Diagnostic Ultrasound High Mechanical Index Impulses Restore Microvascular Flow in Peripheral Arterial Thromboembolism

Ultrasound-Mediated Microbubble Cavitation Transiently Reverses Acute Hindlimb Tissue Ischemia through Augmentation of Microcirculation Perfusion via the eNOS/NO Pathway

Ultrasound-mediated microbubble cavitation improves perfusion in chronic limb and myocardial ischemia. The purpose of this study was to determine the effects of ultrasound-mediated microbubble cavitation in acute limb ischemia and investigate the mechanism of action. The animal with acute hindlimb ischemia was established using male Sprague-Dawley rats. The rats were randomly divided into three groups: intermittent high-mechanical-index ultrasound pulses combined with microbubbles (ultrasound [US] + MB group), US alone (US group) and MB alone (MB group). Both hindlimbs were treated for 10 min. Contrast ultrasound perfusion imaging of both hindlimbs was performed immediately and 5, 10, 15, 20 and 25 min after treatment. The role of the nitric oxide (NO) pathway in increasing blood flow in acutely ischemic tissue was evaluated by inhibiting endothelial nitric oxide synthase (eNOS) with N^ω-nitro-L-arginine methyl ester hydrochloride (L-NAME). In the US + MB group, microvascular blood volume and microvascular blood flow of the ischemic hindlimb were significantly increased after treatment (both p values <0.05), while the microvascular flux rate (β) increased, but not significantly (p > 0.05). The increases were observed immediately after treatment, and had dissipated by 25 min. Changes in the US and MB groups were minimal. Inhibitory studies indicated cavitation increased phospho-eNOS concentration in ischemic hindlimb muscle tissue, and the increase was significantly inhibited by L-NAME (p < 0.05). Ultrasound-mediated microbubble cavitation transiently increases local perfusion in acutely ischemic tissue, mainly by improving microcirculatory perfusion. The eNOS/NO signaling pathway appears to be an important mediator of the effect.

Diagnostic Ultrasound and Microbubbles Treatment Improves Outcomes of Coronary No-Reflow in Canine Models by Sonothrombolysis

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Objectives:

Effective treatment for microvascular thrombosis-induced coronary no-reflow remains an unmet clinical need. This study sought to evaluate whether diagnostic ultrasound and microbubbles treatment could improve outcomes of coronary no-reflow by dissolving platelet- and erythrocyte-rich microthrombi.

Design:

Randomized controlled laboratory investigation.

Setting:

Research laboratory.

Subjects:

Mongrel dogs.

Interventions:

Coronary no-reflow models induced by platelet- or erythrocyte-rich microthrombi were established and randomly assigned to control, ultrasound, recombinant tissue-type plasminogen activator, ultrasound + microbubbles, or ultrasound + microbubbles + recombinant tissue-type plasminogen activator group. All treatments lasted for 30 minutes.

Measurements and Main Results:

Percentage of microemboli-obstructed coronary arterioles was lower in ultrasound + microbubbles group than that in control group for platelet- (> 50% obstruction: 10.20% ± 3.56% vs 31.80% ± 3.96%; < 50% obstruction: 14.80% ± 4.15% vs 28.20% ± 3.56%) and erythrocyte-rich microthrombi (> 50% obstruction: 8.20% ± 3.11% vs 30.60% ± 4.83%; < 50% obstruction: 12.80% ± 4.15% vs 25.80% ± 3.70%) (p < 0.001). Percentage change of myocardial blood flow in left anterior descending artery-dominated region, left ventricular ejection fraction, fractional shortening, and ST-segment resolution were higher, whereas infarcted area, troponin I, and creatine kinase MB isoenzyme were lower in ultrasound + microbubbles group than that in control group for both types of microthrombi (p < 0.001). Percentage change of myocardial blood flow, ejection fraction, fractional shortening, and ST-segment resolution were higher, whereas infarcted area, troponin I, and creatine kinase MB isoenzyme were lower in ultrasound + microbubbles and ultrasound + microbubbles + recombinant tissue-type plasminogen activator groups than that in recombinant tissue-type plasminogen activator group for platelet-rich microthrombi (p < 0.05).

Conclusions:

Ultrasound + microbubbles treatment could dissolve platelet- and erythrocyte-rich microthrombi, thereby improving outcomes of coronary no-reflow, making it a promising supplement to current reperfusion therapy for acute ST-segment elevation myocardial infarction.

Augmentation of Tissue Perfusion in Patients With Peripheral Artery Disease Using Microbubble Cavitation

OBJECTIVES

The authors investigated ideal acoustic conditions on a clinical scanner custom-programmed for ultrasound (US) cavitation-mediated flow augmentation in preclinical models. We then applied these conditions in a first-in-human study to test the hypothesis that contrast US can increase limb perfusion in normal subjects and patients with peripheral artery disease (PAD).

BACKGROUND

US-induced cavitation of microbubble contrast agents augments tissue perfusion by convective shear and secondary purinergic signaling that mediates release of endogenous vasodilators.

METHODS

In mice, unilateral exposure of the proximal hindlimb to therapeutic US (1.3 MHz, mechanical index 1.3) was performed for 10 min after intravenous injection of lipid microbubbles. US varied according to line density (17, 37, 65 lines) and pulse duration. Microvascular perfusion was evaluated by US perfusion imaging, and in vivo adenosine triphosphate (ATP) release was assessed using in vivo optical imaging. Optimal parameters were then used in healthy volunteers and patients with PAD where calf US alone or in combination with intravenous microbubble contrast infusion was performed for 10 min.

RESULTS

In mice, flow was augmented in the US-exposed limb for all acoustic conditions. Only at the lowest line density was there a stepwise increase in perfusion for longer (40-cycle) versus shorter (5-cycle) pulse duration. For higher line densities, blood flow consistently increased by 3-fold to 4-fold in the US-exposed limb irrespective of pulse duration. High line density and long pulse duration resulted in the greatest release of ATP in the cavitation zone. Application of these optimized conditions in humans together with intravenous contrast increased calf muscle blood flow by >2-fold in both healthy subjects and patients with PAD, whereas US alone had no effect.

CONCLUSIONS

US of microbubbles when using optimized acoustic environments can increase perfusion in limb skeletal muscle, raising the possibility of a therapy for patients with PAD. (Augmentation of Limb Perfusion With Contrast Ultrasound; NCT03195556).

The Thrombolytic Effect of Diagnostic Ultrasound-Induced Microbubble Cavitation in Acute Carotid Thromboembolism

BACKGROUND

Acute ischemic stroke is often due to thromboembolism forming over ruptured atherosclerotic plaque in the carotid artery (CA). The presence of intraluminal CA thrombus is associated with a high risk of thromboembolic cerebral ischemic events. The cavitation induced by diagnostic ultrasound high mechanical index (MI) impulses applied locally during a commercially available intravenous microbubble infusion has dissolved intravascular thrombi, especially when using longer pulse durations. The beneficial effects of this in acute carotid thromboembolism is not known.

MATERIALS AND METHODS

An oversized balloon injury was created in the distal extracranial common CA of 38 porcine carotid arteries. After this, a 70% to 80% stenosis was created in the mid common CA proximal to the injury site using partial balloon inflation. Acute thrombotic CA occlusions were created just distal to the balloon catheter by injecting fresh autologous arterial thrombi. After angiographic documentation of occlusion, the common carotid thrombosis was treated with either diagnostic low MI imaging alone (0.2 MI; Philips S5-1) applied through a tissue mimicking phantom (TMP) or intermittent diagnostic high MI stable cavitation (SC)-inducing impulses with a longer pulse duration (0.8 MI; 20 microseconds' pulse duration) or inertial cavitation (IC) impulses (1.2 MI; 20 microseconds' pulse duration). All treatment times were for 30 minutes. Intravenous ultrasound contrast (2% Definity; Lantheus Medical) was infused during the treatment period. Angiographic recanalization in 4 intracranial and extracranial vessels downstream from the CA occlusion (auricular, ascending pharyngeal, buccinator, and maxillary) was assessed with both magnetic resonance 3-dimensional time-of-flight and phase contrast angiography. All magnetic resonance images were interpreted by an independent neuroradiologist using the thrombolysis in cerebral infarction (TICI) scoring system.

RESULTS

By phase contrast angiography, at least mild recanalization (TICI 2a or higher) was seen in 64% of downstream vessels treated with SC impulses compared with 33% of IC treated and 29% of low MI alone treated downstream vessels (P = 0.001), whereas moderate or complete recanalization (TICI 2b or higher) was seen in 39% of SC treated vessels compared with 10% IC treated and 21% of low MI alone treated vessels (P = 0.001).

CONCLUSIONS

High MI 20-microsecond pulse duration impulses during a commercial microbubble infusion can be used to recanalize acutely thrombosed carotid arteries and restore downstream flow without anticoagulants. However, this effect is only seen with SC-inducing impulses and not at higher mechanical indices, when a paradoxical reversal of the thrombolytic effect is observed. Diagnostic ultrasound-induced SC can be a nonsurgical method of dissolving CA thrombi and preventing thromboembolization.

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.

The Role of Nitric Oxide during Sonoreperfusion of Microvascular Obstruction

Rationale: Microembolization during PCI for acute myocardial infarction can cause microvascular obstruction (MVO). MVO severely limits the success of reperfusion therapies, is associated with additional myonecrosis, and is linked to worse prognosis, including death. We have shown, both in in vitro and in vivo models, that ultrasound (US) and microbubble (MB) therapy (termed “sonoreperfusion” or “SRP”) is a theranostic approach that relieves MVO and restores perfusion, but the underlying mechanisms remain to be established.

Objective: In this study, we investigated the role of nitric oxide (NO) during SRP.

Methods and results: We first demonstrated in plated cells that US-stimulated MB oscillations induced a 6-fold increase in endothelial nitric oxide synthase (eNOS) phosphorylation in vitro. We then monitored the kinetics of intramuscular NO and perfusion flow rate responses following 2-min of SRP therapy in the rat hindlimb muscle, with and without blockade of eNOS with LNAME. Following SRP, we found that starting at 6 minutes, intramuscular NO increased significantly over 30 min and was higher than baseline after 13 min. Concomitant contrast enhanced burst reperfusion imaging confirmed that there was a marked increase in perfusion flow rate at 6 and 10 min post SRP compared to baseline (>2.5 fold). The increases in intramuscular NO and perfusion rate were blunted with LNAME. Finally, we tested the hypothesis that NO plays a role in SRP by assessing reperfusion efficacy in a previously described rat hindlimb model of MVO during blockade of eNOS. After US treatment 1, microvascular blood volume was restored to baseline in the MB+US group, but remained low in the LNAME group. Perfusion rates increased in the MB+US group after US treatment 2 but not in the MB+US+LNAME group.

Conclusions: These data strongly support that MB oscillations can activate the eNOS pathway leading to increased blood perfusion and that NO plays a significant role in SRP efficacy.

Intravascular forward-looking ultrasound transducers for microbubble-mediated sonothrombolysis

Effective removal or dissolution of large blood clots remains a challenge in clinical treatment of acute thrombo-occlusive diseases. Here we report the development of an intravascular microbubble-mediated sonothrombolysis device for improving thrombolytic rate and thus minimizing the required dose of thrombolytic drugs. We hypothesize that a sub-megahertz, forward-looking ultrasound transducer with an integrated microbubble injection tube is more advantageous for efficient thrombolysis by enhancing cavitation-induced microstreaming than the conventional high-frequency, side-looking, catheter-mounted transducers. We developed custom miniaturized transducers and demonstrated that these transducers are able to generate sufficient pressure to induce cavitation of lipid-shelled microbubble contrast agents. Our technology demonstrates a thrombolysis rate of 0.7 ± 0.15 percent mass loss/min in vitro without any use of thrombolytic drugs.

Sonoreperfusion Therapy Kinetics in Whole Blood Using Ultrasound, Microbubbles and Tissue Plasminogen Activator

Coronary intervention for myocardial infarction often results in microvascular embolization of thrombus. Sonoreperfusion therapy (SRP) using ultrasound and microbubbles restored perfusion in our in vitro flow model of microvascular obstruction. In this study, we assessed SRP efficacy using whole blood as the perfusate with and without tissue plasminogen activator (tPA). In a phantom vessel bearing a 40-μm-pore mesh to simulate the microvasculature, microthrombi were injected to cause microvascular obstruction and were treated using SRP. Without tPA, the lytic rate increased from 2.6 ± 1.5 mmHg/min with 1000-cycle pulses to 7.3 ± 3.2 mmHg/min with 5000-cycle ultrasound pulses (p < 0.01). The lytic index was similar for tPA-only ([2.0 ± 0.5] × 10^-3 mmHg^-1 min^-1) and 5000 cycles without tPA ([2.3 ± 0.5] × 10^-3 mmHg^-1 min^-1) (p = 0.5) but increased ([3.6 ± 0.8] × 10^-3 mmHg^-1 min^-1) with tPA in conjunction with 5000-cycles ultrasound (p < 0.01). In conclusion, SRP restored microvascular perfusion in whole blood, SRP lytic rate in experiments without tPA increased with ultrasound pulse length and efficacy increased with the addition of tPA.