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

Abstract

Distal embolization of microthrombi during stenting for myocardial infarction (MI) causes microvascular obstruction (MVO). We have previously shown that sonoreperfusion (SRP), a microbubble (MB)-mediated ultrasonic (US) therapy, resolves MVO from venous microthrombi in vitro in saline. However, blood is more viscous than saline and arterial thrombi that embolize during stenting are mechanically distinct from venous clot. Therefore, we tested the hypothesis that MVO created with arterial microthrombi are more resistant to SRP therapy compared with venous microthrombi and higher viscosity further increases the US requirement for effective SRP in an in vitro model of MVO. Lipid MB suspended in plasma with adjusted viscosity (1.1 or 4.0 cP) were passed through tubing bearing a mesh with 40 μm pores to simulate a microvascular cross-section; upstream pressure reflected thrombus burden. To simulate MVO, the mesh was occluded with either arterial or venous microthrombi to increase upstream pressure to 40±5 mmHg. Therapeutic long-tone-burst US was delivered to the occluded area for 20 min. MB activity was recorded with a passive cavitation detector (PCD). MVO caused by arterial microthrombi at either blood or plasma viscosity resulted in less effective SRP therapy, compared to venous thrombi. Higher viscosity further reduced the effectiveness of SRP therapy. PCD showed a decrease in inertial cavitation when viscosity was increased while stable cavitation was affected in a more complex manner. Overall, these data suggest that arterial thrombi may require higher acoustic pressure US than venous thrombi to achieve similar SRP efficacy, increased viscosity decreases SRP efficacy, and both inertial and stable cavitation are implicated in observed SRP efficacy.

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

“…This increased viscosity results in damped MB oscillations compared to PBS, as confirmed in a recent study using high speed imaging (Helfield et al 2016). Reduced SRP efficacy was also found when plasma viscosity was adjusted to mean blood viscosity of 4 cP for venous and arterial type microthrombi in the same model of MVO (Black et al 2016). Our results also support that the addition of tPA was necessary to achieve SRP efficacy similar to that obtained in PBS perfusate as our control experiments without US, MB and without the addition of tPA suggested that endogenous tPA was insufficient to cause effective SRP.…”

“…The energy in the peak at the ultraharmonic band (3.48–3.52 MHz) above the broadband signal, integrated over the whole tone-burst, was defined as the stable cavitation dose (SCD) (Chen et al 2003). The bandwidth chosen for SCD corresponded to the −6 dB bandwidth in the fundamental peak (Black et al 2016; Datta et al 2008). …”

Inertial Cavitation Ultrasound with Microbubbles Improves Reperfusion Efficacy When Combined with Tissue Plasminogen Activator in an In Vitro Model of Microvascular Obstruction

“…(Acconcia et al 2014) demonstrated that in fibrin only clots, there was minimal bubble penetration through fibrin at 0.2 MPa, but significant penetration at 0.4 MPa and above. Work from our group further examined sonoreperfusion efficacy without the use of fibrinolytics as a function of clot architecture from either arterial or venous sources (Black et al 2016) and determined that higher acoustic pressure regimes (e.g. IC regime) may be necessary for effective reperfusion with arterial microthrombi.…”

“…However, known structural differences between venous and arterial type microthrombi, in particular differences in fibrin content, may influence the efficacy of sonoreperfusion. Specifically, thin and highly branched fibrin networks as seen may be observed in arterial type thrombi are typically less permeable and susceptible to lysis (Undas and Ariens 2011; Black et al 2016). Indeed, differences in sonoreperfusion between thrombus origins have been recently reported possibly due to the tighter fibrin network and greater fibrin density of arterial microthrombi when compared to that of venous microthrombi (Undas and Ariens 2011; Black et al 2016).…”

“…Specifically, thin and highly branched fibrin networks as seen may be observed in arterial type thrombi are typically less permeable and susceptible to lysis (Undas and Ariens 2011; Black et al 2016). Indeed, differences in sonoreperfusion between thrombus origins have been recently reported possibly due to the tighter fibrin network and greater fibrin density of arterial microthrombi when compared to that of venous microthrombi (Undas and Ariens 2011; Black et al 2016). However, the efficacy of optimal sonoreperfusion parameters, whether SC or IC regime, in the presence of fibrinolytic agents may further depend on thrombus type, which has not be well studied.…”