Treatment of Microvascular Micro-embolization Using Microbubbles and Long-Tone-Burst Ultrasound: An in Vivo Study

Abstract

Despite epicardial coronary artery reperfusion by percutaneous coronary intervention, distal micro-embolization into the coronary microcirculation limits myocardial salvage during acute myocardial infarction. Thrombolysis using ultrasound and microbubbles (sonothrombolysis) is an approach that induces microbubble oscillations to cause clot disruption and restore perfusion. We sought to determine whether this technique could restore impaired tissue perfusion caused by thrombotic microvascular obstruction. In 16 rats, an imaging transducer was placed on the biceps femoris muscle, perpendicular to a single-element 1-MHz treatment transducer. Ultrasound contrast perfusion imaging was performed at baseline and after micro-embolization. Therapeutic ultrasound (5000 cycles, pulse repetition frequency = 5 0.33 Hz, 1.5 MPa) was delivered to nine rats for two 10-min sessions during intra-arterial infusion of lipid-encapsulated microbubbles; seven control rats received no ultrasound–microbubble therapy. Ultrasound contrast perfusion imaging was repeated after each treatment or control period, and microvascular volume was measured as peak video intensity. There was a 90% decrease in video intensity after micro-embolization (from 8.6 ± 4.8 to 0.7 ± 0.8 dB, p < 0.01). The first and second ultrasound–microbubble sessions were respectively followed by video intensity increases of 5.8 ± 5.1 and 8.7 ± 5.7 dB (p < 0.01, compared with micro-embolization). The first and second control sessions, respectively, resulted in no significant increase in video intensity (2.4 ± 2.3 and 3.6 ± 4.9) compared with micro-embolization (0.6 ± 0.7 dB). We have developed an in vivo model that simulates the distal thrombotic microvascular obstruction that occurs after primary percutaneous coronary intervention. Long-pulse-length ultrasound with microbubbles has a therapeutic effect on microvascular perfusion and may be a valuable adjunct to reperfusion therapy for acute myocardial infarction.

Microbubble-Mediated Sonothrombolysis Improves Outcome After Thrombotic Microembolism-Induced Acute Ischemic Stroke

“…Twentyfour hours after the rats were embolized with the white or red microthrombi, MRI and TTC-stained sections showed that the infarcts were multiple with most distributed in the cortex and few in the subcortical regions, in consistent with histological findings. The cerebral infarct volumes were smaller in the US+MB group than in the CON group (white microthrombi: 9.52±2.99% versus 26 Figures 5A and 5B and 6A and 6B). After treatment of rats with white microthrombi, the infarct volumes were larger in the r-tPA group than in the US+MB group (17.12±2.98% versus 9.52±2.99% for MRI, 16.08±3.20% versus 8.46±2.92% for TTC; P<0.01), whereas they were similar in the US+MB and US+MB+r-tPA groups.…”

“…Only one study has evaluated the efficacy of the treatment for dissolving erythrocyterich microthrombi in a rat hindlimb model. 26 Animal studies have not addressed the utility of US+MB treatment for dissolving platelet-rich microthrombi, which account for the largest proportion of thromboemboli in stroke. 9 Furthermore, neither platelet-nor erythrocyte-rich microthrombi have been used to evaluate the stroke outcomes after the treatment.…”

Effect of Thrombus Composition and Viscosity on Sonoreperfusion Efficacy in a Model of Micro-Vascular Obstruction

“…Using ultra-high-speed imaging, our group has demonstrated that inertial cavitation causes pitting on the surface of a thrombus through direct mechanical effects (Chen et al 2014). We have also demonstrated successful reperfusion with SRP therapy in an in-vitro model of arteriolar MVO (Leeman et al 2012) and in a rodent hind limb model of MVO (Pacella et al 2015). SRP has also been shown to open thrombus occluded venous catheters (Kutty et al 2010), and coronary arteries in a porcine model of acute myocardial infarction (Xie et al 2009).…”

“…Thrombi were fragmented by shaking (4530±100 oscillations/min) in a dental amalgamator (Lantheus Medical Imaging, Inc., Mississauga, Ontario, Canada). Microthrombi were then filtered through a 200 μm nylon mesh (BioDesign Inc., Carmel, NY, USA) to remove remaining large thrombi and produce a size distribution ranging from 10 to 200 μm, the size range responsible for microembolization and subsequent MVO in vivo (Saber et al 1993; Schwartz et al 2009; Pacella et al 2015). …”

Fluid Viscosity Affects the Fragmentation and Inertial Cavitation Threshold of Lipid-Encapsulated Microbubbles

“…In addition to diagnostic applications, ultrasound stimulated microbubbles have been recently employed to elicit therapeutic benefit. Sonothrombolysis, namely the use of ultrasound and microbubbles to non-invasively dissolve blood clots and restore flow, has been shown to be a feasible strategy both in vitro (Acconcia et al 2014; Leeman et al 2012; Tachibana and Tachibana 1995) and in vivo (Birnbaum et al 1998; Culp et al 2011; Pacella et al 2015), and is also the first therapeutic application of microbubbles to enter clinical trials (Molina et al 2006). Ultrasound and microbubbles have also been employed as an emerging strategy for enhanced drug and gene delivery for the treatment of cancer and vascular disease.…”