Influence of Atherosclerotic Risk Factors on the Effectiveness of Therapeutic Ultrasound Cavitation for Flow Augmentation

Abstract

Shear created by inertial cavitation of microbubbles by ultrasound augments limb and myocardial perfusion and can reverse tissue ischemia. Our aim was to determine whether this therapeutic bioeffect is attenuated by atherosclerotic risk factors that are known to impair shear-mediated vasodilation and adversely affect microvascular reactivity. In mice, lipid-stabilized decafluorobutane microbubbles (2×108) were administered intravenously while exposing a proximal hind limb to ultrasound (1.3MHz, 1.3 mechanical index, pulsing interval 5seconds) for 10 minutes. Murine strains included wild-type mice and severely hyperlipidemic mice at 15, 35, or 52 weeks of age as a model of aging and elevated cholesterol, and obese db/db mice (≈15weeks) with severe insulin resistance. Quantitative contrast-enhanced ultrasound perfusion imaging was performed to assess microvascular perfusion in the control and ultrasound-exposed limb. An in situ electrochemical probe and invivo biophotonic imaging were used to assess limb nitric oxide (NO) and adenosine triphosphosphate concentrations, respectively. Microvascular perfusion was significantly increased by several fold in the cavitation-exposed limb versus control limb for all murine strains and ages (P<.001). In wild-type and hyperlipidemic mice, hyperemia from cavitation was attenuated in the 2 older age groups (P<.01). In young mice (15weeks), perfusion in cavitation-exposed muscle was less in both the hyperlipidemic mice and the obese db/db mice compared with corresponding wild-type mice. Using young hyperlipidemic mice as a model for flow impairment, limb NO production after cavitation was reduced but adenosine triphosphosphate production was unaltered when compared with age-matched wild-type mice. In mice, ultrasound cavitation of microbubbles increases limb perfusion by several fold even in the presence of traditional atherosclerotic risk factors. However, older age, hyperlipidemia, and insulin resistance modestly attenuate the degree of flow augmentation, which could impact the degree of flow response in current clinical trials in patients with critical limb ischemia.