VASCULAR MEDICINE/LASER THERAPY: Excimer lasers open blocked arteries
Used to unblock coronary and peripheral arteries for placement of stents, excimer lasers are now being tested for use to literally stop heart attacks in their tracks.
Used to unblock coronary and peripheral arteries for placement of stents, excimer lasers are now being tested for use to literally stop heart attacks in their tracks. Another emerging application: treatment of renal artery stenosis.
By Peter Gwynne
The excimer laser, a device that generates cool light in the ultraviolet (UV) spectrum, is finding increased use in the treatment of arterial diseases. Emitted from the tips of catheters threaded into diseased arteries, the 308-nm wavelength light helps to open up blockages and restore blood flow. Such a system transmits laser pulses along flexible optical fibers encased in the catheters. Once it reaches a blockage, the light ablates cholesterol plaque and other detritus, converting it into tiny particles that the bloodstream absorbs and carries away.
The technology has established itself in treating coronary artery disease (CAD) and peripheral artery disease (PAD).And research groups are exploring new applications–for patients in the midst of heart attacks, for those facing problems with stents inserted into their leg arteries, and for artery blockage-related kidney disease. “It is a useful niche application,” says On Topaz, a cardiologist and cardiovascular specialist at the McGuire Veterans Affairs Medical Center in the Virginia Commonwealth University School of Medicine. “It debulks the blockages very well.”
At the cool end
The idea of using lasers to clean the interiors of arteries emerged more than 20 years ago. However, lasers suggested initially for that purpose created enough heat to damage arterial walls. Then excimer laser manufacturer Spectranetics Corp. (Colorado Springs) entered the arena. The company originally considered applying its technology to semiconductor production, but settled on cardiovascular uses. “Since the excimer laser is on the cool end of the UV light spectrum, we do not generate sufficient heat to damage the arterial wall,” explains CFO Guy Childs. (See sidebar “How photoablation clears arteries”)
The Spectranetics CVX-300 laser system is the only one approved by the U.S. Food and Drug Administration–and overseas regulatory authorities–to treat cardiovascular conditions. Most of the nearly 700 American hospitals and medical centers that use excimer lasers rent their systems, at a cost of $3,000 to $5,000 per month. Institutions can also buy the devices for $175,000 to $200,000.
The excimer laser’s two main uses involve cleaning arteries in the heart and elsewhere prior to the placement of stents. Blocked coronary arteries provide one key application: The cleaning effect of the excimer laser prepares the artery for accurate placement of a drug-coated stent. Patients with PAD have also benefited from the technology. Physicians usually treat both conditions using balloon angioplasty; this involves inflating a balloon placed inside the blocked artery to tamp down the cholesterol plaque prior to introducing a stent. An alternative technique is bypass surgery.
As a minimally invasive technology, the excimer laser has an obvious benefit over traditional, intrusive surgical methods. “Additional advantages,” Topaz says, “are precision, safety, and the dual effect on the atherosclerotic plaque and the associated thrombi.” The method also permits physicians to treat vessels across the size range from the femoral artery to arteries in the toes. Antonio Pratsos, interventional cardiologist at the Bryn Mawr Hospital, summarizes the advantages: “The laser enables you to deal with blockages that you can’t do with just a balloon,” he says. “The more complex the disease, the more you use the laser.”
The technology has some drawbacks. The laser generator is large and expensive. “And if you’re not careful in choosing the size of the catheter and the energy of the laser you use,” Pratsos says, “you could cause damage to the artery.” Childs adds one more perceived disadvantage. “To be effective, it requires cardiologists to advance the laser catheter at very slow speeds–approximately 0.5 mm/s,” he says. “The procedure times are still quite short, but it is counterintuitive for a cardiologist to do anything slowly.”
The technology can work in concert with traditional methods. “When coronary arteries are substantially blocked, we often can’t get the catheter where we need it to go,” Pratsos says. “By directing the laser’s UV light at the blockage first, we can dissolve enough of the plaque to create a channel for the placement of the balloon and stent. The pairing of laser technology with angioplasty can spare patients from bypass surgery.”
Research teams continue to expand the laser’s applications. In spring, Spectranetics announced that it had made a pre-investigational device exemption submission for use of excimer laser ablation in treating in-stent restenosis in the leg, a condition in which tissue regrows inside a stent and causes fresh blockages in the affected artery. And during the past 30 months, Pratsos and his team at Bryn Mawr Hospital have used excimer lasers on about 250 patients suffering heart attacks. “I’m using it to dissolve clots and literally stop heart attacks in their tracks,” Pratsos says. “I’m finding it extremely effective in delivering stents.”
Topaz, meanwhile, is about to publish two papers on new applications. One, to appear in Laser and Medical Science, focuses on use of the laser to open blockages of the left main coronary artery. He will also report, in Lasers in Surgery and Medicine, on the first ever application of the laser in patients with critical renal artery stenosis. The team has applied the technology to 12 patients with conditions such as hypertension, congestive heart failure, and abnormal kidney function that threaten to close the artery that feeds blood to the kidneys. “We used the laser to debulk the obstructive plaque and then placed a stent in the artery,” Topaz explains. Plainly the cool laser has the potential to generate more warmth in the clinic.
Peter Gwynne is a BioOptics World contributing editor.
How photoablation clears arteries
An excimer laser typically combines an inert gas and a reactive and applies electrical stimulation to produce an excimer (a pseudo-molecule) that generates ultraviolet laser light. Spectranetics’ CVX-300 system generates 308nm laser energy by electrically charging xenon and chloride (XeCl). The energy is transmitted along flexible fiber-optic strands encased in catheters and sheaths, which a physician passes through arteries and veins and focuses on the cardiovascular lesion or tissue requiring treatment. The treatment process is called photoablation.
Excimer lasers are unique in that their ultraviolet energy is stronger than the molecular bonds of the tissue. The laser breaks billions of molecular bonds per nanosecond pulse (pulsing keeps the tissue cool), and the light penetrates to 50 microns. The tissue breaks down into particles mostly <10 microns in diameter, which are carried off in the bloodstream.
As the tissue absorbs the laser energy, water within the cells heats up and produces molecular vibration. The water vaporizes, rupturing cells. This process creates gaseous byproducts that form a vapor bubble. Rapid expansion and implosion of the bubble produces kinetic energy: cavitation and pressure that further break down tissue and help clear ablated debris from the catheter’s tip. Secondary cavitation bubbles can form after the implosion of the first vapor bubble, further assisting in tissue ablation and debris removal.