Intro to some things CRM

Speaker: Alan Fryer

Date: 05-03-06

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Goal/Agenda

Present information on pacemakers and ICD’s to give everyone an idea about the technologies involved.

Overview of the Heart

Information about the technology in pacemakers

•Pacemaker algorithms

•Ventricular only pacemaker

•Rate Adaptation

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Caveats

Not an expert on everything I am going to talk about.

Whole books exist on these subjects so cannot cover anything in great detail.

Contains opinions that are not necessarily the opinions of others. Disagreement and debate are a natural part of the engineering process.

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Hearts Job

Part of the circulatory system.

Pumps:

•de-oxygenated blood to the lungs

•oxygenated blood to the body

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Anatomy of the heart

Heart has 4 Chambers.

2 Phase “controlled“ contraction

Atrium’s job: Assist in filling the ventricle.

Ventricle’s job: move the blood.

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How it works

•Heart is a muscle but unique

– can contract rhythmically and automatically without fatigue.

•Made up of cells that rhythmically depolarize and repolarize on their own, or on electrical stimulus.

– contract when the depolarize

– relax when they repolarize

•One cell deplolarizing will cause its neighbors to depolarize in a chain.

•Heart also contains specialized cells that form pathways for rapid conduction

•Some cells depolarize/repolarize at a more rapid rate than others are on the hearts natural “pacemakers”

++++++++++++-- - - - - - - - -- -

cell

intracellularintracellular

extracellularextracellular

V

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Changing potentials for different cells

Sinus node

AV node

PURKINJEfibers

Myocardialcells

70 bpm

40-60 bpm

30-40 bpm

SDD

SDD

SDD

<30 bpm

Shows “action potentials” for various cell types.

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Controlled Contractionby electrical conduction

Atrium beats first assisting the ventricle in filling.

Ventricle beats.

Conduction paths ensure controlled contraction.

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PQRST waves

Conduction process visible external to the heart on the ECG.

•P is the atrium depolarizing•QRS is the ventricle depolarizing•T is the ventricle re-polarizing

Note: Look how big the QRS complex is.

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Heart Control system

Circulatory Control Centers of the Brain

Heart RateContractility

Vascular Resistance

Baroreceptors

Stress, fight or flight, etc

Sympathetic Parasympathetic

Cardiac Output

workload, etc

Mean arterialblood pressure

Adjusts rate and contraction strength to meet the body’s

need.

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Cardiac Rhythm Management

CRM Devices – Treat conduction problems of the heart.

Bradycardia

Too slow

not synchronized

irregular beating

heart does not rate adapt (chronotropically incompetence)

treat with a Pacemaker (IPG) to Stimulate the Heart

Tachycardia

Heat beats too fast

irregular beating (at fast rates)

treat with a Implantable Cardiac Defibrillator (ICD)

Congestive Heart Failure

Heart does not beat efficiently

treat with Cardiac Resynchronization Therapy Device (3 lead ICD or IPG)

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Where do they go?

Implanted by the clavicle.

Leads go through the veins directly into the right side of the heart.

Can also attach leads to the outside of the heart.(epicardial lead)

A 3rd lead is also used to stimulate the left side of the heart (Coronary Sinus) for CRT.

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Where did they start from

Earl Bakken's invented the first wearable, battery-powered, transistorized cardiac pacemaker.

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Big Picture Requirements

•Lifetime requirements

– 10 years, translating to inhibit currents of < 10 uA

– Commercial HC11 MCU quotes 25 uA when all clocks are stopped !!!

•Size (always pushing for smaller)

– 10 cc now (dominated by the battery)

– 30 cc now for ICD’s (dominated by battery and caps)

• Cost

– beginning more cost sensitive, price erosion starting …

– volumes are a lot smaller than other industries changing the economics of decisions

• safety/reliability

– safety critical

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Technologies

•Multi-chip modules

– pushed the manufacturing/packaging industry with regard to miniaturization, now get help from cell phones and digital cameras

• Mixed Signal Design

– analog design a must for power supplies, switches, signal acquisition, communication circuits, charge pumps, etc.

•switched capacitor technology is used throughout the industry.

– strong push to digital to take advantage of shrinking process geometries

– lots of debate as to the ideal partitioning, and the answer changes with time.

• 12 years ago analog required 2 um processes which meant big power hungry digital circuits.

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HW / SW in CRM Devices•For at least the last 10 years, all modern CRM devices incorporate some sort of processor

– 6502, 6808, 6805, Z80, 8051, HC11 have all been used BUT USUALLY CUSTOMIZED

• lower power, lower clock speed ( ~2 Mhz)

• reduced number of clock cycles per instruction (8051)

• added instructions MUL, DIV, MOVE, etc.

• increased address range and added addressing modes

• sometimes even added registers (extra index registers)

• always have a STOP instruction (stopped > 95 % of the time)

• peripherals, IRQ maps, also always custom

– most companies have also built custom processors (DSP processors, controllers, Harvard architecture, micro-coded state-machines, you name it)

• SW becoming critical path, thus creating sw tool chains makes this expensive.

• Different philosophies for HW/SW decomposition

– Things we know, put in HW.

– Things that take a lot of cpu cycles put in HW

– Time critical things, put in HW.

– Things you don’t know always go in SW…

– Safety critical stuff put in HW (this is changing…)

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Applying some of these rules

Features that lean toward HW:

• For sensor data: signal acquisition, filtering, auto gain, event detection, feature extraction, waveform recording, waveform compression.

• For communications: time division multiplexing of comms. channel, packet transmission, packet reception, primitive command encoding, error detection

• For therapy: Bradycardia pacemaker timing, interval measurement, filtering, therapy classification (rate criterions, X of Y, Sudden Onset)

• For General use: DMA transfers, timers, signature calculators, time of day

Features that lead towards software:

•High level control algorithms like communication protocols, feature control, Progression through Tiered therapies.

• New features

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RAM / ROM PartitioningInitially RAM only used for updates late in the development process. Safety demanded code in ROM for fear of corruption.

Now differing opinions:

SW Engineers prefer ram based architectures with minimal code in ROM for safety critical behavior.

HW Engineers prefer as much rom as desired

Factors:

• ROM takes up less chip space, meaning you could have more memory for a given chip size. (effects cost, leakage current, etc)

•Executing code out of ROM takes less current than out of RAM

• Changing ROM means a new part which has significant cost implications for manufacturing and inventory management.

• ROM adds time constraints on development to have code early to meet tape-out dates.

• RAM can be changed any time allowing new products to be developed without IC turns.

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Special Considerations - Clocks

Multiple clocks for low-power operation

•CPU clock consumes too much power, so it is off when not use ( 2 Mhz)

•Main clock is much slower 32 kHz is used often

•clock gating used everywhere

•also gate power to blocks as needed

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Special Considerations – Charge Pumps

Multiple Battery Types

•Battery Voltage vary between 1.6 and 3.4 Volts, impedance can be 1 ohm to > 20 K ohms.

(Nuclear batteries where used in the 70’s but fell out of favor quickly…)

However need to generate:

• up to 7.8 V for 2 ms for pacing

– Voltage multipliers.

– effects IC process choice

• > 700 Volts for charging shock electrodes

– transformer based pump, also used for charging a flash in a camera and in tasers.

– also effects IC process choice

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Patient Safety

• IPGs and especially ICDs are safety critical

– device must be “active” to ensure patient safety

– termed as a safety critical device without a static safe state

• Also have severe constraints: Size, Cost, Life-time

– a safe device that can’t be sold does nobody any good

» 2 batteries?, 2*shock caps?, 2 * the leads?– rules out the use of redundancy as a global solution (triple

modular redundancy)

» not an ideal solution anyway, similar systems tend to fail in similar ways …

• Complexity also an important design constraint

– Requirements/Design Errors much more likely with increased complexity

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Design Process

“Cannot test in quality” means “quality needs to be designed in”.

Safety engineering and risk management are a central part of the design process throughout the product life-cycle.

• formal process considering Hazard, Cause, Probability, Severity, Mitigations, Resulting Probability, Resulting Severity

• Combined with formal design techniques Req, Des, Verif, Validation, etc.

Medical devices are regulated by the FDA which requires manufactures to follow its Good Manufacturing Practices as well as meet any standards appropriate for the industry.

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Modern Pacemaker Safety Architecture

Control System

Monitor

Reset

Therapy

Patient

Temporary Overide

Therapy

Patient Device

Fault Resolution Lifecycle:

1) Error Detection

• monitors job

2) Damage Confinement

• reset to known minimal state safe state

3) Error Recovery and continued service

• test and try to recover

4) Fault treatment

• record diagnostics, log the failure, communicate it to the physician.

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Monitor design crucial

Implemented in HW or Software

Standard protections:

• Write Protection, Stack Checks, Illegal Instruction, Reset on Rom Write, Watch Dogs, Key Protection Registers, Range Checks on Parameters, Signature Checks on Data, Algorithm Sanity Checks, Low Voltage, High Current, etc

Therapy Tests:

•High Rate Monitor – if the pacemaker ever exceeds the maximum rate, a reset occurs.

•Low Rate Monitor - if the pacemaker ever falls below a certain rate, a reset occurs (watch dog can be used for this)

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Control System Design

All other factors being equal, complex systems are more likely to fail due to design/requirements errors than simple ones.

Failures can leave systems in unknown states.

Failures should be rare and may be caused by transitory environmental conditions, thus the device should try to recover.

Some failures are correctable (memory corruption).

Therefore:

• from reset, start operation in a minimal safe state

– Single Chamber Pacemaker

– Single Chamber Pacemaker + Fib Detection and Shocking

• on passing self tests (consistency checks), recover to full functionality

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Start Simple – VVI Back-up Pacemaker

RALA

RV LV Pacemaker Timing Engine

Signal Conditioning

Amps/Filters

SenseDetection

Pace PulseGeneration

Leads

Heart

PhysicianInterface

Initial Focus

Earliest pacemakers just blindly stimulated the ventricle (VOO Mode), here we will stimulate it only when needed.

Ventricular Pacing, Ventricular Sensing, Inhibited

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VVI Timing Diagram and Rules

Signals Seen onventricular electrode

V Pace V Pace V Pace

V Sense

Skipped V Pace

Ignored VSense

Pacing intervals

Refactory intervals

V Pace

•wait, if detect nothing, pace

•if detect something, restart waiting period

•ignore detections that occur within a certain period of a detection/pace

•avoids T-wave and other unwanted noise

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A VVI Pacemaker Design

VVI Pacemaker Engine

Vrefractory Block

Clock Reset

Base Interval

refractory interval

ventricle pace request

ventricle event detection

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VVI Pacemaker Block

base_interval

start = ‘1’

clock

sense detectpace_request

timercontroller

wait

PacingReset

timeout

TRUETRUE

reset

vsense/reset timer

timeout /reset timer

signal pace_request

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VRefractory Block

refractory interval

restart_timer

clock

reset

pace_request sense detectsense

timercontroller

sensing

Signal SenseSignal Pace

waiting

sense detect / restart timer,

signal sense

pace_request/restart timer

timeout

timeout

TRUE TRUE

reset

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Simulation Results

The simulation shows the following behavior:•An initial pace occurs which starts a refractory period.•2 sense events occur and are ignored because they are in the refractory period.•A 3rd sense outside the refractory causes the pacemaker to restart its timing cycle •3 more paces occur as no more sense events are injected.

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What is missing

Real pacemakers have:

• more pacing modes (DDD, DDI, DVI, DDD/T, AAI, VDD, etc.)

• rate hysteresis to search for the intrinsic rhythm

• AV delay adapts to rate, different AV delays for paced vs sensed events

• AV delay hysteresis to search for intrinsic events

• protection against false senses due to cross talk (safety window)

• high rate limiting mechanisms to stop atrial tracking

• mode switching for high rates

• PVC detection

• Far-field avoidance

• rate smoothing on rate drops

•etc…

Gist: Pacemaker timing is a complicated sequence of connected state-machines.

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Another Missing Element - Rate Adaptation

Heart compensates for this by increasing stroke volume.

-> can be bad if allowed to continue over a long time

-> limits as to how well the heart can adapt.

-> not good enough 50% increase in cardiac output vs 200% possible for a normal heart

Circulatory Control Centers of the Brain

Heart RateContractility

Vascular Resistance

Baroreceptors

Stress, fight or flight, etc

Sympathetic Parasympathetic

Cardiac Output

workload, etc

Mean arterialblood pressure

Sometimes HR does not increase with need or has insufficient dynamic range.

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PH Sensor

(Not in use)

Senses metabolic need by measuring changes in acidity due to exercise.

Requires special lead.

Sensor life-time a concern.

Fiboritic tissue growth.

O2 Saturation

(Not in use.)

Senses SO2 by measuring reflectivity against red light.

Requires a special lead.

Would consume power.

Fiboritic growth/position changes will effect effectiveness.

Ventilation Rate

(limited use)

Measured using impedance measurements across the chest.

Holding breath will cause rate to decrease.

MV is more successful.

Minute Ventilation

(in use)

Impedance measurements, this time look for rate and amplitude of the signal

Guidant + St Jude have one, patent is expiring…

Often combined with activity.

Venus Temperature

(in use)

Use a thermistor in the lead to measure

Requires special lead.

Slow in response due to small slow signals measured.

Rate Adaptive Sensors (part 1)

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Part IIQT Interval

(successful for a time)

Measures the time from the QRS complex to the T wave

T-Wave peak is difficult to determine because it is so broad.

Slow to respond.

Ventricular Depolarization gradient

(used but not popular)

Measures the areas under the QRS.

Effected by drugs and electrode polarization.

Systolic Indices

(in use)

Measures stroke volume, Pre-ejection Phase, or contractility. Pacemaker adapts rate to minimize change in stroke volume.

Biotronik Closed Loop Stimulation.

Responds to emotions in addition to exercise.

Pressure

(never tested)

Minimize changes in mean arterial blood pressure.

Special lead.

Needs to be inserted in the left side of the heart.

Activity

(in use)

Measure activity and increase heart rate to match.

All companies sell activity based pacemakers.

Bad for walking down stairs, swimming, etc.

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Close Loop Stimulation – EXTREMELY SIMPLIFIED

Impedance Wave

Acquisition

SlowFilter

FastFilter

AreaCalc

Differential Area to

Rate Transform

Activity Present

Freeze

Raw SensorRate

Response Adaption

Heart Rate Information

Current Wave

Reference Wave

Base Rate

•Impedance measurements detect localized changes in geometry of the heart at the site of the lead.

•Indirectly measures contractility changes

•transform a differential area between the current and reference waveform into a rate