In vivo evaluation of an intrathoracic ventricular assist device

Cholesterol-modified polyurethane valve cusps demonstrate blood outgrowth endothelial cell adhesion post-seeding in vitro and in vivo

BACKGROUND

The clinical and experimental use of polyurethane heart valve prostheses has been compromised by thrombosis and calcified thrombus. This is caused in part by the lack of an intact endothelium on these implant surfaces. We hypothesize that endothelial seeding of a polyurethane heart valve leaflet with autologous sheep blood outgrowth endothelial cells (BOECs) could be achieved with cholesterol-modified polyurethane (PU-Chol) to promote BOEC adhesion, thereby resulting in an intact, shear-resistant endothelium that would promote resistance to thrombosis.

METHODS

Cholesterol-derivatized polyurethane was formulated by bromoalkylation of the urethane nitrogens followed by reactive attachment of mercaptocholesterol. In vitro shear flow studies were carried out comparing BOEC retention on control surfaces versus PU-Chol using forces comparable to those observed in vivo with cardiac valves (75 dyne/cm2). Autologous sheep BOECs were seeded onto PU-Chol before pulmonary leaflet replacement surgery under cardiopulmonary bypass. Studies were terminated at 30 and 90 days followed by retrieval analyses.

RESULTS

Blood outgrowth endothelial cell seeding of PU-Chol surfaces resulted in an endothelial monolayer that was positive for von Willebrand factor. Polyurethane-cholesterol demonstrated significantly greater BOEC adhesion under 75 dyne/cm2 shear force in vitro than control polyurethane (75.3% +/- 12.3% versus 5.8% +/- 3.9%, respectively; p < 0.001). Sheep pulmonary cusp replacements demonstrated retention of seeded BOECs on PU-Chol leaflets with no significant differences in the extent of cellular density comparing unimplanted specimens with explants. Control explants (nonseeded PU-Chol and nonseeded polyurethane) demonstrated no evidence of endothelial recruitment.

CONCLUSIONS

Polyurethane-cholesterol represents a polyurethane formulation with very high adhesive properties for BOECs under heart valve level shear forces both in vitro and in vivo.

The HeartSaver left ventricular assist device: an update

BACKGROUND

Ventricular assist devices have been shown to be effective as bridges to transplantation and recovery for patients with end-stage heart failure. Current technology has been limited because of the need for percutaneous connections with controllers. The HeartSaver ventricular assist device (VAD) (World Heart Corporation, Ottawa, Ontario, Canada) was developed with the intention of having a completely implantable, portable VAD system. The system consists of an electrohydraulic blood pump, internal and external battery power, and a transcutaneous energy transfer and telemetry unit that allows for power transmission through the skin. Control of the device may be achieved locally or remotely through a variety of communication systems.

METHODS

The device has been modified with the Series II preclinical version being available for in vitro (mock loop) and in vivo (bovine model) testing.

RESULTS

Seventeen Series II devices have been functional on mock loops or other testing trials for an accumulated 900 days of operation. There have been eight acute experiments using a bovine model to test various components as they have become available from manufacturing. Mean pump output was 10.4 +/- 1.1 L/min in full-fill/full-eject mode. Changes in the last 24 months include (1) cannula redesign for better port alignment and integration of tissue valves; (2) battery redesign to convert to new lithium-ion cells; (3) optimized infrared information and electromagnetic inductance energy transmission through various skin thicknesses and pigmentation; and (4) improved reliability of internal and external controller hardware and software.

CONCLUSIONS

Modifications have been required to optimize the HeartSaver VAD's performance. The final HeartSaver VAD design will be produced in the near future to allow for formal in vitro and in vivo testing before clinical implantation.

Miniature circulatory support power transformer

Direct power delivery to intracorporeal circulatory support devices risks infection. Electrical transformers spanning the integument (skin or mucosa) have long been attractive means of circumventing this risk. Yet all existing skin surface transformers leak substantial magnetic flux, with an intrinsic risk of battery draining cross-coupling by any nearby conductor, requiring strict control of surroundings. In a progression of designs, we have used the walls of a small pouch, surgically formed from ileum, rather than skin, as a barrier. Previously, we reported a torroidal design with complete magnetic circuits, having zero flux leakage and unmatched heat dissipation in a 2 week canine trial. This work was in three parts: (1) Devices were placed in eight dogs for 9-12 weeks to document performance of the existing design. (2) Based on these results, the device was further miniaturized. Iterative computational modeling was applied to material selection and design. Simultaneously, practical techniques for surgically constructing tiny accommodating ileal pouches were explored in cadavers, by using polymer "mock-ups" of potential designs. (3) The final design was tested by acute implantation in a live 52 kg goat and by in vitro testing of electrical transfer function. IN VIVO TRIALS: The earlier design demonstrated 97.2% AC/AC and 84.2% DC/DC efficiency with contiguous tissue warming of 0.1 degrees C. DESIGN AND PERFORMANCE: A transformer with a Square Permalloy torroidal core (6 cm3, 23 grams, both coils eight turns) was designed, by using human cadaver trials to optimize geometry. Input and output were 6 V, 12W at 9.2 kHz with AC-AC efficiency 96%. OPERATIVE PLACEMENT TRIAL: The primary coil easily fit into an isolated, vascularized, but otherwise disconnected "blind" ileal pouch < 2 cm long with a slender extension to a miniature stoma. With secondary turns, the pouch and transformer fit easily into the abdominal wall. This tiny system seems compatible with near "forgettable" power delivery allowing unprecedented freedom of environment and activity in circulatory support dependent people.