Treatment of Limb Ischemia with Conducted Effects of Catheter-Based Endovascular Ultrasound

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

BACKGROUND

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.

METHODS

In mice, lipid-stabilized decafluorobutane microbubbles (2 × 108) were administered intravenously while exposing a proximal hind limb to ultrasound (1.3 MHz, 1.3 mechanical index, pulsing interval 5 seconds) 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 (≈15 weeks) 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 in vivo biophotonic imaging were used to assess limb nitric oxide (NO) and adenosine triphosphosphate concentrations, respectively.

RESULTS

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 (15 weeks), 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.

CONCLUSIONS

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.

Augmentation of Pulmonary Perfusion by Conducted Effects of a Pulmonary Artery Ultrasound Catheter

Ultrasound (US) generated by catheters used clinically for US-facilitated thrombolysis can release shear-dependent vasodilators from endothelial and red blood cells. We hypothesized that catheter-based US in the pulmonary artery (PA) decreases downstream vascular resistance and increases pulmonary blood flow. In rhesus macaques, a U.S. Food and Drug Administration-approved multi-element US catheter was placed in a pulmonary artery. Comprehensive echocardiography was performed (i) at baseline, (ii) during hypoxemia (12% FIO2) to increase pulmonary vascular resistance (PVR) and (c) 15 min after initiating US during hypoxemia. Reduced FIO2 produced intended reductions in oxygen saturation (69 ± 3%) and PaO2 (34 ± 5 mm Hg), yet on echocardiography, hypoxemia did not create the intended model, with only modest hypoxia-related increases in PA systolic pressure (24 ± 4 to 28 ± 4 mm Hg, p = 0.05) and no significant change in PVR or multiparametric right ventricular (RV) function. Although US did not further change total PVR, on 99mTc-macroalbumin aggregate single-photon-emission computed tomography imaging, lung perfusion was significantly higher in the lung ipsilateral to the US catheter versus the contralateral control lung (133 ± 48 cpm vs. 103 ± 43 × 103 cpm, p = 0.01). We conclude that PA catheter-based US increases regional lung perfusion, most likely from vasodilators that are conducted downstream.