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Ann Thorac Surg 1996;61:452-456
© 1996 The Society of Thoracic Surgeons
Cardiovascular and Pulmonary Research Center, Allegheny-Singer Research Institute, Pittsburgh, Pennsylvania
Abstract
Background. In 1991, Allegheny General Hospital and Allegheny-Singer Research Institute purchased a centrifugal pump, then a 2-year-old technology, from Medtronic Bio-Medicus, as part of its research program for novel treatments of acute and chronic heart failure. During a 4-year development program, we then established and met goals of durability, performance, thromboresistance, and low cost.
Methods. In vitro testing involved extensive hydraulic characterizations using Penn State mock loops. Calorimetry was used to determine efficiency. Durability studies used heated (37°C) seawater for 28 to 45 days. In vivo studies used 46 sheep to test performance and engineering changes and to determine myocardial oxygen consumption, thromboresistance, and long-term durability. A left atrium-to-aorta circuit was used in all.
Results. Hydraulic testing showed no preload sensitivity but moderate afterload sensitivity at all impeller speeds (2,000 to 6,000 rpm). The heat load was low, and overall efficiency was 13% to 15%. Bench durability studies showed no electrical malfunction of the stator or console without degradation of the biomaterials used. Acute in vitro studies showed a near-linear relationship of myocardial oxygen consumption and left ventricular stroke work, pump flow, and pump speed. At speeds of 2 to 3 L/min (50% bypass), left ventricular stroke work and myocardial oxygen consumption were decreased approximately 50%. Additionally, 5 animals have had implants for 28 to 154 days with no macroemboli or microemboli detected in any animal. Hematologic and biochemical studies became normal 3 to 7 days after implantation. Hemolysis was low at less than 10 mg/dL. Clinical costs of the device are estimated to be 80% less than those of currently available devices.
Conclusions. We conclude that an old technology has been made into new technology by application of sound engineering design principles, microchips, and new biomaterials. Qualifying trials for a Food and Drug Agency investigational device exemption application are in progress.
The Allegheny Implantable Centrifugal Pump technology builds on research conducted by Dr Frank Dorman at the University of Minnesota and the work of Sanford Reich and William H. Gates at Medtronic, Inc (Minneapolis, MN). The early work was protected by patents with all rights initially assigned or licensed to Medtronic, Inc. In the early 1970s, various versions of a pump to assist heart function were produced and tested in animals. More than a decade later, the Bio-Medicus Corporation (Eden Prairie, MN) purchased the technology from Medtronic and began various modifications including the use of an infusate into the base of the pump to lubricate the shaft of the impeller at its fitting through a flexible polymeric seal. Allegheny General Hospital supported animal research conducted at Allegheny-Singer Research Institute (collectively ``Allegheny'') under the direction of George J. Magovern, Sr, MD, for several years to test the feasibility of using the small device as either an extracorporeal assist or an implantable ventricular assist device (VAD). In 1991, Medtronic, Inc granted to Allegheny sublicenses under the University of Minnesota and Medtronic, Inc, to use their patents for the implantable use of this technology. Allegheny has conducted a series of studies that has led to the current state of development of the pump. This report is a compilation of previously presented investigations [1–3].
Device Description
The centrifugal pump is powered by a direct-current brushless motor, which consists of a stator and a rotor containing rare earth magnets. The stator is composed of a laminated stack of thin steel plates wound with copper wire. No Hall sensors are used. The shaft of the six-bladed pyrolitic carbon impeller passes through a polymeric seal bonded to a polycarbonate journal and is threaded into the stainless steel rotor. This entire mechanism is fitted into a polycarbonate lower housing, the base of which contains the rotor and fits equatorially in the midst of the rotating torridal magnetic field generated by the stator. The upper housing is threaded into the lower housing, using a compression O-ring for liquid sealant. An exploded diagram is shown in Figure 1
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is the left-atrial inflow cannula, which is a custom-ordered, wire-wrapped polyvinyl chloride 46F lighthouse-tipped caval cannula, ordinarily used for cardiopulmonary bypass. The outflow cannula (not shown) is a polypropylene-reinforced, commercially available 6-mm-diameter expanded polytetrafluorethylene vascular graft. At the most proximal position of the outflow graft, the polypropylene wrap is removed and a 7-mm-diameter polyvinyl chloride tube containing a polyurethane balloon on the inner wall is placed to occlude the vascular graft in case of pump or console failure to prevent backflow. In Vitro Tests
We have presented and published a series of studies concerning the hydraulic behavior of the Allegheny pump [1–3]. Centrifugal pumps are nonoccluding in design and hence sensitive to downstream resistance. Impeller speed, stator current, and perfusate flow were measured as a function of resistance. The family of curves is shown in Figure 2. Typically, at clinical conditions of normal systemic vascular resistance of 1,000 dyne•s/cm5 and a mean arterial pressure of 70 mm Hg, a speed of 4,000 rpm would be required to achieve a perfusate output of 5.6 L/min. Current through our stator ranged from 550 to 650 milliamps for this load. To achieve a 50% reduction of normal cardiac output with a normal systemic vascular resistance and mean arterial pressure, a speed of 3,200 to 3,000 rpm is required with a current draw of 410 to 480 milliamps. Thus, for a 12-volt operating system, the electrical range of power required for half and full flow is 4.9 to 5.8 and 6.6 to 7.8 watts, respectively. These power requirements are significantly lower than those required by pulsatile systems. Other studies concerning heat load used calorimetry and external and internal surface measurements. Our experiments reveal that the maximum stator surface temperature, measured with surface thermistors, is 38.6°C with the pump submerged in a 37°C water bath. Pump efficiency determined by calorimetry was 16%, and that based on mechanical output and electrical input was 13%. The amount of heat transfer to the blood, based on efficiency calculations, was found to be approximately 1.2 W, corresponding to a 0.01°C temperature increase in blood passing through the pump at 2 L/min.
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In Vivo Studies
Acute
From 1992 to the present, we performed acute animal studies to determine the effect of pump function on cardiovascular physiology, particularly with respect to reduction in stroke work and myocardial oxygen consumption. Previous investigations by others [4–9] with roller pumps and pneumatic pulsatile assist devices have demonstrated that nearly complete capture of normal left ventricular end-diastolic volume was necessary for appreciable reductions in oxygen consumption and stroke work. We tested the hypothesis that a centrifugal pump would decrease left ventricular stroke work and oxygen consumption as a function of pump flow.
Ten sheep (35 to 50 kg) were instrumented and placed on left atrium-to-descending aorta bypass with the small, light-weight (254 g), implantable centrifugal pump. The relationships between pump flow as a percent of cardiac output, left ventricular stroke work, and oxygen consumption were studied. Left ventricular stroke work was calculated from the pressure-volume loops obtained with micromanometer and conductance catheters and was indexed per 100 g of left ventricular wet weight. Oxygen consumption was calculated from left main coronary artery blood flow and the arterial-coronary sinus oxygen content difference, normalized to 100 g of left ventricular wet weight and a heart rate of 100 beats/min. Measurements were made in stepwise increments of pump flow from zero to the maximum obtainable and then reversed in similar decrements. Analyses were made for 27 complete runs.
Our data demonstrate that reductions in left ventricular stroke work and oxygen consumption were achieved from zero to maximal bypass. There was an approximate 66% and 50% reduction in left ventricular stroke work and oxygen consumption, respectively, at 60% bypass (Figs 3, 4). Thus, in contrast to other types of assist devices, our centrifugal pump in a left atrium-to-aorta circuit provides predictable and significant reductions in left ventricular stroke work and oxygen consumption at moderate levels of bypass. These data establish the physiologic basis for use of this device as a predictable temporary assist in patients with postcardiotomy low-flow states.
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We have used the swivel system to support adult sheep with the Allegheny implanted centrifugal left ventricular assist device for more than 5 months. Throughout this period, the animals moved freely within a 2.8 m2 pen. Continuous electrical power in the range of 3 to 6 watts was relayed to the implanted pump via the swivel's high-current leads. In addition, the system allowed continuous monitoring of instantaneous pump flow rate and speed as well as arterial, central venous, and pulmonary artery pressures. Pressurized saline lubrication to the pump was transmitted via the swivel. Extension cables allowed remote transmission and sensing of these parameters out of the animal's view. There were no malfunctions of the swivel/tether system or pump for the duration of these experiments.
This unique system has proven its efficacy in chronic studies with an implanted left ventricular assist device, but could easily be adapted to any powered device experiment requiring electrical or fluid-filled leads. It offers the advantage of greatly increased animal mobility, which may improve the recovery and overall health of animals involved in long-term studies.
PHYSIOLOGIC STUDIES.
A variety of studies has been performed on these animals. One 28-day study was typical of our data to date. The pump was implanted in the left thorax of a healthy adult sheep. Electrical power was provided via a thin percutaneous cable. The pump inlet cannula was placed in the left atrium and the outlet graft was anastomosed to the descending aorta. Ultrasonic flow probes were placed on the pump inlet cannula and on the aorta just proximal and distal to the anastomosis. Central venous pressure and pulmonary capillary wedge pressure were measured via a pulmonary artery catheter. Mean arterial pressure and pulse pressure were monitored by a fluid-filled catheter in the left common carotid artery. A thermocouple was secured to the motor casing before implantation to monitor pump temperature and was compared simultaneously with rectal temperature measurements. Pump speed was varied intraoperatively to determine acute hemodynamic effects (Table 1).
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We have performed extensive serial hematologic and biochemical testing on all animals with chronic implants. Our data demonstrate an abrupt alteration in renal and hepatic function associated with the surgical procedure, which became normal after 3 to 5 days and remained so for the duration of the experiment. All animals achieved or exceeded preoperative values of hematocrit, hemoglobin, and red blood cell count, and received iron dietary supplements. White cell and platelet concentrations returned to normal values in 7 to 10 days, and remained within the normal range except for the infrequent episodes of sepsis, which were treated aggressively with multiple intravenous antibiotics.
Coagulation studies showed the expected increase in partial thromboplastin time values with heparin infusion into the pump. The partial thromboplastin time was maintained in the 40 to 65 seconds range. Several sheep were given prophylactic streptokinase and aspirin. We performed control studies in unoperated normal sheep, which showed virtually no plasminogen; hence thrombolytics have no use in this species. Aggregation studies of normal sheep platelets showed no effect of aspirin or dipyridamole. We currently use only heparin in sheep.
Thrombosis and thromboembolus have not occurred in any acute or chronic animal with our current design pump and infusion methods. Explant examinations have been encouraging because no formed thrombus has been found in any part of the pump. We have found and reported a thin 1- to 2-mm doughnut-shaped accumulation of white proteinaceous material around the impeller shaft at the seal after 5 months.
Two in vivo studies of occluder function have been performed, one acutely and one in a chronic animal. Both showed that with the pump turned off, there was a rapid decrement in blood pressure and cardiac output, with an increase in right-sided pressures. In the acute experiment with an open chest, the left heart dilated rapidly as documented with instantaneous left ventricular pressure-volume data. Cardiac arrest occurred in approximately 10 minutes. These experiences have impressed us with the need for an occluder system to prevent back-flow from the aorta to the left atrium through the pump. The occluder has functioned in vitro and in vivo without failure.
Comment
Postcardiotomy catastrophic heart failure unresponsive to intraaortic balloon pumping and industrial doses of inotropes occurs in approximately 1% of all adult cardiac surgical operations, estimated to be 400,000/year. The ventricular assist device registry, maintained at Penn State for many years and recently taken over by The Society of Thoracic Surgeons, has shown a 30% to 40% survivorship rate, with equal results from pulseless and pulsatile devices. The latter are complicated and highly expensive. Only one pulsatile device is approved by the Food and Drug Administration for postcardiotomy failure, and only one for bridge-to-transplantation applications.
We believe that the Allegheny system is suitable for short-term use, because it is simple, has low thrombogenicity, is durable, can be implanted without cardiopulmonary bypass, and, importantly, is inexpensive. We estimate that its price when marketed will be 80% less than that of other assist devices. We conclude that an implantable centrifugal pump will be efficacious and safe when used in the operative setting of acute profound cardiac failure.
Acknowledgments
We appreciate the efforts of Nancy Lynch, MA, BSN, and Jane Etherington, BS, in the preparation and editing of the manuscript.
Supported by the Allegheny Health, Education and Research Foundation.
Footnotes
Presented at The Third International Conference on Circulatory Support Devices for Severe Cardiac Failure, Pittsburgh, PA, Oct 28-30, 1994.
Address reprint requests to Dr Clark, Cardiovascular and Pulmonary Research Center, Allegheny-Singer Research Institute, 320 E North Ave, Pittsburgh, PA 15212.
References
This article has been cited by other articles:
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  J. A. Magovern, M. J. Sussman, A. H. Goldstein, G. W. Szydlowski, E. B. Savage, and S. Westaby Clinical results with the AB-180 left ventricular assist device Ann. Thorac. Surg., March 1, 2001; 71 (2007): S121 - S124. [Abstract] [Full Text] [PDF]
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 A. Parolari, F. Alamanni, M. Naliato, R. Spirito, V. Franze, G. Pompilio, M. Agrifoglio, and P. Biglioli Adult cardiac surgery outcomes: role of the pump type Eur. J. Cardiothorac. Surg., November 1, 2000; 18(5): 575 - 582. [Abstract] [Full Text] [PDF]
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S. Westaby, T. Katsumata, D. Pigott, X. Y. Jin, K. Saatvedt, M. Horton, and R. E. Clark Mechanical bridge to recovery in fulminant myocarditis Ann. Thorac. Surg., July 1, 2000; 70(1): 278 - 282. [Abstract] [Full Text] [PDF]
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E. B. Savage, R. E. Clark, W. P. Griffin, S. A. Davis, S. Hughson, C. J. Conway, and G. J. Magovern Sr The AB-180 circulatory support system: summary of development and plans for phase I clinical trial Ann. Thorac. Surg., August 1, 1999; 68(2): 768 - 774. [Abstract] [Full Text] [PDF]
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S. Ashraf, K. Bhattacharya, S. Zacharias, P. Kaul, P. H. Kay, and K. G. Watterson Serum S100{beta} release after coronary artery bypass grafting: roller versus centrifugal pump Ann. Thorac. Surg., December 1, 1998; 66(6): 1958 - 1962. [Abstract] [Full Text] [PDF]
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