LASER: Coronary Angioplasty

History:

LASER or Light Amplification by Stimulated Emission of Radiation has a variety of commercial uses and is finding a remerging niche in interventional cardiology. The early lasers were argon and Nd: YAG and generated thermal energy to vaporise tissue and plaque material. The disadvantage was excessive thermal injury and vascular damage. Newer lasers such as the FDA approved EXCIMER LASER (Spectranetics) generate cool 308 nm laser energy by electrically charging two gases, Xenon and Chloride (XeCl). The first Excimer laser coronary angioplasty (ECLA) was performed in 1988.

Mechanism of action:

The Excimer Laser causes photo-ablation of tissue. There are three stages in this process: photo-chemical, photo-thermal and photo-mechanical. In the photochemical stage UV light pulses collide with tissue for 125 billionths of a second resulting molecular bonds being fractured. Billions of minute sub cellular sized fragments filter through the blood stream, minimizing the risk of distal embolisation. In the second stage the photochemical energy is generated by absorption of the ultraviolet waves. This creates molecular vibration in the tissue heating the intracellular matrix. This results in rapid vapourisation of water and rupture of cells; the steam rapidly dissipates minimising vessel trauma. The final, photo-mechanical stage, creates kinetic energy from the expansion and collapse of vapor bubbles which breaks down tissue and clearing by-products away from tip. The by-products of ablation are water, gas, and small particles 90% of which are smaller than red blood cells.

There needs to be a continuous infusion of normal saline as the presence of blood or contrast in the lasing medium can produce shock waves increasing the risk of dissection and sub optimal tissue ablation. The early catheters were concentric flexible over the wire devices but their limitation was exposed in treatment of eccentric lesions, bifurcations and lesions over bends. Numerous reports of perforation and dissection led to the development of directional laser catheters which allow greater flexibility and improved tracking. A new catheter technology (termed ‘optimal spacing’) was introduced in 2001 and provides a 30% increase in plaque ablation and thrombus dissolution in comparison with the previous generation of laser catheters. (3)

Indications for ECLA:

The indications for ECLA are as follows:

1. Long diffuse lesions

2. Chronic total occlusions

3. Aorto- ostial lesions

4. Saphenous vein grafts

5. Non dilatable or non crossable lesions

6. Restenotic lesions

The laser should be avoided in very tortuous vessels and if there is a history of previous dissection or perforation.

Clinical Trials

There has been limited number of clinical trials with the Excimer laser.

A multicentre surveillance trial published in 1999 (8) evaluated safety and efficacy of Excimer laser angioplasty for treatment of restenosed or occluded coronary stents. A total of 440 patients with restenoses or occlusions in 527 stents were enrolled for treatment with concentric or eccentric laser catheters and adjunctive balloon angioplasty. The investigators concluded that Excimer laser angioplasty with adjunctive balloon angioplasty was a safe and efficient technology to treat in-stent restenoses.

Laser and AMI

The CARMEL investigators performed Excimer Laser in 151 patients with AMI and found it to be is feasible and safe and provided efficient debulking of thrombus and underlying plaque. The exceptionally low rates of distal embolization and ‘no reflow’ phenomenon encountered in this study has raised the intriguing possibility that, when excimer laser energy is applied, there is, no need for distal embolization protection devices. (9) Recently a single centre trial in Italy “Excimer laser in acute myocardial infarction: Single centre experience on 66 patients” concluded that Laser angioplasty is feasible, safe and effective for the challenging treatment of patients with AMI and thrombus-laden lesions. The acute effects on coronary epicardial and myocardial reperfusion are excellent. In August 2003 the US FDA removed the restrictions on the use of LASER in cases of acute myocardial infarction (AMI), acute thrombosis and depressed LV ejection fraction. (10)

Summary

In summary the Excimer laser has an important role in debulking lesions which are difficult to cross with conventional equipment, however, its efficacy is reduced in heavily calcified lesions. Its role in acute myocardial infarction is potentially exciting but requires randomised trials.

References:

1. Practical Interventional Cardiology : ED Grec & DR Ramsdale

2. www.spectranetics.com

3. Excimer Laser Atherectomy in Acute Myocardial Infarction – Evidence-based Treatment Approach ; A report by On Topaz

4. Practical Interventional Cardiology : ED Grec & DR Ramsdale

5. Textbook of Interventional Cardiology : 5th Edition : EJ Topol

6. Textbook of Interventional Cardiology : 5th Edition : EJ Topol

7. Textbook of Interventional Cardiology : 5th Edition : EJ Topol

8. Laser angioplasty of restenosed coronary stents: results of a multicenter surveillance trial: JACC 1999; 34:25-32

9. Excimer Laser Atherectomy in Acute Myocardial Infarction – Evidence-based Treatment Approach ; A report by On Topaz

10. Excimer Laser Atherectomy in Acute Myocardial Infarction – Evidence-based Treatment Approach ; A report by On Topaz

Images Courtesy Spectranetics