Читать книгу Interventional Cardiology - Группа авторов - Страница 139

The angioplasty balloon consists from proximal to distal of a hub, a proximal shaft, and a distal shaft. It has a cylindrical body with proximal and distal conical tapers and a distal tip (Figure 5.10). Early balloon catheters had a fixed wire proximal to the balloon, as dual lumen catheters were typically bulky and difficult to advance into the coronary circulation. Contemporary balloon catheters are dual lumen with separate ports for the guidewire and balloon inflation. OTW balloons have a lumen for the guidewire extending along the length of the catheter, a feature very useful and sometimes essential for procedures requiring wire exchange such as in the treatment of CTO, crossing of very tortuous lesions, advancement of poorly steerable wires such as those for rotational atherectomy. In most of these applications, however, the bulky OTW balloon catheters have been replaced by microcatheters. The principle of the Monorail or rapid exchange technique is that the wire lumen is limited to a short segment (20–30 cm) at the distal tip which allows the rapid exchange of balloons with no need for long wires or wire extensions. The shaft of the catheter only contains a lumen for balloon inflation and deflation (i.e. can be thinner) and often consists of a reinforced hollow metal tube providing great pushability.

Figure 5.10 The primary curve is shaped to fit the tightest angle to be wired and the secondary curve to reflct vessel size.

The parameters considered when selecting a balloon are the crossing profile, balloon diameter when inflated at nominal pressure, length, and compliance.

Balloon diameter is normally selected to match the vessel size with balloon to artery ratios of 1:1 in general. Vessel size can be measured using quantitative coronary angiography (QCA) or intravascular imaging such as intravascular ultrasound (IVUS) or optical coherence tomography (OCT). For predilatation, “undersizing” may be acceptable whereas for postdilatation balloon to vessel ratios are typically equal or greater than 1:1. For long tapering lesions, the diameter of the vessel at the distal end of the segment to be dilated is typically used as the reference vessel diameter for balloon selection. An appropriately sized balloon for postdilatation is a critical step to achieve better expansion and apposition when the initial balloon deployment fails, despite the high pressures allowed by modern stent delivery balloons, to fully expand the stent.

Balloon length is selected depending upon lesion and stent length. Especially after the introduction of drug eluting stents when the principle is to avoid injuring segments that will not be covered by stents, a situation known as geographic miss, smaller balloons tend to be used for predilatation, just aiming to create a passage for stent insertion and exclude the presence of truly undilatable lesions [8]. Postdilatation balloons should be shorter than the stent and short balloons (8 or also 6 mm long) are recommended for effectively postdilating resistant diaphragm lesions.

The first angioplasty balloons were composed of flexible polyvinyl chloride (PVC), a material characterized by great compliance. Subsequent generations were made of cross‐linked polyethylene, polyethylene terephthalate (PET), nylon, Pebax, and polyurethane. Most modern balloons allow controlled limited expansion, burst resistance up to high pressure, and have a low crossing profile. The tip style (tapering, length, flexibility) varies substantially among different balloons, and is one of the factors contributing to a successful crossing. Compliant balloons show a linear increase in diameter with increasing inflation pressure whereas the diameter increase tends to plateau in semi‐ or non‐compliant balloons until reaching the rated burst pressure. More compliant balloons have a limited pressure range whereas non‐compliant balloons have a limited diameter range and are useful for treating resistant lesions requiring high pressure inflation or postdilatation. Semi‐compliant balloons fall between these two extremes and tend to be multipurpose “workhorse” balloons. Familiarity with the compliance charts of balloons is necessary to reduce the risk of trauma to the healthy vessel or of exceeding the vessel elasticity and induce dramatic vessel ruptures. Terms encountered on these charts include the following:

1 Nominal: the pressure at which the balloon reaches its nominal diameter (diameter on the label).

2 Rated burst pressure: the pressure below which in vitro testing has shown that 99.9% of the balloons will not burst with 95% confidence.

3 Mean burst pressure: the mathematical mean pressure at which a balloon bursts.

Wall stress within a cylindrical balloon can be represented by the following equations:

where σ_radial = radial stress, σ_axial = axial or longitudinal stress, p = pressure, d = diameter, and t = wall thickness. It can be seen that wall stress is linearly proportional to diameter which means that higher dilatation pressure is possible with smaller diameter balloons. Furthermore, axial stress is half of radial stress which means that balloon rupture is usually longitudinal rather than circumferential and therefore less likely to result in vessel trauma.

Balloons have proximal and distal radiopaque markers to allow positioning (one central marker for some small diameter balloons). Rewrap refers to the ability of the balloon to regain its original folded state following deflation. Deflation and rewrapping can take time when large and long balloons are used. Rewrapping is essential to allow safe withdrawal of the balloon into the catheter. Stent deployment balloons tend to rewrap less well, have more variable expansion characteristics, and should ideally not be used for postdilatation.

The past decade has heralded the development of several specialty balloons including ultralow crossing profile balloons, cutting balloons, focal force balloons, and drug‐coated balloons (Figure 5.10) with specific applications for each type of balloon.

Ultralow crossing profile balloon catheters have been recently developed, aimed to improve crossability and pushability to treat the most complex lesions. As mentioned above, the “crossing profile” is one of the parameters that has to be considered when selecting a balloon, especially when facing very severe calcific lesions that may represent a challenge for the operators. In such cases, using an ultralow profile balloon, characterized by optimal tracking and crossing properties, may help to solve the problems almost in all cases. Among these, balloon catheters such Ikazuchi (Kaneka, Osaka, Japan) and Tazuna (Terumo, Tokyo, Japan) have become routinely available. The Ikazuchi Zero catheter balloon is a specialized semi‐compliant balloon which presents good crossability in CTO or calcified lesions with its very low entry‐ and balloon‐profile. Tazuna balloon catheter crossing performance is gained by low entry profile (0.40 mm/0.016”) for the most challenging lesions, a flexible and robust tip to cross tight lesions and stent struts and by the presence of a hydrophilic coating on the distal shaft able to facilitate navigation through tortuous vessels and calcified distal lesions. Moreover, the mid‐shaft construction balances flexibility and stiffness, maximizing the transmission force to the distal section enhancing the pushability of the catheters.

The Glider balloon catheter is a rapid‐exchange PTCA catheter compatible with 0.014” coronary guide wires and 6 F guiding catheters. The most distinctive feature of the Glider balloon is the presence of an oblique cut of the distal tip, which can be rotated from the proximal hub because of its unique braided reinforced and lubricious shaft. The balloon diameters currently available are 2.5 and 3.0 mm, the length is 4.0 mm with a single mid marker and a rated burst pressure of 16 Atm with low compliance. This short balloon is specifically designed for optimal side branch ostium expansion without damaging the distal side branch, and thus, minimizing the risk of side branch dissection [19].

A list of the most commonly used ultra low profile PTCA balloon is reported in Table 5.6.

Table 5.6 List of commonly used ultra low profile PTCA balloon.

	Lesion Crossing profile(“) /balloon diameter (mm)
Nic Nano Hydro CTO Balloon, SIS medical, Swiss	0.0195” / 0.85mm
Tazuna, Terumo, Tokyo, Japan	Not reported / 1.25mm
Ikazuchi (Kaneka, Osaka, Japan	0.0157” / 1mm
Sapphire II Pro CTO, Orbus Neich, Hong Kong	0.016” / 1mm

Cutting balloon and scoring balloon catheters represent two different strategies developed in the context of resistant lesion preparation, enabling to prepare the coronary plaque and to create cracks in the calcium or dense fibrotic wall before stenting.

The cutting balloon has been available for almost 30 years. It is a semi‐compliant balloon with three thin sharp blades mounted on its body, designed to cut the continuity of fibrocalcific plaque once the pressure of the balloon forces them against the vessel wall, creating fissures on the plaque. The Flextome cutting balloon (Boston Scientific, Malborough, MA, USA), has three blades equally spaced around its circumference which come into contact with the arterial wall and score the vessel wall. The balloon is specifically indicated for discrete lesions with resistance to conventional balloon angioplasty without heavy calcification. Despite its theoretical advantages, in the Cutting Balloon Global Randomized Trial the primary endpoint of six‐month binary restenosis did not differ between CBA and POBA (31% vs. 30%, p=0.75) and the rate of perforation was higher with CBA (0.8% vs. 0%, p=0.03) [20]. These negative results, together with the difficulties associated with cutting balloon delivery due to its high crossing profile (0.041‐0.046”), led to the development of the newer generation of cutting balloon Wolverine™ Cutting Balloon (Boston Scientific, USA). In the Wolverine CBA, the atherotome’s support thickness has been reduced, becoming compatible with 6 Fr catheters for all diameters, without affecting the functional height of the blade, resulting in an overall smaller crossing profile and improved crossability.

Scoring balloons, also known as “focal force balloons”, are semicompliant balloons encircled by scoring elements. These scoring elements allow focal concentration of the force during inflation and decrease balloon slippage. Scoring balloons have similar indications to cutting balloons, but scoring balloons are more flexible, have a better profile and can achieve a full expansion with a low inflation pressure, with consequently less trauma to vessel walls and a minor risk of coronary dissections [21,22]. Several types of scoring balloons are now available for treatment of mild to moderate calcified lesions. The AngioSculpt (Spectranetics‐Philips) is a semicompliant balloon with three spiral rectangular Nitinol scoring elements, also available in a drug‐coated version (AngioSculpt X, Spectranetics‐Philips). Developed for both coronary and peripheral vascular applications, the Angiosculpt is similarly used in highly resistant lesions when conventional balloons are unable to dilate the vessel. In a feasibility trial, the AngioSculpt balloon was used for the treatment of de novo lesions prior to BMS implantation and showed very high procedural success and a target lesion revascularization rate of 10% at 6 months [23]. These results were confirmed in an observational study, in which 37 patients treated with AngioSculpt before stent implantation were compared to 145 patients treated with direct stenting and 117 patients with traditional plain old balloon angioplasty before stent implantation. IVUS assessment showed greater stent expansion in the AngioSculpt group than in the other two groups (89% versus 74% of vessels with an area >5.0 mm2, respectively) [24].

Another focal force balloon is the Chocolate balloon (TriReme Medical, Pleasanton, CA, USA), which is a traditional semi‐compliant balloon within a nitinol cage. When the balloon is inflated, the cage restrains the balloon expansion, and the balloon protrudes from between the struts of the nitinol cage applying focal pressure to discrete areas of the plaque. Theoretically, this will result in more controlled plaque fracture. The NSE Alpha scoring balloon (B Braun, Berlin, Germany) has three triangular flexible nylon elements on the balloon surface attached only at the proximal and distal edges of the balloon. Promising results for predilatation of severe calcified lesions were shown with the leopard‐crawl technique [25]. The Scoreflex (Orbus Neich) is a semicompliant balloon with two fixed Nitinol wires on opposite sides of the balloon surface [26]. Otsuka et al. reported a case series where prolonged inflation of the Scoreflex balloon allowed adequate dilation of severe calcified plaques as shown by the ‘creep phenomenon’, whereby prolonged inflation of the balloon produces a distortion force capable of expanding a resistant calcified lesion [27]. Scoring balloons have been considered by cardiologists as an alternative to cutting balloons and, in recent years, have been preferred because of major flexibility and deliverability, although no specific randomized control trials are reported in the literature.