Fundamental Electrical Relationships: Fundamental Electrical Relationships
Ohm’s Law, Strength Duration Curve & Factors Affecting Thresholds
Mark W. Sweesy, FHRS,CCDS,CEPS
y,
,
,
Arrhythmia Technologies Institute
O tline
Outline
z Ohm’s Law
Ohm s Law
z Voltage, current and resistance (impedance)
z E
Energy and power
d z Clinical relevance
z Strength duration curve
z Rheobase
z Chronaxie
z Clinical relevance
Cli i l l
V
I
R
Ohm’s La
Ohm’s Law
V = I x R
V
I = V / R
R = V / I
I xR
The equation uses current in the form of Amps
In pacing current is expressed in mA
V
I
R
F
Fundamentals of Electronics
ndamentals of Electronics
z Voltage (V): force moving the current: electromotive push
z Current (I): flow of electrons through a circuit Current (I): flow of electrons through a circuit mA
mA
z Resistance (R) / Impedance: opposition to current flow: unit nit of measure: Ohm ( of measure: Ohm ( Ω
Ω)
V
I
R
Fundamentals of Electronics
Low Resistance = high current flow
z PPP{
V
g
High Resistance = low current flow
V
PW
PW
R
I
I
Ohm’s La
Ohm’s Law: Pacers Ohm’s Law: Pacers vsICD’s
Pacers vsICD’s
sICD’s
Typical range for pacemaker voltage 1.0‐5.0 V
yp
g
p
g
Typical range for ICD voltage 50‐800 V
Typical range for pacemaker current 2‐16 mA
Typical range for ICD current 1 000 75 000 mA
Typical range for ICD current 1,000‐75,000 mA
Typical range for pacing lead impedance: 300‐1200 Ohms
T
i l f i l d i
d
Oh
Typical impedance range for a high voltage lead 10‐60 Ohms
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Ohm’s La
Ohm’s Law
What is the current in a 2.5 volt system with a
500 Ω resistance?
V
I
R
Ohm’s La
Ohm’s Law
What is the current in a 2.5 volt system with a 500 Ω resistance?
I = V
R
=
2.5 V
500 Ω
=
.005 A
.005 A x1000 mA
1 A
= 5 mA
5
When the impedance is 500 ohms the absolute value of the current is 2x s the voltage
When the impedance is 500 ohms the absolute value of the current is 2x’s the voltage
18. A lead fracture would result in which of the following?
18
A lead fracture would result in which of the following?
a. decreased battery longevity
b decreased conductor impedance values
b.
c. decreased electrode impedance values
d decreased current drain
d.
d
d
d
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Energ
Energy Formula
Form la
E = V x
E V x
E = V V x I I x
xPW
E = V
E = V2x PW
R
E = I
E = I2x R R x
x PW
If you leave current in mA
y
and time in
Msec your answer will be in uJ
15. A patient’s stimulation threshold was 1.0 Volt & 0.2 msec pulse duration. The pacemaker is programmed p
p
p g
to 2.0 Volt & 0.4 msec duration. Lead impedance measures 1000 Ohms The output energy for this measures 1000 Ohms. The output energy for this patient would be:
a 2X threshold
a.
b. 4X threshold
c. 6X threshold
h h ld
d. 8X threshold E=
12
x .2 = .2
1000
2
V
xt
R
22 x .4 = 1.6
1000
Doubling the voltage is 4 times the energy and
d bli
doubling
th
the PW iis 2 ti
times th
the energy
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Lead Impedance and Energ
Lead Impedance and Energy
Energ
Energy = V x I x T
V I T
Pacemaker Pulse Widths= .1 ‐1.5 ms 4 0 V x 8.0 mAx
4.0 V x
8 0 mAx .5 ms 5 ms = 20 uJ
20 uJ
ICD Pulse Widths = 3‐15 ms range1
75
750 V x
14,000 mAx
4,
33 ms = 31.5 J
3 5
or 31,500,000 uJ
1. Thammanomai A, et al. Heart Rhythm 2006; 3:1053-1059
Charge
z In In pacing
In pacing, charge describes the amount of electricity pacing, charge describes the amount of electricity pacing charge describes the amount of electricity delivered to the delivered to the myocardium during an atrial or myocardium during an atrial or ventricular output.
ti l t t
z Charge (Q) Charge (Q) is the product of pulse is the product of pulse duration (
duration (tt) ) in in milliseconds & output current (I) milliseconds & output current (I) in in milliamperes
milliamperes. In . In pacing it is usually expressed in microcoulombs
pacing it is usually expressed in i it i ll d i microcoulombs.
i
l b .
z Q = I x T
Power
Po er
z Power is the rate of doing work or transmitting energy.
z Power P = I Power P = I x
x V: where P is in watts, I is in amperes and V is in volts and V is in volts. 22. What is the estimated battery service life of a pacemaker with a 1.0 Amp‐hour battery and a 20 k ith A
h b tt d microamp current drain?
a. 4.0 years
5 y
b. 5.0 years
c. 5.7 years
d 10.0 years
d.
10 0 years
1 0 Amp-hour
1.0
Amp hour battery and a 20 microamp current drain
Time = Q battery
y capacity
p y in Ah
I current drain in amperes
Time = 1,000,000
1 000 000 uA/hr = 50,000
50 000 hr = 5.7
5 7 yrs
20 uA
8,760
,
hr/yr
y
Factors Affecting Current Drain
acto s ect g Cu e t a
z Lead/tissue interface
z Stimulation threshold & safety margins desired
z Quiescent current
z Impedance
z Electrode design & materials
z Pacemakers programmed settings
z % pacing
Quiescent Current
z Housekeeping H
Housekeeping –
k
i – current utilized when not pacing; i.e., t tili d h t i i current used for ongoing tasks like monitoring the intrinsic rhythm & logging data. Typically in the 8‐
rhythm & logging data. Typically in the 8
h th & l
i d t T i ll i th 8‐12 uA range for A f pacemakers
z Reducing the output from 4.0 to 2.0 V may increase the battery longevity from 4 to 6 yrs, still have housekeeping y
g
y
4
y ,
p g
needs
Bipolar Unipolar
Fracture / Connector / F t / C
Fracture / / Connector / Uni
C
t / U
/ U
Unii Leads L d Causes
Causes of High & Low Impedance
o g & o
peda ce
z High Impedance
z Conductor Fracture, Loose Connector or Unipolar leads programmed to Bipolar
z Low Impedance
z Insulation degradation, Y‐adaptor or parallel pathways, loose connection maintainingcontact with body fluids1
1. In Ellenbogen K, et al. Clinical Cardiac Pacing, Defibrillation & CRT. 2007; pp 834
Measured Data
If you could only pick 3 values on which to base your device
If ld l i k l hi h b d i
evaluation; which 3 would you pick?
Measured Data
If you could only pick 3 values on which to base your device
If ld l i k l hi h b d i
evaluation; which 3 would you pick?
Stimulation Threshold
Stim lation Threshold
z The minimal amount of electrical stimulation that consistently produces a cardiac depolarization The threshold can be depolarization. The threshold can be expressed in terms of amplitude (milliamperes or volts), pulse duration (msec
or volts), pulse duration (msec), ), charge (microcoulombs
charge (
microcoulombs), or energy ), or energy (microjoules).
microjoules).
Strength Duration Curve
z A graphic representation of the pulse amplitude, in voltage or current, required to produce an i lt
t i d t d
action potential in the cells. The curve is plotted as a function of pulse duration and can be defined f
ti f l d ti d b d fi d by 2 points; the Rheobase
by 2 points; the Rheobase and the Chronaxie
and the Chronaxie. As . As th l d ti i
the pulse duration increases the voltage th lt
requirements decline (to a point).
capture
Rheobase
z The value derived from a strength‐
The value derived from a strength‐duration curve that indicates the minimum output that will stimulate the myocardium irrespective of the pulse duration The curve myocardium irrespective of the pulse duration. The curve typically levels out around a pulse duration of 1 msec
typically levels out around a pulse duration of 1 msec
Chronaxie
z The pulse duration value that is twice the rheobase.
The pulse duration value that is twice the rheobase
26. The term 6 Th or fibrous fib
capsule surrounding the electrode:
a. is a normal foreign body reaction
p
b. is an unexcitable capsule of tissue c. decreases the current density at the lead tissue interface
d. all of the above
^AfA
^AfA
^AfA
Virtual Electrode
z
is a normal foreign body reaction which results from acute injury to the cell membrane
z
collagen is formed resulting in anunexcitable collagen is formed resulting in anu
g
g
nexcitable capsule of capsule of p
tissue which decreases the current density at the lead tissue interface
^AfA
^AfA
^AfA
17. Which of the above 3 month stimulation threshold graphs would be most characteristic of steroid eluting tined atrial lead?
a A
a.
b. B
c. C
d. D
^f
^AfA
Electrode Threshold Characteristics
1.0
0.8
Polished platinum
0.6
A
Porous platinum
0.4
B
Activated carbon
02
0.2
C
Steroid eluting tined lead
D
I l t ti (
Implant time (weeks)
k )
^AfA
^
f
14
13
12
11
10
9
8
7
6
5
4
3
2
0.0
1
Pulsse width
h (ms)
12
1.2
The difference noted in thresholds when decrementing until loss of capture versus going from sub threshold to until loss of capture versus going from sub‐threshold to capture is known as?
a. latency
l t
b. Wedensky effect
c. intermittent threshold
p
d. polarization
Wedensk
Wedensky Effect
Effect
z Also called capture hysteresis
z The difference noted in thresholds when decrementing g
until loss of capture versus going from sub‐threshold to until loss of capture versus going from sub‐
capture
p
z Ellenbogen suggests the phenomena is probably explained by varying rates during gain or loss of capture l i d b i t d i i l f t and that effects may be greatest at narrow pulse durations
Ellenbogen & Woods. Cardiac Pacing &ICD’s, p 53
Output Programming
zRule of thumb
R l f h b
zDouble Double Voltage
Voltage
zTriple PW
p
Doubling Voltage
z Chronic Chronic threshold
threshold
.5ms & 2.5V so voltage 5ms & 2 5V so voltage was doubled
z Tripling PW would give an inadequate g
q
safety margin
63 . If the threshold was determined to be at point C in the above 3
p
figure; which of the following would result in an efficient and appropriate safety margin?
pp p
y
g
a. Double voltage
b. Triple the pulse width
c. Triple the voltage
d. Double the pulse width
Voltage Strength Duration Curve
g
g
best programming
O tp t Settings & Longe it Output Settings & Longevity Antiarrhythmics That Increase y
Pacing Thresholds
z Class IA
z Quinidine
z Disopyramide
z Procainamide
z Class IC
z Flecainide Flecainide –– notorious
z Propafenone
p
z Class III
z Amiodorone
z *Sotalol
*Sotalol‐‐ decreases ICD thresholds
z Prapafenone
Electrolyte Abnormalities That y
Increase Pacing Thresholds
z Acidosis
z Alkalosis
z Hyperkalemia
z Hypocalcemia
z Hyperglycemia
z Hypercarbia
yp
z Hypoxemia
Temporal Conditions That p
Increase Pacing Thresholds
z Sleeping
z Acute viral illness
z Eating
z Lead maturation
z Excessive alcohol –
Excessive alcohol – increases defibrillation increases defibrillation thresholds
thresholds1
Papaioannou GI, et al. PACE 2002; 25:1144
Factors That Decrease Pacing g
Thresholds
z Corticosteroids Corticosteroids ‐‐ dexamethasone
z Exercise
z Catecholamines
z Hypocapnia
z Sympathomimetic Sympathomimetic agents agents ‐‐ Isuprel
Isuprel, epinephrine, ephedrine, , epinephrine, ephedrine, atropine
z Hyperoxia
What’s the point?
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Ohm’s La
Ohm’s Law
What would be the calculated voltage in
a 10 mA system with 500 Ω of impedance?
V
I
R
Ohm’s La
Ohm’s Law
What would be the calculated voltage in a 10 mA system with 500 Ω of impedance?
V = I x R
10 mA x 500 Ω = 5000
5000 mV x 1 V
1000 mV
5V
Tripling Pulse Width
z Chronic threshold 2.5V & .2ms PW, PW programmed d
to .6ms providing a 2.1x safety margin.
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Ohm’s La tattoo
Ohm’s Law tattoo
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Polari ation
Polarization
Capacitance effect: positively charged ions resist ion flow
eee-
Na+
e-
Na+
N
Na+
e-
Na+
e- e
Electrode
ClCl-
Na+
Cl- Cl
Cl-
Electrode-Tissue
Interface
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Polari ation/Afterpotential
Polarization/Afterpotential
z Voltage that persists for a time after a paced impulse
z Could cause oversensing
g
z Polarization can represent 30‐
Polarization can represent 30‐40% of total pacing i
impedance
d
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Garden Hose Analog
Garden Hose Analogy
Normal Resistance = normal current flow
Low Resistance = high current flow
High Resistance = low current flow
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Fundamentals of Electronics
z Terminology
T
i l
z Current: flow of electrons through a circuit Current: flow of electrons through a circuit mA
mA
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Current Flow
e‐
P
Pacemaker
k
Conductor
Wire
Wi
Heart
e-
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Voltage: Electromotive push
e‐
Pacemaker
Conductor
Wire
Heart
Provided by The batteryy
V = IR
Oh
Ohm’s
’ L
Law
V = 55.0
0 mA x .500
500 Kohms
V = 22.5
5 Volts
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Ohm’s La
Ohm’s Law
What would be the impedance of a 2.5 volt system with a 10 mA
current drain?
R = V =
I
2.5 V
10mA
= 2.5 V
.01 A
10 mA x 1 A
1000 mA
.01 A
=
250 Ω
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F
Fundamentals of Electronics
ndamentals of Electronics
z Energy: Voltage Energy: Voltage x
x Current Current x
x Time
E = V x
E = V x I I xt
xt
Current in mA
Current in mA
Time in ms
Unit = Joule (J)
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46. Match the units: watt‐second
a. energy
b. power
c. capacitance
d d. current drain
d i
z
z
z
joule: unit of energy or work
henry: unit of inductance (induced by a magnetic field)
coulomb: unit of charge
l b it f h
34. The rate at which work is done is is:
34
a. joule unit of energy or work
b. farad basic unit of capacitance
c. power
g
d coulomb unit of charge
d.
62. In the above figure line B represents the:
a Chronaxie
a.
b. Pulse width threshold
c Knee of the curve
c.
d. Rheobase
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Fundamentals of Electronics
Low Resistance = high current flow
PW
High Resistance = low current flow
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Fundamentals of Electronics
ndamentals of Electronics
z System Impedance
z Cathode Electrode Impedance
z Anode Electrode Impedance
z Polarization Polarization Impedance
Impedance
z Tissue Impedance
z Typical range for pacing lead impedance: 300 to over 1200 Ohms
h
z Typical impedance range for a high voltage lead 10
Typical impedance range for a high voltage lead 10‐‐60 Ohms
Energy, Power, Watts & Current Drain
gy,
,
z Energy: the ability to do work; for batteries usually expressed in 1
watt‐‐hours
watt
z Energy density: the energy content of a battery or capacitor based on volume or mass. For a battery is expressed in units of watt
on volume or mass. For a battery is expressed in units of watt‐
‐hrs 1
per cubic‐‐centimeter or watt‐
per cubic
centimeter or watt‐hrs per gram
z Power: the rate of doing work or transmitting energy, a product of 2
voltage & current usually expressed in watts
z Watt: international unit of electrical power, watts are equivalent to 2
joules per second
z Watt
Watt‐‐hour: the amount of energy at a rate of 1 watt per hour, 1 hour the amount of energy at a rate of 1 watt per hour 1 2
joule = 1watt delivered for 1 second, 1 watt‐
joule = 1watt delivered for 1 second, 1 watt‐hour = 3,600 joules
z Current drain: the average amount of current drawn from a battery by the external load; typically pacemakers draw from 10 by the external load; typically pacemakers draw from 10 –
– 30 2
microamperes
1. In Ellenbogen K, et al. Clinical Cardiac Pacing, Defibrillation & CRT. 2007; pp 236
2. In Hayes &Asirvatham. Dictionary of Cardiac Pacing, Resynchronization, & Arrhythmias. 2007
C rrent I V/R
Current I = V/R
Pacemaker 4.0 V = (I) 500 ohms
P
k V (I) h
4.0 V/.500 Kohms 8.0 mA
4.0 V/.500 Kohms = 8.0 mA
ICD 600‐830 V i,e. 750 V = (I) 50 ohms
ICD 600
830 V i e 750 V = (I) 50 ohms
75 / 5
750 V/.050 Kohms = 15,000 mA
5,
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Pacemaker Longe
Pacemaker Longevity Factors
Pacemaker Longevity Factors
Longe it Factors
z Battery Capacity
z Rate of Drain
z Circuit Efficiency
z Pacing Rate
z Programmed output & Pulse Duration
z % of time pacing
% f ti i
45. The horizontal asymptote of a strength duration curve defines which of the following:
g
a. charge b. pulse width threshold
c. rheobase d. chronaxie
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