Cardiovascular Devices

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Source: STANDARD HANDBOOK OF BIOMEDICAL ENGINEERING AND DESIGN
CHAPTER 20
CARDIOVASCULAR DEVICES
Kenneth L. Gage and William R. Wagner
University of Pittsburgh, Pittsburgh, Pennsylvania
20.1
INTRODUCTION 20.1
20.6
ARTIFICIAL KIDNEYS 20.20
20.2
ARTIFICIAL HEART VALVES 20.1
20.7
INDWELLING VASCULAR CATHETERS
20.3
STENTS AND STENT-GRAFTS:
AND PORTS 20.24
PERCUTANEOUS VASCULAR
20.8
CIRCULATORY SUPPORT
THERAPIES 20.6
DEVICES 20.28
20.4
PACEMAKERS AND IMPLANTABLE
20.9
ARTIFICIAL LUNGS 20.35
DEFIBRILLATORS 20.11
REFERENCES 20.39
20.5
ARTIFICIAL VASCULAR
GRAFTS 20.17
20.1
INTRODUCTION
Cardiovascular disease remains the leading cause of mortality among men and women in Western
countries. Many biomedical engineers have focused their careers on the study of cardiovascular
disease and the development of devices to augment or replace function lost to the disease process.
The application of engineering principles to device design has improved device function, while
minimizing some of the detrimental side effects. Progress to date has allowed complex, challenging
cardiovascular surgical procedures (e.g., open-heart surgery) and medical therapies (e.g., dialysis) to
become routine, although limitations remain. In this chapter, eight major categories of cardiovascular
devices are addressed, including cardiac valves, stents and stent grafts, pacemakers and implantable
defibrillators, vascular grafts, hemodialyzers, indwelling catheters, circulatory support devices, and
blood oxygenators. For each topic, the market size, indications for device use, device design,
complications and patient management, and future trends are covered. The intent is to provide a brief
introduction to the current status of cardiovascular device development and application and to
identify challenges that remain in the field.
20.2
ARTIFICIAL HEART VALVES
20.2.1
Market Size
There were at least 60,000 valve replacement operations performed in the United States during 1996
(Vongpatanasin et al., 1996). About two-thirds of the artificial heart valve market in the United States
20.1
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CARDIOVASCULAR DEVICES
20.2
DESIGN OF MEDICAL DEVICES AND DIAGNOSTIC INSTRUMENTATION
consists of various mechanical valves, with the remaining one-third being distributed among the
bioprosthetic, or tissue-based, models. Worldwide, the mechanical valve market is slightly less,
approximately 60% of market share (Akins, 1995).
20.2.2
Indications
The medical indications for valve replacement are thoroughly described in a report from the
American College of Cardiology/American Heart Association Task Force on Practice Guidelines,
which address the management of patients with valve disease (Bonow et al., 1998). The etiology of
valve disease differs, depending on the patient group and the valve location, as does the preferred
corrective treatment. In general, the young suffer from congenital valve defects, while older adults
exhibit acquired valve disease. Valve replacement can be performed upon all valves of the heart but
most cases involve the aortic or mitral valves. Common reasons for native valve replacement are
severe stenosis and regurgitation with or without symptoms, which may include chest pain, shortness
of breath, and loss of consciousness. The reduction in effective orifice size associated with a stenotic
lesion results in a large transvalvular pressure gradient that may exceed 50 mmHg in the aortic
position for severe cases (Bonow et al., 1998). In regurgitation, the blood pumped forward into the
recipient vessel or ventricle spills back into the adjacent pumping chamber through an incompetent
valve, minimizing forward movement. The end effects of chronic stenosis or regurgitation are
compensatory anatomic changes that accommodate, for a limited time, the reduced pumping
efficiency due to restricted blood movement. In general, heart valve replacement is performed when
repair is not possible, as the implantation of an artificial heart valve brings with it another set of
problems. Total replacement and removal of native valve components in the mitral position is
particularly limited, as the mitral valve is anatomically and functionally integral to the left ventricle
(David et al., 1983; Yun et al., 1999). Concomitant illnesses such as congestive heart failure, atrial
fibrillation, and coronary artery disease can alter the indication for valve replacement, as can the
surgical need to correct other cardiac disease.
20.2.3
Current and Historical Device Design
Artificial heart valve design has a long and colorful history, with more than 80 different versions of
valves being introduced since the 1950s (Vongpatanasin et al., 1996). The two general types of
replacement valves, mechanical and biologic, each have their own set of indications, complications,
and performance factors. The mechanical valve can be further categorized into three major design
lines: caged ball, single tilting disk, and bileaflet (Vongpatanasin et al., 1996). Caged-ball valves have
been largely supplanted by the more modern single-tilting-disk and bileaflet valves. Biologic valves
are divided according to the source of the tissue material, with the term
bioprosthetic
reserved for
valves constructed from nonliving, animal-source tissue. Homograft biologic valves are preserved
human aortic valves or pulmonary valves surgically placed within a recipient patient (Bonow et al.,
1998). Heterograft bioprosthetic valves consist of porcine heart valves or bovine pericardial tissue
formed into a valve over a support structure (Vongpatanasin et al., 1996). Because mechanical and
bioprosthetic valves have different design considerations, the categories are discussed separately.
Mechanical Valves.
The assorted mechanical valve designs use different approaches to achieve the
same functional goal. Caged-ball valves use a free-floating polymeric sphere constrained by a metal
cage to periodically occlude the valve orifice. Single-disk valves possess a central disk occluder that
is held in place by struts projecting from the housing ring. The disk opens through a combination of
tilting and sliding over the struts to reveal primary and secondary orifices. Bileaflet valves feature
leaflets that are hinged into the housing ring. The opened valve presents three orifices, two along the
housing ring edge and one central orifice between the leaflet mount points. Figure 20.1 shows both
orifice and profile views of representative tilting-disk and bileaflet mechanical valves.
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CARDIOVASCULAR DEVICES
CARDIOVASCULAR DEVICES
20.3
FIGURE 20.1
Orifice and profile views of representative bileaflet and tilting-disk valves are provided in
the upper and lower halves of the figure. Note the larger percentage of open orifice area present in the
bileaflet valve. The tilting disk projects farther into the downstream ejection zone than the leaflets on the
bileaflet valve.
Mechanical valves are expected to perform flawlessly for decades with minimal patient burden.
Criteria used to evaluate designs during development and clinical use can be divided into
structural
and
hemodynamic
groups, although there is considerable overlap. Structural considerations involve
fatigue and device integrity, valve profile, rotatability, and occluder interference (Akins, 1995). To
accommodate the wear associated with operating hundreds of millions of times, current mechanical
valves are manufactured with durable metal and carbon alloys (Vongpatanasin et al., 1996; Helmus
and Hubbell, 1993), and include a polymer fabric sewing ring for surgical placement. Rotatability of
the valve is desirable, as evidence suggests that optimum orientations minimizing turbulence and
microembolic signals exist for mechanical heart valves in vivo (Kleine et al., 2000; Laas et al., 1999).
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CARDIOVASCULAR DEVICES
20.4
DESIGN OF MEDICAL DEVICES AND DIAGNOSTIC INSTRUMENTATION
Concerns regarding valve profile and occluder interference focus on possible negative interactions
between the valve, adjacent ventricular structures, native valve remnants, and surgical suture material.
The impingement of these structures into the valve could prevent complete closure of the valve or
cause binding of the occluder. Although not a structural requirement, devices tend to be radio-opaque
to aid in visualization during radiographic procedures.
Hemodynamic performance factors that should be considered during functional evaluation of a
valve design are the transvalvular pressure gradient, rate and duration of valve opening, dynamic
regurgitant fraction, and static leak rate (Akins, 1995). The transvalvular pressure gradient is a
function of the effective orifice area of the valve and the flow regime (turbulent or laminar)
encountered. The mass of the occluder and mechanism of action play a significant role in the rate and
duration of valve actuation, and similarly in the dynamic regurgitant fraction, which represents the
percentage of blood that flows back into the originating chamber prior to valve closure. Finally, some
leak is expected in the closed valve position. Consistent convective flow over these surfaces is
believed to aid in the minimization of thrombotic deposition.
Bioprosthetic Valves.
Engineering design concerns have little influence on homograft valves
because of their anatomic origin, thereby limiting the focus to heterograft bioprostheses. The
bioprosthetic tissue found in heterografts is treated with glutaraldehyde to cross-link the proteins
that make up the tissue structure. The treatment is cytotoxic, disrupts the antigenic proteins that can
cause an immune response, and improves the toughness of the tissue by cross-linking the structural
collagen (Bonow et al., 1998). Some bioprosthetic valves are further treated with surfactants,
diphosphonates, ethanol, or trivalent metal cations to limit the rate of calcification and associated
structural deterioration (Schoen and Levy, 1999).
Porcine heterograft valves can be mounted on a support scaffold with a sewing ring, although
unmounted designs have been introduced to improve flow characteristics and structural endurance.
Heterograft valves constructed of bovine pericardium are formed over a scaffold with a sewing ring
to mimic an anatomic valve shape. Because the pericardial valves are constructed to design criteria
rather than harvested, the orifice size can be made larger to improve flow characteristics, while the
higher collagen content may allow improved graft resilience when cross-linked (Bonow, 1998).
Figure 20.2 shows representative porcine and bovine pericardial heterograft valves.
Design Performance Evaluation.
The design of artificial heart valves has benefited from the advent
of computational fluid dynamics and other computationally intensive modeling techniques.
Simulations have been used to predict the performance of both bioprosthetic (Makhijani et al., 1997)
and mechanical (Krafczyk et al., 1998) valve designs. Results from computer modeling can be
FIGURE 20.2
Two tissue-based artificial heart valves are shown above with a U.S. quarter dollar for size comparison.
The valve on the far left is a porcine heart valve, while the other valve is constructed of bovine pericardium. Both valves
are intended for aortic placement.
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CARDIOVASCULAR DEVICES
CARDIOVASCULAR DEVICES
20.5
compared with findings from experimental studies using such methods as particle image velocimetry
(PIV) (Lim et al., 1998) and high-speed photography of valve structure motion (De Hart et al, 1998).
Such comparisons provide necessary model validation, revealing inadequacies in the numerical
model and capturing phenomena not predicted using existing model assumptions.
20.2.4
Complications and Patient Management
Mechanical and bioprosthetic valves suffer from complications that dictate follow-up care and
preventive measures. Possible complications facing heart valve recipients include thromboembolism,
hemolysis, paravalvular regurgitation, endocarditis, and structural failure of the valve (Vongpatanasin
et al., 1996). Some preventive measures are indicated for both mechanical and biologic valve
recipients, such as the use of antibiotics during dental surgery and invasive procedures to avoid
infective endocarditis (Dajani et al., 1997). Other preventive treatments, such as long-term
anticoagulation, are administered differently, depending on the type of valve implanted.
Because of the high incidence of thromboembolic complications associated with mechanical
artificial heart valves, chronic anticoagulation is required. Anticoagulation with warfarin and an
antiplatelet agent such as aspirin is indicated for both mechanical and heterograft bioprosthetic valves
for the first 3 months after implantation (Stein et al., 2001; Bonow et al., 1998; Heras et al., 1995).
After that time warfarin is discontinued for heterograft bioprosthetic valves unless the patient
possesses a risk factor that increases susceptibility to thromboembolic complications (Bonow et al.,
1998). Despite the use of chronic anticoagulant therapy, a recent review reported that between 0.4
and 6.5 percent of mechanical heart valve recipients will experience a thromboembolic event per
year, a range that is dependent upon valve type, number, placement, and other risk factors, as well as
the level of anticoagulation (Stein et al., 2001). The long-term risk of thromboembolism in
bioprosthetic heart valve recipients is comparatively low, ranging from 0.2 to 2.6 percent per year
(Stein et al., 2001).
In contrast to the anticoagulation requirement associated with mechanical valves, the largest
problem facing patients with bioprosthetic valves is progressive structural deterioration due to
calcification, which can eventually require valve replacement and the risks of a reoperation
(Hammermeister et al., 2000; Schoen and Levy, 1999). Heterograft bioprosthetic valves exhibit
accelerated deterioration in younger patients, which promotes the use of mechanical valves in this age
group when homograft valves are unavailable (Bonow et al., 1998).
Although the literature is rife with comparisons between valve designs in regard to their
complication rates, the general lack of large randomized trials using standardized methods makes
valid comparisons problematic (Horstkotte, 1996). To reduce some of the confusion surrounding
outcome reporting in heart valve studies, the Society of Thoracic Surgeons and the American
Association of Thoracic Surgery have updated guidelines for reporting common surgical and
nonsurgical artificial valve complications (Edmunds et al., 1996). The guidelines distinguish between
structural and non-structural valve dysfunction, thrombosis, embolism, bleeding events, and
infection. In addition, various types of neurologic events are graded, and methods of statistical data
analysis are suggested on the basis of the type of data being collected and analyzed. Adherence to
such guidelines should allow valid comparisons to be made between manuscripts reporting on
outcomes with different valve types and clinical approaches (Edmunds et al., 1996).
20.2.5
Future Trends
Increasing efforts in the area of tissue engineering hold great promise for the development of
replacement valves with improved biocompatibility. As reviewed in a recent article (Jankowski and
Wagner, 1999), researchers have attempted to stimulate the growth of endothelial cells on existing
bioprosthetic valves to limit valve degradation and thromboembolic complications, while others have
endeavored to grow valves and individual leaflets de novo using cell-seeded polymeric scaffolds.
Endothelialization of commercial bioprosthetic valve tissue is hampered by the cytotoxic aftereffects
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