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Intro to some things CRM
Speaker:
Alan Fryer
Date:
05-03-06
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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V
How it works
extracellular
++++++++++++
-- - - - - - - - -- -
intracellular
cell
•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”
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Changing potentials for different cells
Shows “action potentials” for various cell types.
Sinus node
70 bpm
SDD
AV node
40-60 bpm
SDD
PURKINJE
fibers
30-40 bpm
SDD
Myocardial
cells
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<30 bpm
Controlled Contraction
by 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
Adjusts rate and
contraction strength
to meet the body’s
need.
Stress, fight or flight, etc
Circulatory Control Centers of
the Brain
Sympathetic
Contractility
Baroreceptors
Mean arterial
blood pressure
Vascular
Resistance
workload, etc
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Cardiac Output
Parasympathetic
Heart Rate
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, batterypowered,
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
the economics of decisions
• safety/reliability
– safety critical
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changing
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 Partitioning
Initially 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
Fault Resolution Lifecycle:
Patient Device
1) Error Detection
•
monitors job
2) Damage Confinement
•
Control System
reset to known minimal
state safe state
3) Error Recovery and continued
service
•
test and try to recover
4) Fault treatment
•
Reset
Temporary
Overide
record diagnostics, log the
failure, communicate it to
the physician.
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Therapy
Monitor
Therapy
Patient
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
Heart
RA
RV
LA
Signal
Conditioning
Amps/
Filters
Sense
Detection
Pacemaker
Timing Engine
LV
Physician
Interface
Leads
Pace Pulse
Generation
Earliest pacemakers just blindly
stimulated the ventricle (VOO
Mode), here we will stimulate it
only when needed.
Initial
Focus
Ventricular
Pacing, Ventricular Sensing, Inhibited
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VVI Timing Diagram and Rules
Ignored VSense
Skipped V Pace
V Sense
Signals Seen on
ventricular electrode
V Pace
V Pace
V Pace
Pacing intervals
Refactory intervals
•wait, if detect nothing, pace
•if detect something, restart waiting period
•ignore detections that occur within a certain period of a
detection/pace
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•avoids
T-wave and other unwanted noise
V Pace
A VVI Pacemaker Design
Clock
Reset
VVI Pacemaker Engine
Base Interval
Vrefractory Block
ventricle pace
request
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ventricle event
detection
refractory interval
VVI Pacemaker Block
pace_request
base_interval
sense detect
start = ‘1’
controller
timer
reset
timeout
clock
vsense/
reset timer
timeout /
reset timer
signal pace_request
wait
Pacing
Reset
TRUE
TRUE
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VRefractory Block
sense
refractory interval
sense detect
pace_request
reset
reset
controller
timer
restart_timer
timeout
clock
pace_request/
restart timer
sensing
sense detect /
restart timer,
signal sense
timeout
Signal Sense
Signal Pace
TRUE
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waiting
TRUE
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…
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Gist: Pacemaker timing
is a complicated
sequence of connected
state-machines.
Another Missing Element - Rate Adaptation
Sometimes HR does not
increase with need or has
insufficient dynamic range.
Stress, fight or flight, etc
Circulatory Control Centers of
the Brain
Sympathetic
Contractility
Parasympathetic
Heart Rate
Baroreceptors
Mean arterial
blood pressure
Vascular
Resistance
Cardiac Output
workload, etc
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
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for a normal heart
Rate Adaptive Sensors (part 1)
PH Sensor
(Not in use)
O2 Saturation
(Not in use.)
Ventilation Rate
(limited use)
Minute Ventilation
(in use)
Venus Temperature
(in use)
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Senses metabolic need by
measuring changes in
acidity due to exercise.
Requires special lead.
Senses SO2 by measuring
reflectivity against red
light.
Requires a special lead.
Measured using impedance
measurements across the
chest.
Holding breath will cause rate to
decrease.
Impedance
measurements, this time
look for rate and amplitude
of the signal
Guidant + St Jude have one, patent is
expiring…
Use a thermistor in the
lead to measure
Requires special lead.
Sensor life-time a concern.
Fiboritic tissue growth.
Would consume power.
Fiboritic growth/position changes will
effect effectiveness.
MV is more successful.
Often combined with activity.
Slow in response due to small slow
signals measured.
Part II
QT Interval
(successful for a
time)
Ventricular
Depolarization
gradient
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.
Measures the areas under the
QRS.
Effected by drugs and electrode
polarization.
Measures stroke volume,
Pre-ejection Phase, or
contractility. Pacemaker
adapts rate to minimize
change in stroke volume.
Biotronik Closed Loop Stimulation.
Minimize changes in mean
arterial blood pressure.
Special lead.
Measure activity and increase
heart rate to match.
All companies sell activity based
pacemakers.
(used but not
popular)
Systolic Indices
(in use)
Pressure
(never tested)
Activity
(in use)
Responds to emotions in addition to
exercise.
Needs to be inserted in the left side of
the heart.
Bad for walking down stairs, swimming,
etc.
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Close Loop Stimulation – EXTREMELY SIMPLIFIED
Activity Present
Base
Rate
Freeze
Slow
Filter
Reference
Wave
Impedance
Wave
Acquisition
Area
Calc
Fast
Filter
Differential
Area to
Rate
Transform
Raw
Sensor
Rate
Current
Wave
Response
Adaption
Heart Rate
Information
•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
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