INTRODUCTION

Laser is an acronym for ‘light amplification by stimulated emission of radiation’ and describes the process by which an intense monochromatic coherent light beam is produced. This beam can be delivered to a small area with great precision through fibreoptics allowing tissue to be ablated by a combination of mechanical and chemical methods. These properties suggest that lasers would find clinical application in some of the challenges of interventional cardiology such as guidewire passage through chronic occlusions, dissection following balloon angioplasty and restenosis. The ability to create channels in myocardium with great precision has also been used for myocardial laser revascularisation, both percutaneously and via thoracotomy, in patients with chronic refractory angina.

The presence of a chronic total coronary artery occlusion continues to be a major problem for interventional cardiology. Around 10% of all attempted coronary angioplasties are performed for chronic total coronary occlusion.¹ The procedural success rate is lower for patients with chronic total occlusions who undergo coronary angioplasty when compared with patients who have stenosed, but patent, vessels. Depending to some extent on the nature of the occlusion and its duration, the success rate is usually in the order of 50–70%^2,3 – this is clearly much lower than for stenosed vessels. Failure to treat a chronic total occlusion may be due to the inability to cross the lesion with a guidewire or to inability to advance the balloon catheter across the lesion once the wire has crossed successfully. When treating a chronic total occlusion, the conventional technique uses a stiff guidewire and advancement of the balloon catheter close to the tip of the guidewire for additional rigidity. A variety of technologies have been introduced without a major improvement in the success rates. These include guidewires with olive-shaped tips, drills of various velocities, radio frequency heat applicators and laser devices. Two laser technologies will be considered further: first the laser guidewires and second the over-the-wire Excimer laser catheter.

LASER GUIDEWIRES

An Argon laser heated balltip (hot tip) guidewire has been used for the initial passage through chronic total occlusions.⁴ In some cases, it has been successful when conventional systems had failed. Vessel perforation is a possible complication of the technique. Further developments in hot tip recanalisation have not occurred and this technology is now rarely used.

The results of the bare Argon laser instrument Lastac are again comparable to those achieved with less costly mechanical means, although the cases undertaken may have been slightly more complex.⁵ A beneficial mechanical component is undoubtedly present with these catheters, which have many of the features which may be useful when treating chronic total occlusions (e.g. stiffness, bluntness). Randomised prospective trials against conventional technologies have not been carried out and this equipment now has very limited application.

EXCIMER LASER CATHETER

Excimer laser coronary angioplasty may help to solve some of the problems associated with treating chronic total occlusions. The technique requires the passage of a guidewire through the lesion, which clearly is not always possible. Once the guidewire has been advanced to the distal vessel, the Excimer laser catheter can be used to ablate tissue.

The Excimer laser system can be used with over-the-wire multifibre catheters of different diameters. Generally, the larger the diameter of the vessel being treated, the larger the diameter of the laser catheter utilised. Once the occlusion has been crossed with a conventional guidewire, the laser catheter is usually advanced to a position about 5 mm proximal to the lesion. The laser treatment is then initiated and during laser ablation the catheter is advanced slowly – at about 1 mm per second or less. Following the initial laser procedure, an acceptable angiographic result may have been achieved, or it may be necessary to sequentially increase the catheter size, or it may be necessary to use conventional angioplasty technology (i.e. balloon catheters and coronary stents). Periprocedural medications include aspirin, heparin (bolus dose), intravenous nitrate and calcium channel blockers.

In the Excimer Laser Coronary Angioplasty Registry, 172 chronic total occlusions were treated in 162 patients (10.3% of the 1569 patients entered). Once a guidewire crossed an occlusion, the overall laser success rate for treatment of chronic total occlusions was 83%. The extent of stenosis decreased from 100% to 55 *b+ 26%.⁶ In 74% of patients, adjunctive percutaneous balloon coronary angioplasty was used after laser treatment. A final procedural success, defined as less than 50% residual stenosis and no major complication (death, myocardial infarction or CABG) was achieved in 90%. Major complications were infrequent: one death, 1.9% myocardial infarction and 1.2% requirement for emergency bypass surgery. The results suggest that Excimer laser angioplasty may be useful for treating chronic total occlusions that can be crossed with a guidewire but not with the balloon catheter. A role has also been suggested when the occlusion has been confirmed to be extremely long.⁶

Excimer laser angioplasty continues to develop, particularly as an adjunct to conventional balloon angioplasty or coronary stenting in the treatment of chronic total occlusions. The data suggest that failure of laser angioplasty occurs because of low catheter flexibility and the need for good guidewire support when treating total occlusions – once the catheter has reached the target area, the morphology of the lesion seems to be of only minor importance for the success of the procedure.⁷ An alternative strategy when treating totally occluded arteries, which can be crossed with a guidewire, but not the balloon catheter is to use the Rotablator system. This technology, which uses a high-speed rotational burr, produces ablation of tissue. By using burrs of increasing diameter, the lesion may be successfully treated by the rotablator technique alone. Frequently, coronary balloon angioplasty or coronary stenting are required after the initial rotablator therapy. Currently there are no randomised studies comparing Excimer laser angioplasty with Rotablator in the treatment of chronic total occlusions which can be crossed with a guidewire but not a balloon catheter.

LASER BALLOON ANGIOPLASTY

Conventional balloon coronary angioplasty improves luminal dimensions by producing fracture of the atheromatous plaque and by stretching the plaque-free wall. It is associated with dissection of the media and the formation of intimal flaps. These changes create local flow abnormalities which are associated with varying degrees of mural thrombus formation. In the vast majority of cases, dissection and thrombus formation do not result in acute vessel occlusion. During the next few months, however, the vascular reponse to injury may lead to restenosis. The aim of the laser balloon angioplasty is to create a large, smooth vascular lumen, which may lead to better short- and long-term results than conventional balloon angioplasty. In theory, this could potentially be achieved by thermal welding of dissection flaps, the elimination of vascular recoil, the elimination of coronary vasospasm, the reduction in platelet activation and the inhibition of smooth muscle cell proliferation.⁸

Laser balloon angioplasty, therefore, permits the application of heat (generated by the laser source) and pressure (by balloon inflations) to thermally weld tissue during coronary angioplasty. The system uses an Nd: YAG laser and a modified coronary balloon angioplasty catheter. The dose of laser is usually delivered over a period of around 20 seconds, which results in adventitial temperatures of between 90 and 110*b∞C. The technique is very similar to conventional balloon angioplasty. The laser balloon catheter is positioned over a 0.014” guidewire. Once the balloon is in the appropriate location, it is inflated to low pressure (usually about 4 atmospheres) and the programmed laser dose is delivered over around 20 seconds. The balloon inflation is continued for an additional 20–40 seconds while the temperature of the arterial wall returns to normal. It is unusual for further balloon dilatation to be necessary in order to improve the immediate angiographic result.

Laser balloon angioplasty has been shown to be effective in the management of acute failure of balloon coronary angioplasty, due to either immediate vessel recoil or severe dissection with impaired flow (‘impending closure’). Despite this early success rate, however, the late angiographic restenosis rate of laser balloon angioplasty is very similar to conventional balloon angioplasty.⁸ This is despite laser balloon angioplasty producing larger vessel lumens in the acute stage. With the introduction of coronary stents, the potential role for laser balloon angioplasty seems to have been taken over. The use of coronary stents will usually solve the acute problem of vessel recoil or severe dissection resulting in threatened occlusion. In addition, coronary stents are associated with a lower angiographic restenosis rate. Therefore, there would now appear to be no role for the technique of laser balloon angioplasty.

In the vast majority of patients with angina pectoris due to coronary artery disease, antianginal medication, balloon coronary angioplasty/coronary stenting, or CABG can achieve successful treatment. Some patients, however, have angina refractory to medical therapy and have diffuse disease in the distal part of their coronary circulation, which is not amenable to treatment by either coronary angioplasty/stenting or coronary artery bypass surgery. Such patients have often undergone several revascularisation procedures in the past. In this group of patients, who present a difficult management problem, new laser techniques have been utilised in recent years. These include transmyocardial laser revascularisation (TMLR) and percutaneous myocardial revascularisation (PMR).