• Ohm’s Law
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ICD Electronics – Building Blocks
• ICD Shocking / Pacing Outputs
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• Sensing
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• Battery Technology
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Ohm’s Law
Voltage = Current x Resistance
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V (Volts)
(V lt ) = I (A
(Amps)) x R (ohms)
( h )
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Voltage is the “pressure”
pressure that pushes the current through
the circuit. It is also called electromotive force (emf).
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Current is the flow of charge through the circuit.
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Resistance
R
i t
i a measure off the
is
th amountt off currentt you will
ill
get based on a certain Voltage (pressure). Resistance is
basically the availability of charge carriers – the fewer
charge carriers, the higher the resistance
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Ohm’s Law
1 Amp = 1,000 milliAmps = 1,000,000 microAmps
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1 milliAmp = 1,000 microAmps
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Sensing currents are typically 5 – 20 microamps
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Pacing currents are typically 1 – 20 milliamps
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Defibrillation currents are typically 1 – 20 Amps
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Power and Energy
Power = Voltage x Current
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P(watts) = V(Volts) x I(Amps)
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Energy = Power x Time (sec)
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E(watt-sec)
E(watt
sec) = V x I x Time
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E (joules) = V x I x Time
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Power and Energy
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Energy is the ability to do work.
1 Joule = 1 Volt x 1 Amp x 1 sec
1 Joule = 1 Watt x 1 sec
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If the ball has a mass of 1
kilogram (2
(2.2
2 pounds) and it
was dropped 1 meter, its
kinetic energy
gy would be
approximately 10 joules
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Ohm’s Law – Pacing
During pacing, the current flow
will be between the cathodal tip
and the anode ring or shocking
electrode. Typical impedance is
400 - 1200 ohms
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Ohm’s Law – Defibrillation
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IIn an active
ti can ICD system,
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current flow will be between the
ICD metal can and the shocking
electrode(s) in the heart. Typical
impedance is 20 – 80 ohms
When the shock is delivered
delivered,
sufficient current density much
be achieved to depolarize a
critical
iti l mass off cardiac
di cells.
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Ohm’s Law – Series Circuit
Pacing Switch
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R - Lead
Pacing
Capacitor
R @ Lead
Tip / Tissue
Interface
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Current
Flow
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Series Circuit – when
the pacing switch
closes, current flows
from the pacing
capacitor through the
resistance of the lead
and through the
resistance at the tip of
the lead. All of the
current goes through
the same path, so the
resistances ADD.
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Ohm’s Law – Series Circuit
Pacing Switch
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R - Lead
Pacing
Capacitor
R @ Lead
Tip / Tissue
Interface
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Current
Flow
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NUMBERS – If the resistance
in the lead is 50 ohms and the
resistance at the tip / tissue
interface is 550 ohms, the
series resistance is the
summation of the resistances
or 550 + 50 = 600 ohms. Since
the current must go through
each resistance in series, the
current or flow “sees” the
total of all the individual
resistances.
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Ohm’s Law – Parallel Circuit
Pacing Switch
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RLead
Pacing
Capacitor
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Current
Flow
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RLead
R @ Lead
Tip /
Tissue
Interface
R@
Lead Tip
/ Tissue
Interfac
e
Parallel Circuit – when
the pacing switch
closes,
l
currentt flows
fl
from the pacing
capacitor through the
resistance of the first
lead and through the
resistance at the tip of
the lead
lead. Also
Also, current
goes through the
second lead that is in
parallel to the first
lead. This is an
example of BiV pacing
the RV and LV with a
tied output device.
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Ohm’s Law – Parallel Circuit
Pacing Switch
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RLead
Pacing
Capacitor
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Current
Flow
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R @ Lead
Tip /
Tissue
Interface
NUMBERS – If the resistance
in the first lead is 50 ohms
and the resistance of its tip /
tissue interface is 550 ohms,
the series resistance is the
summation of the resistances
or 550 + 50 = 600 ohms. If the
resistance of the second lead
is 100 ohms and the
resistance of the tip / tissue
interface is 900 ohms, the
series resistance is the
summation of the resistances
or 100 + 900 = 1000 ohms.
So we now have 600 ohms in
parallel with 1000 ohms
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NUMBERS – So we now have 600 ohms in parallel with 1000 ohms.
The current will travel down both the 600 ohm path and down the
parallel path of 1000 ohms
ohms. The total resistance would then be the
parallel path of 1000 ohms and 600 ohms. Mathematically, the resulting
total resistance of both paths in parallel will always be LOWER than
either path single path
path. Sort of like pouring water down two long tubes
in parallel to each other – you get more total water flow (the resistance
to water flow has gone down).
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Ohm’s Law – Parallel Circuit
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Here is the math to calculate the total resistance when you have two
parallel paths
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Total Resistance = 600 ohms x 1000 ohms =
15
375 ohms
600 ohms + 1000 ohms
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Shocking Outputs
Programmable Stored Energy - The energy stored in the
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output capacitors is programmable.
programmable The pulsewidth of
the output may also be programmable. Typically
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0 –15%
5% o
of tthe
e sto
stored
ed e
energy
e gy iss lost
ost due to ccircuitry
cu t y in
the ICD and losses in the waveform.
Programmable Delivered Energy - The energy that will be
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delivered to the shocking lead system is programmable
and the AICD stores sufficient energy to ensure the
delivered output.
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Shocking Outputs
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During ICD charging, the battery generates
low Voltage
g and high
g current which is
transformed to a high Voltage and a low
current and this current then charges up the
capacitors
it
tto specific
ifi voltage
lt
llevels
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depending on programmed energy
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During energy delivery, current flows from
the output capacitors similar to how water
empties from a column.
column Initially,
Initially the current
flow is high, but it decreases as the voltage
decreases – the capacitor empties
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Shocking Outputs
Once a specific
Voltage is reached,
energy delivery
d l
is
stopped
Monophasic pulse
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Voltage and current start
high and then decrease
as energy in the
capacitors flows out
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Typically, biphasic
DFTs are 30 – 40%
lower than
monophasic DFTs
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Shocking Outputs
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Biphasic pulse – Polarity of waveform is
reversed during the shock delivery
ICD Electronics – April 2007 - wks
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Sensing and Detection
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Amplitude is Typically > 5 mV
i the
in
th ventricle
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Slew Rate = Amplitude / Time = dV/dT
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NOTE: “Far-field potentials arise from electrical activity distant
from the electrode and include contralateral ventricular activation,
skeletal
muscle
potentials,
and
external
electromagnetic
interference (EMI). “
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Sensing and Detection
Pacemakers
P
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typically
i ll use programmable
bl fixed
fi d sensitivity
ii i
circuits for detecting intrinsic activity.
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2.5 mV
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Blanking
Programmable Refractory
ICD Electronics – April 2007 - wks
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Sensing and Detection
ICDs use auto-adjusting sensitivities or auto-adjusting
amplification for detecting intrinsic activity.
activity
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2.5 mV
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Blanking
Refractory
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NSR
Lead Signal
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AICD Filtered
Signal
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Sensing and Detection
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R f
Refractory
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NSR
Lead Signal
2 mV R-wave and
4 mV T-wave
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AICD Filtered
Signal
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Sensing and Detection
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Refractory
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Sensing and Detection
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Rate Sensing Electrogram Showing Sensing Post-Pacing
Rate Sensing EGM
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Markers
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Battery Technology
Since 1972, lithium batteries have been the primary power
source for pacemakers. Today, Lithium-Iodine batteries are
used
d in
i virtually
i t ll allll pacemakers.
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Other chemistries that have been used include Lithium
cupric sulfide, Lithium thionyl chloride, Lithium lead and
Lithium silver chromate. The type of chemistry determines
the battery’s voltage and discharge characteristics.
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• 1958 - Rechargeable
R h
bl batteries
b tt i
• 1958 through 1960 - zinc-mercury batteries. Pacemaker
longevity of about 2 years.
years
• Nuclear Batteries - half-life of 87 years.
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Battery Technology - LiI
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Voltage curve
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BOL = 2.8 V
ERT
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ERT Impedance is
yp
y 5K to 10K ohms
typically
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Impedance curve
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BOL = 3.2 V
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Battery Technology - Lithium
Silver Vanadium Oxide
Voltage curve
ERT
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ERT Impedance is
typically 0.5 to 0.6 ohms
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Impedance curve
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QUESTION: What typically happens to the charge time
for a high energy shock as an AICD approaches ERT?
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Very Highly Accelerated
Battery
Run Down
Highly Accelerated
Test
Time
2 Years
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October 1990
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Highly Accelerated Test Time - 2 Years
Charge Cycles Every TWO months (4 charge cycles per test)
3.5
3
•
2.5
Charge
g Time
18 seconds
2
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Vo
olts / Charge T
V-drop
1.5
V(CHARGING)
Charge Time / 10 seconds
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V(MONITORING)
0
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0.5
200
400
600
800
1000
1200
1400
1600
1800
Battery Capacity
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Accelerated Test Time - 3 Years
Charge Cycles Every THREE months (4 charge cycles per test)
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3.5
3
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2
1.5
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Vo
olts / Charge tim e
2.5
3 times V (MON)
per year V(CHRG)
per year Charge T
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Charge
g Time
18 seconds
0
50
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0.5
200
00
350
520
5
0
675
6
5
825
8
5
950
1100
00
1250
50
1375
3 5
1525
5 5
Battery Capacity
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“Non-Accelerated” Test Time - 5 Years
Charge Cycles Every SIX months (4 charge cycles per test)
3.5
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3
V-drop
2
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Vo
olts / Charge T
2.5
Charge Time
30 seconds
1.5
V(CHRG)
Charge Time / 10 seconds
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V(MONITORING)
0
0
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200
400
600
800
1000
1200
1400
1600
1800
Battery Capacity
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