Intraaortic Balloon Pump

The intra-aortic balloon pump (IABP), introduced into clinical medicine by Kantrowiti in 1967, is the most commonly

FIG. 8- Percutaneous intra-aorric balloon pump. Left, balloon dellated lor insertion Itight, balloon inflated. iDatascope Corp.. Oakland, N), Courtesy S. Wolvek.;

FIG. 8- Percutaneous intra-aorric balloon pump. Left, balloon dellated lor insertion Itight, balloon inflated. iDatascope Corp.. Oakland, N), Courtesy S. Wolvek.;

balloon rapidly deflates before systole (the period of contraction of the heart), reducing the resistance to ejection and allowing the heart to work more efficiently at a smaller end-systolic volume (lower wall stress). For a number of clinical situations, this reduction in workload allows recovery of normal heart function and the ability to support the patient's circulation without the balloon. It is useful in the treatment of cardiogenic shock and other cardiac disorders and in the support of patients undergoing coronary artery bypass surgery. Currently, it is used primarily to treat cardiac failure developing after cardiac surgery.

Complications following 1ABP use are primarily due to surgical accidents associated with the procedure rather than to device failure or bioincompatibility of materials used. While the blood-contacting surface is large, the high blood flow velocity in the aorta prevents the accumulation of critical concentrations of procoagulants that might lead to thromboembolic complications. Carbon dioxide or helium is used to inflate the balloon, and leakage of both these gases by diffusion through the balloon necessitates periodic refilling. Patients have been supported for up to a year with the IABP, although the duration is more commonly a few days.

ventricular assist devices and total artificial hearts

Heart transplantation has become an effective form of therapy for patients with intractable heart failure; at present 1-and 5-year survival rates are approximately 80% and 70%, respectively. However, the number of hearts available for transplant, about 2000 per year in the United States, falls far short of the number of patients who develop intractable heart failure, which is estimated to be between 17,000 and 35,000 per year in the United States. This discrepancy between supply and need has stimulated research and development of both ventricular assist devices (VADs) and total artificial hearts (TAHs).

VADs and TAHs are used to replace the mechanical functions of part or all of the heart when those functions have failed irreversibly and heart transplantation is not possible. It was demonstrated in the early 1980s that patients with irreversible heart disease could survive for several months to over a year with a totally implantable (Jarvik-7) artificial heart replacing their own heart. However, these patients all succumbed to thromboembolic complications (e.g., stroke), infections in and around the device, and the failure of other organs (lungs, kidneys) (Didisheim et al,, 1989; DeVries, 1988). For these reasons, the clinical approach has shifted away from chronic implantation of a TAH to replace the heart, to shorter term (days to weeks) implantation of a TAH or VAD to support the patient until a donor heart becomes available for transplant ("bridge to transplant"). Improved design and better modes of antithrombotic and antibacterial therapy may allow these devices to be used for more prolonged periods. Meanwhile, research is supporting the development of improved, totally implantable TAHs intended for long-term clinical use. It is projected that these devices will be fully developed and tested during the next decade. A substantial fraction of their design features will

FIG. 9. Totally implantable left ventricular assist device (LVAD). (Novacor Division, Baxter Healthcare Corp., Oakland, CA. Courtesy P. M, Portner.)

evolve from the clinical studies that are planned for implantable VADs.

Beyond the totally implantable artificial heart, an implantable ventricular assist system (VAS) (Fig. 9) is the most complex cardiovascular device to be implanted in humans. Table 2 lists typical biomaterials used in a VAS. The design and enabling biomaterials of the blood pumping chamber represent a substantial challenge to the bioengineer. The blood pumping must (1) be biocompatible (an objective not yet fully realized), (2) possess necessary mechanical properties (structure, flex pattern), (3) be impermeable to water, (4) allow gas transfer for barometric and altitude equilibration, (5) prevent bacteria adhesion (not yet achieved), and (6) not degrade during the useful life of the implant.

Other design factors consist of possible local tissue response to biomaterial extractables and erosion caused by device motion or vibration. Continuous relative motion between the VAS and the surrounding tissue produces inflammation and a thick fibrous encapsulation of the implant. This can lead to a site of pocket infection that is untreatable because oral antibiotics are walled off from the implant. This process must be minimized so that only a thin, stable capsule is formed. Integration of the VAS with surrounding tissue is desirable but has not yet been shown in animals or humans.

VASs designed for long-term use (months, years) have now been implanted in hundreds of patients as a bridge to cardiac transplant. Patients are routinely supported for 3-4 months, with a few VASs functioning for nearly 18 months before transplantation. These results are significant when compared to the early experience of the 112 days Dr. Barney Clark was maintained using the Jarvik-7 TAH (DeVries, 1988). These


Device component

Blood-contacting materials (Inner surface) Pump/drive unit

Inflow and outflow conduits

Tissue-contacting materials (Outer surface) Pump/drive unit

Inflow and outflow conduits Variable volume compensator

Energy control and power unit Belt skin transformer

Interconnecting leads

Special structural materials Pump/drive unit

Variable volume compensator Energy control and power unit

Belt skin transformer (Secondary)

Device subcomponent

Blood pump sac Inflow and outflow valves Luminal surface

Encapsulation shell

Outer surface (Graft) External reinforcement Flexing diaphragm, connecting tube Rigid housing

Hermetic encapsulation shell Belt body

Outer encapsulation

Solenoid energy Converter

Blood pump

Gas-filled (Replenishable) reservoir Hybrids

Application specific-integrated circuits, rechargeable batteries Multistrand wire


Segmented poivether, polyurethane (Biomer)

Porcine valve (with silicone flange) Urethane elastomer (Adiprene L-1Q0) Dacron vascular graft

Titanium :(.!' I Medical grade adhesive A (silicone) Expoxy (polyamine) Dacron vascular graft Polypropylene Segmented polyether, polyurethane (Biomer), Dacron velour fabric Titanium (6Al, 4 V) Titanium (CP-1)


Medical grade adhesive A (silicone), Silver contacts Silicone, Medical grade Adhesive A (silicone)

Titanium decoupling

Spring (6A1, 4 V), Vanadium permendur, magnetic core, Copper coils Lightweight structural composite

Ni-Cad Silver, copper patients are ambulatory and exercise regularly, and preexisting organ dysfunction is often resolved. They perform moderately in sports shooting a basketball and playing volleyball. One interesting case involved a blues musician who continued playing his guitar at concerts, while singing and dancing. These patients seem to have an excellent quality of life, manage their own care, and rapidly recover from their subsequent heart transplant operation.

The Heartmate VAS manufactured by Thermo Cardiosystems, Inc. (Woburn, MA) has demonstrated an interesting bio-compatibility concept (Dasse etal., 1987; Meconi etal., 1995). The blood pump's blood-contacting surface is lined with sintered titanium alloy of 75- to 100-micron microspheres and the flexing silicon elastomer diaphragm with 25 x 300-micron polyurethane fibrils that form an interdigitated matrix. The design rationale is to create a surface that stimulates the development of a firmly attached fibrin layer that in time matures into a thin, collagenous, fibroblast-lined, blood-compatible lining. Both the rigid and the flexing blood surfaces provide the anchoring function.

The Heartmate was the first implantable, vented VAS approved by the Food and Drug Administration for clinical use in the United States. A vented VAS uses a small percutaneous conduit which provides volume compensation during the pulsatile pumping cycle. The patient must protect this conduit from fluids and other contaminants which could cause blockage or infection. This VAS is also approved in Europe in a battery-powered version as an alternative to cardiac transplantation.

The future for implantable ventricular assist and artificial heart systems appears quite bright. Clinical trials will provide data on patient quality of life, function, survival, and cost. Extending device lifetimes from 2 to 5 years is a reasonable extension of existing knowledge. Newer innovative designs use electrically transformed autologous skeletal muscle which is fatigue resistant to actuate the pumping chamber. Rotary blood pumps are much smaller devices which will anatomically fit in nearly all adults and many children. A rotary or continuous-flow blood pump is equivalent to a pulsatile blood pump but with a very small stroke volume and a very high beat rate. Those new concepts are under study in a new program of the National Heart, Lung, and Blood Institute (Kraft et al., 1994).

blood substitutes

Because normal human blood is dependent on the availability of healthy, willing donors, its supply has not always been adequate to meet the demand. Furthermore, the recognition that blood transfusions can be a means of transmitting diseases such as hepatitis or AIDS (current donor tests make this possibility unlikely) has heightened the interest in developing blood substitutes which would not carry this risk (Cheng, 1993). Although blood contains many separable therapeutic components (erythrocytes, leukocytes, platelets, and plasma proteins including gamma globulin, albumin, fibrinogen, and other coagulation factors), the term "blood substitutes" usually refers to oxygen carriers to substitute for erythrocytes (red blood cells). One group of compounds that have shown promise are perfluorochemicals, some of which can carry as much oxygen as hemoglobin. Fluosol, first developed in Japan, is an emulsion of perfluorodecalin and perfluorotripropylamine; it is undergoing clinical trials. Fluosol has been approved for use during percutaneous transluminal coronary angioplasty (PTCA) for high-risk patients. It is also being evaluated as an adjunct to cancer therapy and in the treatment of myocardial infarction in conjunction with thrombolytic therapy.

Another approach has been to encapsulate or cross-link hemoglobin following its separation from stroma. Compared with erythrocytes, microencapsulated hemoglobin has a markedly shortened survival in the circulation and is rapidly taken up by the reticuloendothelial system. Hemoglobin is cross Jinked to polymers to prolong its survival in the circulation. These red blood cell substitutes are effective in the treatment of acute experimental hemorrhagic shock. However nonoxygen-carrying solutions such as isotonic saline and Ringer's lactate may be equally effective for replacement of 30—40% loss in blood volume. With larger losses, cross-linked hemoglobin is more effective. Safety and efficacy in clinical trials have not yet been determined for microencapsulated or cross-linked human hemoglobin.


Most of the devices described in this chapter have advanced from the animal experiment stage to the demonstration of major clinical value within the past 40 years. TAHs and VADs have not yet attained acceptance for long-term therapy. Problems remaining to be solved with these devices are primarily related to biological events at the interface, especially thromboembolism and infection. The small-diameter prosthetic vascular graft requires further development. In this case, the principal problems are thrombosis and intimal hyperplasia, both of which can result in narrowing or occlusion, distal ischemia (reduced blood flow), and tissue injury.

Current Research

Investigative approaches being pursued to overcome the problem of thrombosis and intimal hyperplasia on device surfaces include

1. Binding and controlled release of heparin and other antithrombotic compounds; both heparin and nonanticoag-ulant fractions of heparin inhibit smooth muscle cell replication and intimal hyperplasia; binding of other growth factor inhibitors.

2. Seeding of surfaces with endothelial cells, fetal fibroblasts, or other cells (fetal cells have the advantage over adult cells of inducing a diminished antigenic response).

3. Binding of synthetic peptides that stimulate endothelial cell adhesion and spreading.

4. Using the techniques of genetic engineering, inserting into endothelial or other cells the gene for tPA enhancing their capacity to lyse clots; seeding such cells on the luminal surface of devices, or producing an "organoid" (a hybrid organ composed of a network of biomaterial fibers lined by such cells) capable of producing a systemic clot-inhibitory effect. Genes controlling the expression of other thrombus-inhibiting or hyperplasia-inhibiting molecules could also be used.

5. Development of bioresorbable materials from which to fabricate small-diameter vascular grafts that become replaced by tissues of suitable strength as the polymers are resorbed; development of bioresorbable stents whose stimulus for thrombosis or intimal hyperplasia diminishes as they are resorbed or which incorporate inhibitors of these processes.

6. Improved hemodynamic design aimed at eliminating zones of stasis, stagnation, recirculation, flow separation, turbulence, or excessively high shear stress (Slack and Turitto, 1993).

Approaches to the problem of infection include:

1. Study and modification of receptors to bacterial adherence and colonization on polymeric and metallic surfaces.

2. Binding and controlled release of antibacterial agents.

3. Seeding of surfaces with endothelial or other cells known to have antibacterial properties.

4. Genetic engineering of endothelial or other cells to increase their antibacterial properties; seeding of such cells onto the luminal and outer surface of implanted devices.

Future Directions

In the future, there will be less emphasis on solving problems related to short-term exposure of devices to blood and tissues, since the complications can usually be tolerated or pharmacologically suppressed to acceptable levels. Instead, attention will be focused on interfacial events occurring after long-term implantation of devices, an area of increasing interest in device development. In the cardiovascular field, the approach in long-term implants is shifting away from attempts to develop materials that are increasingly inert, and towards the development of a cardiovascular implant which in time becomes integrated with its biological environment. Such a device would contain on its luminal surface or release molecules similar to the organ it replaces, encourage entry and organization of tissue cells within its structure, and become partially or completely replaced (by bioresorption) by the host's cells that provide the mechanical and functional properties possessed by the organ it replaces. This strategy is an application of the evolving discipline called tissue engineering. Tissue engineering is the application of engineering principles to create devices for the study, restoration, modification, and assembly of functional tissues from native or synthetic sources (Anderson et al„ 1995).


The authors thank the following for assistance and suggestions in preparingthis manuscript: A. S. Berson, Ph.D.; M.J. Domanski, M.D.; G. Nemo, Ph.D.; M. L. Offen, M.D.; R. M. Hakim, M.D., Ph.D.; R. J. Turner, Ph.D.; and Ms. Kendra Brown and Ms. Gloria Dean for typing the manuscript.

Glossary of Terms

Angioplasty Surgical reconstruction of a blood vessel. Anticoagulation The administration of a drug (anticoagulant) such as heparin or Coumadin, which prevents or delays blood coagulation. Balloon angioplasty The process whereby the narrowed segment of a vessel is dilated by inflating a balloon directed to the site of narrowing by a catheter.

Bifurcation Division into two branches.

Cardiogenic shock Shock or failure of the circulation resulting from diminution of cardiac output in heart disease, as in myocardial infarction.

Descending thoracic aorta The portion of the aorta in the chest distal to the ascending or proximal segment that arises from the left ventricle and distal to the arch of the aorta.

Dura mater Outermost tough fibrous membrane encasing the brain and spinal cord.

Endothelial-derived relaxing factor (EDRF) A labile factor produced by and released from endothelial cells and having the properties of dilating blood vessels and inhibiting platelet aggregation. Extracorporeal membrane oxygenation (ECMO) A system whereby unoxygenated blood from a peripheral (arm or leg) blood vessel is oxygenated by flowing it through a synthetic membrane oxygenator outside the body and returning it to the circulation. Fascia lata A sheet of fibrous tissue that encases the muscles of the thigh.

Fibrinolysis The dissolution of fibrin by an enzyme, such as tPA, urokinase, or streptokinase.

Fibroblast A connective tissue cell that plays a role in supporting and binding tissues.

Hemocompatibility Compatibility of a material with blood to which it is exposed.

Hemolysis Destruction of red blood cells.

Hemostasis The mechanisms whereby blood coagulation, platelet functions, vascular integrity, and vasoconstriction (constriction of blood vessels) control excessive bleeding following vascular injury. Heparin An acid mucopolysaccharide present in many tissues and having the property of preventing blood from coagulating. Clinically it is used to prevent thrombosis.

Hyperplasia Abnormal increase in number of cells in normal arrangement in a tissue.

Hypotensive Causing a drop in blood pressure; having a low blood pressure.

Intimal Pertaining to the innermost layer of arteries or veins. Inferior vena cava The major vein in the abdominal cavity draining the legs, pelvic, and abdominal organs. Leukocyte White blood cell.

Myocardial infarction Heart attack caused by necrosis (death) of a portion of heart muscle.

Neointimal Pertaining to a newly formed inner layer of a blood vessel. Neutropenia A decrease in the concentration of polymorphonuclear leukocytes (PMNLs) to a subnormal level.

Perianastomotic In the vicinity of the anastomosis or point where two blood vessels, or a natural tissue (i.e., blood vessel) and a prosthesis (i.e., synthetic vascular graft), are sutured together. Pericardium Fibrous sac that encases the heart.

Polymorphonuclear leukocyte (PMNL) Neutrophil; one of several types of white blood cells. Plays important roles in defense against infection and in immune response.

Pulmonary artery (left and right) Major artery carrying blood from the heart to the lungs.

Pulmonary embolism Obstruction of an artery in the lung by thrombotic material that lodged there after flowing from a distant site such as the heart or a vein.

Restenosis Renarrowing of the lumen of a vessel (usually by neointimal hyperplasia) after the original narrowed segment has been reopened, as by angioplasty.

Thrombocytopenia A decrease in the concentration of blood platelets to a subnormal level, commonly associated with bleeding. Thromboembolism The dislodgment of thrombotic material from its site of formation resulting in blockage of blood flow at a downstream site.

Thrombolytic therapy Therapy aimed at lysing clots. See fibrinolysis, tPA, and urokinase.

Thromboresistant Resistant to the initiation or formation of thrombus.

Thrombosis The formation of a clot in flowing blood in the heart or a blood vessel.

Thrombus A blood clot forming in flowing blood in the heart or a blood vessel.

tPA Tissue-type plasminogen activator, an enzyme found in various tissues, capable of activating plasminogen to plasmin and therefore of lysing clots.

Urokinase An enzyme found in the urine of mammals, including humans, capable of activating plasminogen to plasmin, a fibrinolytic enzyme, which dissolves clots.

Vasoconstriction Diminution of caliber of vessels caused by contraction of cells in the vessel wall, resulting in decreased flow.


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