Surgery – Internal organ support or sling
Reexamination Certificate
2001-09-14
2003-07-22
Gilbert, Samuel G. (Department: 3736)
Surgery
Internal organ support or sling
Reexamination Certificate
active
06595912
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to mechanical systems for treating congestive heart failure. Specifically, the invention relates to devices that interface mechanically with a patient's failing heart in order to improve its pumping function.
2. Description of the Related Art
Congestive heart failure (“CHF”) is characterized by the failure of the heart to pump blood at sufficient flow rates to meet the metabolic demand of tissues, especially the demand for oxygen. Historically, congestive heart failure has been managed with a variety of drugs. There is also a considerable history of the use of devices to improve cardiac output. For example, physicians have employed many designs for powered left-ventricular assist pumps. Multi-chamber pacing has been employed to optimally synchronize the beating of the heart chambers to improve cardiac output. Various skeletal muscles have been investigated as potential autologous power sources for ventricular assist. Among these, dynamic cardiomyoplasty using the latissimus dorsi muscle has attracted the most interest. It has been suggested that the beneficial effects of this procedure stem from both an active, dynamic, systolic assistance and a passive, adynamic girdling of the heart that limits diastolic stretch of the ventricle.
To exploit these beneficial clinical features, researchers and cardiac surgeons have experimented with prosthetic “girdles” around the heart. One such design reported in the literature is a prosthetic “sock” that is wrapped around the heart. Others have proposed the application of an intraventricular splint to reduce the volume of the left ventricle. Several design shortcomings are apparent with each.
The intraventricular splint, for example, extends through the left ventricular wall. Consequently, some components of the splint contact the patient's blood. This creates the potential for thrombogenesis, or the generation of blood clots. In addition, splint placement requires perforation of the ventricular wall, which may lead to leakage problems such as hemorrhage or hematoma formation. Furthermore, because one end of the splint extends to the epicardial surface of the left ventricle, options for the orientation of the splint are limited.
Pulling opposite walls of the ventricle closer together may reduce average wall stress via LaPlace's law, by reduction in ventricular diameter. However, this may create an irregular ventricular wall contour. This creates stress concentrations in the regions of the ventricle that are between the localized compression points. Consequently, this may lead to aneurysm formation, fibrosis, and impairment of the contractility and compliance of the ventricle. Also, the resulting irregular contour of the endocardial surface of the left ventricle may lead to localized hemostasis or turbulence, which may in turn lead to thrombus formation and possible thromboembolism.
Coronary artery disease causes approximately 70% of congestive heart failure. Acute myocardial infarction (“AMI”) due to obstruction of a coronary artery is a common initiating event that can lead ultimately to heart failure. This process by which this occurs is referred to as remodeling and is described in the text Heart Disease, 5th ed., E. Braunwald, Ch. 37 (1997). Remodeling after a myocardial infarction involves two distinct types of physical changes to the size, shape and thickness of the left ventricle. The first, known as infarct expansion, involves a localized thinning and stretching of the myocardium in the infarct zone. This myocardium can go through progressive phases of functional impairment, depending on the severity of the infarction. These phases reflect the underlying myocardial wall motion abnormality and include an initial dyssynchrony, followed by hypokinesis, akinesis, and finally, in cases that result in left ventricular aneurysm, dyskinesis. This dyskinesis has been described as “paradoxical” motion because the infarct zone bulges outward during systole while the rest of the left ventricle contracts inward. Consequently, end-systolic volume in dyskinetic hearts increases relative to nondyskinetic hearts.
The second physical characteristic of a remodeling left ventricle is the attempted compensation of noninfarcted region of myocardium for the infarcted region by becoming hyperkinetic and expanding acutely, causing the left ventricle to assume a more spherical shape. This helps to preserve stroke volume after an infarction. These changes increase wall stress in the myocardium of the left ventricle. It is thought that wall tension is one of the most important parameters that stimulate left ventricular remodeling (Pfeffer et al. 1990). In response to increased wall tension or stress, further ventricular dilatation ensues. Thus, a vicious cycle can result, in which dilatation leads to further dilatation and greater functional impairment. On a cellular level, unfavorable adaptations occur as well. This further compounds the functional deterioration.
Some have proposed that an elastic wrap around the heart might attenuate the remodeling process that is actively underway in failing hearts, prompting treatment with latissimus dorsi cardiomyoplasty. Based on experimental work to date, passive latissimus dorsi muscles appear to be best suited for this application. Oh et al. (1997) published experimental work in which they found a relatively inelastic prosthetic fabric wrap to be inferior to adynamic latissimus dorsi in bringing about reverse remodeling in an experimental model of heart failure. This was attributed to the greater elasticity of the muscle wrap.
It is thought that application of a device to provide compressive reinforcement similar to that of adynamic cardiomyoplasty might be therapeutic in treating dilated, failing hearts. Because heart failure is only the clinical end-stage of a continuous remodeling process, such a device might be able to attenuate or stop remodeling after a myocardial infarction far before the onset of heart failure. Such a device would have different functional requirements from a device that is used solely to treat established heart failure.
One requirement is to provide a slight elastic compression to the epicardial surface of the left ventricular wall. The device should allow expansion and contraction of the heart, but continue to apply gentle elastic compression to the left ventricle. This would reduce circumferential and longitudinal wall tension, thereby improving efficiency, lowering energy expenditure, reducing neurohornional activation, encouraging favorable cellular changes, and stabilizing the dimensions of the heart. This mechanical action is often referred to as “myocardial sparing.” The device should effect myocardial sparing without limiting the motion or the dimensions of the heart. Nor should it actively change the shape of the heart by pulling it or squeezing it. In fact, imposing a rigid barrier to limit distension or to squeeze the heart can be potentially dangerous. Shabetai in The Role of the Pericardium in the Pathophysiology of Heart Failure notes that the pericardium exerts 3-4 mm Hg of pressure against the heart. Cardiac function can be adversely affected with just a slight increase in pericardial constraint. For example, cardiac tamponade begins to be seen with pericardial pressures as low as 5-10 mm Hg.
A second requirement of such a device is to provide reinforcement that prevents the further shape change of the left ventricle without acutely changing the shape by its application. The device would act to prevent both global dilatation toward a more spherical shape and local infarct expansion after a myocardial infarction. In fact, if the local infarct expansion can be minimized with such a device, the compensatory global dilatation and increase in sphericity may be prevented. What is needed is a mild compressive support that conforms to the epicardial contour. As the left ventricle or portions of the left ventricle distend outward, they would be met with greater pressure from the device.
Hartigan Bill
Lau Lilip
Gilbert Samuel G.
Knobbe Martens & Olson Bear LLP.
Paracor Surgical, Inc.
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