Interventional Cardiology. Группа авторов

the key ingredients for a successful PCI and reflected the experience and quality of individual operators. There has been a period when direct stent deployment became fashionable. The fact that stents can be inserted without pre‐dilatation in most cases, however, does not mean that this is a good idea, with the possible exception of some primary angioplasty for thrombus containing lesions or frail lesions in SVGs. Lack of predilatation means inability to spot undilatable lesions, leaving unexpanded stents and underestimation of true vessel size. Post‐dilatation has now become standard of care for most lesions in experienced hands. With the advent of drug‐coated balloons, angioplasty balloons are also able to deliver more than acute gain from vessel dilatation, with adjunctive pharmacotherapy able to mitigate neointimal hyperplasia and reduce restenosis.

      Anatomy of a balloon catheter

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 In most of these applications, however, the bulky OTW balloon catheters have been replaced by microcatheters.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.

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