The Potential of Acoustic Energy
A new technology holds the promise of providing greater benefits and utility for treating endovascular occlusive disease.
New approaches for treating endovascular occlusive diseases, including hemodialysis access thrombosis, lower-limb arterial occlusions, and deep vein thrombosis (DVT), are constantly being developed. Today, a new technology—one that uses acoustic energy—holds the promise of providing far greater benefits and broader clinical utility than current methods of treatment.
USES OF ACOUSTIC ENERGY
Acoustic energy uses high-frequency sound waves to treat diseased tissue or endovascular occlusions. The therapeutic properties of acoustic energy have been under investigation for decades. Scientists have long hypothesized that the acoustic property of bioselectivity would enable the targeted treatment of specific tissues while avoiding damage to nontargeted tissues, and that this property could be applied to endovascular applications.1
Research into the use of acoustic energy to resolve thrombus has demonstrated the value of the approach, namely, that targeting thrombus resolution with acoustic energy was possible.2 However, early attempts to utilize acoustic energy in the vasculature were limited by device design and by a reliance on conventional approaches to acoustic energy delivery. These early systems used relatively inflexible, large-diameter catheters and high power levels that tended to create heat.
Although acoustic energy was being successfully applied in other clinical areas, such as cataract treatment, dental scaling, and lithotripsy,3 little progress was made in endovascular applications. The fundamental principles of standing wave formation on small-diameter wires had yet to be fully characterized or applied.
A NEW APPROACH
A new technology that may enable the endovascular delivery of therapeutic acoustic energy is now under development.
The Resolution System (OmniSonics Medical Technologies, Wilmington, MA) operates through the creation of a standing transverse wave on the distal end of an extremely small-diameter (.015-inch or smaller) wire. The transverse wave has the ability to create acoustic energy circumferentially around the diameter of the wire, using very low-power energy (approximately 2 watts). When this acoustic energy encounters thrombotic material, the thrombus is resolved by the sound waves into particulate approximately the size of red blood cells (approximately 10 µm) (Figure 1).4
The device has gone through extensive in vitro and in vivo testing and is currently the subject of a FDA clinical trial for the treatment of thrombosed synthetic hemodialysis access grafts. The Resolution System bears the CE Mark and is approved for sale in Europe and Australia for the treatment of lower-extremity arterial thrombosis and thrombosed hemodialysis access sites (Figure 2).
For any new device to be fully accepted into interventional radiology clinical practice, it must meet several important criteria.
First, the treatment must meet appropriate safety criteria. In endovascular treatments, both damage to the healthy tissue and distal embolization are significant patient risks to avoid. The fact that OmniSonics is using acoustic energy with the property of tissue selectivity addresses the first risk: no damage to vessel walls. OmniSonics’ preclinical work and international clinical work have demonstrated that the low power levels the system uses do not pose a risk of damage to the vessel walls.5
Regarding particulate, the obvious safety goal is to avoid creating emboli and the attendant ischemic tissue damage. The Resolution System has demonstrated, through in vitro research, that it reduces thrombotic materials to particulate approximately the size of red blood cells.4 This suggests superiority over mechanical thrombectomy approaches that sometimes generate larger thromboemboli.
Hemolysis is another undesirable side effect of some percutaneous mechanical thrombectomy devices. Evaluation of the OmniSonics technology in a canine model showed a return to baseline plasma-free hemoglobin levels in 24 hours—faster than with other thrombectomy systems.6
Second, the treatment must be at least as effective as currently used approaches. It must resolve occlusive materials rapidly and make a clinically significant contribution to improving the patient’s condition. In preliminary clinical work with the OmniSonics device, synthetic hemodialysis access grafts were successfully declotted in all patients, even those with a heavy thrombus burden, in less than 3 minutes (Siskin G, oral communication, June 2002). Ongoing international studies also suggest the ability of the Resolution System to be effective in the treatment of lower-extremity chronic total occlusions.
Last, the device must be easy to use in all clinical situations. The OmniSonics approach offers significant advantages over present products because it is wire-based. Virtually no instrument is more universally familiar to interventional radiologists than the guidewires we use on a daily basis.
The Resolution 360 Therapeutic Wire (Figure 3) is a 95-cm-long, flexible, titanium wire with a diameter that tapers from .025 inch to .015 inch. The last 20 cm of the wire is ‘active,’ meaning that the acoustic energy is circumferentially directed around the diameter of this section of the wire. The company plans to introduce a family of wires with varying lengths, handling characteristics, and shapes for a wide variety of procedure-specific indications.
Some future areas of interest for clinicians will be the potential to apply the technology to the treatment of conditions such as critical limb ischemia and DVT (Figure 4).
Critical Limb Ischemia
Approximately nine million people worldwide have critical limb ischemia. Treatment options for occlusions in arteries of the legs include overnight infusion of thrombolytic drugs and angioplasty procedures.7,8 Severe cases may require bypass surgery or amputation. Without effective treatment, these blockages result in amputation in up to 40% of patients.
The future use of a flexible, wire-based device for treating critical limb ischemia could offer a less-invasive alternative for patients who are at the greatest risk of amputation.
Deep Vein Thrombosis
In 2001, two million Americans had DVT, and acute DVT accounts for almost 800,000 hospitalizations in the US each year.9 By offering the potential of resolving the thrombus more rapidly than current lytic approaches, acoustic energy could help patients with DVT avoid problems associated with postphlebotic syndrome.
OmniSonics has not yet begun clinical work on the potential use of the Resolution System for the treatment of DVT. But this will potentially be an area of future investigation for the technology, given the complications inherent in lytic therapy for DVT. Additional work must be done to demonstrate broader applicability of acoustic energy in a range of clinical conditions. The early work with this device, however, suggests a solid underlying technology that holds promise for the treatment of thrombotic occlusive disease.
John Lipman, MD, is Director of the Center for Minimally Invasive Therapy (MICRO) at Radiology Associates of Atlanta, Georgia. He is the principal investigator of OmniSonics Medical Technologies’ US pivotal clinical trial for the treatment of thrombosed synthetic dialysis access grafts. Dr. Lipman may be reached at email@example.com.
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