3. Myocardial StimulationMyocardial Stimulation
An artificial electrical stimulus
excites cardiac tissue by the
creation of an electrical field at the
interface of the stimulating
electrode and the myocardium
The electric field should be strong
enough and should last long
enough to initiate action potentials
in the cells at the electrode-tissue
interface
4. Myocardial StimulationMyocardial Stimulation
The AP’s at the site of stimulation
result in AP’s in the neighbouring
areas of the myocardium
resulting in a wave of AP’s
(depolarization) propagating
away from the site of stimulation
Time (Milliseconds)
100 200 300 400 500
Phase 2
Phase 1
Phase 3
Phase 4
TransmembranePotential
(Millivolts)
-50
0
50
-100
Phase0
Threshold
An artificial electrical stimulus excites
cardiac tissue by the creation of an
electrical field at the interface of the
stimulating electrode and the myocardium
The electric field should be strong enough
and should last long enough to initiate
action potentials in the cells at the
electrode-tissue interface
5. Electric Field – Current DensityElectric Field – Current Density
TheoryTheory
Electric Field Theory
– A Minimum Voltage/cmis required to trigger a
self-propagating wave of depolarization
Current Density Theory
– A Minimum Current/cm2
is required to trigger a
self-propagating wave of depolarization
Stimulation threshold is a function of Voltage/cm or
Current/ cm2
that is induced in the myocardium
beneath the stimulating electrode
The two theories are related by Ohm’s Law
6. The Implantable Pacemaker SystemThe Implantable Pacemaker System
The Implantable Pulse Generator (IPG) : metal can
(titanium) containing electronics/battery & an electrode or
lead connector header
Lead : Electrical connection between the pacemaker & the
heart
7. The Pacemaker CircuitThe Pacemaker Circuit
Stimulation of cardiac tissue using electric pulses
– IPG
– Lead – insulated current conductor (s) &
electrodes to transmit pulses to heart tissue
and measure or sense electrical activity in the
heart
Heart
Lead
IPG
Completion of circuit
through
Lead or Body tissue
8. A Unipolar Pacing System Contains a Lead withA Unipolar Pacing System Contains a Lead with
Only One Electrode Within the Heart; In ThisOnly One Electrode Within the Heart; In This
System, the Impulse:System, the Impulse:
Flows through the tip
electrode (cathode)
Stimulates the heart
Returns through
body fluid and tissue
to the IPG (anode)
Cathode
Anode
-
9. Anode
Flows through the
tip electrode located
at the end of the
lead wire
Stimulates the heart
Returns to the ring
electrode above the
lead tip
A Bipolar Pacing System Contains a Lead with TwoA Bipolar Pacing System Contains a Lead with Two
Electrodes Within the Heart. In This System, the Impulse:Electrodes Within the Heart. In This System, the Impulse:
Cathode
Tip electrode coil
Indifferent electrode
coil
15. One lead implanted
in the atrium and
one in the ventricle
Dual-Chamber Systems Have Two Leads:Dual-Chamber Systems Have Two Leads:
16. A pacing system can be thought of a standard electrical
circuit:
The pacemaker supplies the voltage.
Current (electrons) flow down the
conductor to the lead tip or cathode (-)
Where the lead tip touches the
myocardium, electrical resistance is
produced.
The current then flows through the
body tissues to the anode (+) and back
to the battery.
All of these things are required for
current to flow.
19. VoltageVoltage
Voltage is the force or “push” that causes
electrons to move through a circuit
In a pacing system, voltage is:
– Measured in volts
– Represented by the letter “V”
– Provided by the pacemaker battery
– Often referred to as amplitude
21. CurrentCurrent
The flow of electrons in a completed circuit
In a pacing system, current is:
– Measured in mA (milliamps)
– Represented by the letter “I”
– Determined by the amount of electrons that
move through a circuit
23. ImpedanceImpedance
The opposition to current flow in a circuit
In a pacing system, impedance is:
– Measured in ohms
– Represented by the letter “R” (Ω for
numerical values)
– The measurement of the sum of all
resistance to the flow of current
24. Ohm’s LawOhm’s Law
Voltage = Current X Resistance
Current = Voltage ÷Resistance
Resistance = Voltage ÷ Current
25. The Pacemaker StimulusThe Pacemaker Stimulus
Time
5 Volts 5 Volts
0.5 ms
1 sec
Pacing Stimulus Voltage or Amplitude – 5 Volts
Pulse Width – 0.0005 seconds or 0.5 milliseconds
Pacing Rate – One stimulus per second or 60 stimuli (beats)
per minute
Voltage
26. The Pacing PulseThe Pacing Pulse
tt
Pacing Pulse
Pulse Duration (Width)
OutputVoltage
V = Pulse Amplitude in Volts (V) (say 2.5 V)
t = Pulse Duration or Width in milliseconds (ms)
(say 0.5 ms)
R = Impedance of Pacing Circuit (ohms)
(say 500 ohms)
I = V/R = Current through pacing circuit (mA)
= 2.5 V/ 500 ohms = 0.005 A = 5 mA
E = Energy supplied by Pulse to the Pacing Circuit
and Cardiac Tissue = V . I . t = I2
Rt = V2
t/R
= 2.5 V . 5 mA . 0.5 ms = 6.25 micro Joules
V
t
27. Stimulation ThresholdStimulation Threshold
Pacing Voltage Threshold – The minimum
pacing voltage at any given pulse width
required to consistently achieve myocardial
depolarization outside the heart’s refractory
period
29. The Strength-Duration RelationThe Strength-Duration Relation
The intensity of an electrical stimulus (Energy,
Voltage, Current, Charge) required to capture
(non-refractory) cardiac tissue is dependent on
the duration for which the electrical stimulus is
applied (i.e. pulse width)
30. The Voltage-Strength DurationThe Voltage-Strength Duration
CurveCurve
Stimulus Voltage & Pulse Width
have an exponential relationship
At short pulse widths (<0.25 ms)the
curve rises sharply (i.e. small
reductions in pulse width result in
large increases in the voltage
threshold)
At long pulse widths (>1.0 ms)the
curve is flat (i.e. the voltage
threshold does not reduce with
increasing pulse width) Duration
Pulse Width (ms)
.50
1.0
1.5
2.0
.25StimulationThreshold(Volts)
0.25 1.0 1.5
Capture
31. Rheobase & ChronaxieRheobase & Chronaxie
Rheobase Voltage = Voltage
Threshold at infinite Pulse widths
(e.g. 2 ms) – also known as
fundamental threshold
Chronaxie Point = Pulse Width
threshold at twice the rheobase
voltage (i.e. 1 V & 0.3 ms)
Energy of Pacing Pulse
At rheobase = 0.5V. 2ms. 1mA = 1
MicroJoule
At Chronaxie = 1V. 0.3 ms. 2mA
= 0.6 MicroJoules
At PW=0.25 ms, = 1.5V. 0.25ms.
3mA = 1.125 MicroJoules
The chronaxie point approximates the point of minimum threshold energy on the Strength-Duration Curve
Duration
Pulse Width (ms)
StimulationThreshold(Volts)
0.2 0.6 1.0
Energy Strength Duration
Curve
Capture
0.4
0.5
1.0
2.0
0.8 1.2 1.4
Rheobase Voltage
Chronaxie Point
32. Goals of Pacemaker OutputGoals of Pacemaker Output
ProgrammingProgramming
Consistent capture & Patient Safety is ensured
Battery drain minimized, Pacemaker longevity
maximized
33. Programming Chronic PacemakerProgramming Chronic Pacemaker
OutputOutput
Duration
Pulse Width (ms)
StimulationThreshold(Volts)
0.2 0.6 1.0
Energy Strength Duration Curve
• Capture
Adequate Safety Margin while minimizing
Pulse Energy
Operate around the chronaxie point
Programming long pulse widths (& low
Voltages) increases Pulse energy but
hardly increases safety margin
Programming high voltages (with short
Pulse widths) increases Pulse energy but
hardly increases safety margin
For a 2 times safety margin :
Output = 2 x chronaxie voltage at
chronaxie pulse width (2V, 0.35 ms)
Output = 3 x pulse duration threshold
at twice chronaxie voltage (2V, 0.51 ms)
0.4
0.5
1.0
2.0
0.8 1.2 1.4
Rheobase
Chronaxie
34. Determination of StimulationDetermination of Stimulation
Threshold during Implant or Follow-upThreshold during Implant or Follow-up
Pace the heart at
– Rate higher than sinus or intrinsic rate
– Stimulus amplitude and pulse width that ensure
capture (usually 5 V and 0.5 ms)
Gradually reduce stimulus amplitude while maintaining
pulse width constant till capture is lost
The Stimulation threshold is specified by the minimum
stimulus amplitude at which capture consistently occurs
at a given stimulus pulse width
35. Programming Pacemaker OutputProgramming Pacemaker Output
Acute – Immediately post-implant
Chronic – Approx. 6 to 8 weeks post implant
Goal : To ensure consistent capture despite potential
changes in the SD curve while minimizing the energy
delivered by the pulse
36. Evolution of Pacing ThresholdEvolution of Pacing ThresholdVoltage
Threshold(V)
Observation Time (weeks)
Acute Phase
Chronic Phase
Safety Margin
6
5
4
3
2
1
0 4 8 12 16
38. Pacing Impedance - ConsiderationsPacing Impedance - Considerations
Maximize Pacemaker Longevity
– Reduce current drain
– Maintain relatively high impedance
Ensure consistent capture
– Appropriate voltage & current are available
at the electrode-tissue interface required to
stimulate tissue
39. I pacing
Components of Pacing ImpedanceComponents of Pacing Impedance
Rcoil
Zpolarisation -electrode tissue
interface
Rtissue
Electrode
Tissue
Interface
Pulse
Generator
V pulse
I pacing
40. RRcoil -coil - Lead Conductor ResistanceLead Conductor Resistance
– Lead Conductor
Impedance
• Reduces the voltage &
energy available at the
tissue for pacing
• Generates waste heat
• Designed to be low
conductor
resistance
41. Conductor ResistanceConductor Resistance
Total Coil Voltage Voltage
impedance resistance at tip loss
(Ω)(Ω) (V) %
600 50 2.3 8.3
750 200 1.8 26.7
1200 650 1.1 54.2
1200 50 2.4 4.2
Output Voltage = 2.5 V
Impact of conductor coil resistance upon
available tip electrode voltage
42. Conductor Resistance
V mV
2.5 2500
Total imp 600
Current I =V/R 4.166666667 mAConductor
imp 50
Voltage drop V = IR 208.3333333 mV
0.208333333 V
43. What is the Polarization Effect – PolarizationWhat is the Polarization Effect – Polarization
Impedance ?Impedance ?
As the pacing pulse begins, electrons from the pacemaker battery flow to the lead
tip and positively charged ions from the tissue are attracted to the lead tip.
Initially, the movement of these negatively
charged ions results in the flow of current
from the electrode into the myocardium.
As the pacing pulse continues, positively charged ions surround the electrode tip. This
produces a positively charged layer on the electrode called polarization. Polarization can
impede current flow from the electrode into the tissue.
44. Polarization Impedance &Polarization Impedance &
Electrode Surface AreaElectrode Surface Area
As electrode surface area goes up
– Polarization impedance decreases
As electrode surface area goes down
– Polarization impedance increases
46. Factors affecting PolarizationFactors affecting Polarization
ImpedanceImpedance
Pulse Width
Surface area of the electrode tip
– Larger the surface area – Lower the
Polarization
47. Electrode – Tissue InterfaceElectrode – Tissue Interface
ImpedanceImpedance
Resistance to current flow from body tissue at the
electrode-tissue interface
48. Electrode – Tissue InterfaceElectrode – Tissue Interface
ImpedanceImpedance
Smaller the surface area of the tip electrode, the higher
the current density (or electric field strength) at the
electrode-tissue interface
The higher the field strength, the greater the pacing
efficiency & lower the threshold
49. Electrode – Tissue InterfaceElectrode – Tissue Interface
ImpedanceImpedance
Smaller the geometric size of the tip electrode, the
higher the resistance between electrode and tissue
PacingImpedance(Ohms)
0
500
1000
1500
0 1 2 3 4 5.5 6
Geometric Tip Electrode Surface Area (mm2
)
Size = Impedance
50. The Ideal Stimulating ElectrodeThe Ideal Stimulating Electrode
Needs large surface area to reduce Polarization
Impedance
Needs small size to maximize Electric Field
Strength and Stimulation Efficacy
Needs small size to maximize Electrode-tissue
resistance and minimize pacemaker current
drain
59. Factors that affect StimulationFactors that affect Stimulation
ThresholdThreshold
Eating, Sleeping, Exercise, Medications, Changes in Cardiac condition –
30 to 50 % during the day
Drugs e.g. Steroids reduce stimulation threshold by reducing
inflammation
Sympathomimetic drugs decrease threshold
Amiodarone, Class I A (quinidine, procainamide), Class I B (Mexilitene)
Hypokalemia increases threshold (diuretics)
Hypocalcemia increases threshold
Hypoxia & Hypercapnia – Increase threshold
Electrolyte imbalance & pH e.g. acidosis and alkalosis increase threshold
60. Typical Pacing CircuitTypical Pacing Circuit
ImpedancesImpedances
300 to 1200 ohms
May be measured at implant time with a
Pacing System Analyzer (PSA) at implant time
May be measured through telemetry with a
pacemaker programmer
61. Pacing Impedance Values Will Change DuePacing Impedance Values Will Change Due
to:to:
Insulation breaks
Wire fractures
62. An Insulation Break Around the Lead WireAn Insulation Break Around the Lead Wire
Can Cause Impedance Values to FallCan Cause Impedance Values to Fall
Insulation breaks expose the
wire to body fluids which
have a low resistance and
cause impedance values to fall
Current drains through the
insulation break into the body
which depletes the battery
An insulation break can cause
impedance values to fall
below 300 Ω
Insulation break
Decreased resistance
63. A Wire Fracture Within the Insulating SheathA Wire Fracture Within the Insulating Sheath
May Cause Impedance Values to RiseMay Cause Impedance Values to Rise
Impedance values
across a break in the
wire will increase
Current flow may be
too low to be effective
Impedance values
may exceed 3,000 Ω
Lead wire fracture
Increased resistance
64. Unipolar &Bipolar StimulationUnipolar &Bipolar Stimulation
+
-
+ -
-
+
Anode is IPG
Case
Cathode is lead tip
electrode
-
+
Anode is lead
proximal
electrode
Cathode is lead tip
electrode
65. Unipolar & Bipolar StimulationUnipolar & Bipolar Stimulation
Unipolar Stimulation– Large Stimulus Artifact
Bipolar Stimulation – Small Stimulus Artifact
There is a greater chance of pacemaker pocket
muscle stimulation with Unipolar Stimulation
66. The Pacemaker BatteryThe Pacemaker Battery
Pacemakers typically use Lithium-Iodide batteries
Most LiI batteries have a Beginning of Life (BOL) value
of 2.8 volts
Recommended Replacement Time (ERI/RRT) value of
2.6 V
End of Service (EOL/EOS) value of 2.5 V
As battery depletes its internal resistance goes up – (BOL
= 100 ohms , EOL > 5000 ohms )
67. Recommended Replacement Time
RRT (ERI)
2.8V-
2.6V-
2.5V-
BOL
RRT 2.6V
EOS 2.5V
Time
Lithium Iodine Battery depletion
BOL – Beginning of Life
RRT – Recommended Replacement Time
EOS - End of Service
68. Battery LifeBattery Life
Battery Life
– Battery Life = Battery Capacity/Current Drain
– 2.0 Ah/25microamps = 80,000 hours = 9.3 years
Battery properties
– Reliability – no premature failure
– High volumetric energy density – Small battery
volume with high storage capacity
– Low self-discharge rate
– High hermiticity – no gas generation during
operation
69. Pacemaker LongevityPacemaker Longevity
Energy = (V.I.t) = (V2
/R) . t Joules
Reducing V by a factor of 2 reduces E by a
factor of 4
Increasing Z by a factor by 2 reduces E by a
factor of 2
70. Pacemaker LongevityPacemaker Longevity
High Outputs and pulse widths are the primary
cause for reduced pacemaker longevity
• Output Voltages higher than the Battery Voltage (2.8 V)
require the use of voltage doublers that use high battery
energy
• Long Pulse Widths Reduce pacing efficiency due to
increased polarization impedance
72. A Pacemaker Must Be Able to Sense
and Respond to Cardiac Rhythms
Accurate sensing enables the pacemaker to determine
whether or not the heart has created a beat on its own
The pacemaker is usually programmed to respond with
a pacing impulse only when the heart fails to produce
an intrinsic beat
73. Intracardiac Electrical SignalsIntracardiac Electrical Signals
Electrical currents that arise in the myocardium during
depolarization and repolarization
A myocardial electrode - records voltage difference
wrt reference electrode when the myocardium under
the electrode undergoes depolarization or
repolarization
The electrical activity measured by such an electrode
(which is in direct contact with cardiac tissue) – local
tissue electrical activity- Intracardiac Electrogram or
EGM
75. Depolarization WaveDepolarization Wave
Processed byProcessed by
DeviceDevice
Processed byProcessed by
DeviceDevice
The EGM Signal
The signal from a depolarization wave passing between two electrodes
76. Intracardiac ElectrogramIntracardiac Electrogram
R wave of the EGM indicating
depolarisation of cardiac
ventricular tissue at lead tip –
Roughly corresponds to R
wave of the ECG that
represents depolarisation of the
ventricles
T wave of the EGM indicating
repolarisation of cardiac
ventricular tissue at lead tip –
Roughly corresponds to T wave
of the ECG that represents
repolarisation of the ventricles
77. Voltage Deflections of the SensedVoltage Deflections of the Sensed
EGM in a PacemakerEGM in a Pacemaker
Pacemaker
Stimulus
Paced R wave
Post-pace T wave
Intrinsic R wave
T wave corresponding to
intrinsic R wave
78. Undersensing . . .
Pacemaker does not “see” the intrinsic beat, and therefore
does not respond appropriately
Intrinsic beat
not sensed
Scheduled pace
delivered
VVI / 60
79. Oversensing
An electrical signal other than the intended P or R wave
is detected
Marker channel
shows intrinsic
activity...
...though no
activity is
present
VVI / 60
80. Measured by:
– Amplitude
• Peak-to-peak measurement (height)
of deflection
• Measured in Millivolts (mV)
– Slew Rate
• Speed of deflection change over time
• Measured in volts per second (V/s)
EGM amplitude
81. EGM Amplitude & Slew RateEGM Amplitude & Slew Rate
Amplitude of the EGM R wave = dV
Slew Rate of the EGM R wave= dV/dt
85. Effect of Filtering on EGMEffect of Filtering on EGM
FILTER
•Filters out frequencies below 5 Hz (T waves) and above 50 Hz
(myopotentials)
•Enhances frequencies between 10 Hz and 50 Hz (R waves) with maximum
enhancement at around 30 Hz frequency
• Low Slew Rate implies Low Frequency implies Less enhancement due to
filtering
Level
Detector
86. Level Detector or SensitivityLevel Detector or Sensitivity
SettingSetting
This is a value specified to the pacemaker in millivolts through programming.
All ventricular EGM deflections AFTER FILTERING that exceed the sensitivity
setting will be identified by the pacemaker as intrinsic R waves
The typical sensitivity setting that is programmed for Ventricular sensing is 2.5 mV
89. Sensitivity SettingSensitivity Setting
Sensitivity settings less than 2.5 mv – High sensitivity – can lead
to oversensing
Sensitivity settings greater than 2.5 mV – Low sensitivity – can
lead to undersensing
Amplitude(mV)
Amplitude(mV)
Time Time
5.0
2.5
1.25
5.0
2.5
1.25
90. Factors That May Affect Sensing Are:
Lead polarity (unipolar vs. bipolar)
Lead integrity
– Insulation break
– Wire fracture
EMI – Electromagnetic Interference
91. Unipolar Sensing
Produces a large
potential difference due
to:
– A cathode and
anode that are
farther apart than
in a bipolar
system
_
92. Bipolar Sensing
Produces a smaller potential
difference due to the short
interelectrode distance
– Electrical signals from
outside the heart such
as myopotentials are
less likely to be sensed
93. An Insulation Break May Cause Both
Undersensing or Oversensing
Undersensing occurs when inner and outer conductor coils are in
continuous contact
– Signals from intrinsic beats are reduced at the sense
amplifier and amplitude no longer meets the programmed
sensing value
–
Oversensing occurs when inner and outer conductor coils make
intermittent contact
– Signals are incorrectly interpreted as P or R waves
94. Wire Fracture Can Cause Both Undersensing and
Oversensing
Undersensing occurs when the cardiac signal is unable to get back to the
pacemaker – intrinsic signals cannot cross the wire fracture
Oversensing occurs when the severed ends of the wire intermittently make
contact, which creates potentials interpreted by the pacemaker as P or R waves
Fracture in one filament leads to an
increase in resistance
95. Electromagnetic Interference
Interference is caused by electromagnetic energy with a source that is
outside the body
Electromagnetic fields that may affect pacemakers are radio-frequency
waves
– 50-60 Hz are most frequently associated with
pacemaker interference
Few sources of EMI are found in the home or office but several exist in
hospitals
96. Oversensing May Occur When EMI Signals Are
Incorrectly Interpreted as P Waves or R Waves
Pacing rates will vary as a result of EMI:
– Rates will accelerate if sensed as P waves in
dual-chamber systems (P waves are “tracked”)
– Rates will be low or inhibited if sensed in single-
chamber systems, or on ventricular lead in
dual-chamber systems
97. Electrocautery is the Most Common Hospital
Source of Pacemaker EMI
Outcomes
– Oversensing–inhibition
– Undersensing (noise
reversion)
– Power on Reset
– Permanent loss of
pacemaker output
(if battery
voltage is low)
Precautions
– Reprogram mode to
VOO/DOO, or place a
magnet over device
– Strategically place the
grounding plate
– Limit electrocautery
bursts to 1-second burst
every 10 seconds
– Use bipolar
electrocautery forceps
98. Transthoracic Defibrillation
Outcome
– Inappropriate reprogramming
of the pulse
generator (POR)
– Damage to
pacemaker circuitry
Precautions
– Position defibrillation paddles
apex-posterior (AP) and as far
from the pacemaker and leads
as possible
99. Magnetic Resonance Imaging (MRI) is Generally
Contraindicated in Patients with Pacemakers
Outcomes
– Extremely high
pacing rate
– Reversion to
asynchronous pacing
Precautions
– Program pacemaker
output low enough to
create persistent
non-capture, ODO
or OVO mode
100. Radiation Energy May Cause Permanent
Damage
Certain kinds of radiation energy may cause damage to the semi-
conductor circuitry
– Ionizing radiation used for breast or
lung cancer therapy
Damage can be permanent and requires
replacement of the pacemaker
101. Therapeutic Radiation May Cause Severe
Damage
Outcomes:
– Pacemaker circuit damage
– Loss of output
– “Runaway”
Precautions:
– Keep cumulative radiation
absorbed by the pacemaker
to less than 500 rads;
shielding may be required
– Check pacemaker after
radiation sessions for changes
in pacemaker function (can
be done transtelephonically)
103. Voltage Deflections of the SensedVoltage Deflections of the Sensed
EGM in a PacemakerEGM in a Pacemaker
Pacemaker
Stimulus
Paced R wave
Post-pace T wave
Intrinsic R wave
T wave corresponding to
intrinsic R wave
2.5 mV
105. Refractory PeriodRefractory Period
A programmable period immediately following a
pacemaker stimulus or a sensed intrinsic R wave
during which the pacemaker does not react to sensed
events
To prevent repeated sensing of the same intrinsic R
wave
To prevent misidentification of T waves as intrinsic R
waves
To prevent misidentification of effects of pacemaker
stimulus/evoked R wave
Usually programmed to 325 ms
To Prevent Oversensing
106. Afterpotential due to PolarizationAfterpotential due to Polarization
Afterpotential
107. Blanking PeriodBlanking Period
The first portion of every refractory period
Pacemaker is “blind” to any activity and no events can
be sensed
Designed to prevent oversensing of pacing stimulus &
after-potential
Blanking Period
Refractory Period
108. ConductorTip Electrode Insulation Connector Pin
Pacing Lead ComponentsPacing Lead Components
Conductor
Connector Pin
Insulation
Electrode
109. ConnectorConnector
Purpose
– Connects lead to IPG, and provides a conduit to:
• Deliver current from IPG to lead
• Return sensed cardiac signals to IPG
Connector
110. Connector -- IS-1 StandardConnector -- IS-1 Standard
IS-1 Standard Connectors
Sizes Prior to IS-1 Standard
– 3.2 mm low-profile connectors
– 5/6 mm connectors
116. Battery CapacityBattery Capacity
A battery is a reservoir of electrical charge measured in
Coulombs
Current is the amount of charge delivered per unit time
– 1 Ampere = 1 Coulomb per second
– 1 Coulomb = 1 Ampere x 1 second
Qc = Battery Capacity is specified as the quantity of
charge it can deliver in AmpereHours (0.5 to 3
Amperehours)
117. Battery LifeBattery Life
Battery Life
– Battery Life = Battery Capacity/Current Drain
– 2.0 Ah/25microamps = 80,000 hours = 9.3 years
Battery properties
– Reliability – no premature failure
– High volumetric energy density – Small battery
volume with high storage capacity
– Low self-discharge rate
– High hermiticity – no gas generation during
operation
118. On the figure, the zone of non capture is
indicated by which number?
a)1
b)2
c)3
d)4
119. Which of the following output settings best
represents the Chronaxie point on the strength-
duration curve when the Rheobase is 0.5V @
1.5ms?
a) 0.5V @ 1.5ms
b) 1.0V @ 0.5ms
c) 1.5V @ 0.1 ms
d) 2.0V @ 0.05ms
120. Sensing
Sensing is the ability of the pacemaker to “see” when a natural (intrinsic)
depolarization is occurring
– Pacemakers sense cardiac depolarization by measuring
changes in electrical potential of myocardial cells between the
anode and cathode
Intrinsic deflection on an EGM
occurs when a depolarization wave
passes directly under the
electrodes
Two characteristics of the EGM
are:
– Signal amplitude
– Slew rate
Editor's Notes
Permanent pacing leads have five major components:
1. The conductor(s)
2. The connector pin
3. The insulation
4. The electrode(s)
5. The Fixation mechanism
Each of these components has critical design considerations
We already discussed the conductors and insulation. Now we will address also connector, electrode and fixation mechanism.
Passive fixation in relation to endocardial leads means that no part of the lead itself is actually embedded in the endocardium. Rather, the lead tip is trapped within the trabeculae and/or is held in position by its pre-formed shape (e.g., J-lead in atrium).
Passive fixation leads commonly use tines or fins to &quot;catch&quot; trabeculi in the heart, or the lead is canted or curved to help place and hold the lead tip in a certain position.
Passive anchoring devices, such as flanges or wedge tips, cages and balloons were early attempts to solve the dislodgment problems of early pacing leads. However, only after tined leads were introduced by Medtronic in 1976 did a true solution to dislodgment emerge with regard to passive fixation.
Today, there is approximately a 1-2% dislodgment rate 1, 2, 3, 4, 5, 6 with passive fixation leads:
1 - 2% in the atria,1, 2, 6
1 - 2% in the ventricle.1, 3, 4, 5
References:
1 Gammage MD, Marshall HJ, Harris JI. Five-year experience with polyurethane, passive fixation, steroid-eluting leads. PACE. 1998;(Pt II):842. Abstract.
2 Hua W, Mond HG, Strathmore N. Chronic steroid-eluting lead performance: a comparison of atrial and ventricular pacing. Pacing Clinical Electrophysiology. 1995;20(1 Pt 1):17-24.
3 Kazama S, Nishiyama K, Machii M, Tanaka K, Amano T, Nomura T, Ohuchi M, Kasahara S, Nie M, Ishihara A. Long-term follow-up of ventricular endocardial pacing leads. Jpn Heart J. 1993;34(2):193-200.
4 Mayer DA, Tsapogas MJ. Pacemakers: dual or single chamber implantation. Vasc Surg. 1992;26(5):400-7.
5 Miller GB, Leman RB, Kratz JM, Gillette PC. Comparison of lead dislodgment and pocket infection rates after pacemaker implantation in the operating room versus the catheterization laboratory. Am Heart J. 1998;115(5):1048-51.
6 Mond HG, Hua W, Wang CC. Atrial pacing leads: the clinical contribution of steroid elution. Pacing Clinical Electrophysiology. 1995;18(9 Pt 1):1601-8.
7 Glikson M, von Feldt LK, Suman VJ, Hayes DL. Short- and long-term results with an active fixation, bipolar, polyurethane-insulated atrial pacing lead. Pacing Clinical Electrophysiology. 1996;19(10):1469-73.
8 Stirbys P. Implantation of double screw-in leads. Pacing Clinical Electrophysiology. 1988;11(10):1482-4.
Active fixation means that part of the lead actually embeds in the heart tissue for fixation via ascrew-in helix electrode. Extendible/retractable and fixed screw mechanisms are the most common active fixation methods.
Fixed Screw leads provide excellent stability. The lead body must be turned in a counter-clockwise rotation during insertion of the lead. Sense mapping can be done prior to fixing the screw to the myocardium. The lead body is then turned in a clockwise rotation to fix the screw to the myocardium.
Extendible/Retractable screw-in leads provide excellent stability. The screw is retracted to prevent damage to the veins and cardiac structures during lead advancement. The ability to retract the screw makes entanglement in cardiac structures less likely. The screw is retracted during transvenous introduction and during sense mapping and extended for lead fixation. Care must be taken not to over-extend or over-retract the screw with this type of screw mechanism.
Today, there is approximately a 1 - 4% dislodgment rate 6, 7, 8 with active fixation leads:
1 - 4% in the atria 6, 7, 8
&lt;1% in the ventricle.8
References:
6 Mond HG, Hua W, Wang CC. Atrial pacing leads: the clinical contribution of steroid elution. Pacing Clinical Electrophysiology. 1995;18(9 Pt 1):1601-8.
7 Glikson M, von Feldt LK, Suman VJ, Hayes DL. Short- and long-term results with an active fixation, bipolar, polyurethane-insulated atrial pacing lead. Pacing Clinical Electrophysiology. 1996;19(10):1469-73.
8 Stirbys P. Implantation of double screw-in leads. Pacing Clinical Electrophysiology. 1988;11(10):1482-4.
Myocardial/Epicardial leads require an open-chest, tunneling, or minimally invasively sub-xyphoid to attach the lead tip to the outside of the heart. They are used in special situations, such as:
- pediatric applications,
- congenital malformations,
- when a mechanical heart valve is present,
- where multiple abandoned endocardial leads are present,
- or concurrent with open-chest surgery.
Stab-in and screw-in leads are considered myocardial leads because they are attached to the outside of the heart and the “hook” electrode and the screw electrode actually pass into the myocardium when fixated.
Suture-on leads are considered epicardial leads because the electrode is attached to the outside of the heart and does not penetrate the myocardium.
Speakers Notes
What to say:
In a basic electrical circuit the voltage drops around the circuit at each resistance. Assuming the heart/electrode interface is the only significant impedance, all voltage should be “spent” there
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The terms “amplitude” and “voltage” are often used interchangeably, undoubtedly to express “Voltage Amplitude” which refers to the voltage output.
Current:
Measured in amperes (I)
1 Ampere =1000 milliamps
Movement of electricity or free electrons through a circuit
One ampere is a unit of electrical current produced by 1 volt acting through a resistance of 1 ohm
Impedance is the sum of all resistance to the flow of current. The resistive factors to a pacing system include:
Lead conductor resistance
The resistance to current flow from the electrode to the myocardium
Polarization impedance, which is the accumulation of charges of opposite polarity in the myocardium at the electrode-tissue interface.
Resistance is a term used to refer to simple electric circuits without capacitors and with constant voltage and current. Impedance is a term used to describe more complex circuits with capacitors and with varying voltage and current. Therefore, the use of the term impedance is more appropriate than resistance when discussing pacing circuits.
Possible Causes
Increased threshold
QRS complex not visible
Faulty cable connection
Dislodged/fractured lead
Tissue is refractory
Battery depletion
Corrective Measures
Increased output (mA)
Change EKG lead or gain
Check cable connections
Reposition/replace lead
Switch polarity
Change battery or pulse generator
Turn patient
Pacing impedance refers to the opposition to current flow. Three sources contribute to pacing impedance:
1. pacing lead conductor coil
2. electrode-tissue interface
3. tissue
Impedance at the electrode-tissue interface derives from two factors: electrode resistance and electrode capacitance. The latter is due to polarization.
Increasing the value of any of these three components will decrease lead current and enhance battery longevity. But, the safety and efficiency of increasing each component must also be considered.
The effect of increasing impedance of each component will be examined.
The table shows that increasing the conductor coil resistance reduces the amount of the output voltage that is available at the tip electrode for myocardial stimulation.
When the conductor coil resistance is low, more of the pacemaker’s output voltage (and current) is made available at the cathode for myocardial stimulation.
Not only does the voltage at the tip electrode change as a result of increased coil impedance, but the safety factor also changes.
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A pacing pulse may be thought of as negative ions moving down the lead. These negative ions attract positive ions to the lead tip.
Initially, the electrical current (negative ions) move freely into the myocardium.
As the pacing pulse continues for the programmed pulse duration (pulse width), positively charged ions surround the electrode tip, forming a layer called polarization. This layer may impede the movement of negative ions into the myocardium, and affect the ability of the pacemaker to sense the evoked response.
A smaller geometric size can increase impedance but, as we have seen, it can also increase polarization. By altering the surface structure to be more porous you increase the geometric surface area of the electrode and therefore lower the polarization.
3The platinized porus electrodes have a larger functional surface area. The collective spaces within the electrode pores make up the larger functional surface area, which controls polarization for this geometrically smaller electrode.
In summary, smaller geometric size, larger functional surface area.
We can compare the pacemaker with this system formed by a reservoir with water (the pacemaker’s battery), a pipe (the lead) and a waterwheel which is rotated by the water pressure (the heart muscle captured by the pacing pulse). Notice on the picture the waterwheel turns but a significant amount of water is wasted, (a lot of current depleted from the battery that is not used).
To reduce the water flow (the current stream from the battery), we can narrow the pipeline (increasing the resistance of the entire lead), but this will significantly decrease the water pressure at the waterwheel (the voltage applied directly on the heart muscle).
Finally, the best option is to narrow the conductor only at the end of the pipeline (increasing the impedance to the tip electrode only, and maintaining a small impedance on the lead conductor)
The result in our analogy is to have good water pressure at the waterwheel but also reduce the water flow from the reservoir.
Therefore a high impedance for the tip electrode, and a low impedance for the lead conductor ensures a low the current drain from the pacemaker’s battery.
Steroid benefits begin immediately upon implantation.
Fewer and less active inflammatory cells congregate at the electrode-tissue site.
As a result of the decreased inflammatory response, there is less fibrotic development around the electrode. The intrinsic conductivity of the myocardial cells that surround the electrode appears to be maintained.
A pacing lead is a foreign body that can produce an inflammatory response.
Steroid-eluting electrodes provide a continuous elution of minute doses of dexamethasone sodium phosphate from a silicone rubber binder.
Steroid elution limits the inflammatory process at the electrode-tissue interface.
Steroid in matrix:
This approach provides long-term steroid elution by providing steroid in a silicone rubber plug.
3 Medtronic uses this method.
Steroid coated:
Another approach to steroid elution is to coat the electrode. This approach provides an acute benefit, but is not yet proven for chronic performance.
3 Medtronic uses this method in conjunction with the steroid plug.
This graph compares the stimulation thresholds of contemporary pacing leads. Older electrodes exhibited higher threshold peaking than that of steroid leads shown on the slide.
The different types of electrodes exhibit a wide range of threshold peaking.
Steroid-eluting electrodes continue to show lower chronic stimulation thresholds and no significant peaking.
Threshold changes are shown here over a 12-week period post-implant, where a comparison is made between:
• smooth metal electrode
• textured metal electrode
• steroid-eluting electrode
Traditionally, implant stimulation thresholds are relatively low.
Non-steroid-eluting electrodes exhibit a peaking phase from week 1 to approximately week 6, due to the maturation process at the electrode-tissue interface.
Steroid-eluting electrodes exhibit virtually no peaking.
The chronic phase of stimulation threshold occurs 8-12 weeks post-implant which is characterized by a plateau. This plateau is higher than the acute phase, due to fibrotic encapsulation of the electrode. Steroid-eluting lead chronic thresholds remain close to implant values.
Insulation around the lead wire prevents current loss from the lead wire.
Electrical current seeks the path of least resistance.
An insulation break that exposes wire to body fluids which have low resistance causes:
Lead impedance to fall
Current to drain into the body
Battery depletion
Impedance values below 300
Insulation breaks are often marked by a trend of falling impedance values.
An impedance reading that changes suddenly or one that is &gt;30% is considered significant and should be watched closely.
Insulation may remain intact but the wire may break within the insulating sheath.
Impedance may exceed 3,000
Current flow may be too low to be effective.
If a complete fracture of the wire occurs:
No current will flow
Impedance number will be “infinite”
When suspecting a wire break, look for a trend in an increase in impedance values rather than a single lead impedance value.
Speakers Notes
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When the heart functions normally, there is no need for the pacemaker to deliver artificial pacing impulses.
A pacemaker must be able to sense and respond to normal and abnormal cardiac rhythms
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As the heart muscle depolarizes, a potential difference (measured in millivolts) is created between the tip electrode (cathode or negative electrode) and the ring in a bipolar system, or the coil in an integrated bipolar system (anode or positive electrode). It is this net signal that goes to the device for processing.
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Oversensing will exhibit pauses in single chamber systems. In dual chamber systems, atrial oversensing may cause fast ventricular pacing without P waves preceding the paced ventricular events.
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The intracardiac EGM is characterized in clinical practice in terms of its amplitude (measured in millivolts), and slew rate (measured in volts per second). We will expand on these in the next few slides.
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Typically, slew rate measurements at implant should exceed .5 volts per second for P waves; .75 volts per second for R wave measurements.
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Different signal types occur in different frequency bands. Pacemaker sense amplifiers will attempt to filter out signal that are “non-physiologic”. P waves, R waves and PVC occur in the 5 to 60Hz range.
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Pacemakers have programmable sensitivity settings that can be thought of like a fence: with a lower fence more of the signal is seen; with a higher fence less of the signal is seen.
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If the system is sensing myopotentials, then raise the fence or increase the number of the sensitivity setting. The pacemaker will &quot;see less&quot; of the incoming signal.
If the pacing system is not “seeing” intrinsic cardiac events, set the fence lower or decrease the number of the sensitivity setting. The pacemaker will then &quot;see” more of the incoming signal.
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Unipolar sensing produces a large potential difference due to a cathode and anode that are farther apart than a bipolar system. Because both electrodes may contribute to the electrical signal that is sensed, the unipolar electrode configuration may detect electrical signals that occur near the pacemaker pocket as well as those inside the heart.
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Produces a small potential difference:
Electrodes are close to one another
Intracardiac signal arrives at each electrode at almost the same time
Less likely to sense:
Myopotentials: depolarization of muscles near the heart or the anode (the wave produced after the action potential wave passes along a nerve).
Afterpotentials
Far-field intracardiac signals
Noise
EMI
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When two metals make intermittent contact, a small voltage is produced. This can be inappropriately sensed by the pacemaker. An inner insulation break may allow the two conductors to touch.
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Lead fractures that are intermittent are also referred to as “make and break” fractures, due to the artifacts in the electrogram that are produced as the conductor wires make and break contact.
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Electrosurgery used within six inches of an implanted pacemaker/lead system has the potential to cause permanent loss of pacemaker output. Earlier designed pacemakers are more susceptible to loss of output as the battery voltage decreases.
Precautions:
Monitor the patient’s pulse during application of the cautery.
Program the pacemaker to VOO/DOO if the patient is pacemaker dependent, or secure a magnet over the device.
Place the grounding plate as close to the operative site as possible—usually under buttocks or thighs—and as far from pacemaker as possible (a minimum of 15 cm from pacemaker).
Limit electrocautery to 1-second bursts every 10 seconds.
Use bipolar electrocautery forceps where practical. Speakers Notes
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Defibrillation concerns for IPGs are similar to those mentioned for use with electrocautery.
If possible, position the electrodes so that currents are not passing through the pacing system.
Place the defibrillator electrodes at least thirteen centimeters or five inches from the IPG.
Use the least amount of energy to satisfactorily revert the patient.
Medtronic IPGs are designed to withstand 400 watt-seconds of defibrillation energy.
Always check the operation of the IPG following defibrillator discharges.
Damage may be to various components of the circuitry.
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Diagnostic x-ray exposure poses no risk.
Therapeutic radiation may cause severe damage.
Patients receiving therapeutic radiation in treatment of malignant thoracic disease are particularly at risk. Appropriate shielding of the pulse generator is essential.
If adequate shielding is not possible, repositioning of the IPG may have to be carried out.
Patients should be checked after each session.
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This slide illustrates the essential components of a pacing lead.
The following topics will be discussed for each component:
· Purpose
· Design factors
· Performance factors
This is the connector of the lead. It connects the pacing lead to the IPG.
An accurate mechanical fit between the connector pin and the pin cavity of the pulse generator is crucial for the safe transmission of current from the pulse generator without current leakage.
Pass around samples of IS-1 connector pins and labels.
IS-1 is a formal International Standard for pacing connectors developed jointly by the International Organization for Standardization and the International Electrotechnical Commission in 1992.
Both bipolar and unipolar IS-1 lead connectors are 3.2 mm in diameter and have sealing rings and a short connector pin. Sealing rings provide proper insulation of the pacing lead electrical connection.
The leads are labeled IS-1 UNI or IS-1 BI. You will also notice that unipolar connectors have a blue connector ring as a visual indication that the lead is unipolar.
Prior to IS-1, other lead sizes include 3.2 mm low-profile and 5/6 mm. These connector sizes are only seen today during IPG replacements. Adapters may be required.
Silicone and polyurethane are the most commonly used insulation materials for pacing leads.
Silicone is a soft, flexible material and has been used for over 30+ years for pacing leads. It is used for the outer and inner insulations in today’s pacing leads. Currently, silicone is used on half in the world’s pacing leads.
Polyurethane is a firmer, stiffer material than silicone and has been used for over 20 years for pacing leads. It is used for the outer and inner insulations in today’s pacing leads. Currently, polyurethane is used in half of the world’s pacing leads.
Fluoropolymers (PTFE - poly tetra fluoro ethylene, ETFE - Ethyl tetra fluoro ethylene) is a soft, stiff material and is used primarily as a coating to protect conductor wires from corrosion. In addition, fluoropolymers are used to make the conductors more slippery so as not to cut through the primary silicone or polyurethane insulation . Fluoropolymers are used in less than 5% of today’s pacing leads.
Passive fixation in relation to endocardial leads means that no part of the lead itself is actually embedded in the endocardium. Rather, the lead tip is trapped within the trabeculae and/or is held in position by its pre-formed shape (e.g., J-lead in atrium).
Passive fixation leads commonly use tines or fins to &quot;catch&quot; trabeculi in the heart, or the lead is canted or curved to help place and hold the lead tip in a certain position.
Passive anchoring devices, such as flanges or wedge tips, cages and balloons were early attempts to solve the dislodgment problems of early pacing leads. However, only after tined leads were introduced by Medtronic in 1976 did a true solution to dislodgment emerge with regard to passive fixation.
Today, there is approximately a 1-2% dislodgment rate 1, 2, 3, 4, 5, 6 with passive fixation leads:
1 - 2% in the atria,1, 2, 6
1 - 2% in the ventricle.1, 3, 4, 5
References:
1 Gammage MD, Marshall HJ, Harris JI. Five-year experience with polyurethane, passive fixation, steroid-eluting leads. PACE. 1998;(Pt II):842. Abstract.
2 Hua W, Mond HG, Strathmore N. Chronic steroid-eluting lead performance: a comparison of atrial and ventricular pacing. Pacing Clinical Electrophysiology. 1995;20(1 Pt 1):17-24.
3 Kazama S, Nishiyama K, Machii M, Tanaka K, Amano T, Nomura T, Ohuchi M, Kasahara S, Nie M, Ishihara A. Long-term follow-up of ventricular endocardial pacing leads. Jpn Heart J. 1993;34(2):193-200.
4 Mayer DA, Tsapogas MJ. Pacemakers: dual or single chamber implantation. Vasc Surg. 1992;26(5):400-7.
5 Miller GB, Leman RB, Kratz JM, Gillette PC. Comparison of lead dislodgment and pocket infection rates after pacemaker implantation in the operating room versus the catheterization laboratory. Am Heart J. 1998;115(5):1048-51.
6 Mond HG, Hua W, Wang CC. Atrial pacing leads: the clinical contribution of steroid elution. Pacing Clinical Electrophysiology. 1995;18(9 Pt 1):1601-8.
7 Glikson M, von Feldt LK, Suman VJ, Hayes DL. Short- and long-term results with an active fixation, bipolar, polyurethane-insulated atrial pacing lead. Pacing Clinical Electrophysiology. 1996;19(10):1469-73.
8 Stirbys P. Implantation of double screw-in leads. Pacing Clinical Electrophysiology. 1988;11(10):1482-4.
Active fixation means that part of the lead actually embeds in the heart tissue for fixation via ascrew-in helix electrode. Extendible/retractable and fixed screw mechanisms are the most common active fixation methods.
Fixed Screw leads provide excellent stability. The lead body must be turned in a counter-clockwise rotation during insertion of the lead. Sense mapping can be done prior to fixing the screw to the myocardium. The lead body is then turned in a clockwise rotation to fix the screw to the myocardium.
Extendible/Retractable screw-in leads provide excellent stability. The screw is retracted to prevent damage to the veins and cardiac structures during lead advancement. The ability to retract the screw makes entanglement in cardiac structures less likely. The screw is retracted during transvenous introduction and during sense mapping and extended for lead fixation. Care must be taken not to over-extend or over-retract the screw with this type of screw mechanism.
Today, there is approximately a 1 - 4% dislodgment rate 6, 7, 8 with active fixation leads:
1 - 4% in the atria 6, 7, 8
&lt;1% in the ventricle.8
References:
6 Mond HG, Hua W, Wang CC. Atrial pacing leads: the clinical contribution of steroid elution. Pacing Clinical Electrophysiology. 1995;18(9 Pt 1):1601-8.
7 Glikson M, von Feldt LK, Suman VJ, Hayes DL. Short- and long-term results with an active fixation, bipolar, polyurethane-insulated atrial pacing lead. Pacing Clinical Electrophysiology. 1996;19(10):1469-73.
8 Stirbys P. Implantation of double screw-in leads. Pacing Clinical Electrophysiology. 1988;11(10):1482-4.
Fluorovisibility of pacing leads:
• Visual quality can vary widely among different fluoroscopy units and techniques.
• Plastics and less dense materials, such as titanium, are not visible in fluoroscopy.
• Denser metals, such as platinum, are more easily viewed.
3In this image a SureFix and CapSureFix active fixation lead is show under normal fluoroscopic conditions. You will notice a space is present when the helix is retracted as pointed out in the slide above.
Myocardial/Epicardial leads require an open-chest, tunneling, or minimally invasively sub-xyphoid to attach the lead tip to the outside of the heart. They are used in special situations, such as:
- pediatric applications,
- congenital malformations,
- when a mechanical heart valve is present,
- where multiple abandoned endocardial leads are present,
- or concurrent with open-chest surgery.
Stab-in and screw-in leads are considered myocardial leads because they are attached to the outside of the heart and the “hook” electrode and the screw electrode actually pass into the myocardium when fixated.
Suture-on leads are considered epicardial leads because the electrode is attached to the outside of the heart and does not penetrate the myocardium.
This slide illustrates the effects of lead impedance upon longevity.
Increasing pacing impedance enhances longevity.
3500 ohms approximates the CapSure lead pacing impedance. 600 ohms approximates the CapSure SP Novus lead pacing impedance. The 1,000 to 1,200 ohms approximates the CapSure Z Novus lead pacing impedance.
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Due to the excellent electrode-tissue biocompatibility:
•there is no significant threshold peaking and no substantial increase in chronic thresholds, virtually eliminating exit block.
•the amplitude of sensed intracardiac signals remains relatively constantpost-implant, especially chronic P-wave sensing.