Engineering Parameters And The Coronary System

When faced with the task of designing and testing the devices used in these types of interventional procedures, a thorough understanding of the structural and geometric parameters of the coronary system is crucial for success. The main purpose of the following text is to summarize, at a basic level, the important anatomical parameters needed to design interventional devices and/or associated delivery procedures related to the coronary system.

From an engineering perspective, for the predesign of any medical device, there are a number of important parameters that should be familiar; this is especially true because of the complexity and variation found in the human coronary system. As with any device placed in the human body, an excellent understanding of the fundamental anatomical properties of the tissue with which the device interacts is vital to obtain acceptable results regarding: (1) delivery efficacy, (2) long-term

Stent Mounted on Balloon

Stent Mounted on Balloon

Balloon Inflates, Expanding Stent

Balloon Inflates, Expanding Stent

Stent Implanted in Artery

Fig. 4. An illustration of the stenting procedure. The balloon catheter with a collapsed stent mounted on it is placed in the artery at the location of narrowing. The balloon is inflated to open the artery and deploy the stent. Finally, the catheter is removed, and the stent is left behind.

Stent Implanted in Artery

Fig. 4. An illustration of the stenting procedure. The balloon catheter with a collapsed stent mounted on it is placed in the artery at the location of narrowing. The balloon is inflated to open the artery and deploy the stent. Finally, the catheter is removed, and the stent is left behind.

PlaceLateral.mpg

Fig. 5. Animated movie depicting coronary sinus cannulation and lead placement in basal, midventricular, and apical locations during a biventricular pacing implant procedure. See PlaceLateral.mpg on the Companion CD.

PlaceLateral.mpg

Fig. 5. Animated movie depicting coronary sinus cannulation and lead placement in basal, midventricular, and apical locations during a biventricular pacing implant procedure. See PlaceLateral.mpg on the Companion CD.

device stability, and/or (3) overall performance. This is true not only chronically, but also maybe even more importantly for initial device delivery. Although biological reactions to materials placed inside the human body must be understood to guarantee long-term stability and performance of medical devices, the following discussion focuses on the macroscopic physical properties of the coronary vessels.

To simplify the coronary system down to its basic structure, each vessel branch in the vessel network can be defined in the simple terms of a flexible cylinder or tube. A tube is a hollow cylindrical structure of a known but variable length, radius, and wall thickness. This means the coronary arterial and venous networks can be defined in terms of a large number of interrelated tubes that feed and receive blood to and from one another. The parameters described here are those that must be defined to understand fully the geometry and dynamic properties of this fine network of tubes so optimal devices may be designed to interact with them.

8.1. Diameter

The first and most basic parameter that must be known about the arteries and veins is their diameter. Yet, the diameters of both arteries and veins are not constant along their lengths (1921). Typically, coronary arteries taper and decrease in diameter as they move further away from their source (20,21). This means that the left main and right coronary arteries have generally the largest diameters of the entire coronary arterial network; these diameters are typically around 4-5 mm and 3-4 mm, respectively (20).

The more bifurcations an artery undergoes, the smaller its diameter will become. In the case of the coronary arteries, the vessels located at the very end of the network are the capillaries, which are typically on the order of 5-7 ^m in diameter (22). This is approx 600 times smaller than that of either the right or left main coronary arteries. Conversely, veins increase in diameter as they move from their source to their termination. Thus, the largest diameter vessel in the coronary venous network is the coronary sinus, which is located at the end of the network and has a diameter of approx 6-12 mm at its ostium (7,19). The difference in diameter from one end of the venous system to the other is roughly a factor of 1200. However, in diseased states such as heart failure, the ostium tends to increase in diameter (5). It is also important to recall that, because arteries and veins are made up of compliant tissue, their diameters change throughout the cardiac cycle because of pressure changes that occur during systole and diastole (23).

The design of coronary stents and balloon angioplasty catheters relies heavily on the diameter of the vessels they are meant to enter. If a stent or balloon is designed with too large a diameter, when it is deployed within the artery it may cause a wall strain so high that it could be damaging to the artery. On the contrary, if the design has a diameter that is too small, the device will be ineffective. For example, in the case of an under-size stent, restenosis of the artery will occur much quicker than desired. In the case of the balloon catheter with a diameter that is too small, the lumen will not be opened up enough to cause any significant decrease in the degree of occlusion.

Another device that must be designed with vessel diameter in mind is the left ventricular pacing lead for heart failure. Because this lead is designed for placement in a lateral branch of the coronary sinus, it must have a small enough diameter to fit inside the vein, but also have a large enough diameter to stay in its intended location. It is considered that if such criteria are not met, the leads may not be useful or safe.

8.2. Cross-Sectional Profile

A parameter that is very closely related to vessel diameter is that of cross-sectional shape profile. Cross-sectional shape profile is determined by the shape of the vessel that results after slicing it perpendicular to its centerline. In a hypothetical cylinder, this profile would be a perfect circle. When arteries are diseased and contain significant amounts of atherosclerotic plaque, their cross-sectional profile can change from roughly circular to various different (and often quite complex) profiles, depending on the amount and orientation of the plaque. To date, coronary venous shape profiles have not been well documented, but they can be considered as noncircular in general because of the lower pressures within the vessel as well as the more easily deformable vessel walls in relation to the arteries.

The design of two devices in particular should be considered in relation to the cross-sectional shape of the coronary vessels—coronary stents and angioplasty catheter balloons. Because coronary arteries are typically circular in cross section, stents are designed also to be circular in their cross section. Interestingly, more often than not, the vessel to be stented has a pretreated cross section that is very far from circular. If a similar device were ever needed for placement in the relatively healthy coronary venous network, a different design would probably be initially considered because the cross-sectional profile of a coronary vein is generally noncircular. Angioplasty balloons have been designed with the consideration that coronary arteries are typically circular in cross section. When inflated, the balloon generates a shape that has a uniform diameter in cross section, which may be consistent with what a healthy coronary artery looks like in cross-section.

8.3. Ostial Anatomy

Understanding the anatomy of the ostia of each of the three most prominent vessels in the coronary system (the right coronary artery, left main coronary artery, and the coronary sinus) is especially important when interventional procedures require cannulation of the ostia to perform a specific procedure within the lumen of the vessel. This is true of nearly all procedures done on coronary vessels because they are typically aimed at the lumen of the vessel, but on occasion one may want to block off or place a flow-through catheter in the ostium.

The ostia of the coronary arteries are generally open with no obstructions except when coronary plaques form; in this case, they can become partially or even fully occluded. When occlusion is not present at the ostial origin of the coronary arteries, there are generally no naturally occurring anatomical structures to impede entrance into the vessels.

The coronary sinus ostium, as discussed in Section 4, often has a simple flap of tissue covering its opening into the right atrium; this flap is called the Thebesian valve. This valve can take many different forms and morphologies and can cover the coronary sinus ostium to varying degrees (4,6-8,24,25). When the Thebesian valve is significantly prominent in the manner in which it covers the coronary sinus ostium, cannulation can be much more difficult than in other cases (5,8).

This consideration is important as it specifically applies to the implantation of biventricular pacemaker leads. In the process of delivering a biventricular pacing lead, coronary sinus cannulation is of paramount importance because it is currently considered as the primary point of entry into the coronary venous network for pacemaker lead introduction for eventual pacing of the left ventricle. To design the optimal catheter or lead delivery procedure, the presence of the Thebesian valve should be fully considered in addition to other anatomical features.

8.4. Vessel Length

Each tube that makes up a section of the coronary arterial or venous network is also a branch that arises from a parent vessel. Each of these vessel branches has starting and ending points. Typically, vessel lengths can be measured directly on a specimen after the heart has been extracted. With the advent of 3D medical imaging techniques such as magnetic resonance imaging and computerized tomographic angiography, coronary vessel lengths can be measured in vivo by reconstructing them in space (26-28).

One application for which vessel length is an important consideration is implantation of left ventricular leads in the lateral or posterior branches of the coronary sinus. Optimal lead designs should take into account the average length along the coronary sinus of the normal and/or diseased human heart where a candidate lateral branch enters. Foreknowledge of this parameter, either in a specific patient or across a population, might improve ease of implant. This information could also be useful in understanding the likelihood that a lead will not dislodge after initial fixation. Furthermore, when percutaneous transluminal coronary angioplasty procedures are performed, it is critical that the physician knows exactly where along the length of an artery the occlusion occurs and the relative distance needed from catheter entry to the site. These parameters are often measured using contrast angiography. When contrast is injected and fluoroscopic images are acquired, the location of the occluded arterial region can be quickly identified.

8.5. Tortuosity

Because the vessels in the coronary system course along a nonplanar epicardial surface, they are by nature tortuous. Thus, they have varying degrees of curvature along their lengths according to the topography of the epicardial surfaces on which they lie. If vessels were simply curvilinear entities such that they only lie in a single plane, their tortuosities would be much more easily defined. But, in reality, the vessels of the coronary system are not curvilinear. Rather, they are 3D curves that twist and turn in more than two dimensions. When the third dimension is added, the definition of tortuosity becomes much more complex. Not only must the curvature of each segment element be defined, but also the direction in which that curve is oriented.

The levels of tortuosity encountered in the coronary vessels may significantly influence device delivery and chronic performance. When a device such as a catheter or lead must be passed through a tortuous anatomy, such as that of the coronary vessels, the greater the curvature and change in curvature over the length of a vessel, the more difficult it will be to pass the device through it. For vessels that more closely resemble a straight line, these devices will pass through much more easily. It should also be noted that vessel tortuosity in humans is considered to increase with age. Therefore, patient age may be another important consideration when designing these types of devices and/ or the mechanisms by which they are to be delivered.

8.6. Wall Thickness

All coronary vessel walls have a certain thickness. When a device is placed into the vessels of the coronary system, there is always a danger of perforation. In general, perforation takes

Fig. 6. Diagram of the branching angle between a parent vessel and its daughter. Angle 0 represents the branching angle generated between the parent vessel and daughter 2.

place when a device is inadvertently introduced into the vessel lumen with a level of force and angle of incidence to the vessel wall that causes the device to perforate the wall and generate a hole through which blood can flow. This situation, although not very common, is not only very dangerous but can be lethal if not dealt with appropriately. Perforation is usually more often fatal when it happens in arteries as opposed to veins for two reasons:

(1) more blood is lost under higher pressures in the arteries; and

(2) loss of oxygenated blood to the body and the heart itself is more immediately detrimental than if deoxygenated blood were to exit the coronary veins.

Although it is clear that no device is meant to perforate the vessels of the coronary system, each should be developed with the worst-case scenario of perforation in mind, such that they will not be problematic for patients or physicians. It should be noted that the wall thickness of the larger coronary arteries (they are the thickest) is roughly 1 mm (29,30). Interestingly, coronary venous wall thicknesses have not as yet been clearly defined in the literature.

8.7. Branch Angle

As a vessel bifurcates, at least those of the daughter branches, it is diverted in a different direction from the parent. This creates a situation in which the smaller vessel has a certain branching angle in relation to the direction of the parent vessel. Branching angles can be measured by calculating the angle between the trajectory of the parent vessel and its daughter. An example of this idea is illustrated in Fig. 6.

The branch angle of a daughter vessel is important to understand as it applies directly to when a biventricular pacing lead enters a posterior or lateral branch of the coronary sinus. Thus, the only way to optimize the design of this type of lead, such that it can easily make the turn into a branching vessel, is to know how gentle or severe that branching angle generally is. The more gradual the turn a lead has to take from a parent vessel to its daughter, the easier it is for an implanter to navigate in general.

8.8. Motion Characteristics

Because the vessels of the coronary system are attached directly to the epicardium, it follows that they are not stationary as the heart beats. Along with the simple 3D displacement that occurs over time because of motion, there are other mechanical parameters that are dynamic, such as curvature, strain, and torsion. Each of these fundamental mechanical parameters can have significant effects on devices placed in the lumen of a deforming vessel. Devices such as stents and leads must be designed to withstand all types of strain, curvature, and torsion changes they are expected to experience over their lifetime within an arterial or venous vessel lumen. Additional necessary considerations include (1) the relative changes in both the 3D path of each vessel and changes in lumen diameter during a given cardiac cycle and (2) the relative influences associated with alterations in contractility states (e.g., the effects of exercise increasing cardiac output four- to sixfold).

9. SUMMARY

This chapter reviewed the anatomical and functional features of the coronary system. The effects of several disease processes on cardiac function relative to flow changes were discussed, as well as associated therapies. In addition, the use of the coronary system to gain access to various regions of the heart for specific clinical needs was described. Finally, pertinent issues that must be considered when designing devices for invasive placement within the coronary vessels were discussed.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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