Excimer Laser Angioplasty for CTOs
An underestimated tool is undergoing a resurgence for treatment of complex PVD.
By Tony Das, MD
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Despite the increasing enthusiasm surrounding endovascular techniques for treating peripheral vascular disease, chronic total occlusions (CTOs) continue to pose significant challenges to interventionists. The search for devices to improve acute procedural success and long-term results has been difficult. The theoretical advantage of plaque ablation or vaporization, especially in long, diffuse, and occlusive disease, remains appealing. Excimer laser angioplasty has been studied since the late 1980s for peripheral vascular disease. However, widespread adoption has been limited by misunderstanding data from various laser studies in which catheters of different wavelengths were generalized under the generic acronym of “laser.” This article describes the currently known data on the excimer (xenon chloride), 308-nm wavelength laser and its use in chronic peripheral vascular occlusions.
HOW DOES AN EXCIMER LASER WORK?
An excimer (excited dimer) laser produces photons of ultraviolet light at a wavelength of 308 nm from the decay of a hydrogen chloride/xenon gas mixture that has been excited by electrical energy.1 The fibers that make up excimer laser catheters are composed of very pure, fused silica (synthetic quartz). These fibers can be bundled in a variety of patterns to attempt to produce lumens larger than the catheter diameter.2 The fibers are 50 µm to 140 µm in diameter and up to 250 fibers per catheter are configured in concentric or eccentric orientations to ablate tissue on direct contact. The unique wavelength of 308 nm is highly absorbed in human tissues, including atherosclerotic plaque and thrombus.3 The long pulses (130 ns) of ultraviolet light emitted by a hydrogen chloride/xenon laser at 308 nm deliver enough energy to maximize tissue ablation while limiting thermal injury (Figure 1A,B). The laser photon energy produced (4 eV) is higher than the enthalpy of a single carbon-carbon bond (3.6 eV) and leads to photochemical ablation, which may theoretically reduce dissection after PTA.4 Improved saline-infusion techniques with lasers have demonstrated lower dissection rates in clinical practice.5 In addition, some of the ultraviolet light from excimer lasers absorbed in the tissues causes cellular water to create steam bubbles that expand and then implode, leading to a “scrubbing effect” of the protein content and thrombus in contact with the laser tip. In addition, direct platelet aggregation inhibition has been demonstrated with excimer lasers.6 The unique properties of photochemical ablation and favorable interaction with thombus make excimer laser angioplasty an appealing choice for treating chronic, often thrombotic, peripheral vascular occlusions.
AVAILABLE LASER CATHETERS
Excimer lasers are available in concentric and eccentric catheter designs. The concentric catheters include Vitesse C (Spectranetics, Colorado Springs, CO) (0.014-inch wire compatible, rapid exchange) in 0.9-mm, 1.4-mm, 1.7-mm, and 2-mm diameters, and the Vitesse OS (Spectranetics) (optimally spaced, 0.014-inch wire, rapid exchange) in 1.4-mm, 1.7-mm, and 2-mm sizes. The Vitesse E (eccentric) (Spectranetics) catheter is available in 1.7-mm and 2-mm diameters. The Extreme (Spectranetics) (0.014-inch and 0.018-inch wire, over-the-wire) catheter is available in 0.9-mm and 2-mm varieties. In Europe, the 2.2-mm and 2.5-mm concentric catheters (0.035-inch wire compatible) wires are also available. Excimer laser therapeutic energy density (fluence) is between 30 mJ/mm2 and 80 mJ/mm2, with 45 mJ/mm2 being a common starting level. Pulse repetition rates are from 25 pulses/s to 40 pulses/s, with calcium-cutting, 0.9-mm catheters being able to deliver 80 mJ/mm2 and 80 pulse repetitions/s. Because less than 3 W is delivered to the tip of even the largest catheter, minimal thermal effects are noted with ablation.
ILIAC ARTERY LASER ANGIOPLASTY
Nearly 30% of peripheral arterial stenosis occurs in the aortoiliac region. Angioplasty has been proven to be effective for focal iliac stenosis.7 However, chronic iliac occlusions produce lower acute procedural success and certainly lower long-term patency than stenotic disease.8 The first task in treating chronic occlusions is safely crossing the lesion with a guidewire. In the iliac artery, occlusions may be up to 10 cm long and often begin at the common iliac, with little to no proximal nub. Occlusive iliac artery disease is often atherosclerotic and thrombotic. Excimer laser treatment may be a safe alternative to thrombolysis because it limits distal embolization in this complex disease.9
SUPERFICIAL FEMORAL ARTERY LASER ANGIOPLASTY
In the superficial femoral artery (SFA), occlusions outnumber stenosis by threefold, and the occlusions are often longer than 10 cm. Percutaneous therapy of SFA disease has variable medium- and long-term results.10 The addition of stents in the femoral circulation has not improved the long-term patency.11,12 In fact, therapy that could reduce stent use may be effective for success in the SFA occlusive lesion subset. Scheinert et al13 reported a 7.3% stent use in more than 400 femoral lesions when pretreated with excimer laser. In addition, the multicenter, prospective, randomized PELA study comparing PTA alone to excimer laser and PTA in chronic occlusions >10 cm showed a significantly reduced use of stents in the laser group (42%) versus the PTA alone group (59%).14 However, the 1-year reintervention rate was the same for both groups (51%), which may have been biased by stenting in the PTA group. In the SFA, short focal stenoses have acceptable 5-year patency rates as high as 75%, but occlusions of >3 cm have significantly lower long-term patency rates.15 In fact, according to the TASC document,16 a consensus panel had no firm recommendation of angioplasty or surgery for lesions between 3 cm and 5 cm, and it deferred complete occlusions of >5 cm to surgery only. However, with a procedural success rate of >90% and a 1-year secondary patency rate of 76% in the hands of careful operators,13 the application of excimer laser to complex, long, occlusive SFA disease may still warrant consideration, particularly when trying to reduce stent use (Figure 2).
TIBEOPERONEAL ARTERY LASER ANGIOPLASTY
Tibeoperoneal disease is more often long, diffuse, and calcified. Each of these anatomical barriers makes safe guidewire passage difficult. The 0.9-mm, high-energy catheters may be advantageous in this setting. Published series on chronic occlusions suggest lower initial procedural success rates for occlusive disease.17 The clinical goals of tibeoperoneal disease therapy are different than those of treatments for iliac or SFA disease. Often, below-the-knee occlusive disease is the source of nonhealing ulcers and resting pain. Clinical success is therefore measured in terms of wound healing and not long-term patency. In addition, the patients often have more chronic illnesses, including diabetes and cardiovascular disease, making them poor surgical candidates. In light of these factors, devices such as excimer lasers make theoretical sense (Figure 3). Poor surgical candidates with critical limb ischemia were studied in the LACI phase II prospective, multicenter, clinical registry. Patients with Rutherford category 4-6 were treated with excimer laser of the SFA, popliteal and/or infrapopliteal arteries with adjunctive PTA, and optional stenting. The primary endpoint was no need for amputation at or above the ankle. Straight line flow to the foot was achieved in 89% of patients. The rate of limb salvage was 93% at 6 months.17
Chronic occlusions of the peripheral vessels pose increased challenges and higher complication rates than stenotic disease. The long-term patency and procedural success rates are historically lower than the nonocclusive angiographic subsets. Many clinical devices have been tested to improve outcomes, but the results have been disappointing. Excimer laser treatment has a unique photochemical interaction with plaque and thrombus, which makes it appealing to consider for chronic, diffuse, and thrombotic occlusions. Laser clinical trials have been very convincing in the tibeoperoneal region for improving limb salvage. For SFA and aortoiliac disease, single-center results of secondary patency continue to be encouraging. Further study is certainly warranted.
Tony Das, MD, is Director of Peripheral Interventions, Cardiology Section Presbyterian Heart Institute, Dallas, Texas. He holds no financial interest in any product or manufacturer mentioned herein. Dr. Das may be reached at (214) 369-3613; firstname.lastname@example.org.
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