Surgery – Cardiac augmentation
Reexamination Certificate
2000-11-28
2003-09-09
Jastrzab, Jeffrey R. (Department: 3762)
Surgery
Cardiac augmentation
Reexamination Certificate
active
06616596
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to cardiac assist and/or resuscitation systems for restoration or augmentation of natural blood flow and, more particularly, to implantable systems and associated methods for assisting the natural contractions of the heart.
Following a heart attack or as a result of other cardiac disease states, the heart's ability to pump blood can be seriously impaired. Conventional cardiac assist systems employ a variety of pumping approaches for assisting a failing natural heart. Generally, there are two categories of cardiac assist systems: those which contact blood, referred to herein as blood-contacting cardiac assist systems; and those which do not, referred to herein as non-blood-contacting cardiac assist systems.
A primary drawback of blood-contacting cardiac assist systems is the associated risk of thromboembolism. Although significant efforts have been made to reduce or eliminate this problem, the continued risk of thrombosis has restricted blood-contacting cardiac support systems to temporary or short-term applications. In addition to the risk of thrombosis, blood-contacting cardiac assist devices typically also experience calcification. The degree of calcification increases with time, again making these devices undesirable for long term applications.
Non-blood-contacting cardiac support systems significantly reduce the risk of thromboembolism and calcification. One conventional approach has been to directly apply forces to the heart so as to facilitate pumping. For example, U.S. Pat. No. 4,304,225 to Freeman discloses a non-contacting cardiac assist system designed to compress all or part of the heart by alternately tightening and releasing a circumferential compression band. Another conventional device, described in U.S. Pat. No. 4,583,523 to Kleinke et al., is an articulated mechanical device for applying an encircling force to the aorta. European Publication No. 0583012 A1 to Heilman et al. teaches the application of a similar device to the heart. Still other conventional systems, such as U.S. Pat. No. 4,411,268 to Cox and U.S. Pat. No. 4,813,952 to Khalafalla disclose an approach of encircling the heart with the latissimus dorsi muscle to achieve a desired compression of the heart.
Another class of non-blood-contacting cardiac assist devices uses hydraulic or gas pressure to displace an equivalent volume of blood in the heart through pressure applied to the outer surface of the heart, the epicardium. One conventional approach has been to use a housing of rigid construction for enveloping, at least partially, the ventricular region of the myocardium. The inner surface of the housing typically has a distensible elastic membrane adjacent to the myocardial wall. Pumping fluids are fed to the chambers defined by the housing and the membrane to apply pressure on the myocardial wall. In some instances the outer portion of the housing is formed of a flexible, non-distensible, member with an elastic distensible inner membrane. In general these conventional approaches utilize one or more compartments, each characterized by an elastic inner wall and an inelastic outer wall. Filling the compartments compresses the myocardium of the ventricle to aid pumping. When pumping is facilitated in this manner, a volume of inflating fluid or gas is required to displace an equal volume of blood. Cardiac assist devices of this general class are described in U.S. Pat. Nos. 2,826,193; 3,371,662; 3,455,298, 3,587,567; 3,613,672; 4,048,990; 4,192,293; 4,506,658; 4,536,893; 4,690,134; 4,731,076; 5,119,804; 5,131,905; 5,169,381; and 5,273,518.
Other approaches have employed a concave, gel-filled compression pad activated by a plate on its outer surface (U.S. Pat. Nos. 4,925,443; 5,098,369; 5,348,528), a cardiac assist envelope designed for minimally invasive implantation (U.S. Pat. No. 5,256,132); or a cardiac assist device having a fluid filled jacket encasing at least the heart ventricles to provide a compliant, completely passive support (U.S. Pat. No. 4,957,477).
A drawback to these cardiac assist devices is that they typically pump blood by displacing the blood with an equal inflation volume of a hydraulic fluid. As a result of this limitation, such systems require large reservoirs of the hydraulic fluid and/or complex pumping protocols.
To overcome this and other drawbacks, cardiac assist devices have been devised which displace blood with an inflation volume smaller than the displaced blood volume. Such cardiac assist devices typically produce higher pumping capacities through the injection of a relatively smaller quantity of fluid or gas under high pressure. Generally, these devices utilize a chamber or wrap having a number of inflatable segments.
For example, commonly owned U.S. Pat. No. 5,713,954 to Rosenberg et al. describes a non-blood-contacting cardiac assist device having tubes that contract a circumference of the heart when inflated. In one embodiment, the Rosenberg device is constructed of vertically-oriented, cylindrical (tube-shaped) inflation chambers arranged to form a ring and surrounded by a nondistensible sheath to form an artificial myocardium or heart wrap. The administration of a fluid under pressure causes the tubes of these conventional devices to have an expanded cross section, which is generally circular. When the fluid is withdrawn, the tubes flatten perpendicular to the direction of force generated by the pressure in the heart. When the tubes are deflated, the circumference of the pumping chamber is equivalent to the value of the number of tubes in the wrap multiplied by one half the circumference of one of the constituent tubes. When the tubes are fully inflated, the circumference of the pumping chamber is equivalent to the product of the number of tubes and the inflated diameter of one of the constituent tubes. When the wrap circumference is minimized there is no dilation of the tube circumference.
The resulting contraction of the circumference of the heart wrap is maximally 36%. This limit is due to the geometry of the device and is independent of the radius of the tubes chosen. Therefore, the volume of each tubes can be made small while maintaining a constant ejection volume. However, the work done is, in all cases, the same. The result is that smaller tubes require a higher pressure to attain a circular cross section. In general, for constant work performed, the inflation pressure is inversely proportional to the inflation volume.
U.S. Pat. No. 3,464,322 to Pequigot also discloses an artificial blood pumping chamber that has walls which are formed from an arrangement of inflatable tubes. A drawback of the Pequigot device is that the inflation chamber tubes are free to dilate when inflated. The Rosenberg device overcomes the drawbacks of the Pequigot device since the circumference of the inflation chambers of the Rosenberg device cannot exceed the dimensions of the fabric pockets in which they are imbedded. Consequently, the pumping action resulting from a contraction of the Rosenberg heart wrap is not defeated by dilation of the radii of the inflation chambers. Therefore, the Rosenberg device is more likely to reach the theoretical maximum contraction of 36%. However, like the Pequigot device, the Rosenberg device cannot exceed this limiting maximum contraction ratio.
One drawback with the above blood pumping devices is that the actual extent of contraction, expressed as a percentage of the circumference of the deflated pumping chamber, is dependent upon the amount of non-contracting space between the tubes. However, in practice, it is very difficult to inflate a sheet of tubes, joined only at a tangent, without inducing high stress in the tubes or in an encircling sheath. Maintenance of the tubes in close proximity at high pressure necessitates some non-contracting space between the tubes. Furthermore, since the pumping chamber is meant to fit snugly to the heart, allocation must be made for fitting the pumping chamber to the heart in situ. Consequently, the tubes must be spaced apart for this p
Abiomed, Inc.
Cahill Ronald E.
Engellenner Thomas J.
Jastrzab Jeffrey R.
Nutter & McClennen & Fish LLP
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