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Axial Flow Blood Pumps

Song, Xinwei*; Throckmorton, Amy L.; Untaroiu, Alexandrina*; Patel, Sonna; Allaire, Paul E.*; Wood, Houston G.*; Olsen, Don B.

doi: 10.1097/01.MAT.0000074117.00332.0D
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Each year, thousands of cardiac patients await healthy donor hearts for transplantation. Due to the current shortage of donor hearts (approximately 2300 per year), these patients often require supplemental circulatory support until a transplant becomes available. This supplemental support is often provided by a mechanical heart pump or left ventricular assist device (LVAD). This article explores one type of LVAD, specifically the design and development of axial flow ventricular assist devices (VAD). We discuss the design details, and experimental or clinical experience with the following axial flow support systems: Hemopump, MicroMed DeBakey VAD, Jarvik 2000, HeartMate II, Streamliner, Impella, Berlin INCOR I, Valvo pump, and IVAP. All of these devices demonstrate promise in providing bridge-to-transplant and ultimately destination therapy for adult cardiac failure patients.ASAIO Journal2003; 49:355-364.

From the *Mechanical and Aerospace Engineering Department, Virginia Artificial Heart Institute University of Virginia, Charlottesville, VA; †Biomedical Engineering Department, Virginia Artificial Heart Institute, University of Virginia, Charlottesville, VA; and ‡Utah Artificial Heart Institute, Salt Lake City, UT.

Submitted for consideration November 2002; accepted for publication in revised form January 2003.

Reprint requests: Amy L. Throckmorton, Biomedical Engineering Department, Virginia Artificial Heart Institute, 415 Lane Road, MR5 Building, University of Virginia, Charlottesville, VA.

In the United States, approximately 40,000 new cardiac failure patients each year await a healthy donor heart for transplantation.1-3 Unfortunately, only approximately 2300 donor hearts become available each year.1-3 Recent statistics indicate that only 1 in 24 cardiac patients who desperately need a transplant actually receive a donor heart.3 Because of this shortage of healthy donor hearts, cardiac failure patients must rely on alternative means of circulatory support, such as using a mechanical artificial heart pump or left ventricular assist device (LVAD).

During the past 20 years, blood pumps have gained widespread acceptance as prospective devices for bridge-to-transplant as circulatory support of cardiac failure patients. Over the years, a number of research groups have developed a significant number of ventricular assist devices (VADs).4-12 Generally, blood pumps fall into two main categories: positive displacement pulsatile and rotary continuous flow pumps. Several studies have demonstrated that pulsatile flow was not critical to maintaining the integrity of the pulmonary or systemic circulation and organ function, whereas other experiments have suggested evidence to the contrary.13 The implications of continuous flow (nonpulsatile) conditions remain unclear to the medical community, and additional research may provide insight into design considerations for both categories of blood pumps.13

Compared with continuous flow systems, pulsatile artificial hearts typically experience many mechanical failures, such as failing membranes caused by repetitive flexions and malfunctioning valve systems as a result of continual opening and closing.4,13,14 A number of components force the positive displacement pumps to be larger in size than a rotary pump. For example, a rather sizable solenoid valve is used in the Novacor VAD, and the HeartMate I VAD includes an electric motor with a cam system. The compressed air chambers for the Thoratec VAD and CardioWest total artificial heart contribute to the bulkiness of these pulsatile support systems. Continuous flow VADs universally include fewer moving parts and require less power to operate.

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Centrifugal and Axial Flow Blood Pumps

Rotary continuous- flow blood pumps can be further subclassified into two categories: centrifugal and axial pumps. In consideration of pump design theory, centrifugal pumps are capable of producing higher pressures at lower flows, whereas axial flow pumps typically generate higher flows at lower pressure rises.3-14 Axial flow pumps, although far more compact than centrifugal pumps, operate at much higher rotational speeds to produce the desired head pressure and flow. Because of their smaller size and tubular configuration, axial pumps require less time to implant, thereby decreasing the cost and invasiveness of the procedure.3-14 Centrifugal pumps typically weigh more than axial flow pumps, and this may lead to patient discomfort after installation beyond normal recovery complications. In addition, axial flow pumps generally consume less power, which allows for more compact and lighter power supply components and eventually implantable batteries.

Researchers have competed over the years to develop a number of axial flow VADs for the adult population. Several of the latest designs include the Hemopump, MicroMed DeBakey VAD, Jarvik 2000, HeartMate II, Streamliner, Impella system, Valvo pump, the IVAP VAD, and the Berlin INCOR I. The following sections discuss each of these devices and detail its experimental or clinical experience.

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Hemopump

Unique in its design, the Nimbus, later the Medtronic Hemopump, is a catheter-mounted, left ventricular assist system. This pump was originally invented by Dr. Rich Wampler and experimentally tested at the Utah Artificial Heart Institute.15-17 It was the very first axial pump to be developed and used in Food and Drug Administration (FDA) approved clinical studies. Medtronic then purchased the technology to employ it intraoperatively. The Hemopump (Figure 1) was developed specifically as an acute cardiac assist device for patients during minimally invasive coronary bypass surgery and for pathologies in which the heart must be relieved of approximately 80% of its workload.18-26 Those pathologies include, but are not limited to, cardiogenic shock, postcardiotomy left ventricle failure, and acute myocardial infarction. Intended for shortterm use, the axial flow pump assembly was inserted through the femoral artery and positioned to accelerate blood movement from the failing left ventricle into the ascending aorta and great arteries. Therefore, patients suffering from peripheral vascular disease, aortic aneurysm, aortic valve stenosis or insufficiency, and left ventricular thrombosis presented numerous difficulties in implanting the catheter based pump.25

Figure 1.

Figure 1.

The bedside installed console allows regulation of the pump speed from 17,000 to 26,000 revolutions per minute, which yields nonpulsatile flows from 3.5 to 4.5 L per minute.26 In this system, two cannulas are used in series: a femoral device and a transthoracic device; both devices are 8.1 millimeters in diameter. The femoral cannula, however, has a longer inflow tip with a maximum flow of approximately 3.5 L per minute, whereas the transthoracic open chest cannula can deliver a maximum flow of 5.1 L per minute.22,23

In vitro and in vivo testing of the Hemopump began in 1983.22 Cooper et al.25 reported its use to provide right ventricular support to six pigs. Statistics suggest that 20% of cardiac patients receiving left ventricular mechanical circulatory support will likely experience right ventricular failure as well. The axial pump was placed through the pulmonary artery into the right ventricle to relieve its workload. This group investigated the pump's ability to maintain perfusion in a pig with an acute partial pulmonary arterial obstruction.25 The Hemopump returned the right ventricle's stroke volume and output to normal physiologic levels and maintained perfusion. In addition, Meyns et al.18,24 analyzed the hemodynamic changes, peripheral vascular perfusion, and myocardial perfusion in 12 sheep supported with the Hemopump. The pump operated at 5.1 L/min for maximum flow at 26,000 revolutions per minute and was able to maintain adequate perfusion.

In 1988, the FDA approved clinical studies for the Hemopump in patients suffering from cardiogenic shock.22 The initial acute studies involved use in seven patients with a 42% survival rate after 1 year. For these patients, no incidence of infection, thrombosis, or vascular injury occurred. Shortly thereafter, a multi-institutional study was conducted with 41 patients over a 30 day time period. The survival rate for the patients was approximately 32%.22 From 1989 to 1997 in Belgium, approximately 61 patients suffering from postcardiotomy left ventricular failure were supported with the Hemopump.24 The mean age of patients ranged from 64 • 7.9 years with 43% initially suffering from a myocardial infarction. The axial pump support increased the cardiac indices and mean arterial pressures of the patients and decreased their left atrial pressures. Of the 61 patients, 20 patients suffered from acute renal failure, and 6 of these 20 patients demonstrated immediate improvement with the axial pump's supplemental support. Approximately 20% of the patients died from inadequate organ perfusion and brain emboli.24 The Hemopump cardiac assist system has also been clinically used in Europe to successfully support coronary bypass patients.20,21 This device is no longer available but established the basis for all subsequent axial flow VADs that are undergoing clinical tests and development today.

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Micromed DeBakey VAD

The Micromed DeBakey VAD, an implantable miniaturized axial-flow blood pump, supplements the pumping action of the left ventricle.27-34 Developed in collaboration with NASA Johnson Space Center, Baylor College of Medicine, and MicroMed Technology Incorporated, this pump measures 25 mm in diameter by 75 mm in length. The axial VAD requires less than 10 watts of input power for a rotational speed of 10,000 revolutions per minute. This rotational speed enables the pump to produce 5 to 6 L per minute at a pressure of 100 ml of mercury.30 The impeller is capable of rotating from 7,500 to 12,500 revolutions per minute and generating flow up to 10 L per minute.27-34Figure 2 illustrates the MicroMed DeBakey VAD.35,36

Figure 2.

Figure 2.

The components within the titanium pump housing consist of an inducer-impeller, a stationary flow straightener acting as the front/mechanical pivot bearing support for the inducerimpeller, and a stationary diffuser, which is downstream of the impeller and contains the rear mechanical pivot bearing. The bearings of this pump have proven extremely reliable in bench tests, exceeding 2 years of operability. Eight earth magnets are embedded into the three blades of the impeller. Placing these magnets in the blades helped to reduce the “fluid gap” of the brushless direct current (DC) motor. The diffuser slows the high tangential velocity of the blood as produced in the impeller region by redirecting it axially, resulting in an increase in the pressure of the fluid.

Before clinical use in the adult population, experimental trials in calves were completed to assess the pump's overall performance as well as hemodynamic effects. For these experiments, the index of hemolysis after 3 months of implantation in calves remained below 0.0002 g per 100 L of blood, which is well within acceptable limits for blood pumps.30 The calves also demonstrated normal renal and liver function for all levels of physical activity, including exercise conditions. The trials also revealed no evidence of thrombus formations caused by stagnant or irregular flow patterns.30

In 2000, MicroMed received conditional approval from the FDA to begin its multicenter clinical study of the DeBakey VAD in the United States.31-34 A report issued in September of 2000 by the Baylor College of Medicine indicated that 51 adult cardiac patients had successfully received a bridge-to-transplantation on the DeBakey VAD.36-38 After evaluation of the first 32 patients, the survival rate after 30 days of circulatory support averaged 81%. The median support duration was 47 days, with a cumulative patient support extending 1876 days.33 Patients experiencing complications primarily suffered from multiorgan failure. The pump maintained cerebral perfusion and end organ function during its support duration and demonstrated few signs of hemolysis and thrombosis.

More recent statistics released in September of 2002 at the International Society of Rotary Blood Pump Conference in Osaka, Japan indicated that the pump has supported 169 patients at 14 different heart centers and in 7 countries around the world.37-40 The mean flow rate for these trials ranged from 3.6 to 5.0 L/min.40 In addition, MicroMed applied the Carmeda CBAS biocompatible coating to the internal pump housing and internal components that contact the blood to reduce the incidence of thrombus formations.40

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Jarvik 2000

Since 1988, Transicoil Incorporated, the Oxford Heart Center, and the Texas Heart Institute (THI) have been developing the Jarvik 2000 for the adult population.41-48 This intraventricular axial flow pump (Figure 3) measures 25 mm in diameter by 55 mm in length with a mass of approximately 85 g.49,50 The pump speed is regulated by a pulse width-modulated brushless DC motor controller, which enables the impeller to rotate at speeds of 8,000 to 12,000 revolutions per minute to deliver 2 to 7 L/min at a pressure of 100 mm of mercury.

Figure 3.

Figure 3.

Figure 3 illustrates the power supply system that was designed for clinical use. This system includes a power cable that extends through the abdominal wall and is connected to the external controller. Lithium-ion or lead acid batteries provide the power for the system. The pulse width-modulated control circuit allows the user to adjust the rotational speed of the pump manually. This design has thus far been used in most of the implants.

At the THI from 1991 to 1999, approximately 37 healthy calves received a Jarvik 2000 VAD to evaluate the performance of the pump.48 These calves were supported for a mean duration of 70 days. Hemolysis levels averaged 0.00082 • 0.00054 g per 100 L of blood with heparin and warfarin administered. Neurologic, renal, hepatic, and pulmonary function remained normal throughout the experiments.48 Similarly, in vivo experiments performed in 1997 showed the plasma free hemoglobin levels to be 0.0007 • 0.00051 g per 100 L of blood with no evidence of thromboembolic events, except for a minimal ring of fibrin (approximately 1 mm) on the inflow bearing.44 As of March of 2001, the Jarvik 2000 had been implanted into 67 animals for performance testing and to enable design improvements.

With promising results from animal experiments in early 2000, clinical trials began in April at the THI and in Oxford, United Kingdom, shortly thereafter.41-48 The first three patients included two males and one female who suffered from chronic heart failure because of cardiomyopathy. The mean support duration was 65 days, with minimal signs of postoperative hemorrhaging. With the Jarvik 2000 support, the patients' hemodynamic conditions significantly improved. The average cardiac index increased to almost 2.5 times the level before implantation, and low levels of retrograde flow were observed. Echocardiographic data further revealed a decrease in the cardiac silhouette, thereby suggesting a reduction in left ventricular workload.

At the International Society of Rotary Blood Pump Conference in September 2002, the latest update of the Jarvik 2000 pump was presented.51 In total, 37 clinical implants have taken place, with 22 occurring in the United States and 12 in European facilities. All of these patients survived the initial implant of the pump, and the 1 year survival rate is approximately 56%. The virtually noiseless pump operated in conjunction with the native left ventricle to generate pulsatile blood flow. Twelve of the 37 patients were successfully weaned from support and received a heart transplant. Four of the permanent implants continue to thrive today, which demonstrates an operability of between 1 and 2 years.51

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HeartMate II

Developed by Thoratec Corporation, formerly ThermoCardiosystems, the University of Pittsburgh's McGowan Center (UPMC), and Nimbus Company since 1991, the HeartMate II is an axial flow blood pump. It has mechanical bearings, percutaneous electrical leads, an external power source, and a battery powered or an alternating current (AC) supplied power driver.52-58 Approximately the size of a D cell battery, the VAD measures 4 cm in diameter and 6 cm in length with a mass of approximately 375 g. As shown in Figure 4, this axial pump can generate flows up to 10 L/min at physiologic pressures.53 Experimental testing revealed that the pump operates successfully at 8000 to 9000 revolutions per minute for a pressure rise of 80 to 100 mm of mercury and a flow of 3 to 4 L per minute generating reported levels of plasma free hemoglobin from 3 to 6 g per day.56 The impeller includes three airfoil-shaped vanes to guide flow through the pump. Under normal operating conditions, the rechargeable batteries provide power for 2 to 4 hours.

Figure 4.

Figure 4.

Since 1997, this support system has been tested in more than 40 calves.56 These implant experiments evaluated the pump's performance, biocompatibility, controller response, and mechanical wear.54,57 The longest support duration was 225 days, with other experiments lasting as long as 184 to 195 days. The autopsies of the calves showed no signs of thromboemboli, and hemolysis levels remained well within acceptable levels according to the reports. Unfortunately, no normalized indices of hemolysis or plasma free hemoglobin concentrations were published. As of 2001, the pump had been implanted in 51 animals for a mean support duration of 47 days. For this study, the longest support duration lasted 226 days. Further testing of the VAD demonstrated a ball-andsocket bearing life of at least 5 years.56

Clinical trials of the HeartMate II were initiated in 2000, and the first human implant occurred in July 2000 in Israel. Similarly, in Europe, the first implant occurred in Germany for a 61-year-old patient suffering from ischemic cardiomyopathy. The HeartMate II is currently an investigative device in Europe only and is not available in the United States.54-57 Thoratec Corporation hopes to soon complete a multicenter clinical trial in Europe. This trial will evaluate the performance of the HeartMate II as a therapy for endstage heart failure, ultimately as an alternative to transplantation.

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Streamliner

Formerly under development at the UPMC, the Streamliner is a magnetically suspended axial VAD intended for permanent implantation.59-62Figure 5 displays the Streamliner's impeller with stationary flow straightener blades, rotor, and stationary diffuser blades.59 Hybrid passive radial and active axial magnetic bearings support and suspend the impeller, which extends approximately 20 mm. Shock testing demonstrated the suspension system's ability to withstand a peak acceleration of 8 times that of gravity along all three axes. The rotor body contains electromagnetic components of the motor and magnetic bearings and is suspended on a long post along its centerline. Because of the suspension system's design, two flow paths exist through the pump. The main flow path includes the region between the rotor and interior of the housing. A secondary interior annular gap is present between the rotor, and the internal stationary spindle is situated in the center of the pump. This secondary gap allows for retrograde flow toward the inlet region of the pump as driven by the pressure differential. The hydraulic design involves five impeller blades, four interior annular gap refinements (flow straightener), and six stator blade refinements (diffuser). To accommodate electrical components, the diffuser's blades were designed with an unusual thickness.59 Designers used computational fluid dynamics and oil streaking technology to evaluate the shear stress within the fluid paths.61,62 These methods enabled the designers to improve the flow paths and reduce the likelihood of hemolysis or regions of stagnant flow.

Figure 5.

Figure 5.

In vitro hydrodynamic experiments with a water/glycerol solution demonstrated a hydraulic efficiency of 28% and the pump's ability to operate over a wide range of flow conditions. Another in vitro evaluation with bovine blood demonstrated very low hemolysis levels for operating conditions of 7000 revolutions per minute and a flow of 6 L per minute against 100 mm of mercury. In 1998, an in vivo calf implant experiment was conducted with a titanium-nitride coated pump connected between the left ventricular apex and the descending aorta. This experiment showed a mean plasma free hemoglobin of 0.00073 • 0.00039 g per 100 L of blood and normal platelet levels. All blood-contacting surfaces of the pump were found to be free of thrombus with the exception of crevices at the inflow cannula junction and between the leading edge of the stator blades and housing. Thrombus formations in these areas emphasize the importance of eliminating grooves and crevices during the manufacturing process.60

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Impella

Designed by Impella AD in Aachen, Germany, the intracardiac Impella circulatory assist system measures 6.4 mm in diameter by 60 mm in length as seen in Figure 6.63 These systems are fully capable of providing biventricular support to adult patients.64-69 The left ventricular Impella pump nominally rotates at 30,000 revolutions per minute to deliver 7 L/min at 100 mm of mercury of head with a hydraulic efficiency of approximately 30%. The impeller has only two blades of 0.3 mm in thickness and a tip clearance (from the tip of the blade to housing) of 0.1 mm. Designers employed miniature pressure sensors, only a few tenths of a millimeter in diameter, for control purposes. These sensors are capable of assuring reliable pressure data transmission for a number of days.64,65

Figure 6.

Figure 6.

The compact size and sophisticated design of the Impella systems enabled several successful animal and clinical trials. At the Helmholtz Institute, Siess et al.65 reported that the Impella pump was employed to support 6 sheep with a mass of 50 to 80 kg. Intended for biventricular support, the axial pumps generated 3.5 to 4.5 L/min at a mean arterial pressure of 49 to 65 mm per mercury. Stolinski et al.67 further evaluated the effects of this axial pump in supporting 14 sheep. A myocardial infarction was induced in each animal for these experiments. The VAD reduced left ventricular workload, decreased the infarct size, and maintained perfusion levels. This group also performed another animal study evaluating the performance of the Impella VAD without anticoagulants.68 In this study, four sheep received support from the Impella pump for 10 days without continuous infusions of heparin. One pump was replaced during this study because of a mechanical failure. Autopsies of the animals revealed no signs of thrombosis, no emboli in the liver and kidneys, and no evidence of hemolysis.68

Since these successful animal experiments in 2000, approximately 120 patients have received supplemental support by the Impella pump while undergoing coronary bypass grafting.68 More specifically, Autshbach et al.69 reported a study comparing conventional cardiopulmonary bypass (CPB) tech- niques with the Impella performance in a multicenter study of patients undergoing coronary artery bypass grafting (CABG) surgery. The hemodynamics of 92 patients were compared with conventional CPB versus implanting the Impella pump. This study, unfortunately, demonstrated no significant differences in the mechanical circulatory support techniques. A comparison of the plasma free hemoglobin, concentration of red and white blood cells, platelet count, bilirubin concentrations, and creatinine levels showed no variance between techniques.69 Stable perfusion and physiologic conditions were maintained during both support techniques.

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Additional Axial Support Devices Under Investigation

INCOR I

The INCOR I is currently being developed by the Berlin Heart group.70 Developers initially sought to provide bridgeto-transplant and bridge-to-recovery mechanical circulatory support. They hoped to ultimately provide an alternative support system to heart transplantation. This impeller, suspended by magnetic bearings, rotates at speeds of 8000 revolutions per minute, thus enabling the pump to deliver a flow of 5 L/min at 100 mm of mercury. The INCOR I consists of an inducer region with guide vanes, impeller section, and stationary diffuser to enhance the fluid's pressure rise.70 The impeller region is magnetically levitated by passive radial and axially active bearings.71 It requires only 2 to 4 watts of power for the motor with the bearings consuming as much as 1 watt for all operating conditions. The pump measures 30 mm in diameter with a mass of only 200 g. Computational fluid dynamics was used to develop the optimal flow path. Hydraulic performance testing in vitro and in vivo confirmed a successful design. Further experimentation demonstrated hemolysis levels of 0.006 g per 100 L of blood. Animal trials revealed no increase in plasma free hemoglobin.

In June 2002, the first two clinical implantations of the INCOR I took place at the German Heart Institute in Berlin in patients suffering from coronary heart disease.70 Specifically, one recipient suffered from ischemic cardiomyopathy, whereas the other patient suffered from idiopathic dilated cardiomyopathy. During implantation, the inlet cannula was inserted into the apex of the left ventricle and the outlet cannula connected directly to the ascending aorta. The pumps delivered approximately 5.5 L/min. No obvious signs of hemolysis occurred during patient support. Experimental measurements indicated that one patient's pulmonary arterial pressure (PAP) in systole decreased from 70 mm of mercury to 44 mm of mercury whereas the other patient's PAP was reduced from 45 mm of mercury to 28 mm of mercury. This reduction in the pulmonary pressure demonstrated improvement in left ventricular function and fluid movement.71 With the pump's success in supporting these patients, developers announced the beginning of a third clinical implant at the International Society for Rotary Blood Pumps conference in September of 2002.

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Valvo Pump

The intracardiac Valvo pump is approximately 40 mm in length by 38 mm in outer diameter.72 This device consists of an impeller, a DC brushless motor, and a guide vane. The motor and impeller assembly are fixed to a stainless steel casing by way of the guide vane. With four blades, the impeller extends 22 mm in diameter with an outer discharge angle of 19.2°.72 The impeller's hub diameter measures approximately 13 mm with a radial clearance between the impeller and the housing of 0.5 mm. This axial pump uses a ferrofluidic seal as the shaft's seal to protect the bearings, which experimentally demonstrated no leaks for 41 days under a pressure of 150 mm of mercury.72,73 Designed to deliver 5 L/min of blood flow, the Valvo pump generates 100 mm of mercury at 7000 revolutions per minute.

Mitamura et al.72 reported the results of a blood bag experiment for the Valvo VAD, which appears to be in its initial developmental stages. This experiment revealed a normalized index hemolysis of 0.03 • 0.003 g per 100 L of blood for 5 L/min at 100 mm of mercury. This test also allowed for pressure flow performance curves and hemolysis levels to be obtained and analyzed for varying rotational speeds.72 To date, no additional publications were found to document mock loop, animal, or other experiments using the Valvo pump.

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IVAP

The IVAP is an intraventricular axial flow, left ventricular assist pump by SUN Medical Technology Research Corporation in Nagano, Japan.74 This three-stage pump consists of a guide vane, tube housing, and DC motor. It can deliver more than 8 L/min at a rotor speed of 13,000 revolutions per minute. The housing diameter is 13.5 mm, and the hub diameter is 8 mm with a rotor tip clearance of 0.1 mm.74 The flow Reynolds number ranges from approximately 1300 to 2500 when the flow rate varies between 3 and 6 L/min. Advantages of this axial flow pump include size and compactness, minimally stagnant flow in the left ventricle and within the pump, and minimal blood contact with the surfaces of the pump compared with other support systems. Despite its continuous flow design, there is a small pressure drop across the pump allowing a pulsatile effect almost in sync with the beating heart.74

Fluorescent image tracking velocimetry (FITV) technology has been employed at two regions of interest in the pump: downstream of the rotor-stator interface and the pump outlet area.75 Results from FITV for the rotor-stator interface do not show any preliminary signs of hemolysis or stagnation. At the pump's outlet, there is evidence of slight unsteady flow conditions based on measured results. Unfortunately, no further information was found about this pump.

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INTEC

The fundamental design of the INTEC axial flow pump is based on the Jarvik 2000. Developers have improved the overall design by adding magnetic bearings to completely suspend the impeller. This system consists of bearing rings that are located at each end of the impeller to ensure stability.76 The pump housing diameter extends 25.4 mm and is surrounded by motor coils.76,77 Flow enters the pump and immediately encounters a set of stator vanes before the impeller region. The impeller includes three helical curved blades followed by a set of flow straightening stator vanes.77 The fluid gap region is 0.5 mm to prevent as much hemolysis and thrombosis as possible. The brushless motor, which rotates the impeller, is 12.7 mm in diameter. The flow of this pump at 8500 revolutions per minute is 6 L/min with a mean pressure of 100 mm of mercury.

The developer speculates that advantages of this pump include physiologic control without the use of external sensors, but this is as of yet unproven. Goldowsky et al.77 suggests that control is possible on the basis of the real time axial position of the rotor and estimated differential pressure caused by rotor position. The differential pressure is directly measured using the coil inductance. Systolic and diastolic pressures can be obtained as these parameters change over the course of a heartbeat.77 Currently, no other articles have been found that discuss experimental validation for this pump.

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Conclusions

With a rapid increase in the number of patients suffering from congestive heart failure and donor heart availability remaining stagnant each year, the artificial heart industry strives to provide an alternative means of long-term circulatory support for these patients. Axial flow pumps comprise the latest effort by research organizations and artificial heart technologies to offer effective and reliable mechanical circulatory support options.

The industry continues to reach design milestones, with the DeBakey VAD and Jarvik 2000 axial pump receiving conditional approvals from the FDA for clinical trials in the United States. The DeBakey VAD, Jarvik 2000, Hemopump, and Heartmate II have further been employed to support patients in clinical trials throughout Europe with reasonable success rates. These clinical trials often include the collaboration of several institutions. The Hemopump has also proven successful for biventricular support and received FDA approval for patients suffering from cardiogenic shock in the United States. In addition, the Impella VAD has shown promise in supporting CPB and CABG patients. The INCOR pump has also recently been used for clinical implantations in June of 2002, with results pending. The Valvo pump and IVAP system, however, are in their initial developmental stages according to existing publications.

During the past 20 years, all of the aforementioned axial flow VADs were designed, developed, manufactured, and experimentally tested by various techniques. Each device demonstrated promising results as a bridge-to-transplant system and potentially as an alternative to heart transplantation. However, there are many technical challenges and obstacles to overcome. As identified in the literature, the lifespan of the mechanical bearings and the suspension system durability presents ongoing challenges for designers. Additional challenges include the axial flow VADs ability to deliver a nominal flow rate of 10 L/min without needing to increase its inherent design capacity and size. This increased capacity will allow patients to participate in various levels of physical activity, which may promote muscle gain and an overall improvement in health. Current axial flow pump designs may lead to blood trauma and thrombosis, which can be reduced with further understanding of the mechanisms causing such damage and activating the coagulation cascade. As these axial flow VADs reach clinical use at a rapid pace, research organizations and developers will continue to persevere until the most effective and reliable blood pump is available for long-term mechanical circulatory support.

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Acknowledgment

The authors wish to acknowledge the financial support for this work provided by Utah Artificial Heart Institute, Department of Health and Human Services, National Institutes of Health, and the National Heart, Lung, and Blood Institute Grant number R01 HL64378-01.

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