in three dogs mechanical passage of the unactivated probe failed to recanalize the artery, and the arteries remained thrombotically occluded. After passage of the activated ultrasound probe, angiography revealed widely patent coronary arteries in 13 of 15 dogs and partial recanalization with filling defects indicative of residual thrombus in 2 of 15 dogs. Three of 15 coronary arteries were histologically free of residual thrombi. Mural thrombi extending to < or = 10% of the vessel circumference were seen in 10 of 15 dogs. Residual thrombi > or = 50% of the vessel circumference were found in two cases. There was no histologic evidence of ultrasound-mediated vessel damage.

CONCLUSIONS

Catheter-delivered therapeutic ultrasound effectively dissolves clots in vitro and in canine coronary arteries in vivo. Thus, therapeutic catheter-delivered ultrasound has the potential to serve as an adjunct or alternative treatment for thrombus-mediated coronary ischemic syndromes or myocardial infarction.

Ultrasound accelerates urokinase-induced thrombolysis and reperfusion

We have shown that ultrasound accelerates TPA-induced thrombolysis in vitro as assessed by release of labeled fibrinogen from radioactive labeled clots. Others have shown that ultrasound shortens the time to recanalization of TPA treated thrombi in animal models. The aim of this study was to test the hypothesis that ultrasound enhances thrombolysis and reperfusion by using urokinase in an in vitro flow system. An in vitro flow system of a branching tubing circuit was developed. Flow in one branch was obstructed by a thrombus. Five control clots were exposed to continuous wave ultrasound at a frequency of 1 MHz and intensity of 2.5 W/cm2 only without any thrombolytic agent (group 1). Twenty clots were exposed to a bolus of 80,000 U of urokinase and randomized to either ultrasound exposure (group 2) or to urokinase only without ultrasound (group 3). Flow distal to the clot and the rate of release of radiolabeled fibrin were used as indexes of reperfusion and thrombolysis, respectively. Exposure to ultrasound significantly accelerated urokinase-mediated reperfusion, with 40.6% +/- 11.8% of maximal flow in group 2 versus 1.3% +/- 0.7% in group 3, p < 0.0015 after 25 min. The maximal difference in flow between groups 2 and 3 was achieved at 40 minutes (67.4% +/- 11.1% vs 13.1% +/- 5.6%, p < 0.0009). Thrombolysis was significantly higher after 25 minutes of ultrasound exposure (24.1% +/- 4.6% in the ultrasound-treated group vs 9.7% +/- 3.5% in group 3, p < 0.013). The maximal difference in thrombolysis between groups 2 and 3 was 60 minutes. (52.5% +/- 5.1% vs 18.7% +/- 6.2%, p < 0.00015).(ABSTRACT TRUNCATED AT 250 WORDS)

Use of therapeutic ultrasound in percutaneous coronary angioplasty. Experimental in vitro studies and initial clinical experience

BACKGROUND

Previous studies have shown the feasibility of peripheral arterial ultrasound angioplasty.

METHODS AND RESULTS

In this report, we describe the use of percutaneous therapeutic ultrasound for coronary angioplasty. In vitro, 11 postmortem, atherosclerotically occluded coronary arteries were obtained to assess catheter-delivered ultrasound for arterial recanalization as well as for assessment of the size of particulate debris. Clinically, coronary ultrasound angioplasty was performed in 19 patients (mean age, 56 years) to assess safety and feasibility for the treatment of obstructive coronary atherosclerosis. Three patients with unstable angina and 16 with exercise-induced myocardial ischemia were treated with a prototype 4.6F coronary catheter ultrasound ablation device with a 1.7-mm diameter ball tip. The ultrasound coronary catheter delivered ultrasound energy at 19.5 kHz, with a power output of 16 to 20 W at the transducer. Energy is delivered in a pulsed mode with a 50% duty cycle of 30 milliseconds. Patients were treated for a mean of 493 seconds (range, 130 to 890) with intracoronary ultrasound ablation. All lesions were treated with adjunctive balloon angioplasty. All 11 postmortem coronary occlusions were recanalized, and 99% of the particulates generated were < 10 microns in diameter. We found that after ultrasound, mean (+/- SD) coronary arterial stenosis fell from 80 +/- 12% to 60 +/- 18% (P < .001) and to 26 +/- 11% (P < .001) after adjunctive balloon angioplasty. Mean pressures required to achieve full balloon inflation were 2.7 atm (range, 1 to 5.5) with a median of 3.0-mm balloon size (2.5 to 3.5). No ultrasound-related complications were identified.

CONCLUSIONS

Intracoronary ultrasound plaque ablation appears to be safe. Our findings suggest that catheter-delivered high-intensity, low-frequency ultrasound may be useful for lesion debulking and enhancing arterial distensibility, allowing balloon dilation at relatively low pressures.

Feasibility of recanalization of human coronary arteries using high-intensity ultrasound

To investigate the feasibility of ultrasonic recanalization of obstructed human coronary arteries in vitro, high-intensity ultrasound was applied to 16 coronary arteries obtained at autopsy, using a prototype instrument enabling insonification through a catheter tip. It was a 119 cm long, 0.95 mm thick wire in an 8Fr catheter connected to an external ultrasonic transformer and power generator. A 5 MHz phased-array 2-dimensional echocardiography instrument was used to determine minimal luminal diameter and percent diameter narrowing before and after ultrasound application. The ultrasonic energy was delivered at 21.5 kHz and with a 52 +/- 19 micrometer average amplitude of tip displacement. The mean percent luminal diameter narrowing, flow rate and mean pressure gradient before ultrasound exposure were 74 +/- 11%, 97 +/- 61 ml/min, and 92 +/- 18 mm Hg, respectively. After recanalization, the mean percent luminal diameter narrowing decreased to 45 +/- 17% (p < 0.001), the mean flow rate increased to 84 +/- 92 ml/min (p < 0.001), and the mean pressure gradient was reduced to 45 +/- 24 mm Hg (p < 0.001). Of the debris particles, 95% had a diameter < 9 microns (range 5 to 12). Arterial perforation occurred in 5 of 16 arteries (31%) and all 5 occurred due to stiff wire manipulation and without ultrasound application. Mechanical fracture of the wire occurred in 8 cases (50%). No signs of thermal injury were found on histology. Thus, ultrasonic recanalization of human coronary arteries in vitro is feasible. It may reduce obstruction and improve blood flow. Debris sizes are sufficiently small to minimize the hazard of peripheral embolization.