Development of Smaller Artificial Ventricles and Valves Made by Vacuum Forming

Implantable prosthetic ventricles and trileaflet valves made by vacuum forming have been developed and implant tested. All components are made from Pellethane®. Recognizing the need for smaller as well as larger ventricles, designs with effective stroke volumes of 50, 85, 100 and 130 cc have been tested with several valve types. The pneumatically driven Utah ventricular assist device (UVAD) can be used as a total artificial heart (TAH) or ventricular assist device (VAD) by using the appropriate inflow and outflow adapters. In vitro durability testing has demonstrated ventricular lifetime beyond two years and valve lifetime to nearly one and one half years. The polymer valves have lower regurgitation than mechanical valves.

Animal implantation experience includes 21 TAH implants and 16 left ventricular assist device (LVAD) implantations. TAH survival ranges from 2 to 210 days. LVAD animals have lived up to 116 days before elective termination. The animal were healthy and grew normally. The devices exhibit a “Starling's Law” response. One TAH animal survived 72 days before successful explantation followed by transplantation. At autopsy, this animal had no renal infarcts. Hematology data has demonstrated the existence of little or no intravascular hemolysis (PF Hb < 5 mg%).

The “Philadelphia” version of the UVAD vacuum formed ventricles are small enough to be implanted without thrombus provoking connectors. Eight animals have received this TAH and survived up to 120 days. Vacuum forming offers a rapid and inexpensive way to produce reliable and effective total artificial hearts and valves for widespread, temporary clinical application in any size adult human.

Old, new and Revised Aspects of Artificial Organs

Retrograde perfusion of the coronary veins with oxygenated blood is effective. Closed chest transarterial left ventricular bypass was developed by Hans Zwart to support the failing left ventricle. Transapical left ventricular bypass started by Alex Kralios, later studied by Jeffrey Peters et al, became effective when filters were introduced into the system –- first surviving patient! One must be prepared to support both ventricles. Retrograde transpulmonary bypass (Kralios) is as yet only experimental. The blood is pumped from one occluded pulmonary artery into the aorta. Plasmapheresis removes particles with a molecular weight of 7 to 45,000 which may be harmful in patients with burns or oxygenators since they blind the leukocytes. Starling's Law will regulate the total artificial heart. The future is for the electrohydraulic artificial heart by Robert Jarvik. However, air driven hearts may offer a life of sufficient quality to people now doomed to die. All types of hemodialyzers or peritoneal dialyzers should be made portable or wearable. The «mouse» is a good peritoneal access device. It makes recirculating peritoneal dialysis practical.

Hemodynamic and Energetic Assessment of Calves Implanted with a Left Ventricular Assist Device (LVAD)

Hemodynamic and ventricular energetic parameters were measured in calves implanted with the air driven Utah Ventricular Assist Device (UVAD). Uptake site was varied to determine the effect of control mode and vacuum augmentation of filling. Uptake was drawn solely from the left atrium or combined with a left ventricular apical vent. LVAD outflow returned to the descending, thoracic aorta. Control modes examined included asynchronous pumping as well as 1:1 and 1:2 synchronous diastolic counterpulsation. The 85cc LVAD, vacuum formed from PELLETHANE®, was implanted acutely in four animals and chronically in six (7, 49 and 116 days paracorporeally, 1, 28 and 32 days intrathoracically). Instantaneous blood pressures, intramyocardial pressure, aortic outflow, oxygen consumption, LVAD output and drive parameters were recorded.

LVAD output was independent of control mode when the natural heart rate was ≥ 80 beats per minute. Intrathoracically positioned LVADs pumped a mean flow of ≈5 liters/min without vacuum augmentation of filling. Paracorporeally positioned LVADs pumped ≈3 liters/min mean flow without vacuum augmentation and up to ≈6 liters/min with 38 mm Hg of vacuum augmentation of filling. Instantaneous ascending aortic pressure and flow showed distinct beat-to-beat variation depending on LVAD control mode. Lower average ventricular afterload was observed when pumping the LVAD asynchronously or 1:2 synchronously. In one acute preparation, left ventricular myocardial oxygen consumption was reduced from the unassisted average control level by 37% for the asynchronous and 1:1 synchronous control modes with left atrial uptake. With combined uptake, oxygen consumption was reduced an additional 30% during asynchronous control or 11% during 1:1 synchronous control without any change in LVAD output. Endocardial/epicardial blood flow ratio was similar and ≥1.12 for all test conditions. Renal and brain blood flow was maintained, or slightly elevated during ventricular assistance. Intramyocardial pressures were monitored using Millar catheter tip transducers. In an acute preparation, left ventricular assistance reduced peak intramyocardial pressure. Changing from atrial to combined uptake cannulation further reduced peak intramyocardial pressure for asynchronous and 1:1 synchronous LVAD control. Reduced end-diastolic intramyocardial pressures were seen with all modes of LVAD control.

These data demonstrate excellent UVAD pumping function and suggest that left ventricular assistance does not compromise endocardial blood flow while sustaining blood flow to other major organs. Regardless of the uptake site, asynchronous or 1:2 synchronous LVAD control may be clinically preferable for effective reduction of left ventricular myocardial oxygen consumption.

Development of a New Inflow Valve for a 20cc Semisoft Ventricle: Preliminary Results

We remodeled and tested our semisoft 20cc ventricle and made a new bileaflet flap inflow valve. Housings, bases, outflow valve, and a newly designed diaphragm were all made by vacuum forming and put together by radiofrequency welding or glue. In vitro, the ventricle produced a cardiac output of 2.5 to 3.0 L/min and showed reliable durability results. Hematological testing showed no important thrombogenecity of the new valve. Cardiac output was higher than expected for the volume of the ventricle, perhaps because of stretching or flow through. Animal experiments with the left ventricular assist device (LVAD) version was done at Ohio State University. Earlier in Utah, we did 20 cc total artificial heart (TAH) implantations and LVAD experiments in lambs and recently in calves with the 60cc TAH version. A soft ventricle is easy to implant and low in production costs.

A computer controlled pulsatile pump: preliminary study

A Stepper Motor Driven Reciprocating Pump (SDRP) can replace roller pumps and rotary pumps for cardio pulmonary bypass, hemodialysis and regional perfusion. The blood pumping ventricles are basically the same as ventricles used for air driven artificial hearts and ventricular assist devices. The electric stepper motor uses a flexible linkage belt to produce a reciprocating movement, which pushes a hard sphere into the diaphragm of the blood ventricles. The SDRP generates pulsatile flow and has a small priming volume. The preset power level of the motor driver limits the maximum potential outflow pressure, so the driver acts as a safety device. A double pump can be made by connecting two fluid pumping chambers to opposing sides of the motor base. Each pump generates pulsatile flow. Pressure and flow studies with water were undertaken. Preliminary blood studies showed low hemolysis, even when circulating a small amount of blood up to 16 hours.

Elastomeric Valves, a New Design

The convex bileaflet valve replaces the flat biflap inflow valve designed by Long Sheng Yu and the tricusp semilunair outflow valve. One reason is easier manufacturing. Convex bileaflet valves are developed for the 11, 20, 40, 70 and 140cc ventricles. Testing included curves (Cardiac Output versus Venous Pressure, Cardiac Output versus Heart rate), flow visualization studies, paint and bloodbag studies. The curves and flow visualization were done by connecting ventricles to one of our standard mock circulations. Paint and bloodbag studies were done by connecting the hearts to a bloodbag, but the bag was filled with water for the paint studies. The curves show high cardiac output, even with pumping at high heart rates (150 BPM+). The flow visualization shows a good stream through the sinus Valsalvae. No stagnating flow is visible. The bloodbag studies which provoke thrombosis show it on the edges of the heart valves, and little in the groove between the valve and the sinus Valsalvae. Heparninzation prevents the thrombosis. Results of our tests were good. The convex bileaflet valve seems to have good future.

Photoelastic stress analysis of silastic poppets in prosthetic heart valves

Some prosthetic heart valves with Silastic ball poppets undergo degeneration and variance. The problem is avoided with metal poppets; however, their impact can damage the cloth coverings on the struts, resulting in distorted blood flow paths. Consequently, renewed interest in mild-cured Silastic poppets and inserts is developing. With photostress equipment, we analyzed two spherical poppet models to study poppet degeneration. The first analysis simulated the ring type of loading transmitted from the valve seat to the poppet, and the second analysis involved hydrostatic pressure loading and a detailed stress analysis to simulate cage confrontation. Results showed that regions of the poppet were susceptible to stress cycling with relatively large tensions below the upper surface but above the ring support. This same location could experience large compressive stress, depending on how the poppet was positioned during a seating stroke. Shear stresses near the valve support ring were at least four times the hydrostatic pressure load. Such subsurface stress cycling should play a role in the formation of internal fissures that surface and fracture the poppet.