Laser type
Wavelength (nm)
Absorption depth (mm)
Absorption mechanism
XeCla (Excimer)
308
0.05
Protein-lipids
Nd:YAGb
1060
2.0
Protein-water
Dye
480
0.5
Protein
Argon
488
0.5
Protein
Ho:YAGc
2060
0.3
Water
Nd:YAG
1320
1.25
Water
Fig. 24.1
The spectrum of light. The wavelength of light emitted from a laser is determined by the lasing medium and it is an important factor in determining the properties of the system. Laser light emitted from the Spectranectics CVX-300 excimer (XeCl) laser system is “Cool” (308 nm) which is similar to laser light employed for LASIK (193.3 nm) used in Ophthalmic surgery. In contrast to infrared lasers, the excimer laser has a shallow penetration of depth (50 um) and ablates tissue precisely without excessive heat production and minimises inadvertent tissue damage
Excimer laser tissue ablation is mediated through three distinct mechanisms: photochemical, photo-thermal, and photomechanical. UV light is rapidly and effectively absorbed by intravascular tissue and thrombus, and the absorbed light breaks carbon-carbon bonds, weakening the structure of the cells (photochemical). In addition, laser light elevates the temperature of intracellular water, eventually producing water vapor causing the cells to rupture. The generation of a vapour bubble cloud at the tip of the catheter enables controlled disruption of the atherosclerotic material (photo-thermal). Expansion and implosion of these vapor bubbles generates the photomechanical effect as the pressure is released from the vapour bubble, further disrupting the obstructive intravascular material as well as sweeping the freed particles downstream (photomechanical). The vast majority of the fragments released during laser atherectomy are >10 microns in diameter and are easily filtered by the reticuloendothelial system downstream which avoids microvascular obstruction and no-reflow phenomena.
The minimum energy needed to penetrate the UV light into the adjacent tissue and consequent creation of a steam bubble called “fluence” (range 30–80 mJ/mm2). “Pulse repetition” is the amount of pulses produced during 1 s. “Pulse width” is the length of each pulse. Pulse width can be adjusted according to needs of the lesions (Fig. 24.2).
Fig. 24.2
The Spectranetics CVX-300 Excimer laser system. Panel i illustrates the pulse generator. The controls are located on the top of the device, and illustrated in panel ii. Two numbers are visible, with fluence on the left (indicated with an orange box, in this case set at 45 mJ/mm2), and pulse repetition frequency on the right (in this example, 25Hz). Panel iii illustrates the two available configurations of laser catheter – concentric and eccentric – referring to the orientation of the laser fibres within the catheter. These are directed at the calibration port (Panel iv) before being introduced into the body
24.2.1 Excimer Laser Equipment and General Technique
The CVX-300 cardiovascular laser excimer system (Spectranetics, Colorado Springs, CO, USA, Fig. 24.2) uses Xenon chloride (XeCl) as the active medium. Consequently, the light produced is pulsed and remains in the ultraviolet B (UVB) region of the spectrum with a wavelength of 308 nm and a tissue penetration depth between 30 and 50 microns. The excimer laser light is produced by a transportable generator that is powered by mains electricity with a standard plug suitable for each country. Its is the only device currently available with FDA approval.
ELCA catheters are delivered using a monorail segment that is well-matched with any standard 0.014″ guidewire and are presented in four diameters for use in the coronary artery 0.9 mm, 1.4 mm, 1.7 mm, and 2.0 mm sizes. The catheters most commonly used have a concentric array of laser fibres at the tip. Alternatively eccentric laser catheters are available where the laser fibres are focused toward one hemisphere of the catheter tip which potentially serves as a better device for debulking In-stent Restenosis (ISR – Fig. 24.2). The larger diameter devices (1.7 mm & 2.0 mm catheters, and the eccentric catheters) are primarily used in straight sections of large diameter vessels for example in treating saphenous vein grafts. They necessitate 7 F and 8 F guide catheters correspondingly, which limits their deliverability via the radial route. The remainder can be used effectively via a 6 F guiding system. From the radial route, care should be taken to select a guiding catheter that provides adequate support, and that remains coaxial during lasing. The use of sheathless large calibre guides (eg Ashai sheathless 7.5 F system, Japan) is a solution for when large calibre laser catheters are being considered.
It is essential that certain safety procedures should be observed in the catheter lab when performing laser atherectomy. Prior to the excimer laser being activated all persons in the room, even the patient, must wear protective tinted spectacles to minimize the risk of retinal exposure to the ultraviolet light and all windows should be covered and the doors should be locked. Following this safety checklist, the laser unit is warmed up and then the selected catheter is plugged into the generator and calibrated prior to being introduced into the body. Even when the catheter is in-vivo, all staff in the vicinity should wear eye protection in case the laser catheter breaks and releases UV light. Laser catheter size selection is primarily based on (a) the severity of the lesion, (b) the reference vessel diameter, and (c) consistency of the target material [12].
The selected catheter is advanced to the lesion over a standard 0.014″ guide wire which is supportive and well placed in the distal coronary vessel, Lasing can commence with the catheter on or just proximal to the lesion. Both the 0.9 mm and 1.4 mm are 6 F compatible catheters, along with the 0.9 mm, X-80 ELCA catheter. This device is constructed to enhance deliverability as the radio-opaque marker is set back from the tip, containing 65 fibers of 50 μm diameter. In general the 0.9 mm X80 catheter is used in non-crossable, non-dilatable fibro-calcific lesions given it’s deliverability characteristics and because this catheter can emit the most power (80 mJ/mm2) at the highest repletion rate (80 Hz) necessary for ‘balloon resistant’ coronary lesions. The larger (1.4–2.0 mm) catheters are used in larger diameter vessels with straight segments and are therefore particularly useful when dealing with heavy intra-coronary thrombus or in the treatment of saphenous venous grafts (SVG). In some circumstances more than one catheter may be required, gradually increasing size based on the result obtained from the initial laser procedure.
24.2.2 Saline Infusion Technique
Both blood and iodinated contrast media, in comparison to water or saline, absorb the majority of delivered excimer laser energy. If not removed from the catheter tip, this results in the formation of cavity micro-bubbles at the site of energy delivery. This can potentiate the effect of the pressure waves at the catheter tip, and this increases the likelihood of traumatic vessel dissection [13]. In order to keep the tip free from contrast/blood during lasing. it is recommended that a saline flush/infusion technique is used [14]. This maximizes the delivery of laser energy directly into the atherosclerotic material, limiting vascular dissection rates [15].
In order to clear blood from the catheter/tissue interface, a 1 l bag of 0.9 % saline is connected to the manifold via a three-way tap, and a clean 20 ml Leur-lock syringe replaces the standard contrast syringe. Once contrast has been cleared from the flush tubing, confirmed by direct screening whilst purging the system with saline, 5 ml of saline should be infused prior to laser activation followed by a slow injection continued at a rate of 2–3 ml/s throughout the lasing process (usually 5–10 s). It is important to ensure that the guide catheter is well intubated into the coronary artery to ensure saline delivery to the laser catheter tip. It is important to directly visualize under fluoroscopy that all contrast is flushed from the guide catheter in the selected starting position for lasing, and that the saline does not get mixed with contrast. For the coronary catheters the duration of each lasing train is preset so for the standard catheters activation will automatically stop after 5 s with a 10 s rest period before the next laser train can commence. The X80 0.9 mm catheter permits 10 s activation and 5 s rest period reflecting its use in more complex lesions subsets. Continue the lasing, until the catheter has gone through the lesion or enough alteration has been made to allow balloon expansion.
The excimer laser energy is carried in pulses as the catheter is slowly [0.5 mm/s] advanced through the lesion. Since the depth of the excimer laser penetration is shallow [35–50 micron] the slow progression along the target lesion provides enough absorption of the emitted light energy into the lesion and subsequent ablation of the atherosclerotic plaque and thrombus. Upon completion of several trains of emission along anterograde laser propagation the laser catheter should perform retrograde lasing particularly in severe lesions when there is resistance for anterograde crossing. Continuous saline flushes accompany all stages of the procedure to reduce adverse expansion of acoustic shock waves from interaction between the contrast media or blood and the laser light. Application of laser in blood or contrast media is rarely performed in certain specific situations, but should only be undertaken by experienced operators (see later sections).
24.2.3 Specific Radial Considerations
When planning ELCA via the radial approach, there are a number of a factors that should be considered:
- 1.
Route of access – In general, the native coronary vessels can be easily accessed from the right radial approach. Access to vein grafts anastomosed onto the ascending aorta can be technically challenging from this route, with guide support often poor. The left radial approach is these circumstances offers more guide support, and in general preferred for patients in whom graft intervention is being undertaken.
- 2.
Size of the Laser catheter: its diameter being employed will determine the diameter of the guiding catheter that should be used. Both 0.9 mm and 1.4 mm catheters can be used via a 6 F guiding system, with the 1.7 mm and 2.0 mm requiring 7 F and 8 F diameter guiding catheters respectively. If it is felt that a larger diameter is required, then solutions for delivery of large calibre guiding systems include the use of sheathless guides or balloon tracking to deliver such equipment to the aortic root.
- 3.
Shape of the guide – In general, a supportive guide is preferred. It must align co-axially with the coronary ostium to allow adequate flow of saline during injection. Operators should gain experience in PCI using the radial approach and selecting appropriate guiding catheters prior to embarking upon complex lesion subsets that may require ELCA.
24.2.4 Contraindications
Laser coronary atherectomy can be safely performed in PCI institutions without on-site cardiothoracic surgery but as with all PCI procedures, arrangements for offsite surgical cover must be in place. Other than lack of informed consent and unprotected left main disease [a relative contraindication] there are no absolute coronary contraindications to laser.
24.2.5 Avoiding Complications – Tips and Tricks (Fig. 24.3)
Fig. 24.3
Tips and tricks for successful laser atherectomy. Once the indication for laser has been defined, the appropriate catheter selected, the access route can be determined. Almost all devices can be delivered via the radial route, although delivery of an 8 F catheter (or equivalent sheathless guide) requires care and many need techniques such as balloon tracking. Once an appropriate coaxial guide has been selected, that maintains good co-coaxial support, then ELCA can be undertaken once adequate safety precautions have been employed
ELCA complications are on the whole similar to those encountered during routine PCI. Specific issues arise from interruption of the continuous saline flush/contamination with contrast, which can generate excessive heat, and increase the risk of vascular perforation. ELCA is not recommended when the operator is conscious of presence of large length sub-intimal guidewire positioning (as is the case in typical dissection re-entry case). In such cases, any anterograde injection may lead to propagation of a dissection plane and ultimately resulting in a longer length of stenting if not immediate no-reflow phenomena. Furthermore the saline infusion is unlikely to reach the intended target given the lack of run off from the lesion and will be therefore ineffective. In addition, ELCA catheters are relatively indiscriminate in performing tissue ablation and will essentially ‘modify’ any tissue in their field of delivery. Within the sub-intimal space the catheter would lie in closer proximity to the media and adventitia of the vessel that may cause perforation. This should be reserved for operators experienced in both CTO and ELCA intervention.
24.3 Clinical Indications for ELCA
As the application of ELCA has been refined, a number of indications have emerged for the technique. These are summarised below, with a brief summary of the evidence that underpins the use of ELCA. Each indication is illustrated with a case, with ELCA being applied via the radial route.
24.3.1 Acute Coronary Syndromes and Myocardial Infarction
Patients presenting with acute myocardial infarction (AMI) represent a medical emergency. There is marked activation of the clotting cascade with production of large amounts of platelet and fibrin rich thrombus within the coronary arterial vasculature. In most developed countries the recommended treatment for AMI associated with ST segment elevation on the ECG is immediate emergent PCI (Primary PCI) [16, 17]. The preferred route of access in primary PCI should be radial, with a mortality advantage being demonstrated in a number of randomised trials and meta-analysis [18]. In such circumstances, ELCA may be a beneficial revascularisation modality given its potential for effective thrombus removal [19], promotion of fibrinolysis [20], platelet stunning effects [21], and concomitant plaque debulking [22].
However, randomized clinical data regarding for the use of ELCA in AMI remains limited. The largest study to date, The CARMEL [Cohort of Acute Revascularization of Myocardial infarction with Excimer Laser] [23] multicenter registry, registered 151 AMI patients from 6 centers in the USA, 1 in Canada and 1 in Germany. One in five cases involved a SVG, and large thrombus burden was present in IRA in 65 % of the patients. Adjunctive glycoprotein IIb/IIIa inhibitor (GPI) was administered in 52 % of the cases. Following ELCA, TIMI flow grade was significantly increased from 1.2 to 2.8, with an associated reduction in angiographic stenosis (83–52 %). Overall a 91 % procedural success rate, a 95 % device success rate and a 97 % angiographic success rate were reported [23]. There was a low rate (8.6 %) of associated major adverse coronary events (MACE) (3 % dissection, with just 0.6 % rate of distal embolization and associated no-reflow). The greatest laser effect was observed in lesions associated with a outsized angiographic thrombus burden. Further data has suggested that ELCA is accomplished of removing up to 80 % of the thrombus burden from the treated targets [24]. Two other small registries examining the effects of ELCA in ACS suggested a greater outcome with regards to TIMI flow and myocardial blush grade compared with manual thrombus aspiration devices [25, 26]. There is a single randomized trial, the Laser AMI study. This included just 66 patients, and demonstrated safety and feasibility. A second, larger Laser AMI study is ongoing in patients treated with Primary PCI, with a 1:1 randomization to either ELCA or manual thrombus aspiration followed by standard PCI strategies. The primary endpoint in this study will be MACE at 6 months follow-up and is due to report in 2016 [27].
In the majority of cases, a 6 F guide catheter is most frequently used catheter diameter when treating AMI. This immediately limits the choice of ELCA catheter to either the 0.9 mm or 1.4 mm diameter device. Practically, a longer lasing duration (10 s) is preferred due to the enhanced thrombus ablation that occurs. Therefore, the 0.9 mm X80 catheter is often used in preference to the 1.4 mm device due to its ability to deliver a longer duration of Laser pulses, for relatively little loss in luminal diameter (Fig. 24.4).
Fig. 24.4
ELCA for thrombus (and calcium). The use of ELCA in intra-coronary thrombus. This 68 year old male presented with inferior ST elevation myocardial infarction having previously had a tissue aortic valve replacement 6 years before. Primary PCI was undertaken form the right radial artery. Panel i illustrates a large dominant right coronary artery (RCA) containing a large amount of organised thrombus associated with calcific culprit tandem lesions. Noting the tortuous nature of the proximal vessel, the guide was switched for an amplatz shape (box A), and a guideliner (box B) was used to deliver the 0.9 mm laser catheter(box C) to the lesion. The effect of laser can been seen in panel iii, with a dramatic reduction in organised thrombus. The additional benefit of laser was also to modify the calcified lesions to permit balloon and subsequent stent passage to complete the case. The guideliner was then engaged deeply into the vessel to allow delivery of overlapping drug eluting stents, with an excellent overall final result (panels v and vi)
24.3.2 ELCA for Non-crossable/Non-dilatable Lesions (Balloon Failure)
In contemporary PCI practice and with an expanding elderly patient cohort, it is not unusual to find that a coronary lesion can be crossed with a 0.014″ guidewire but either a low profile balloon fails to cross or once across the lesion fails to fully expand. This situation is considered as balloon failure, and a situation where ELCA may be applied. The success rate in uncross able or un dilatable stenoses is high, approaching 90 %, using the X80 catheter. However, when these targets are calcified, the response is less favourable to laser debulking than that of non calcified lesions [79% vs. 96%, p<0.05] [28, 29].
However, in cases of balloon failure, the default technique for the majority of PCI operators remains rotational atherectomy (RA). Even for the proficient ELCA user, RA would be preferred if there was heavy calcification as it is more effective at debulking. This method necessitates delivery of a dedicated 0.009″ guidewire (Rotawire™) into the distal coronary vessel. This wire is less deliverable directly, and it may not be possible either independently or through a micro-catheter exchange system when the lesion is very stenotic/calcific. Here, ELCA can be used upstream to modify the lesion and create a channel through which a Rotawire™ can be delivered distally (usually via a micro catheter) and RA then used for lesion debulking. We first termed the combination of ELCA and RA as the RASER technique and we have developed this use of combined atherectomy devices to great clinical effect in a number of challenging cases [30]. It is particularly effective for non-crossable, non-dilatable calcified stenosis and has been demonstrated to have a low complication rate in experienced hands [30, 31].
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