Basics of Percutaneous Coronary Interventions

1 Basics of Percutaneous Coronary Interventions



On September 16, 1977, Andreas Grüntzig performed the first human percutaneous transluminal coronary angioplasty in Zurich, Switzerland. Until then, coronary artery bypass surgery was the only alternative to medicine for the treatment of coronary artery disease. Over the past 35 years, PTCA has evolved into more sophisticated techniques involving predominantly stenting and is now called percutaneous coronary intervention (PCI). PCI is one of the most successful methods of coronary revascularization with more than 1,500,000 procedures done in the United States alone. PCI is the treatment of choice for discrete single- and double-vessel coronary lesions and plays an important role in complex revascularization in patients with multivessel coronary artery disease, including left main narrowings with and without depressed left ventricular function. Techniques and technology used in PCI extend into the treatment of peripheral arterial disease. Further evolution of PCI into the treatment of structural heart disease (valves, septal defects, etc.) is emerging as a separate discipline within interventional cardiology.


PCI encompasses various coronary techniques, such as balloons, stents, cutters, lasers, grinders, suckers, filters, and other tools. The term percutaneous transluminal coronary angioplasty (PTCA) is used when describing techniques and outcomes related to use of the original balloon inflation technique first used by Grüntzig.


This chapter is the extension of the PCI chapter from The Cardiac Catheterization Handbook, fifth edition, and presents the basic method and mechanisms of balloon angioplasty and stenting as an introduction to the practice of interventional cardiology. The various techniques of PCI can be placed into niche applications for specific devices (Table 1-1).




Overview of the Basic PCI Method


PCI was derived from the basic procedures used for diagnostic cardiac catheterization and coronary angiography. PCI begins with vascular access by means of the same techniques for the insertion of an arterial sheath through the arm (radial artery) or leg with Seldinger’s method (needle and guidewire). In contrast to diagnostic catheters, specialized large-lumen “guiding” catheters engage the coronary artery in the same manner but are designed to provide stabilization or backup for delivery of PCI equipment.



Steps in the PCI Procedure


The steps in the PCI procedure are shown in Figure 1-1. First, a guiding catheter is seated in the coronary ostium. A thin, steerable guidewire is introduced into the guide catheter and then into the coronary artery and positioned across the stenosis into the distal aspect of the artery. A very small angioplasty balloon catheter is placed on the guidewire, inserted through the guiding catheter, and is positioned in the artery across the stenotic area by tracking it over the guidewire. Once correctly placed within the narrowed area to be treated, the balloon on the PCI catheter is inflated several times for brief periods (10–60 seconds). The inflation and deflation of the balloon expand the stenosis and restore blood flow to an area of the heart previously deprived by the stenosed artery.



After the balloon expands the stenotic area, the balloon catheter is exchanged for a stent-carrying balloon catheter. The stent is a metal scaffold, mounted in a compressed form on another balloon catheter, and delivered in the same manner as the first balloon catheter was delivered. The stent is deployed by inflation of the balloon as was performed for dilating the stenosis. The stent should be carefully positioned. It is inflated with the same pressure gauge syringe (8–16 atm pressure) for 10 to 20 seconds. A full opening of the stent with complete strut apposition to the vessel wall is important for good short- and long-term results.


After the stent struts have been expanded and implanted into the artery wall, the balloon is deflated, and the delivery catheter and guidewire are removed. Intravascular ultrasound (IVUS) imaging is often used to confirm appropriate vessel-stent matching and full stent strut apposition (contact without space against the wall). After IVUS and final angiography have been performed, the guide catheter is removed. The femoral or radial arterial sheath is removed, and hemostasis is obtained in the laboratory. The patient is then transferred to a recovery area and then to the patient’s room. If no complications occur, the patient is discharged the next morning. The patient usually returns to work shortly (<2 days) thereafter. The definitions of a successful PCI procedure are summarized in Table 1-2.


Table 1-2 Definitions of PCI Success

















Percutaneous coronary intervention (PCI) success may be defined by angiographic, procedural, and clinical criteria.
Angiographic Success

Procedural Success

Clinical Success




Indications for PCI


Guidelines and recommendations for the performance of PCI are provided in extensive detail in the updated (2009) PCI guidelines written by the American Heart Association (AHA), the American College of Cardiology (ACC), and the Society for Cardiovascular Angiography and Interventions (SCAI). Specific anatomical and clinical features for each patient should be considered for the likelihood of success, failure, and risk of complications with vessel closure, morbidity, mortality, and restenosis. Restenosis and incomplete revascularization must also be weighed against the outcome anticipated for coronary artery bypass graft (CABG) surgery.


In general, PCI is indicted for patients with the following:






PCI Equipment


The most commonly used PCI equipment consists of four basic elements: a guiding catheter, a balloon catheter, a coronary guidewire, and a stent (Fig. 1-2 and Table 1-3 list approximate costs of such equipment).



Table 1-3 Approximate Costs of Coronary Angioplasty Equipment













































Equipment Cost ($US)
Balloon dilatation catheter 300 OTW/250 Rx
Guiding catheter 65
Guidewire 80
Exchange guidewire (300 cm) 120
Indeflator 55
Y connector 31
Sheath introducer 8
Torque tool 18
Nonballoon devices  
Stent (noncoated) 1300
Stent (drug-eluting) 2300
Rotablator 1330
IABP 840

IABP, intra-aortic balloon pump; OTW, over-the-wire; Rx, rapid exchange.


There are three major operational challenges with PCI: (1) placing the guide catheter in a stable position, (2) negotiating tortuous vessel segments with the guidewire, and (3) delivering the stent through tortuous segments. To complete the PCI, the operator must control the three principal movable components (guide catheter, balloon catheter, and guidewire) simultaneously.



Guiding Catheter


A special large-lumen catheter is used to deliver the coronary balloon catheter and other interventional devices to the vessel that contains the lesion to be dilated. The features of the guide catheter noted in Figure 1-3 differentiate it from diagnostic catheters.




Functions


A guiding catheter serves three major functions during angioplasty:




Balloon Catheter Delivery and Guidance


To deliver the balloon catheter to the coronary artery over the guidewire, the guiding catheter should be seated with the tip parallel to the long axis of the artery (coaxial) at its origin. Coaxial alignment permits safer transmission of force needed to advance the balloon across a stenosis. This act may require guide catheter repositioning or occasionally deep seating into the artery.


Adequate contrast injection through the guide catheter is critical to position the balloon/stent and depends on the size of the guide catheter lumen with the angioplasty device in place. A guiding catheter must be large enough to permit adequate contrast administration with the PCI catheter in place to opacify the target vessel and visualize the lesion. Large, nonballoon PCI devices (Rotablator, thrombus aspiration catheters, etc.) in small guide catheters may not allow adequate vessel visualization during angiography. This problem has been overcome with large-lumen, small guide catheters and contrast media power injectors in some laboratories.


Operators should select a guide catheter with a lumen diameter large enough to allow enough contrast flow around the PCI device to permit clear angiographic imaging of the lesion. As balloon and PCI catheters have become smaller, the size of internal diameter of the guiding catheter has become less important for achieving adequate visualization. A large guide catheter lumen, however, is critical to facilitate easy passage of double balloon/stent systems for complex or bifurcation lesions.



Backup Support for Balloon Catheter and Stent Advancement


Support or “backup” for stent advancement is achieved after seating (cannulation) the guide catheter in the coronary ostium. The guiding catheter provides a platform from which one can push the stent over the guidewire through the artery and across the stenosis.


Inadequate backup support will result in failure to cross a lesion and an unsuccessful procedure. Backup support requires a combination of correct coaxial (in-line with the artery ostium) alignment and the ability to provide carefully controlled advancement (deep seating) of the guiding catheter into the coronary ostium.


The improved quality and size of currently used stents have reduced the need for robust backup support in most situations. For more complex and technically difficult lesions, the choice of an appropriate guiding catheter for extra support and lesion visualization remains essential (see Chapter 3). When there is insufficient backup in crossing a very tight stenosis, the guiding catheter will disengage from the coronary ostium and back out into the aortic root. When pressure is applied to the stent catheter during attempts to cross the lesion, repositioning the guide catheter in a stepwise fashion while the stent is advanced may overcome this loss of support. However, aggressive intubation of the coronary ostium may damage the vessel, stopping the procedure prematurely, or may require additional stenting for an ostial dissection.


Deep seating of the guide catheter is achieved by manipulating the guide catheter over the balloon catheter shaft, past the aortocoronary ostium, and farther into the vessel. This maneuver is used to obtain increased backup support for crossing difficult lesions and is typically a last resort maneuver because of the increased chance of guide catheter–induced dissection of the left main or proximal vessel.




Characteristics


Compared with the diagnostic catheters, the guiding catheters have thinner walls, larger lumens, and stiffer shafts (Fig. 1-3). A large catheter lumen is achieved at the expense of catheter wall thickness and thus may result in decreased catheter wall strength, less torque control, or catheter kinking. The guiding catheters are generally stiffer to provide backup support during the PCI catheter advancement into the coronary artery and, therefore, respond differently to manipulation than diagnostic catheters. The guiding catheter tip is not tapered. Pressure-wave damping upon engaging the coronary ostium is seen more often than with similar-sized diagnostic angiographic catheters. Some guide catheters have relatively shorter and more flexible tips to decrease catheter-induced trauma.


Guiding catheters with small side holes near the tip permit blood to enter the coronary artery when the ostium is blocked by the guide catheter. Side holes are used when the guide catheter either partially or totally occludes blood flow into the coronary artery. The guide catheter coronary occlusion is noted by the change in the arterial pressure waveform to one of “damping.” Catheter side holes reduce ischemia when the guiding catheter is seated in a small artery. However, side holes may lead to inadequate artery visualization from loss of contrast media exiting the catheter before entering the artery. Although side holes may provide reliable aortic pressure, coronary flow can still be compromised during the angioplasty procedure. The guide catheter and side holes act as a “second stenosis” at the coronary ostium.


Small shaft diameter guide catheters (e.g., 6F) are the most frequently used size of guiding catheter. The use of small-diameter guide catheters results in fewer femoral vascular complications and allows earlier ambulation of patients. Small-size (<5F) guide catheters do not allow for the use of some stents. Guide catheters sized 7F or 8F are used for complex procedures involving larger PCI devices or two stents for treatment of bifurcation lesions. Use of 6 F (or, in some patients, 7F) guide catheters from the radial artery approach may become the favored access because of the markedly reduced vascular complications associated with radial PCI procedures.



Balloon Dilatation Catheter Systems



Types


There are three types of PCI balloon catheters (Fig. 1-4):




The OTW and monorail balloons, but not fixed-wire balloons, are also used to deliver stents that are mounted by the manufacturer on a specific balloon. The advantages and limitations are summarized in Table 1-4.


Table 1-4 Advantages and Limitations of Angioplasty Balloon Types




































Advantages Limitations
Over the wire
Distal wire position Two experienced personnel required
Distal port available for pressure measurement or contrast media injection Larger profile
Accepts multiple guidewires  
Rapid exchange
Distal wire position
Enhanced visualization
Low-profile balloons
Single-operator system
Exchanging balloons at hemostatic valve may be technically demanding
Fixed wire
Enhanced visualization Lack of through lumen
Single-operator system Inability to recross lesion without removing
Use with small guiding catheters system  
Low-profile balloons  

Modified from Kern MJ, ed. The cardiac catheterization handbook, 2nd ed. St Louis, MO: Mosby, 1995.



OTW Angioplasty Balloon Catheters


Historically, the OTW balloon was the first introduced and has remained popular in a few centers. A standard OTW angioplasty balloon catheter has a central lumen throughout the length of the catheter for the guidewire and another, separate lumen for balloon inflation (Fig. 1-5). These balloons are approximately 145 to 155 cm long and are designed to be used with guidewires of various dimensions (0.010–0.014 inches). The major OTW advantage is the ability to maintain distal artery access with the balloon beyond the lesion while one guidewire is exchanged for another. The OTW system tracks very well because the whole balloon length has a wire lumen. It permits long guidewire exchanges, and because of the through lumen, it allows for delivery contrast and drugs distally in an artery. To exchange PCI catheters, the balloon is advanced over the wire to a distal position. The standard short (145-cm) wire is then removed from the balloon. A longer guidewire (300 cm) is then inserted to maintain distal wire position while the balloon catheter is completely withdrawn over the guidewire and another balloon catheter is introduced over the same long guidewire for additional dilatations. OTW catheters can accept multiple guidewires, which allows for exchanging additional devices that may require stronger, stiffer, or specialized guidewires.



Limitations of OTW balloon catheters include a slightly larger diameter than the rapid-exchange (monorail) and fixed-wire catheters and the need for additional personnel to help with long guidewire catheter exchanges.



Rapid-Exchange (Monorail) Balloon Catheters


“Rapid-exchange” or monorail catheters were developed to permit the exchange of angioplasty balloon catheters by a single operator. Rapid-exchange catheters have only a short (30–40 cm) length of the catheter shaft containing two lumens (Fig. 1-6). One lumen, in the distal 30-cm portion of the catheter shaft, houses the guidewire. The remaining lumen runs the entire length of the catheter and is used for balloon inflation. Because only a limited portion of the balloon requires dual lumens, rapid-exchange catheters are smaller in diameter than are OTW balloon catheters.



Rapid-exchange balloon catheters address certain inherent limitations of OTW catheters. First, OTW balloon exchanges require a long (or extension) guidewire, which is unnecessary for the rapid-exchange balloon. Second, a single operator can use rapid-exchange balloon catheters without the aid of other assistants to maintain distal guidewire position.


Limitations of monorail catheters include the need for more care in manipulation of the guidewire, balloon catheter, and guiding catheter. Excessive blood loss at the rotating hemostatic valve during removal of the balloon catheter (back-out) maneuver may occur, but valved Y connectors have reduced this problem. More caution when moving the balloon is needed. If the monorail balloon is advanced beyond the distal end of the guidewire, the wire may come out of its short lumen, necessitating catheter withdrawal and reassembly of the balloon and guidewire. This is especially true when catheters with relatively short “rail” segments are used. If the balloon catheter requires force to advance beyond a lesion, a loop of guidewire may sometimes form outside the guide catheter in the aorta. This loop is nearly invisible but should be considered if the operator advances the catheter without seeing motion at the balloon tip.



Fixed-Wire Angioplasty Balloon Catheters


The fixed-wire catheter was the first catheter designed by Grüntzig. It has the balloon mounted on a central hollow wire with a distal flexible steering tip. The proximal end of the catheter consists of a single port connected to a thin metal tube (hypotube) used to inflate the balloon. A core wire extends from the hypotube to the end of the distal steerable tip. This assembly is coated with a thin plastic shaft that enhances flexibility. Fixed-wire balloons have only one enclosed lumen for balloon inflation.


In the balloon on-the-wire catheter system, the guidewire cannot be advanced independently of the balloon and the balloon cannot be exchanged without removing the entire system. Because the wire is attached to the distal end of the balloon, there is no central balloon lumen, resulting in a lower total profile than an OTW or monorail system. Its principal advantages relate to its low profile, enabling passage through very tight stenoses, and good contrast visualization of the lesion being dilated around the balloon catheter.


The small shaft size provides excellent coronary visualization. Because the balloon is mounted on the distal guidewire, the device was designed to be used by a single operator. Fixed-wire balloon catheters are particularly useful for distal lesions, subtotal stenoses, and lesions located in tortuous vasculature.


The limitations of fixed-wire catheters include lack of stent capability and the loss of the inherent safety advantage of OTW and rapid-exchange systems because there is no movable wire available to exchange for a stent if a dissection occurs. To exchange this catheter for another, the operator either must remove the entire system and recross the stenosis or dissection anew or, while leaving the fixed-wire balloon in place, advance another guidewire next to it to secure distal position and proceed with stenting in the usual fashion. A dissected lesion may not permit recrossing with a guidewire or advancing another balloon catheter.



Characteristics


The plastic material of the balloon determines its compliance (defined as the amount of expansion or diameter size for given amount of pressure) and strength. Compliance is the main differentiating feature among balloon catheters. Inflation of a compliant balloon above factory-determined average mean pressure (also called nominal or a set pressure for a known balloon size) will lead to further expansion of the balloon size approximately 10% to 20% over the predicted diameter. Noncompliant balloons, on the other hand, remain very close to their rated diameter even when inflated several atmospheres above nominal pressure. The advantages and disadvantages of balloon materials remain controversial. A compliant balloon may produce oversizing, particularly on second and third high-pressure inflations, resulting in dissections. After most stents are deployed, post-deployment high-pressure inflations are performed with low-compliance or noncompliant balloons to implant the stent struts into the vessel wall. Most balloons are also coated with low-friction surface polymers to facilitate lesion crossing.






Selection


The selection of a balloon catheter is highly subjective and less critical in the current era of stents. The balloon size is selected to achieve a 1:1 size match with the vessel. Balloon-to-artery ratios of more than 1.2:1 are associated with increased complications. Longer balloons (30–40 mm) are useful for dilating long and diffuse narrowings. Short (10–15 mm) balloons are used for stent re-expansion to avoid stretching the vessel wall outside the stent.


The balloon size is determined with the distal arterial reference segment diameter as gauged by the size of the guiding catheter (e.g., 7F guide = 2.31 mm, 8F = 2.64 mm, 9F = 2.97 mm, 11F = 3.63 mm). Visual estimation of artery diameter is less accurate than quantitative angiographic and IVUS imaging approaches, but it is the method used by most interventionalists. From IVUS studies, most stents selected by visual sizing are 0.5 mm smaller than true vessel dimensions.


Important technical considerations for selecting PCI systems include catheter profile, trackability, pushability, and ease of exchange. The size of a deflated catheter has been emphasized in catheter selection. Stent profiles are now very small (profile size of 0.035 and 0.033 inches). There appears to be no practical difference among equipment sizes. Stent profile is not the only factor in facilitating a stent to cross a lesion.


Trackability

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Jun 4, 2016 | Posted by in CARDIAC SURGERY | Comments Off on Basics of Percutaneous Coronary Interventions

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