ENT 318/3 Artificial Organs

CONTROL DESIGN OF VAD

Lecturer

Normahira Mamat @ Mohamad Nornormahira@unimap.edu.my 1

VAD Design Consideration• What is design ?

“Designing is the intellectual attempt to meet certain demands in the best possible way. It is an engineering activity that impinges on nearly every sphere of human life, relies on the discoveries and laws of science, and creates the conditions for applying these laws to the manufacture of useful products”Pahl, G. and Beitz, W. (1977). Engineering Design, Berlin Heidelberg, Springer-Verlag

1. DEFINE THE PROBLEM• How to define a problem?• Begin with writing specific design specification and requirement.• General statement for heart assist device problem :

“To develop a device that when implanted in the human will provide a longer and better quality of life than conventional pharmacologic or transplant.”

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VAD Design ConsiderationA. FIT OF THE SYSTEM• Decide who the device intended for.• Device fit in patient.• Cause minimal or no pathologic conditions.• Consider volume, mass, dimension, location of tubes, conduits, connectors.• Physical attributes – hard, soft, smooth, rough, sharp corners damage

tissues.• Should not project heat.• Effect of device movement and vibration.• Acceptable sound levels at various frequencies.• Meet EM interference, susceptibility standard.

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VAD Design ConsiderationB. PUMP PERFORMANCE• Performance specified in cardiac output range.• Cardiac output performance obtained at physiologic pressure (left atrial

pressure ~7mmHg, aortic pressure~100mmHg etc).• Control of device must be included.• Device must respond to patient’s cardiac output requirement.

C. BIOCOMPATIBILITY• Device must not cause excessive damage to biologic system.• Minimally thrombogenic and minimally hemolytic.• Minimal effect on immune system.• Should not promote infection, calcification, tissue necrosis.

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VAD Design ConsiderationD. RELIABILITY

• Design specification must have :– Target reliability for device (e.g 80% confidence for 2 yrs)– Clarification on components that could be changed if necessary– Deal with any service that device may require.

• Example : device design life is 5 yrs but battery replacement at 2 yrs interval is allowed

E. QUALITY OF LIFE• Designer must specify what is a satisfactory quality of life.• Quality of life must be considered in relation to patient’s quality of life

without the device.• How much weight older patient can carry?• How often power source required to recharge?• What sound level is acceptable?

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VAD Design Consideration2. CONCEPTUAL DESIGN• Plan treatment of the problem.• Thorough review of literature.• Design of proposed solution is examined• Consider various design that meet specification.

e.g. : Pulsatile or non-pulsatile flow ? Types of pump?

• Consider other non-traditional solutions.e.g. : employ micro-machines, MEMS, magneto- hydrodinamics

• Source of energy. Batery, fuel cell, piezoelectric crystal? The performance ?• Control of the device. Should include sensor? Consider performance of

sensor. Consider less or no sensor.

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VAD Design Consideration

• Pump performance and interaction with biological system.• Selection of materials.

2. DETAILED DESIGN• Initial prototyping and testing.• Evaluation of performance.

- System, manufacturing, cost, etc.

3. LEARN AND GENERALIZE• After design complete.• Learn and generalize the design for further design or implemented the

concept on other device.

Reference :

G. Rosenberg , “Artificial Heart and Circulatory Assist Devices” in Tissue Engineering and Artificial Organs, J.D. Bronzino, Ed. Florida : CRC Press, 2006

Go to www.engnetbase.com and find in the search 7

Control techniques of VAD

Principle goal – blood pump controller is to respond the body’s demand for cardiac output. Since it is not realistic to place sensors in the human body for long term applications. Feedback controller automatically adjust the pump speed for perfusion for different levels of patient activity.

Control Problem for Ventricular Assist Devices

• Earliest pulsatile types VADs actuated by air.- used in a limited and critical care setting- open-loop mode that allows one or two manual adjustments to prescribe the eject rate and duty cycle (systole/diastole ratio)

• Open-loop design for critical care, supervised by human.• Control adjustment made by operator which is human.• When patient condition change, pump will operate with current setting

until operator manage to identify the changes and make proper adjustment.

• Patient condition might change due to change in blood volume, inotropic level or stress.

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Control techniques of VAD• For certain critical event, operator must decide whether to adjust pump or

to call clinician for treatment.• Thus operator is required to provide feedback control and fault detection

to the system. • Problem – the inability of the devices to respond automatically to

changes in demand for cardiac output impact the quality of patient life.• For pulsatile pumps, the challenge is to specify pumping rate and

control of blood pressure.• For axial pumps, blood might flow backwards through the pump if

running slow since there is no valve.• If run to fast, pump might draw more blood from ventricle. This will cause

suction in the ventricle.

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Manual and auto control system

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Control techniques of VAD

How to Eliminate the Problems?• Human operated device should be abolished if long-term implantation are

to be implemented.• Eliminate the need for human to monitor the device

• VAD control system must be able to respond to changes.

• Patient’s status should be monitored to notice change in patient’s requirement.

• The VAD condition itself should also be monitored to detect hardware failure, changes in assist device parameters, uncontrollable situation and dangerous control commands by the controller.

• Control a blood pressure to maintain adequate haemodynamics

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Control techniques of VAD

Research done by Pittsburgh University, USA

PROBLEMS• Axial pump run continuously to draw blood from heart.don’t have

valve.• If pump run slow, blood flow backward (regurgitant).• If pump run fast, more blood will drawn from heart, produce

negative pressure in ventricle (suction).• Motor speed 9000 rpm, minimum pump flow close to zero and

ventricle pressure peak reach 70 mmHg.• Motor speed 10,000 rpm, pump flow less pulsatile with minimum

greater than zero, ventricle pressure peak at 30mmHg.• Figure in next slide shows how suction occurs.

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Control techniques of VAD

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Control techniques of VAD

• At 11,000 rpm, the pulsatile pattern is lost. Ventricular pressure is consistently less than zero.- a dangerous condition that must be detected quickly and corrected by reducing the pump speed before the heart muscle damaged.

• Lower limit pump speed must avoid regurgitant flow.

• Upper pump speed limited to speed that induce suction in ventricle.

• The control problem of rotary pump is to determine appropriate reference speed between this limits.

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Control techniques of VADPULSATILITY INDEX • Pulsatility in pump flow signal can be used to define pulsatility index.• Hemodynamic signal (aortic pressure, ventricle pressure) produce

varying degree of pulsatility.• Thus, pump load will be pulsatile which make pump flow and motor

drive current pulsatile.• As pump speed increase, pulsatility decrease.• Pulsatility index used as control signal.• Speed change until pulsatility index equal reference value.• Reference value will hold speed above minimum pulsatility value

corresponds to suction.

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Control techniques of VAD

• Thus, this technique attempt to operate the pump at a speed just below suction.

• Provide maximum safe cardiac output.• Fuzzy-logic controller has been used for this task.• In this controller, the law is that when index higher than reference,

speed increased to reduce index & vice versa.

PROBLEM WITH PULSATILITY INDEX• Does not consider other hemodynamic parameters.• When systemic vascular resistance increase, pump output reduce

which will increase pump speed.• Increasing speed of pump might increase arterial pressure to

region outside physiological range.

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Cardiac output

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• Speed changes made to keep the pump operating just below suction.

• SVR = systemic vascular resistance• At speed 1, the pump operating is intersection of the pump

pressure-flow with SVR1 line-normal physiologic range.• At speed 2, SVR2 increase which reduces pump output. Arterial

pressure may increase and operating point outside the physiologic range.

• In this case, pump operation at the lower speed might be desirable.

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Control techniques of VADMULTIOBJECTIVE OPTIMIZATION• Common constraint for VAD :

1. Cardiac output above minimum value (3-6 L/min).

2. LAP below 10-15 mmHg to avoid pulmonary edema and above 0 mmHg to avoid suction.

3. Systolic arterial pressure maintained between patient-specific limits to assure adequate perfusion

• Formulate control problem solution to meet constraints as optimization.

• There is three control outputs i.e. cardiac output K, arterial pressure A, left atrial pressure F.

• Penalty functions applied on those three control outputs.• If controlled output far from desired point it receive large penalty, if

close to desired point it receive small/zero penalty.

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Control techniques of VAD

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Control techniques of VAD

Multiobjective optimization• Multiobjective optimization considers all criteria of interest.• Provide most satisfactory approach to specifying pump speed.• Requires the most information about patient.• Performance measure defined as :

J(ω) = [J1(ω), J2(ω), J3(ω)]• Where ω is the speed of pump and J1, J2 and J3 are penalties

assigned to cardiac output K(ω), arterial pressure A(ω) and left atrial pressure F(ω) respectively.

• To determine the penalties associated with different pump speeds, the functions K(ω), A(ω), and F(ω) are needed.

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Control techniques of VAD

• These relations depend on pump characteristics, the state of the natural heart, and the systemic circulation parameters, and they can be expected to vary with time.

• Model of the patient’s cardiovascular system is used to relate pump speed to hemodynamic variables.

• If natural heart very weak, VAD provide most of blood flow, ventricular pressure close to zero, total cardiac output is essentially equal to the pump flow and arterial pressure approx equivalent to the pressure developed across pump.

• If natural heart provide some output, total flow may exceed pump flow, and arterial pressure depends on natural heart.

• A simple model with minimum number of parameters must be used, since the are patient-specific and must be estimated for each individual.

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Multi-objective optimization

• Most satisfactory approach to specifying pump speed.• Additional crtiteria can always be added as the clinicians

feel appropriate for the patient or as sensor inputs.

• Requires the most information about the patient.

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Control architecture

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• A local control algorithm built into the pump maintains pump speed at a reference value.

• The reference speed is determined by one of several algorithms (optimal, heuristic, or default), depending on the patient’s physical condition, device status, and confidence that accurate measurements or estimates of hemodynamic variables are available

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• The default control mode can be used in the event of extensive sensor failure, software failure, or uncertainty concerning the reliability of control actions or functioning of the assist device itself.

• The default mode provides a constant pump speed that is low enough in most cases to avoid suction while still providing a nominal flow output.

• This mode attempts to provide safe operation of the device without requiring any sensory information, state variables, or model parameter estimates.

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Control techniques of VAD

• The controller should respond to changes in demand for cardiac output.

• Cardiac signals may be of limited use in early implantation, since patients will often have abnormal cardiovascular status.

• However, if cardiac recovery is obtained, these signals will become of greater value.

• The potential contribution for multiple sensors, but they caution against incorporating multiple sensors that do not contribute independent information, due to the increased complexity.

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Control techniques of VAD

• The control system, as well as the assist device itself, must be reliable, and it must be able to react appropriately to hardware failures.

• Device and patient-adaptive cardiovascular models can be used to determine the reference pump speed and evaluate device performance, and the hierarchical control structure can decide which model approach to use.

• However, obtaining adequate information to identify the models in real time remains a significant challenge.

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Control techniques of VAD

REFERENCES/SUPPLEMENT READING• J.F. Antaki, J.R. Boston, and M.A. Simaan, “Control of Heart Assist

Devices,” Proceedings of the 42nd IEEEConference on Decision and Control, pp. 4084-4089, 2003.

• J.R. Boston, J.F. Antaki, and M.A. Simaan, “Hierarchical Control of Heart-Assist Devices,” IEEE Robotics & Automation Magazine, pp. 54-64 , March 2003.

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