Excimer laser angioplasty was developed in the early 1980s in an ef fort to solve two of the limitations of balloon angioplasty, recanalization and restenosis. Excimer lasers ("cool" lasers) generate nanosecond, high power, UV pulses that strip electrons from the nucleus of an atom. This causes bonds to break and plaques to vaporize with shock waves or photoabolative effect. In its best essence, laser angioplasty technology offers the potential for moving a fiberoptic catheter through the entire length of the coronary circulation to vaporize all plaques along the arterial wall. While thermal damage has been sufficiently reduced in excimer laser angioplasty, this procedure is still used only in minority of the cases as a stand-alone operation due to high-pressure waves and bubble formation caused by excimer laser.
 

INTRODUCTION:

Serious stake in laser angioplasty began in the early 1980s in an effort to solve two of the limitations of balloon and stent angioplasty, restenosis and recanalization. By demonstrating the capability of laser irradiation to vaporize atherosclerotic tissue, it was reasonable to presume that this new technology may allow recanalization of lesions that could not be crossed by conventional balloon insertion. With refinements in laser fiber optics, FDA approved excimer laser procedure in 1992 for clinical use as recanalization device in both coronary and peripheral arteries. However, the requirement to follow laser angioplasty with balloon angioplasty in the majority of cases and the insufficiency of an effect of laser tissue removal on restenosis has limited the usage of this procedure. Perhaps, improved techniques discussed later in this paper such as better fiber-optic lens systems or saline fusion will allow laser angioplasty to offer a true niche in interventional cardiology (Ref. 6). This paper has been written to give a general overview of laser angioplasty, its procedures, and its applications.
 
 

TEXT:

Angioplasty is a method used to open coronary arteries blocked by obstructing plaques. Plaque by definition is a build up of cholesterol and other fatty molecules inside an artery. Over time, plaques can slow or even stop the flow of blood to and from the heart. In order for the heart to function properly, the normal flow of blood must be retained, by cleaning or bypassing the blockage. One procedure used to remove a plaque is laser angioplasty. In laser angioplasty, a thin tube (a catheter) is inserted into an artery and moved through the blood vessels to the blocked artery. The laser emits short pulses of photons that cause the plaque to vaporize. Alternative methods for removing a plaque from an artery include coronary artery bypass surgery, mechanical removal surgery, and balloon angioplasty. The method used depends on the location of the plaque, size of the plaque, and the number of plaques. Laser angioplasty, however, is still not used alone very often (Ref. 1). Typically, it is used with balloon angioplasty. When a plaque has totally blocked an artery, laser angioplasty can be used to drill a hole in the plaque, so the balloon angioplasty can be successfully performed. Therefore, laser angioplasty has become a safe and a cheap alternative for expensive, bypass, open-heart surgery (Ref. 1)

Laser angioplasty does not replace balloon angioplasty, however, when used with the balloon catheter, more patients can be operated successfully at low medical costs. Today, the usage of laser and balloon angioplasty has become so popular that bypass operation is barely used. So what exactly is balloon angioplasty? In balloon angioplasty, the cardiologist inserts a long, hollow, narrow tube, catheter, into an artery through an incision made in the groin or the arm. Using the X-ray technology, the doctor can monitor the exact place of the catheter inside an artery. The cardiologist advances the tube through the blood vessels until he gets to the blocked area of the artery. The second thinner catheter is then inserted into the first hollowed tube. At the tip of the second catheter exists a small miniature, a deflated balloon. Once in proper position, the miniature is inflated, causing the narrowed area of the artery to become slightly wider. In some cases, a device known as stent is inserted via another narrow catheter. Stent reduces the likelihood that the artery becomes narrowed again (Ref. 1). Figure 1 illustrates the usage of balloon and stent.
 
 

As Figure 1 shows, balloon angioplasty can be used only when the miniature is placed inside the plaque. If the plaque has completely blocked the artery, then before inserting the balloon catheter, a laser catheter must be inserted to drill a hole through the plaque to open a space for the miniature.

In order for laser angioplasty to be a successful operation, it is essential that a laser with the right wavelength and energy be chosen. The laser should remove the plaques without damaging the blood cells or the blood vessel tissues. Thermal damage caused by the laser energy being absorbed by the tissues depends on the wavelength of the laser, duration of the beam, tissue color, consistency, and water content. As the tissue absorbs the laser energy, more and more heat energy is produced. The temperature continues to rise over 100 C. This will cause the protein denaturization in the cells and cause the water inside the cells to evaporate. Since water vapors occupy more volume than the liquid water, the cells burst and the cell membranes break open. To minimize this thermal damage, short pulses of laser is instead used which reduces the heat energy produced by the radiation; therefore, decreases tissue damage (Ref. 1). Table 1 shows the biological and visual changes that takes place as the temperature increases.

As Table 1 notes, the laser used in angioplasty should produce a temperature of about 65-90 degrees. At this temperature, protein is denaturized, atomic and molecular bonds are broken, and plaques can be removed. At higher temperatures, thermal damage of the tissues is expected while at lower temperatures, plaques cannot be removed.
 

There are three different methods of laser angioplasty: thermal, photothermal, and photoablative. In thermal laser angioplasty, an argon, or Nd: YAG laser is used to heat a biocompatible metal ally tip attached to a fiberoptic waveguide. The highest temperature is 400C, which is the temperature of the very tip of the probe. The surgeon guides the probe through the obstruction causing a small lumen for the blood to flow through. This method is not very useful since it can cause thermal damage of the tissues and carbonization and necrosis of the vessel walls. Photothermal laser angioplasty involves a contact of a Nd: YAG laser probe that rapidly heats the plaque to the point of vaporization using a photo-optical effect. This method is more productive and safer than the thermal method, but still can cause damage to vessel walls. The third method is photoablative laser angioplasty that involves the usage of excimer laser. The excimer, excited dimer, laser uses either two noble gas atoms or a noble gas atom in conjunction with a halogen atom to lase at different wavelengths, mostly Ultraviolet (Ref 3). Table 2 shows different examples of excimer laser and their output wavelengths.
 
 

The energy source of an excimer laser can be a beam of few electrons at high energy or many electrons at high current discharge (100,000 A). XeCl is the most common excimer laser that is used for angioplasty. In XeCl laser, an electron collides with a Xe atom causing it to go to an unstable electron configuration, which allows it to bind with a Cl atom and produce XeCl. Since there are no XeCl molecules at a low energy state, population inversion takes place (Ref. 3).

Xe + Cl ======> XeCl + Cl^-
e^- + Cl2 ======> Cl^- + Cl
Cl^- + Xe⁺ + M ======> XeCl + M

Excimer lasers require special UV mirrors and optics to operate. Mirrors and lenses of these lasers apply fused silica, magnesium fluoride, or calcium fluoride substrates with aluminum or magnesium fluoride coatings. The excimer laser generates nanosecond, high power pulses that strip electrons from the nucleus of an atom. This causes bonds to break with shock waves or photoabolative effect. The laser light is fired in short blasts through fiber optic bundles. This laser method, “cool” laser, used since 1988, became the first laser to be approved by FDA for angioplasty in 1992. The precise accuracy of these pulses of UV light is made possible by series of magnetic switches developed by the Jet Propulsion Laboratory. This method is so precise that it can remove exactly 9 millionths of an inch of tissue in 12 billionths of a second (Ref. 1). One example of laser angioplasty is Smooth Excimer Laser Coronary Angioplasty (SELCA) which uses a XeCl laser with the following properties:

Pulse Duration = 115 ns
Repetition Rate = 100 Hz
Wavelength = 308 nm
Area of radiation = 1 mm2
Energy Density = 50 mJ/mm2
Energy of Pulse = 50 mJ

f = c/lam (Equation 1)
f = frequency of light
lam = wavelength of light
c = speed of light
Speed Of Light = 3 * 10^8 m/s
f = (3 * 10^8 m/s) / (308 nm * 10^-9 m/nm) = 9.703 * 1014 s-1

Ep = hf (Equation 2)
Ep = engergy of photon
h = plank’s constant
f = frequency of light
Plank’s Constant = 6.626 * 10^-34 J.s
E = (6.626 * 10^-34 J.s) * (9.703 * 10^14 s-1) = 6.43 * 10^-19 J

N = Et / Ep (Equation 3)
N = number of photons emmited in each pulse
Et = total energy of pulse
Ep = energy of photon
N = (50 mJ * 10^-3 J/mJ) / (6.43 * 10^-19 J) = 8 * 10^16

P = E * R (Equation 4)
P = power
E = energy of each pulse
R = repetition rate
P = (50 mJ) * (150 Hz) = 7500 mW = 7.5 W

D = P / A
D = power density
P = power
A = spot size
D = (7.5 W) / (1 mm2) = 7.5 W/mm2

The excimer laser generates less heat than thermal and photothermal lasers. This reduces the carbonization and thermal damage of the blood vessels. While safer, excimer laser method still raises some safety concerns. The gases that are used for this method are typically very toxic and even fatal. No leakage of these gases can, therefore, be allowed. The UV radiation generated by the excimer laser can also cause genetic mutations of the DNA by breaking the hydrogen bonds between the two strands. Overall, laser angioplasty is also associated with many potential problems including, including reocclusion, vascular spasm, perforation, cardiac arrhythmias, and intimal dissection (Ref. 3).

To increase the precision of the operation, the physician must be able to distinguish the plaque, blood cells, and vessel walls. Chromophore-tagged monoclonal antibodies that selectively attach to plaque molecules have been developed. When the plaque is tagged, only the tagged molecules absorb the therapeutic laser beam. Therefore, the chance of damaging the surrounding vessel walls has been greatly decreased (Ref. 1).

Studies have shown that during XeCl excimer laser ablation of tissue, small, short lived bubbles, predominately containing water vapors, are formed that can damage the blood vessel tissues (Ref. 2). These rapidly expanding and imploding bubbles are less than 3 mm in diameter and have a lifetime shorter than 300 microseconds. For a given radiant exposure, the size of these vapor bubbles can be decreased either by decreasing the surface area of the tip of the fiber or by reducing absorption coefficient of the hemoglobin solution (Ref. 5).
 
 

CONCLUSION:

From 1988 to 1998, approximately, 14,000 patients were treated with excimer laser angioplasty (Ref. 4). The data underline the workability and safety of this procedure. However, laser angioplasty so far has been used as a stand-alone technique only in minority of the cases. Clinical studies have also shown that excimer laser angioplasty does not reduce the incidence of restenosis. While safer, laser angioplasty still can injure vessel walls by high-pressure waves or bubble formation. Technical and procedural progress including homogeneous light distribution, infusion of saline, and the concept of smooth laser ablation have been pushed forward to make excimer laser angioplasty safer, more predictable, more precise, and more effective.
 

Researchers continue to work for the development of new laser methods to be used in angioplasty. Promising new lasers are expected to have less thermal tissue damage. Solid-state, pulsed-wave, mid-infrared (2.1 microns) holmium:YAG laser has recently been used for angioplasty. It is still too early to predict the success rate of this new laser technology (Ref. 8).
 

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excimer laser angioplasty: short and long term effects in the rabbit femoral artery.
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dimensions, theory, and implications for laser angioplasty. Lasers Surg Med
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(ENGLAND), 1996 Jun, 1(2):117-9.
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9. http://www.mech.gla.ac.uk/~davidb/lectures/laserapp/angiop.htm#I1
10. http://mayohealth.org/mayo/9307/htm/angiopla.htm

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