Mechatronic Artificial Heart Doesn't Beat

For all their success at extending the lives of a fewheart-failure patients, artificial hearts still suffer from a couple of designflaws that have limited their use. These electromechanical hearts currentlyrely on positive displacement blood pumps, which tend to be bulky. So bulky, infact, that the most recent self-contained heart designs simply won't fit in smallerchest cavities. Positive displacement pumps also have more moving, cyclically-loadedparts than engineers like to see from a reliability standpoint. A new kind ofmechatronic heart under development at the Texas Heart Institute

uses a much simpler pump and sophisticated control algorithmsto address both issues.

Rather than positive displacement pumps which mimic the pulsations of a natural heart, theTexas Heart Institute's total artificial heart (TAH) design employs a pair ofcontinuous-flow axial pumps. A pulmonary-loop pump carries oxygen-depletedblood away from the heart to the lungs and returns oxygenated blood to theheart. And a systemic-loop pump carries oxygenated blood away from the heart tothe body and returns deoxygenated blood back to the heart. A controller allowsthe pumps to work in tandem, adjusting their outputs relative to one anotherand to changing physiological demands.

According to Steve Parnis, assistant technology director for the TexasHeart Institute's Center for Cardiac Support, these continuous flow pumps areessentially repurposed DeBakey ventricular assist devices (VADs) from MicroMedCardiovascular.Usually a VAD does what its name suggests – it assists the natural heart with itspumping duties. "In this case, the two VADs would completely replace thenatural ventricles," Parnis says.

It's a novel idea that's been around for a few years now. Dr. BudFrazier, the Texas Heart Institute's director of research and chief ofcardiopulmonary transplantation, published papers on a continuous flow TAH backin 2006.  His idea took a step closer toclinical reality this year, when the National Institutes of Health awarded a $2.8 million grantto the Texas Heart Institute to fund the development of the continuous flowheart design.

VADs have alot going for them in a total heart application. For one, they're each aboutthe size of a C-cell battery, versus a 2-lb chunk of titanium and plasticfor a self-contained pulsatile pump. "VADs will fit in the majority of patientsversus a minority of patients for the pulsatile pumps currently in use," saysParnis.

Foranother, VADs have a proven clinical track record. About 500 of the current generationof DeBakey VADs are in use right now, according to Bob Benkowski, MicroMed'schief operating officer and one of the engineers who helped develop theoriginal 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 ofMicroMed's axial pump, whose single moving part, the impeller, is directlydriven by the electrical windings.

Parnis putsthe lifecycle expectation for even the most modern positive displacement bloodpumps, whose pulsations create cyclical loading conditions not seen by theaxial models, at two years. Continuousflow pumps will also likely require less power and cost less than the morecomplex pulsatile models, he says.

So if acouple of continuous flow VADs would make such a great TAH, why aren't they inuse yet? It turns out these devicesneed a significant amount of controls engineering to make the jump from hearthelper to total heart replacement.

And that's where Matthew Franchek and Ralph Metcalfe, both Ph.D mechanicalengineers and professors at the University of Houston's Cullen College of Engineering, enter the picture. As part ofthe NIH grant, they're working on a feedback controller that will allow twoVADs to work together as a TAH. Franchekand other university researchers have developed similar auto-regulating controlsystems for automotive applications, most recently working on a diesel enginegovernor for Cummins Engine.

In someways, Franchek and Metcalfe have had a head start in the controls developmentwork thanks to the use of the proven VAD technology. MicroMed's VADs alreadyhave their own controls. Benkowski describes them as feedback controllers, whichtake an actual flow measurement from an ultrasonic sensor, compare it with adesired flow output and generate an appropriate PWM control signal to regulatethe impeller speed.

Yet the two engineering professors still have their work cut outfor them. VADs normally operate individually as support for a remaining naturalheart. In the TAH, they have to operate in close coordination to emulate thebalanced flow provided by a natural heart's left and right ventricles. "Pairingthe pumps creates a complex multivariable control problem," Franchek says. "Each pump's loadingconditions and flow output affects the loading conditions and flow output ofthe other pump."

The TAHcontroller also has to tie these interrelated flow and loading conditions – whichinclude both inlet pressure and outflow resistance – back to the changing needsof the human body. Franchek says everyday activities such as standing orwalking change flow and loading conditions. So do cardiovascular events such asvascular restrictions, hypertension or changes in blood viscosity. And so dointrinsic physiological differences between individual patients. "Our challengeis to maintain a steady-state cardiac output as physiological conditionsfluctuate for whatever reason," says Franchek.

Axial flow pumps inherently lend themselves to meeting thischallenge. They can auto-regulate transient events because their flow output issensitive to both inlet pressure and outflow resistance. And Benkowski says the VAD pump's impeller geometry and flow passages can be tweaked to comeup with optimized flow-pressure behavior for the TAH application. "We can alter the pressuresensitivity of the pumps to make it a little easier for the control algorithmsto do their thing," he says.

Thosealgorithms, meanwhile, will be based on an analog integral controller whichmeasures actual output flow, compares it to the desired value and adjusts thevoltage to the pumps accordingly.Franchek and Metcalfe picked a seemingly simple integral controlstrategy for this application because it does a good job at maintaining steady-state conditions in systems whose dynamic behavior is both well understood andcharacterized by cooperative transients. Understanding that dynamic behaviorgiven the influence of physiology on pump conditions is not so simple. And alarge part of the control development work under the NIH grant involves thecreation of a lumped parameter mathematical model of the human circulatorysystem. According to Franchek, this model will ultimately be incorporated intothe TAH control algorithms (see block diagram below).

Franchek expects the first pass at the TAH controlalgorithms won't be ready until this summer. "Right now, we're at the verybeginning of the controls engineering," says Franchek. And there are still some fundamental decisions to bemade about how the pumps will operate. For example, the researchers have yet todecide whether one or both of the pumps should be operated in a quasi-pulsatilemode. Franchek says a repetitive control strategy would let the pump motors"whirl up and whirl down" to emulate the pulsating action of the natural heartif need be.

Otherdevelopment work includes the possible addition of blood-viscosity monitoringto the system. "We believe we'll be able to infer the effective viscosity ofthe blood from our flow measurements and voltage signals," Franchek says.

He andMetcalfe are using a variety of simulation tools to do their developmentwork, including MATLAB and Simulink to develop themathematical models. They're simulating the resulting control algorithms andprototyping the controller hardware in dSPACE, a set of development tools formechatronic systems.