Progress on development of the Nimbus-University of Pittsburgh axial flow left ventricular assist system.

Nimbus Inc. (Rancho Cordova, CA) and the University of Pittsburgh have completed the second year of development of a totally implanted axial flow blood pump under the National Institutes of Health Innovative Ventricular Assist System Program. The focus this year has been on completing pump hydraulic development and addressing the development of the other key system components. Having demonstrated satisfactory pump hydraulic and biocompatibility performance, pump development has focused on design features that improve pump manufacturability. A controller featuring full redundancy has been designed and is in the breadboard test phase. Initial printed circuit layout of this circuit has shown it to be appropriately sized at 5 x 6 cm to be compatible with implantation. A completely implantable system requires the use of a transcutaneous energy transformer system (TETS) and a diagnostic telemetry system. The TETS power circuitry has been redesigned incorporating an improved, more reliable operating topography. A telemetry circuit is undergoing characterization testing. Closed loop speed control algorithms are being tested in vitro and in vivo with good success. Eleven in vivo tests were conducted with durations from 1 to 195 days. Endurance pumps have passed the 6 month interval with minimal bearing wear. All aspects of the program continue to function under formal quality assurance.

Recovery of sheep hearts after perfusion preservation or static storage with crystalloid media.

BACKGROUND: These experiments were designed to evaluate the viability of large hearts after preservation by use of procedures that have shown good results with small animal hearts. Efficacy of novel long-term preservation protocols should be documented with a large animal model before such procedures can be adopted for clinical use. We studied the recovery of sheep hearts that were perfusion-preserved in media containing two different substrate mixtures and hearts stored without perfusion either in University of Wisconsin solution modified to maintain tissue adenosine triphosphate content or in Stanford solution. METHODS: Six groups of sheep hearts were studied: I, fresh nonpreserved controls; II, hearts perfusion-preserved at 11 degrees C for 24 hours by use of an oxygenated extracellular-type medium with pyruvate + glucose substrate; III, hearts preserved as for II but with aspartate + glutamate + glucose substrate; IV, hearts stored without perfusion at 3 degrees C for 24 hours in University of Wisconsin solution containing 2,3-butanedione monoxime 30 mmol/L, CaCl2 1 mmol/L, and fresh reduced glutathione 3 mmol/L; V, hearts stored without perfusion at 3 degrees C for 4 hours in Stanford solution; VI, hearts preserved as for II but without perfusion. Recovery was measured for 6 hours in a Langendorff model, perfused with an erythrocyte + albumin medium. RESULTS: Hearts that were perfusion-preserved with both substrate mixtures and hearts stored in modified University of Wisconsin solution recovered function that was not significantly different from control subjects. Hearts stored in Stanford medium did not recover as well as did groups II, III, and IV. Left ventricular pressure and peak rate of left ventricular relaxation of the Stanford group were lower, and left ventricular enddiastolic pressure was higher, than those values for controls (repeated measures analysis of variance; Dunnett's procedure). The group VI hearts did not recover function at all. CONCLUSION: The results suggest that large hearts preserved with medium containing either aspartate + glutamate + glucose or pyruvate + glucose have comparable recovery after long-term perfusion preservation. Aspartate + glutamate may offer advantages for clinical use because of their lower production of lactate and better chemical stability compared with pyruvate. Static storage in modified University of Wisconsin solution also produced viable hearts with recovery comparable to perfusion-preserved aspartate + glutamate + glucose hearts. Tests of these preservation media and procedures with large transplanted hearts are warranted.

Continuing development of the Cleveland Clinic-Nimbus total artificial heart.

A completely implanted total artificial heart (TAH) is under development by Nimbus, Inc., and the Cleveland Clinic Foundation (CCF). Key features of the system include an electrohydraulic energy converter, an automatic control system that produces a Frank-Starling response, and dual ventricles composed of graphite-epoxy and titanium with gelatin blood contacting surfaces. The system is controlled by a single substrate, hybridized microcircuit (the hybrid). Fabrication of the TAH control hybrid has recently been completed and testing begun. Its design emphasizes simplicity, reliability, and efficiency. Particular attention was given to optimizing thermal management. Externally controlled TAH systems have been used in eight in vivo experiments of up to 120 days' duration. In the last two of these experiments, a variable volume device was also implanted with excellent results. In vivo use of the system has demonstrated the Frank-Starling pump response, but the systems quickly reach maximum output with the bovine animal models. Human fitting studies, including adult patients undergoing heart transplantation, demonstrated satisfactory fit of the pump within the pericardium without compression of the vascular structures or chest wall. Measurements of chest circumference, plain chest films, and transesophageal echocardiograms should provide reliable predictions of pump fit in the majority of patients.

An intrathoracic left ventricular assist system: utilization of results from a development program.

An intrathoracic, electrohydraulically actuated, left ventricular assist system (LVAS) was subjected to formal device readiness testing. Endurance testing was initiated on eight systems before testing was halted due to failure of four of the systems. Three failed due to environmental leakage. Solutions were straightforward, involving gasket changes and o-ring resizing. The fourth failure involved a magnetic coupling piston swelling and seizing. The failure was attributed, after long investigation, to hydrogen adsorption by the samarium-cobalt magnets. An unknown number of coupling magnets were affected in this fashion, necessitating complete replacement of magnets to resolve the problem. However, this was beyond the scope of the program, and no further endurance testing was accomplished. The test experience of the Nimbus/CCF LVAS has demonstrated all functional aspects of the complete LVAS, both in vitro and in vivo, and the endurance and reliability potential is indicated as well. Although the LVAS program is currently inactive, its legacy of technical innovations continue to drive the development of other medical devices.