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Ann Thorac Surg 1999;68:637-640
© 1999 The Society of Thoracic Surgeons

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Supplement: Circulatory Support

A miniature implantable axial flow ventricular assist device

^a DeBakey Heart Center, Department of Surgery, Baylor College of Medicine, Houston, Texas, USA

Address reprint requests to Dr DeBakey, Department of Surgery, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030

Presented at the Fourth International Conference on Circulatory Support Devices for Severe Cardiac Failure, Houston, TX, October 3–5, 1997.

Abstract

Background. Since 1984, in collaboration with NASA engineers, we developed an axial flow pump that is 86 mm long, 22 mm wide, weighs 95 g, produces a flow of 5 to 6 L/min against a 100-mm Hg pressure at about 10,000 rpm, and requires less than 10 W of power.

Methods. The pump has been implanted in 9 calves with the inlet cannula inserted into the left ventricle and the outlet cannula, consisting of an albumin-coated Dacron graft, attached by end-to-side anastomosis to the descending thoracic aorta.

Results. All animals showed normal behavior until they were killed 1 to 3 months after operation. At autopsy, systemic studies of vital organs demonstrated no evidence of thromboembolism; the rpm of the pump was maintained between 9,000 and 10,000; the wattage ranged between 7 and 9; the output between 4 and 5 L/min; the hemoglobin was maintained between 32 and 35 mg/dL; the plasma-free hemoglobin ranged between 0.5 and 3 mg/dL; the BUN ranged between 8 and 14 mg/dL; the creatinine remained less than 1 mg/dL; and bilirubin studies were within normal limits. Bearing wear-tear tests up to about 5 months have been negative.

Conclusions. The performance characteristics of the pump implanted in calves up to 90 days are highly gratifying, particularly in terms of pump output of 5 L/min, an index of hemolysis well within normal limits, and absence of thromboembolism.

The concept of mechanical ventricular assistance to support a profoundly failing heart evolved from clinical experience with the heart-lung machine in the 1950s, when some patients required more than 1 to 2 hours of support in order to be weaned from cardiopulmonary bypass. This observation suggested that in other patients in whom this failed, weaning might be successful by longer support for a few days to a few weeks. During the next few decades, both experimental and clinical studies firmly established the validity of this concept [1–6]. Of particular importance in this regard was application of mechanical ventricular assistance as a bridge to transplantation. Thus, in patients with end-stage heart failure requiring cardiac transplantation, this device may be the only means of support until a donor heart becomes available [7–9]. It is estimated that this bridge to transplantation concept has been performed worldwide more than 600 times, and for many of these patients it was lifesaving. Of particular interest in this connection is the observation that some patients with idiopathic dilated cardiomyopathy who were on such a mechanical support system could be weaned after prolonged support. Müller and associates [10] recently reported that of 17 patients with nonischemic idiopathic dilated cardiomyopathy on such mechanical support for a mean duration of 230 ± 201 days, 6 patients died, 4 had cardiac transplantation, 2 are still on support, and 5 patients with significant recovery were weaned after 160 to 794 days and are now device-free for 51 to 592 days with normal cardiac function. I was recently informed by Dr Hetzer, head of Deutsche Herzzentrum Berlin, that at present, the number of explanted patients has increased to 19 (Hetzer R, personal communication, October 1997). Thus, the validity of the concept of mechanical ventricular assistance in profound heart failure, which was open to question several decades ago, is now well established for temporary support such as bridge to transplantation, but may also be useful in providing myocardial functional recovery in some forms of end-stage heart failure [11, 12].

Our own studies on the artificial heart and ventricular assist devices have spanned more than three decades [1, 4–6, 13–15]. Although we directed our investigative efforts toward both purposes, it became increasingly evident that development of a mechanical pump for total cardiac replacement is a highly complex bioengineering problem. For this reason and because of early impressive clinical experience with a left ventricular assist device, I began increasingly to direct my investigative efforts toward this latter approach [1, 4, 6, 15]. Our first impressive clinical experience with a left ventricular assist device was in 1963, in a patient in whom severe left ventricular failure with pulmonary edema had developed after resuscitation from cardiac arrest 1 day after aortic valve replacement. A left ventricular assist device was implanted with attachment of the inlet tube to the left atrium and the outlet tube to the descending thoracic aorta. During the next few days the pulmonary edema cleared impressively. Unfortunately, the patient died on the fourth postoperative day [4], probably from brain damage during cardiac arrest.

Our next and most impressive case was in 1966 in a 37-year-old woman suffering from heart failure, caused by severe aortic insufficiency and mitral stenosis, in whom both valves were replaced with ball valves [1]. Because she could not be weaned from the heart-lung machine, a left ventricular assist device, fabricated in our laboratory, was installed. It consisted of a gas-energized, synchronized pump of hemispherical design with a Dacron-reinforced, Silastic, molded diaphragm separating the gas chamber from the blood chamber with a Dacron velour surface lining the blood chamber. The inlet cannula was attached to the left atrium, and the outlet to the right axillary artery. With a pump flow of 1,200 mL/min, we were able to wean the patient from the heart-lung machine. Although from time to time it was necessary to increase the pump flow, during the next 10 days the heart had sufficiently recovered to permit explantation of the pump [1]. For the next 6 years, the patient enjoyed full normal activities but was then tragically killed in an automobile accident.

Our investigative efforts during the past 15 years have been primarily directed toward studies of ventricular assist devices, and since 1984, entirely on an axial flow pump [16]. This began after a NASA engineer, on whom we had performed cardiac transplantation, showed an interest in our artificial heart laboratory research activities and arranged a meeting with some of his engineering colleagues at the NASA Johnson Space Center. Following this meeting they began to work with us first on a volunteer basis, and subsequently on a formal joint collaborative NASA/Baylor project.

These collaborative efforts led to development of an axial flow pump that is 86-mm long, 25-mm wide (about the size of an AA battery), and weighs 95 g. Within the flow tube, the components consist of an inducer-impeller (the only moving part); a fixed flow straightener that acts as the front-bearing support for the inducer-impeller; and a fixed rear diffuser, which provides the rear bearing of the inducer-impeller (Fig 1). The diffuser retards the highly tangential blood flow velocity by redirecting it axially, an action that results in fluid pressure build. Rare earth magnets, which are embedded in the blades of the impeller, act as the rotor of the brushless motor, causing it to spin in a magnetic field. This pump can produce a flow of 5 to 6 L/min against a 100-mm Hg pressure at about 10,000 rpm and requires less than 10 W of power [16]. Extensive computational fluid dynamics analyses were performed jointly with NASA-Ames Research Center to optimize the hydraulic performance and hemolysis of the pump.

View larger version (26K): [in this window] [in a new window]   Fig 1. Essential components of the DeBakey/NASA axial flow ventricular assist pump.

The pump was tested both in vitro and in vivo for thrombogenicity. Using a paracorporeal implantation model in calves, we screened the pumps for thrombus formation for periods of 2 to 30 days. By this means, it became possible to eliminate from the test matrix pumps that did not exhibit desirable antithrombogenic properties [18, 19].

The next set of experiments required the implantation of the pump in the chests of calves. The inlet cannula of the pump was inserted into the left ventricle, and the outlet cannula, consisting of an albumin-coated Dacron graft, was attached to the descending thoracic aorta by end-to-side anastomosis. This procedure was performed in 12 calves, 3 of which did not survive the operation for technical reasons. Postoperative observations of the remaining 9 calves showed normal behavior until they were killed 1 to 3 months after operation. At autopsy, systemic studies of vital organs showed no evidence of thromboembolism.

During the 1- to 3-month observation period on these 9 calves, the rpm of the pump was maintained between 9,000 and 10,000, the wattage ranged between 7 and 9, and the output between 4 and 5 L/min (Fig 2). Hematologic studies during this 3-month period showed that the hemoglobin was maintained between 32 and 35 mg/dL, and that the plasma-free hemoglobin ranged between 0.5 and 3 mg/dL (Fig 3). Renal function studies showed both the blood urea nitrogen (BUN) range to be within normal limits (8 to 14 mg/dL) as well as the creatinine (less than 1 mg/dL) (Fig 4). In addition, results of bilirubin studies were normal (less than 1 mg/dL). The results of these studies on the 9 calves in which the pump was implanted (periods ranging from 30 to 90 days), are highly gratifying in terms of critical performance characteristics. These include sustained and stable pump output (4 to 5 L/min), an index of hemolysis well within normal limits, and absence of thromboembolism. In this connection, previous experience has shown that such stable performance characteristics of a pump over a period of several months provides strong indications that these performance characteristics can be maintained for a much longer period. Nonetheless, we are planning similar implantations in calves up to 6 months. In addition, bearing wear and pump durability endurance tests, currently at 5 months, have been negative and will be extended for a much longer period. Further studies concerned with the inflow cannula both for the left ventricle and the atria are planned.

View larger version (22K): [in this window] [in a new window]   Fig 2. The rpms, flow output, and wattage of the implantable pump in the 9 calves observed from 30 to 90 days.

View larger version (22K): [in this window] [in a new window]   Fig 3. Hematologic studies after implantation of pump in 9 calves observed 30 to 90 days.

View larger version (21K): [in this window] [in a new window]   Fig 4. Renal and liver function studies after implantation of pump in 9 calves observed 30 to 90 days.

View larger version (71K): [in this window] [in a new window]   Fig 5. Potential clinical application of axial flow pump with attached batteries and controller.

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