Ann Thorac Surg 2005;79:1378-1383
© 2005 The Society of Thoracic Surgeons
New technology
MagScrew Total Artificial Heart In Vivo Performance Above 200 Beats Per Minute
Soren Schenk, MDa,
Stephan Weber,
Dipl Inga,
Viviane Luangphakdy, MSBMEa,
Ryan S. Klatte, BSBMEa,
Christine R. Flick, BSBMEa,
Ji-Feng Chen, BSa,
Michael W. Kopcak, Jr, BAa,
Yoshio Ootaki, MD, PhDa,
Keiji Kamohara, MDa,
Gordon B. Hirschman, MEng EEb,
Nicholas G. Vitale, BSMEb,
Peter A. Chapman, Jr, BSME PEb,
William A. Smith, D Eng PEa,
Kiyotaka Fukamachi, MD, PhDa,*
a Department of Biomedical Engineering, The Cleveland Clinic Foundation, Cleveland, Ohio,, USA
b Foster-Miller Technologies, Albany, New York, USA
Accepted for publication March 15, 2004.
* Address reprint requests to Dr Fukamachi, Department of Biomedical Engineering, ND20, The Cleveland Clinic Foundation, 9500 Euclid Ave, Cleveland, OH 44195, USA
fukamach{at}bme.ri.ccf.org
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Abstract
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PURPOSE: Downsizing pulsatile devices requires an increase of beat rate if flow capacity is to be maintained. We applied this concept to the preclinical MagScrew total artificial heart (TAH).
DESCRIPTION: The device fills passively with a stroke volume of 45 ml and beat rates up to 250 beats per minute (bpm).
EVALUATION: Stable hemodynamics were observed during a 30-day bovine implant with a flow of 8.7 ± 1.2 L/min at beat rates of 204 ± 18 bpm. Device filling was exceptional up to 250 bpm generating flow of greater than 12 L/min. Beat rate adapted to preload in a way similar to a Frank-Starling response. Left and right atrial pressures were balanced. The aortic pulse pressure was 49–70 mm Hg, which translates to a pulsatility index of 0.49–0.77. Organ functions were preserved and blood damage did not occur.
CONCLUSIONS: Increasing the beat rate while downsizing the MagScrew TAH was successful with strong flow generation by passive filling. Pulsatility was maintained at high beat rates. This innovative approach may be used to develop small pulsatile pumps.
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Introduction
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| Mssrs. Hirschman, Chapman, and Vitale disclose that they have a financial relationship with Foster-Miller Technologies.
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Pulsatile ventricular assist devices and total artificial hearts (TAH) can maintain physiologic blood flow. However the large size of current devices contributes to serious complications and limits the patient population because of anatomic fit [1]. These complications include compression of abdominal viscera resulting in visceral perforation or obstruction, wound dehiscence, malnutrition and bleeding in the large pocket that, at times, occurs with infection. Increasing the device beat rate allows for a smaller stroke volume pump thereby decreasing size while maintaining high-flow capacity. Here we describe physiologic interactions with the current MagScrew TAH, a compact pulsatile device capable of beat rates up to 250 beats per minute (bpm). We examined device flow and filling, atrial balance, pulsatility, level of blood damage, organ functions, and functional activity during a 30-day bovine implant. Blood pressure amplitude and pulse curves were compared with the normal bovine physiology.
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Technology
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The MagScrew TAH has been developed by The Cleveland Clinic Foundation and Foster-Miller Technologies (Fig 1). Compared with its predecessor [2, 3] the displaced volume of the current pump/actuator package is reduced by 10% to 550 ml. The new pump possesses reduced thickness and an improved anatomic fit but maintains flow and pressure capability. The maximal beat rate capability increased nominally from 160 to 250 bpm. The new device possesses a stroke volume of 45 ml (ie, a 25% decrease compared with the previous model). The target design point is 5.4 L/min output at 120 bpm and a left afterload of 100 mm Hg. Flows greater than 11 L/min are generated at beat rates exceeding 210 bpm.
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Fig 1. MagScrew total artificial heart (TAH). (A, B) The actuation mechanism is based on the attracting forces of conventional magnets incorporated in "MagScrew" and "MagNut." The motor/MagNut combination alternately rotates clockwise and counterclockwise. The linear bearing of the MagScrew forces the MagScrew to reciprocate to maintain alignment of the magnetic "threads" with the nut. This alternately pushes the right and left pump to ejection. (C) Device configuration in a potential human implant. The full system includes the MagScrew TAH and supporting devices (ie, electronic control unit, compliance chamber with refill port, internal and external batteries, and a transcutaneous energy transmission system [TETS]).
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Detailed descriptions of surface coating and actuation mechanism have been published previously [2, 4]. A gelatin coating of blood-contacting surfaces and bioprosthetic valves are used to eliminate the necessity for postoperative anticoagulants. The device fills passively in response to venous return. The pump diaphragms float free of the actuator during filling, which prevents suction. Filling the left pump regulates beat rate and also determines pump flow. To maintain atrial balance the right pump stroke flow is limited by 20% in stroke length and by 15% voltage supply in ejection compared with the left pump. The actuator is a nut/screw system that uses spiraling magnets instead of threads to convert rotary motion to linear actuation (Fig 1B). This mechanism reduces friction and wear of this critical interface. The primary loaded components are two ball bearings supporting the MagNut with an estimated 99% survival after more than 10 years of operation.
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Technique
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Orthotopic device implantation followed previously described techniques [3]. Briefly, under cardiopulmonary support, the native ventricles of the calf (body weight 99 kg) were excised. The aorta and pulmonary artery were mobilized and Dacron conduits were anastomosed to each vessel. Next the inflow cuffs were anastomosed to the left and right atria using Teflon felt as reinforcement. The pump was brought to the surgical field, its bioprosthetic inflow and outflow valves (Edwards 6900P; Edwards Lifesciences, LLC, Irvine, CA) were installed, and the device was connected to inflow cuffs and outflow conduits. The pump was first operated in single-stroke mode to allow air removal and then switched to automatic mode with cardiopulmonary bypass suspended. Aortic pressure (AoP) was measured in the carotid artery, whereas left atrial pressure (LAP) and right atrial pressure (RAP) were measured at the inflow cuffs of the device. Pulsatility was quantified by pulse pressure and pulsatility index (pulse pressure/mean arterial pressure) [5]. Pump flow was calculated from preload and afterload, motor current consumption, and beat rate as validated by in vitro tests. No anticoagulants or antiplatelet drugs were used postoperatively.
To compare the pulse pressure curves with normal bovine physiology, data were recorded from a control animal using a fluid-filled pressure line in the carotid artery to obtain AoP. Heart rate was modulated by left atrial pacing up to 140 bpm. The level of blood damage and organ function during 30 days of in vivo testing was assessed by analyzing free serum hemoglobin, creatinine, lactate, bilirubin, and liver enzymes. Functional activity was evaluated by a treadmill exercise.
In vitro device evaluation under various preloads and afterloads followed previously described procedures [2]. Separate water-filled chambers simulated right and left atria as well as pulmonary artery and aorta. The inflow chambers were open to atmosphere while the outflow chambers contained trapped air volume to simulate arterial compliance. Varying the inflow water column assessed preload responsiveness. Aortic flow was measured with an ultrasonic flow probe (20XL121; Transonic Systems, Inc., Ithaca, NY).
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Animal Experiment
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The MagScrew TAH functioned without any problems for 30 days of in vivo testing. Pump output amounted to 8.7 ± 1.2 L/min at a mean left afterload of 93 ± 12 mm Hg. The device beat rate ranged from 157–249 bpm with an average of 204 ± 18 bpm. The mean stroke volume was 44 ± 2 ml corresponding to essentially 100% pump fill up to a device beat rate of 250 bpm. Figure 2 illustrates device flow versus beat rate. The linear slope of the relation demonstrates that device filling was adequate at beat rates of up to 250 bpm. It is noteworthy that beat rates of 200 and 250 bpm corresponded to TAH filling times of 0.15 seconds and 0.12 seconds, respectively. Data from in vitro tests documented adequate device fill up to 270 bpm generating flow of nearly 14 L/min (Fig 2).
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Fig 2. Flow generation versus beat rate. The linear relationship between device flow and beat rate indicates adequate fill up to 250 beats per minute (bpm) in vivo (closed circles) and 270 bpm in vitro (open circles).
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Figure 3 depicts the controller function between postoperative day (POD) 1 and 3. Fluctuations of LAP during this early postoperative period were commonly caused by hypovolemia, administration of volume expanders, and change of animal position. The automatic control mode operates from measured left pump stroke and attempts to maintain 90% left pump filling. Five episodes of decreasing left filling pressure (LAP) were identified, which triggered automatic decreases of device beat rate and flow. Increases of LAP triggered a controller response resulting in higher device beat rates and flow. Notably the two prolonged periods of decreased LAP and device flow, each between 1930 and 0230, coincided with the period of time that the calf was resting quietly and would be expected from the natural heart during sleep. Furthermore Figure 4 illustrates preload sensitivity as tested in vitro. Adequate device performance was demonstrated at various preloads and afterloads.
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Fig 3. In vivo controller response. Five episodes of decreasing left atrial pressure (LAP) are identified (arrows), which mark device beat rate adjustments. Corresponding trends of flow are demonstrated.
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Fig 4. In vitro preload sensitivity. The device response at various preload conditions (left atrial pressure [LAP]) was tested under a wide range of afterload conditions [aortic pressure (AoP) 70–130 mm Hg; pulmonary artery pressure (PAP) 20–40 mm Hg]). All pressures represent mean values.
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Maintaining adequate left and right atrial pressure balance is a key criterion with regard to the development of the MagScrew TAH. The pump response to varying left and right afterloads must prevent overburdening the pulmonary circulation caused by flow imbalance. Atrial balance was well maintained during 30 days of in vivo testing (Fig 5). The average mean LAP and RAP were 12 ± 5 mm Hg and 7 ± 6 mm Hg, respectively.
Increasing the beat rate to downsize the MagScrew TAH did not diminish systemic arterial pulsatility, but rather increased it. Average systolic and diastolic AoP during 30 days of in vivo testing was 124 ± 13 mm Hg and 68 ± 14 mm Hg, respectively. The aortic pulse pressure, averaged over beat rates of 160–250 bpm in 10 bpm increments, ranged between 49 ± 16 mm Hg and 70 ± 20 mm Hg translating to a pulsatility index of 0.49 ± 0.17–0.77 ± 0.13 (Fig 6). Figure 7 compares the shape of AoP waveforms at various beat rates with those of the bovine heart. As expected increasing the natural heart rate to 140 bpm decreased pulse pressure. In contrast pulse pressure of the MagScrew TAH increased with higher beat rates.
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Fig 6. In vivo aortic pulse pressure and pulsatility index versus device beat rate (beats per minute [bpm]).
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Fig 7. Comparison of aortic pulse pressure (AoP) waveforms of the native heart (A) and the MagScrew total artificial heart (B).
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Favorable blood compatibility was maintained throughout the experiment without any sign of hemolysis or thromboembolic events. Free serum hemoglobin remained below 5 mg/dL and organ functions were well preserved with normal values of lactate, creatinine, bilirubin, and liver enzymes after recovery from surgery (Table 1). As expected there was a sustained mild decrease in hematocrit levels caused by an increase in circulating blood volume, but not erythrocyte damage [6]. Weight gain during the study (from 99–115 kg) reflected the favorable health of the animal under cardiac replacement with the MagScrew TAH.
A 20-minute treadmill test on POD 27 at speeds of up to 1.5 mph was well tolerated. Pump flow reached 12 L/min whereas LAP and RAP remained less than 20 mm Hg. It is noteworthy that there was no transition to anaerobic metabolism with lactate levels ranging between 0.9 and 1.4 mmol/L.
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Comment
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This study addresses the performance and physiologic interactions of a high beat rate device—the MagScrew TAH. The device performed well in vitro and 30 days in vivo. Adequate flow and stable hemodynamics were maintained throughout the course of the implant. The linear relationship of flow versus beat rate indicated adequate pump fill and ejection at all times. Beat rate adapted well to variations in preload passively and in a way similar to a Frank-Starling response. Organ functions were well preserved and blood damage did not occur. The preservation of pulsatility is noteworthy. Pulse pressure magnitude and pulsatility index were comparable with physiologic conditions [5]. In addition there was no decrease of pulse pressure with higher beat rates up to 250 bpm as observed in the native heart [7]. Pulsatile flow can reduce systemic vascular resistance to improve microcirculation [8]. In contrast chronic nonpulsatile flow can cause smooth muscle cell hypertrophy in small arteries that eventually decreases tissue perfusion [5]. The impact of nonphysiologic continuous flow on organ functions is subject to controversy. However maintaining pulsatility may be desirable physiologically.
Increasing the beat rate to develop a small pulsatile assist device or TAH that still produces strong support is a useful strategy. Smaller devices should be easier to implant in smaller patients and less apt to cause complications related to the size. Unfortunately high beat rates of up to 250 bpm raise questions regarding device function and durability. Thrombosis resulting from insufficient filling and blood damage are possible complications. Moreover the in vivo flow pattern and pulsatility at high beat rates is unpredictable and their impact on tissue perfusion is unknown.
The concept of increasing beat rates to downsize pulsatile devices is intriguing. Our results indicate that pulsatility is well maintained at high beat rates. Device flow and atrial balance were exceptional. The absence of blood damage and maintenance of favorable organ function are promising but have been demonstrated in only one animal and thus require further evaluation. In addition although the bovine model has become standard in the field of blood pump development, it does not necessarily predict favorable biocompatibility with regard to human application caused by differences in erythrocytes and coagulation systems. However, our study is a promising step forward, although other questions remain unanswered.
Actuator, valve, and pump durability must be extensively tested at high beat rates. To ensure adequate long-term durability of the device and the bioprosthetic valves, several special design considerations were implemented into the MagScrew TAH. The use of magnetic forces to transmit rotational motion into translational actuation without mechanical contact acts to eliminate friction and wear at this critical interface. The magnetic coupling between motor and pump also results in more gradual acceleration. After contact with the pump the magnets must slip relative to each other to develop an axial force. The force varies from zero (aligned magnets) to maximum (magnets offset by 1/2 the magnet width). In addition the "soft" actuation mechanism through speed profiling should reduce dP/dt related fatigue of the bioprosthetic valves.
In conclusion increasing the device beat rate to downsize the MagScrew TAH is technically feasible. Flow output, pump filling, and preload-sensitive beat rate adaptations were well maintained in vitro and for 30 days in vivo. Pulsatility is maintained at device beat rates of up to 250 bpm and organ functions were preserved. This approach may be used to develop small pulsatile pumps.
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Disclosures and Freedom of Investigation
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The Cleveland Clinic Foundation and Foster-Miller Technologies own the MagScrew TAH technology. Engineers, doctors, and other scientists from both institutions are among the study participants and coauthors involved with this article. We hereby confirm that all participants maintained full control of the design of the study, the methods used, outcome parameters, analysis of data, and production of this written report.
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Acknowledgments
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This study was supported by The National Institutes of Health BRP Grant No. 1 R01 HL67628–01. Impra, Inc, generously provided the polyester fabric used for the inflow cuffs and percutaneous drivelines. The following people participated in the development of the MagScrew TAH: Arthur Donahue, Charles Prisco, Dayton Simmons, and William Wetterau. Alex Massiello and Adelaide Jaffe provided editorial advice.
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Footnotes
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The Society of Thoracic Surgeons, the Southern Thoracic Surgical Association, and The Annals of Thoracic Surgery neither endorse nor discourage use of the new technology described in this article.
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References
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- Doi K, Smith WA, Harasaki H, et al. In vivo studies of the MagScrew total artificial heart in calves. ASAIO J. 2002;48:222–225[Medline]
- Kambic H, Barenburg S, Harasaki H, Gibbons D, Kiraly R, Nose Y. Glutaraldehyde-protein complexes as blood compatible coatings. Trans Am Soc Artif Int Organs. 1978;24:426–438[Medline]
- Kihara S, Litwak KN, Nichols L, et al. Smooth muscle cell hypertrophy of renal cortex arteries with chronic continuous flow left ventricular assist. Ann Thorac Surg. 2003;75:178–183; discussion 183[Abstract/Free Full Text]
- Harasaki H, Fukamachi K, Benavides M, Manos J, Wika K, Massiello A. A comprehensive hematologic study in calves with total artificial hearts. ASAIO J. 1995;41:M266–M271[Medline]
- Wilkinson IB, Mohammad NH, Tyrrell S, et al. Heart rate dependency of pulse pressure amplification and arterial stiffness. Am J Hypertens. 2002;15:24–30[Medline]
- Fukae K, Tominaga R, Tokunaga S, Kawachi Y, Imaizumi T, Yasui H. The effects of pulsatile and nonpulsatile systemic perfusion on renal sympathetic nerve activity in anesthetized dogs. J Thorac Cardiovasc Surg. 1996;111:478–484[Abstract/Free Full Text]
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Abstract
Introduction
