For all their success at extending the lives of a few
heart-failure patients, artificial hearts still suffer from a couple of design
flaws that have limited their use. These electromechanical hearts currently
rely on positive displacement blood pumps, which tend to be bulky. So bulky, in
fact, that the most recent self-contained heart designs simply won't fit in smaller
chest cavities. Positive displacement pumps also have more moving, cyclically-loaded
parts than engineers like to see from a reliability standpoint. A new kind of
mechatronic heart under development at the Texas Heart Institute
uses a much simpler pump and sophisticated control algorithms
to address both issues.
Rather than positive displacement pumps which mimic the pulsations of a natural heart, the
Texas Heart Institute's total artificial heart (TAH) design employs a pair of
continuous-flow axial pumps. A pulmonary-loop pump carries oxygen-depleted
blood away from the heart to the lungs and returns oxygenated blood to the
heart. And a systemic-loop pump carries oxygenated blood away from the heart to
the body and returns deoxygenated blood back to the heart. A controller allows
the pumps to work in tandem, adjusting their outputs relative to one another
and to changing physiological demands.
According to Steve Parnis, assistant technology director for the Texas
Heart Institute's Center for Cardiac Support, these continuous flow pumps are
essentially repurposed DeBakey ventricular assist devices (VADs) from MicroMed
Usually a VAD does what its name suggests – it assists the natural heart with its
pumping duties. "In this case, the two VADs would completely replace the
natural ventricles," Parnis says.
It's a novel idea that's been around for a few years now. Dr. Bud
Frazier, the Texas Heart Institute's director of research and chief of
cardiopulmonary transplantation, published papers on a continuous flow TAH back
in 2006. His idea took a step closer to
clinical reality this year, when the National Institutes of Health awarded a $2.8 million grant
to the Texas Heart Institute to fund the development of the continuous flow
VADs have a
lot going for them in a total heart application. For one, they're each about
the size of a C-cell battery, versus a 2-lb chunk of titanium and plastic
for a self-contained pulsatile pump. "VADs will fit in the majority of patients
versus a minority of patients for the pulsatile pumps currently in use," says
another, VADs have a proven clinical track record. About 500 of the current generation
of DeBakey VADs are in use right now, according to Bob Benkowski, MicroMed's
chief operating officer and one of the engineers who helped develop the
original DeBakey VAD model. "VADs have run in patients for as long as eight years,"
he says. And he attributes that reliability in part to the simplicity of
MicroMed's axial pump, whose single moving part, the impeller, is directly
driven by the electrical windings.
the lifecycle expectation for even the most modern positive displacement blood
pumps, whose pulsations create cyclical loading conditions not seen by the
axial models, at two years. Continuous
flow pumps will also likely require less power and cost less than the more
complex pulsatile models, he says.
So if a
couple of continuous flow VADs would make such a great TAH, why aren't they in
use yet? It turns out these devices
need a significant amount of controls engineering to make the jump from heart
helper to total heart replacement.
And that's where Matthew Franchek and Ralph Metcalfe, both Ph.D mechanical
engineers and professors at the University of Houston's Cullen College of Engineering, enter the picture. As part of
the NIH grant, they're working on a feedback controller that will allow two
VADs to work together as a TAH. Franchek
and other university researchers have developed similar auto-regulating control
systems for automotive applications, most recently working on a diesel engine
governor for Cummins Engine.
ways, Franchek and Metcalfe have had a head start in the controls development
work thanks to the use of the proven VAD technology. MicroMed's VADs already
have their own controls. Benkowski describes them as feedback controllers, which
take an actual flow measurement from an ultrasonic sensor, compare it with a
desired flow output and generate an appropriate PWM control signal to regulate
the impeller speed.
Yet the two engineering professors still have their work cut out
for them. VADs normally operate individually as support for a remaining natural
heart. In the TAH, they have to operate in close coordination to emulate the
balanced flow provided by a natural heart's left and right ventricles. "Pairing
the pumps creates a complex multivariable control problem," Franchek says. "Each pump's loading
conditions and flow output affects the loading conditions and flow output of
the other pump."
controller also has to tie these interrelated flow and loading conditions – which
include both inlet pressure and outflow resistance – back to the changing needs
of the human body. Franchek says everyday activities such as standing or
walking change flow and loading conditions. So do cardiovascular events such as
vascular restrictions, hypertension or changes in blood viscosity. And so do
intrinsic physiological differences between individual patients. "Our challenge
is to maintain a steady-state cardiac output as physiological conditions
fluctuate for whatever reason," says Franchek.
Axial flow pumps inherently lend themselves to meeting this
challenge. They can auto-regulate transient events because their flow output is
sensitive to both inlet pressure and outflow resistance. And Benkowski says the VAD pump's impeller geometry and flow passages can be tweaked to come
up with optimized flow-pressure behavior for the TAH application. "We can alter the pressure
sensitivity of the pumps to make it a little easier for the control algorithms
to do their thing," he says.
algorithms, meanwhile, will be based on an analog integral controller which
measures actual output flow, compares it to the desired value and adjusts the
voltage to the pumps accordingly.
Franchek and Metcalfe picked a seemingly simple integral control
strategy for this application because it does a good job at maintaining steady-state conditions in systems whose dynamic behavior is both well understood and
characterized by cooperative transients. Understanding that dynamic behavior
given the influence of physiology on pump conditions is not so simple. And a
large part of the control development work under the NIH grant involves the
creation of a lumped parameter mathematical model of the human circulatory
system. According to Franchek, this model will ultimately be incorporated into
the TAH control algorithms (see block diagram below).
Franchek expects the first pass at the TAH control
algorithms won't be ready until this summer. "Right now, we're at the very
beginning of the controls engineering," says Franchek. And there are still some fundamental decisions to be
made about how the pumps will operate. For example, the researchers have yet to
decide whether one or both of the pumps should be operated in a quasi-pulsatile
mode. Franchek says a repetitive control strategy would let the pump motors
"whirl up and whirl down" to emulate the pulsating action of the natural heart
if need be.
development work includes the possible addition of blood-viscosity monitoring
to the system. "We believe we'll be able to infer the effective viscosity of
the blood from our flow measurements and voltage signals," Franchek says.
Metcalfe are using a variety of simulation tools to do their development
work, including MATLAB and Simulink to develop the
mathematical models. They're simulating the resulting control algorithms and
prototyping the controller hardware in dSPACE, a set of development tools for