Continuing Education Activity

Excimer laser coronary angioplasty (ELCA) is a safe and effective technique as an adjunct to conventional PCI and can improve clinical outcomes when used in the appropriate context. Its primary application currently is for lesions that are uncrossable or un-dilatable with conventional balloons. This article provides a concise review of the ELCA technique and focuses on its role in interventional cardiology. This activity reviews the basic concepts behind excimer lasers, indications and contraindications, procedural steps, potential complications, and highlights the role of the interprofessional team in evaluating and improving care for patients who undergo this procedure.

Objectives:

• Identify the indications for excimer laser coronary angioplasty.
• Outline the equipment, preparation, and technique in regards to excimer laser coronary angioplasty.
• Describe the appropriate evaluation and their potential complications and clinical significance of excimer laser coronary angioplasty.
• Review strategies for improving outcomes from excimer laser coronary angioplasty.

Introduction

Laser coronary angioplasty was introduced in the early 1980s, mainly to manage balloon-untreatable coronary artery lesions.[1] However, due to the huge cost of the laser system, disappointing results, and complications associated with the continuous waveform of argon and Nd: YAG lasers available at that time, it did not gain popularity.[2][3][4][5] Later in that decade, excimer lasers were developed.  Excimer, an acronym for the excited dimer, produces ultraviolet laser energy pulsatile and short wavelength.  The pulsatile nature ensured the precise ablation of plaque tissue with insignificant thermal injury to the vessel.[6] The short wavelength through less depth of penetration, compared to the infra-red range of argon and Nd: YAG lasers, also limited collateral damage. Both of these properties of excimer lasers, in addition to improvement in catheter design, proper selection of patients, and development of safety protocols, played a crucial role in the reintroduction of laser technology in routine practice.[7][8][9][10] In 1988, the first successful excimer laser coronary angioplasty (ELCA) was performed on a human subject at the Cedar Sinai Medical Center, Los Angeles.[11]

Anatomy and Physiology

Light amplification by stimulated emission of radiation, or LASER, in short, refers to the creation of high-energy, single-wavelength light beam from a gas mixture. For excimer lasers specifically, a mixture of xenon gas and diluted hydrogen chloride solution is used. After a high-voltage electrical discharge is passed through this gas mixture, excited dimers or excimers, xenon chloride (XeCl), are produced. These dimer molecules subsequently release photons with an ultraviolet (UV) wavelength. Mirror systems are then utilized to amplify this process and deliver the resulting high-energy laser beam to target tissues. On contact with tissue, this laser beam then modifies it via three major mechanisms, as detailed below:

1. Photochemical: Breakage of molecular bonds.
2. Photothermal: Vibration of molecular bonds generates heat and leads to the vaporization of intracellular water, causing bubble formation. This ultimately leads to cell rupture.
3. Photokinetic/photomechanical: The vapor bubbles generated secondary to the photothermal mechanism coalesce to form larger bubbles, further breaking down plaque tissue.

The breakdown products generated from these biochemical processes are small enough (usually < 10 µm) to be rapidly cleared by the reticuloendothelial system of the body, hence preventing distal embolization.

Indications

The Food and Drug Administration (FDA) currently approved indications for excimer laser coronary angioplasty (ELCA) are[12]:

1. Balloon uncrossable and un-dilatable lesions
2. Multi-focal, thrombotic saphenous vein graft lesions
3. Chronic total occlusion (CTO)
4. Moderately calcified lesions
5. Ostial lesions
6. Eccentric lesions
7. Long lesions (> 20 mm)
8. In-stent restenosis

Besides, there have been reports of ELCA being utilized successfully for other indications, which are listed below:

1. Adjunctive to conventional percutaneous coronary intervention (PCI) for under-expanded/under-deployed stents, or in cases of large thrombus burden.[13]
2. Modification of proximal cap of heavily calcified lesions, when initial attempts to cross with rotablation wire are unsuccessful.[14]

Contraindications

There are no absolute contraindications for excimer laser coronary angioplasty (ELCA). Relative contraindications include:[15]

• Acute angulation (> 45 degrees)
• Coronary dissection
• Unprotected left main coronary artery

ELCA is also currently not recommended for poorly visualized/heavily calcified lesions and those with a diameter of less than the smallest catheter size available (0.9 mm).

Equipment

Excimer laser coronary angioplasty (ELCA) equipment includes an excimer laser-generator and catheters of variable sizes capable of delivering this laser energy. There is currently only one cardiovascular laser system approved for use in the United States by the FDA.[16] It generates a pulsed XeCl laser energy beam, which has a wavelength of 308 mm in the UV–B range, tissue depth of penetration between 0-30 µm, outflow range of 30 to 80 mJ/mm (termed fluence), repetition rate between 25 to 80 pulses per second (Hz), and a duration (termed pulse width) range of 125 to 200 ns.

ELCA catheters are currently available in four diameters (0.9 mm, 1.4 mm, 1.7 mm, and 2.0 mm), and are of two types based on the arrangement of fiberoptic fibers within the catheter. They can either be concentric around the guidewire lumen or eccentrically localized towards one hemisphere. Most commonly, the 0.9 mm catheter is used. Catheters can be conventional over-the-wire or newer rapid-exchange/monorail catheters. Monorail catheters are currently being utilized more commonly, given their advantages of better tip control and axial force transmission.

Preparation

Before the initiation and throughout the procedure's length, all staff present inside the room, and the patient is required to wear tinted eye goggles to protect against corneal or retinal damage by the UV laser. The procedure should be done in a room with tinted windows and locked doors, and unauthorized access prohibited.

Once turned on, the excimer laser generator requires around 5 minutes of start-up time. An appropriate catheter is then selected, and its central guidewire lumen flushed. General concepts to consider when choosing a catheter are a) its size should not exceed two-thirds the diameter of the target vessel, and b) it should be able to deliver the intensity of energy needed to treat the target lesion based on the lesion’s severity and consistency. Concentric catheters are most commonly utilized; however, eccentric catheters are recommended for in-stent restenosis, bifurcation lesions, and eccentric lesions since the laser beam can be rotated towards the target lesion using a torque knob.

The proximal catheter end is then attached to the laser unit and its distal end calibrated. Calibration is an automated process, which is achieved by pointing the catheter tip towards the energy detector on the main laser unit and activating the laser. Subsequently, the laser unit enters into a standby mode.

Technique or Treatment

A standard 0.014-inch PCI guidewire is typically advanced till it crosses the target lesion, after which the catheter is passed over it till its tip is in direct contact with the lesion. This is a major advantage of excimer laser coronary angioplasty (ELCA) over other atherectomy techniques, which usually require dedicated guidewires. The desired fluence, pulse rate, and pulse width settings are then selected. By default, the system calibrates at 45 mJ/mm^2 at 25 Hz, with a pulse width of 135 ns. If resistance is encountered with these default settings, they can be increased in a stepwise fashion. This should be undertaken slowly since higher energies and frequencies can be associated with a higher chance of dissection and perforation complications.[15] The manufacturer recommends that fluence be increased first rather than the frequency.

A saline flush protocol is then employed before initiation of lasing. The concept behind this step is that both blood and contrast media consist of macromolecules, including proteins, that can absorb the bulk of the laser energy and lead to the formation of insoluble gas bubbles. This also increases the risk of complications such as intimal dissection and perforation.[8][17] On the other hand, Saline provides a clear interface for the laser energy to be delivered directly to the target lesion. To perform the saline flush protocol, a 1 L bag of 0.9% normal saline is attached to one of the triple manifold ports via a three-way stopcock. The contrast syringe is replaced with a clean 20 cc syringe used for flushing contrast and blood from the entire system. Thereafter, the operator infuses a 5 to 10 cc bolus of normal saline through the guiding catheter, with the initiation of lasing immediately afterward. This is accompanied by a continuous infusion of normal saline at 1 to 3 mL/s throughout the duration of laser activation. The system is programmed to activate for 5 to 10 seconds, after which goes into a 5 to 10 second rest period. An audible alert will mark the end of this rest period, at which point the next lasing sequence can be commenced. This potentially helps avoid complications from prolonged laser energy exposure to the vessel.

It is also recommended that the catheter be advanced at a slow rate (<1 mm/s) within the vessel lumen to allow the plaque tissue sufficient time to adequately absorb the light energy and result in optimal vaporization and debulking.[15]

Complications

With the optimization of laser catheters and the introduction of safety techniques such as the saline infusion protocol, the incidence of previously seen serious complications such as flow-limiting dissections and vessel perforations has significantly decreased.[8][11][18] Other measures that can prevent these adverse outcomes include avoiding excessive force and lasing on high settings for prolonged periods. Perforations are more likely to occur if an inappropriate size or type of catheter is used (for example, concentric for an eccentrically located lesion), or if energy is applied to a previously dissected segment.[15] If any of these complications occur, the lasing procedure should be aborted, and the complication managed per standard protocol.

Clinical Significance

Excimer laser coronary angioplasty (ELCA) is a safe and effective technique as an adjunct to conventional PCI and can improve clinical outcomes when used in the appropriate context. As noted previously, its primary application currently is for lesions that are uncrossable or un-dilatable with conventional balloons.[19] In cases where even the laser catheter cannot cross the lesion, such as CTOs or heavily calcified lesions, the laser energy has been successfully utilized to modify the lesion's proximal cap.[14] This allows the passage of a standard PCI microcatheter or dedicated guidewire such as RotaWire later on, and the procedure can then be completed in a usual fashion.

Another important application of ELCA is to debulk thrombotic occlusions in saphenous vein grafts (SVGs). These lesions are prone to distal embolization[20] and can cause no-reflow with standard SVG-PCI. Hence, it is recommended that distal protection devices (DPDs) be utilized to prevent this complication.[21] However, these devices are often bulky and unable to be delivered distally. ELCA has been shown to have a low rate of distal embolization and can be used as a safer alternative without DPDs for the majority of cases.[22][23]

The incidence of in-stent restenosis (ISR) has reduced considerably with the advent of drug-eluting stents (DES)[24], compared to bare-metal stents (BMS).[25] In those cases with ISR, ELCA is a safe technique that does not cause collateral damage to the stainless-steel stent.[26] Simultaneously, it achieves greater intravascular ultrasound (IVUS) cross-sectional area, luminal gain, and intimal hyperplasia ablation compared to balloon angioplasty alone.[27][28]

Additionally, since ELCA has been shown to ablate both the luminal and the abluminal plaque tissue[29], in those cases where an under-deployed stent was the cause of ISR, these stents can be better approximated to the vessel wall.[13] This can potentially prevent the need for future revascularization.

Enhancing Healthcare Team Outcomes

Since excimer laser coronary angioplasty (ELCA) is still a relatively new procedure, there are no evidence-based guidelines currently to ensure optimal outcomes. A "5S approach" has been suggested to achieve the best results from ELCA, which are[15]:

1. Selection of patient
2. Size of the laser catheter
3. Settings (fluence and pulse rate)
4. Saline infusion protocol
5. Slow advancement