A New Approach to Treating SFA Disease
Preliminary results suggest that the PolarCath PTA system is a promising new weapon in the battle against SFA disease.
To view the figures related to this article please refer to the print version of our April issue, page 34.
Interventional techniques for treating the diseased superficial femoral artery (SFA), while preferred from the patient's perspective, have poor long-term patency rates. Restenosis after percutaneous transluminal angioplasty (PTA) in the femoropopliteal arteries due to elastic recoil, constrictive remodeling, and neointimal hyperplasia ranges from 38% to 57% at 12 months.1-6 Patency rates range from 25% at 5 years to 14% at 10 years and result in high rates of attempts to revascularize the restenotic vessels.7
Stenting in the SFA adds little benefit to PTA: studies demonstrate 1-year patency rates of 22% to 81% in the femoropopliteal vessels,8-12 and the device adds to the cost of the procedure. The new drug-eluting stents have been shown to be promising in the coronary arteries, but the results have yet to be duplicated in peripheral arteries. Furthermore, drug-eluting stents are quite costly, and in the SFA, multiple stents will likely be required. Also, anecdotal evidence suggests potential late sequelae, including edge effect, aneurysm formation, vessel ulceration, and thrombotic occlusion.
Surgical bypass using saphenous veins offers higher long-term patency rates—83% at 1 year and 60% at 5 years—and is therefore considered the standard of care for long lesions in the SFA.7,13 Bypass surgery, however, is associated with higher rates of procedural morbidity, mortality, and a longer hospital stay.14 Operative mortality rates range from 1.3% to 6%, with a perioperative risk of myocardial infarction ranging from 1.9% to 3.4%. Wound complications occur in up to 10% to 30% of cases, with serious graft infections in 1% to 1.5%.
The PolarCath peripheral balloon catheter (Cryo-Vascular Systems, Inc., Los Gatos, CA) is a novel angioplasty system that simultaneously dilates and cools the plaque and vessel wall in the area of treatment. Cooling is achieved by inflating the balloon with nitrous oxide rather than the usual saline/contrast mixture. It is believed that this cooling induces an acute phase change that triggers apoptosis in smooth muscle cells. This noninflammatory form of cell death can lead to several potentially beneficial effects, including reduced elastic recoil and constrictive remodeling, and reduced neointimal hyperplasia.
MECHANISMS OF ACTION
Cryoplasty involves advancing the balloon catheter to the site of the lesion and delivering liquid nitrous oxide into the balloon, where it expands into gas and inflates the balloon. The evaporation of the liquid draws energy, and the surface of the balloon rapidly cools from 37ºC to -10ºC. Cryoplasty dilatation is theoretically more homogenous and less injurious than standard balloon dilatation. The drop in temperature causes interstitial saline in the wall of the artery to freeze and expand as ice forms, generating high radial, longitudinal, and circumferential forces. The resultant theoretical potential beneficial effects are threefold:
(1) Altering the plaque response with controlled plaque fracture. Microfractures weaken the plaque, resulting in a more uniform dilatation of the blood vessel. This prevents the formation of large tears deep into the wall of the vessel that can occur in standard angioplasty and result in extensive dissection.
(2) Reducing vessel wall recoil. The freeze-induced alteration of the morphology of collagen and elastin fibers results in short-term loss of vessel elasticity. This reduces postdilatation elastic recoil, helps optimize the initial angiographic result, and possibly reduces the need for a stent. Without a stent, the cell proliferation associated with foreign-body immune response is eliminated.
(3) Apoptosis. On a cellular level, osmotic forces in the presence of ice cause smooth muscle cells to eject water. The process of dehydration and subsequent rehydration triggers noninflammatory cell death. Apoptosis of the targeted cells leads to a reduction in neointimal formation and collagen synthesis, providing protection against constrictive remodeling and restenosis. Furthermore, it does not interfere with or delay re-endothelialization.
THE PolarCath PERIPHERAL CATHETER
The PolarCath system consists of three components: a disposable, hand-held microprocessor-controlled inflation unit, a balloon dilatation catheter with dual, coaxial balloons, and a cartridge of liquid nitrous oxide (Figure 1).
Nitrous oxide is used to inflate and chill the balloon, exposing the vessel wall to a predetermined algorithm of temperature (-10ºC), pressure (8 atm), and dwell time (25 sec). The latest generation of the PolarCath inflation unit is programmed to inflate the balloon in a stepped manner at 2-atm increments until the nominal pressure is achieved, which allows for a more controlled dilatation of the lesion.
Using conventional angioplasty techniques, the balloon is positioned at the site of the arterial narrowing. The start button is pressed and the nitrous oxide leaves the cartridge and travels through the catheter to the balloon (Figure 2). The liquid changes to a gas, expanding the balloon. This process of evaporation causes a significant reduction in temperature. Throughout the cycle, internal balloon pressure is regulated via an exit lumen and relief valve, allowing for continuous flow of nitrous oxide and subsequent cooling. At the completion of the cycle, the gas is evacuated, the balloon is deflated, and the vessel returns to its baseline temperature of 37ºC. Each dilatation cycle takes less than 1 minute. Multiple dilatations can be performed with each system—only the liquid nitrous oxide cartridge needs to be changed.
In vitro and in vivo preclinical studies and early clinical data have demonstrated that the device is safe and effective for the intended use of PTA of stenotic lesions in the superficial femoral and popliteal arteries. FDA approval for this application was received in September 2002. A multicenter registry evaluating the safety and efficacy of the PolarCath for lesions in the superficial femoral artery and popliteal artery has been completed. The PolarCath also has also been used in coronary arteries and for saphenous vein grafts and arteriovenous grafts. To date, approximately 300 peripheral and 120 coronary procedures have been performed using the PolarCath system. Trials evaluating use of the PolarCath in the coronary arteries are ongoing.
MULTICENTER REGISTRY RESULTS
In the multicenter registry, 102 patients were enrolled at 15 sites in the US and one site in Germany. All patients were treated for SFA/popliteal stenoses or occlusions of &Mac178;10 cm in length (mean percent diameter of stenosis was 87 ± 10%). The acute procedural success rate (<30% residual angiographic stenosis and &Mac178;50% stenosis by Duplex ultrasound prior to discharge) was 96%.15 Four patients did not meet the study definition of acute procedural success. Two patients had >30% residual diameter stenosis as determined by angio-graphy. Two other patients had what appeared to be an angiographically successful procedure; however, on Duplex imaging prior to discharge, there were elevated Doppler velocities at the lesion site correlating with >50% stenosis. Stand-alone success with the PolarCath was achieved in 87% of cases. Nine percent of patients received stents for what was believed to be a suboptimal angiographic result. Review of the index procedure angiograms demonstrates that the PolarCath achieves excellent angiographic results with a low rate of type C or greater dissections (7%). This compares favorably with a recent publication demonstrating a 43% incidence of significant dissections when the standard practice of short balloon inflations is utilized.16
The 9-month clinical outcome in 45 patients was presented at the SIR meeting on March 31, 2003.15 Only seven patients (15%) required reintervention on the target limb at the 9-month endpoint, yielding a clinical patency rate of 85%. Ankle-brachial indices (ABI) were improved in 75% of patients and claudication symptoms were significantly reduced in 64%. The baseline ABI of 0.72±0.17 increased to 0.87±0.17 at 3 months and was maintained at 0.91±0.16 at 9 months. These encouraging early results will need to be confirmed after completion of 9-month follow-up and analysis of the 9-month Duplex scans.
In the multicenter registry, lesions up to 10 cm in length were treated. This would allow for up to two dilatations with the 6-cm-long PolarCath balloon. Figures 3 and 4 demonstrate two case examples from the multicenter study. In Figure 3, we see a diffusely diseased distal SFA and popliteal artery with tandem high-grade stenoses. An excellent result is achieved after two dilatations with a 6-mm-diameter, 6-cm-long balloon. Figure 4 demonstrates a more complex, severe stenosis of the popliteal artery seen best on a lateral arteriogram. Certainly, this is a region where it would be desirable to avoid stenting if at all possible. An excellent angiographic result is achieved after cryoplasty without dissection or the need for stenting.
Cryoplasty, a combination of angioplasty and endovascular cryotherapy, has been developed as a potential way to improve acute and long-term clinical outcomes in the interventional treatment of arterial disease. Cooling is achieved by inflating the balloon with nitrous oxide instead of the usual mixture of contrast and saline. At the optimum temperature of -10ºC, there is induction of an acute phase change that triggers apoptosis in smooth muscle cells. Through a variety of mechanisms, this noninflammatory cell death may lead to a reduced risk of restenosis. Early studies using this novel angioplasty system in coronary and peripheral arteries are promising, but definitive conclusions must await further data on long-term outcomes.
John R. Laird, Jr, MD, is Director of Peripheral Vascular Interventions at the Cardiovascular Research Institute, and is Assistant Clinical Professor of Medicine at Georgetown University Medical Center, Washington, DC. He also is the Co-Director of the Center of Vascular Care at the Washington Hospital Center, Washington, DC. He has no financial interest in any product or manufacturer mentioned herein. Dr. Laird may be reached at (202) 877-5975; John.R.Laird@medstar.net.
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