The hydraulic system
Control of the hydraulic system is simple and reliable. It does not require the transducers or signal-processing circuitry associated with other designs that rely on electrocardiogram (ECG) signals, pusher plate position transducers, and pressure transducers. A tiny gear pump, Figure 4, operates between 5000 and 10,000 rpm, generating flows of 0.5 to 1.0 in.3/sec. System pressures range from 20 to 100 psi, based on the patient’s aortic blood pressure between 70- and 200-mm Hg.
Figure 4. Gear pump (shown actual size disassembled, left, and assembled) supplies hydraulic muscle to drive left-ventricle assist system.
Pressure fluid is alternately supplied above the driver magnet piston to extend the actuator for blood pump ejection. Conversely, fluid is directed below the piston to retract the actuator for pump filling. When in the ejection position (as illustrated), the main spool is held in place by a spring located at the left of the spool. While in this position, the upper actuator volume is ported to pump discharge; the bottom volume is coupled to pump suction.
When the piston approaches its end-of-extension (EOE) limit, pilot pressure at the left of the displacer piston is blocked, shifting the EOE spool valve to the left. This action sends pressure fluid to the left side of the main spool, which overcomes spring force and shifts to the left. Pressure fluid now is routed to the volume below the piston. But this pressure alone is not enough to initiate blood pump filling; it just counterbalances the main spring force. The pump holds this condition until the patient’s left ventricle contracts.
Blood pressure from ventricle contraction combines with the actuator pressure to supply the force necessary to fill the blood pump. The filling process continues until the natural heart has ejected its entire volume or until the piston uncovers the end-of-fill port. If the port is exposed to pressure fluid, the shuttle valve directs this pressure fluid to the right end of the EOE spool. The EOE spool then shifts
to the left and directs flow to shift the main spool back to the extend position. If the piston stops moving prior to uncovering the port, an abrupt reduction in pressure above the piston takes place.
This event produces a piloting action that shifts the main spool back to the right. With the main spool shifted to the right, the actuator extends to cause the blood pump to eject the volume of blood acquired during filling. Blood pump volume and flow rate are controlled by the rate and volume pumped from the heart; maximum stroke volume is 90 cm3 .
This balanced-force sensing approach provides reliable synchronous counter pulsation with no dependence on ECG signals, pusher plate position sensing, or complex electronic signal processing hardware. The end effect of this control scheme is up to a 90% reduction in the natural left ventricle workload.
The accumulator in Figure 3 serves as a pressure regulator during actuator retraction and as a relief valve at the end of the actuator’s retract stroke. Without the accumulator, slight delays between the end of pump extension and the beginning of left ventricle contraction would cause a pressure increase. This would require large motor controller corrections to maintain optimum torque angle. Wide swings in motor speed would lead to less efficient operation.
Perhaps more importantly, the accumulator stores pressure fluid when the actuator is retracting. This pressure fluid then becomes available during the next heart contraction – an energy-efficient practice that recovers fluid power that would otherwise be wasted.
Fill characteristics of the blood pump are established by the actuator main spring. These characteristics are selected so that the pusher plate begins to retract when blood pressure reaches 2 mm Hg. Blood pressure of at least 15 mm Hg is needed to fully retract the pusher plate. Blood flow from the heart dictates fill time because sufficient hydraulic flow exists to accommodate heart ejection rate with only modest increases in blood pump pressure.
The filter, placed in series with pump discharge flow, traps any particulate contamination circulating in the hydraulic fluid. This is a precautionary device intended to protect the hydraulic system from contamination that might be inadvertently introduced during fabrication, assembly, of testing of the energy converter.
The variable-volume device is an elastomeric diaphragm attached to a rigid backplate. This flexible chamber is attached to the blood pump via a flexible stainless steel duct. Functionally, the device accommodates air displaced by the moving blood pump pusher plate by accepting air during pump filling and allowing it to return during pump ejection.
Electrohydraulic LVASs of this type have been tested in calves for durations of up to 196 days. The systems have been able to synchronize with the left ventricle at over 150 beats per minute and to pump blood at flows exceeding 12 1/min. Two years of multiple-system bench tests and multiple-animal implants of five months are being planned as a precursor to human use.
Hydraulic fluid selection
Initially, silicone oil was selected as fluid for the LVAS’s electrohydraulic energy converter. Selection was based on:
- the ability to tailor the fluid viscosity by blending
- low toxicity
- compatibility with system elastomers, and
- expected resistance to thermal decomposition.
Although satisfactory for nominal operating conditions, resistance to thermal decomposition proved inadequate when gear pump hydrodynamic bearing loads increased when pumping against aortic pressure as high as 200 mm Hg. Because these pressures can occur transiently, alternative fluids with greater thermal stability were evaluated.
Saturated hydrocarbons, which have excellent thermal stability and reasonable lubricity, were investigated, based on discussions with the USAF’s Wright Air Development Center’s Fuels and Lubricants Branch. Pentadecane (C15H32) has the required viscosity of 2.0 cP at the hydraulic fluid operating temperature. The fluid also demonstrated good compatibility with the materials it contacts within the energy converter. Gear pump testing under severe conditions (pressures of 100 psi) showed no tendency toward thermal decomposition. Pentadecane has been substituted for silicone oil in prototype energy converters and endurance units with expected performance as a result.
Richard R. Navarro previously held the position of Senior Research Engineer, Artificial Organs, at Cleveland Clinic Foundation and is currently President and CEO of OrthoData Inc, located in Hinckley, OH. For more information on OrthoData Inc., visit www.orthodatainc.com.
This article was originally published in the June 1996 issue of Hydraulics & Pneumatics. It is reproduced here for its historical value.