Applying Electrical Concepts to Pacemakers
Module 4
1
Objectives
• Upon completion you will be able to:
– Recognize a high impedance condition
– Recognize a low impedance condition
– Recognize capture threshold
– Determine which sensitivity value is more
(or less) sensitive
2
Electrical Information
• Why is this electrical information relevant?
• A pacemaker is implanted to:
– Provide a heart rate to meet metabolic needs
• In order to pace the heart, it must capture the myocardium
• In order to pace the heart, it must know when to pace, i.e., it must
be able to sense
• A pacemaker requires an intact electrical circuit
3
Ohm’s Law
Relevance to Pacemaker Patients
• High impedance conditions reduce battery current drain
– Can increase pacemaker battery longevity
– Why?
• R = V/I If “R” increases and “V” remains the same, then “I” must
decrease
• Low impedance conditions increase battery current drain
– Can decrease pacemaker battery longevity
– Why?
• R = V/I If “R” decreases and “V” remains the same, then “I” must
increase
The Effect of Lead Performance on
Myocardial Capture
What would you expect to happen if a lead was partially
fractured?
•
Impedance (or Resistance) would rise
•
Current would decrease and battery energy conserved
- but Could you guarantee that enough current (I) can flow
through this fractured lead so that each time the
pacemaker fired the myocardium would beat?
5
High Impedance Conditions
A Fractured Conductor
• A fractured wire can cause
Impedance values to rise
– Current flow from the battery
may be too low to be effective
• Impedance values may
exceed 3,000 W
Lead wire fracture
Increased resistance
Other reason for high
impedance: Lead not seated
properly in pacemaker header.
Lead Impedance Values Change as a Result of:
• Wire fractures
• Insulation breaks
Typically, normal impedance reading values range from 300
to 1,000 W
– Some leads are high impedance by design. These leads will
normally show impedance reading values greater than 1,000 ohms
• Medtronic High Impedance leads are:
– CapSure® Z
– CapSure® Z Novus
Low Impedance Conditions
• Insulation breaks expose the
lead wire to the following
– Body fluids, which have a low
resistance, or
– Another lead wire (in a bipolar
lead)
• Insulation break that exposes a
conductor causes the following
– Impedance values to fall
– Current to drain through the
insulation break into the body, or
into the other wire
– Potential for loss of capture
– More rapid battery depletion
Current will follow the path of
LEAST resistance
Capture Threshold
• The minimum electrical stimulus needed to consistently capture the
heart outside of the heart’s own refractory period
Capture
Ventricular pacemaker 60 ppm
Non-Capture
Effect of Lead Design on Capture
• Lead maturation
– Fibrotic “capsule” develops around the electrode following lead
implantation
– May gradually raise threshold
– Usually no measurable effect on impedance
Steroid Eluting Leads
• Steroid eluting leads
reduce the inflammatory
process
Porous, platinized tip
for steroid elution
– Exhibit little to no acute
stimulation threshold peaking
– Leads maintain low chronic
thresholds
Silicone rubber plug
containing steroid
Tines for
stable
fixation
Effect of Steroid on Stimulation Thresholds
5
Volts
4
Smooth Metal Electrode
3
Textured Metal Electrode
2
1
Steroid-Eluting Electrode
0
0
1
2
3
4
5
6
7
8
9
10 11 12
Implant Time (Weeks)
Pulse Width = 0.5 msec
References: Pacing Reference Guide, Bakken Education Center, 1995, UC199601047aEN. Cardiac Pacing,
2nd Edition, Edited by Kenneth A. Ellenbogen. 1996.
Myocardial Capture
• Capture is a function of:
– Amplitude—the strength of the impulse expressed in volts
• The amplitude of the impulse must be large enough to cause
depolarization (i.e., to “capture” the heart)
• The amplitude of the impulse must be sufficient to provide an
appropriate pacing safety margin
– Pulse width—the duration of the current flow expressed in
milliseconds
• The pulse width must be long enough for depolarization to disperse to
the surrounding tissue
Comparison
5.0 Volt Amplitude at Different Pulse Widths
Amplitude
5.0 V
0.5 ms
0.25 ms
1.0 ms
The Strength-Duration Curve
• The strength-duration
curve illustrates the
relationship of
amplitude and pulse
width
1.5
Volts
– Any combination of
pulse width and voltage,
on or above the curve,
will result in capture
2.0
Chronaxie
Capture
1.0
Rheobase
.50
.25
No Capture
0.5
1.0
Pulse Width
1.5
• By accurately determining
capture threshold, we can assure
adequate safety margins
because:
– Thresholds may differ in acute or
chronic pacing systems
– Thresholds fluctuate slightly daily
– Thresholds can change due to
metabolic conditions or
medications
Stimulation Threshold (Volts)
Clinical Utility of the Strength-Duration Curve
2.0
X
Programmed Output
1.5
1.0
.50
.25
0.5
1.0
Duration
Pulse Width (ms)
1.5
Programming Outputs
• Primary goal: Ensure patient safety and appropriate device
performance
• Secondary goal: Extend the service life of the battery
– Typically program amplitude to < 2.5 V, but always maintain
adequate safety margins
• A common output value might be 2.0 V at 0.4 ms
– Amplitude values greater than the cell capacity of the pacemaker
battery (usually about 2.8 V) require a voltage multiplier, resulting in
markedly decreased battery longevity
Pacemaker Sensing
• Refers to the ability of the pacemaker to “see” signals
– Expressed in millivolts (mV)
• The millivolts (mV) refers to the size of the signal the
pacemaker is able to “see”
0.5 mV signal
2.0 mV signal
18
Sensitivity
The Value Programmed into the IPG
5.0 mV
2.5 mV
1.25 mV
Time
Sensitivity
The Value Programmed into the IPG
5.0 mV
2.5 mV
5 mV sensitivity
1.25 mV
Time
At this value the pacemaker will not
see the 3.0 mV signal
Sensitivity
The Value Programmed into the IPG
5.0 mV
But what
about this?
2.5 mV
1.25 mV
1.25 mV Sensitivity
Time
At this value, the pacemaker can see both the 3.0 mV and the
1.30 mV signal. So, is “more sensitive” better, because the
pacemaker sees smaller signals?
Sensing Amplifiers/Filters
• Accurate sensing requires that extraneous signals are filtered out
– Because whatever a pacemaker senses is by definition a P- or an R-wave
– Sensing amplifiers use filters that allow appropriate sensing of P- and Rwaves, and reject inappropriate signals
• Unwanted signals most commonly sensed are:
– T-waves (which the pacemaker defines as an R-wave)
– Far-field events (R-waves sensed by the atrial channel, which the
pacemaker thinks are P-waves)
– Skeletal muscle myopotentials (e.g., from the pectoral muscle, which the
pacemaker may think are either P- or R-waves)
– Signals from the pacemaker (e.g., a ventricular pacing spike sensed on the
atrial channel “crosstalk”)
Sensing Accuracy
• Affected by:
– Pacemaker circuit (lead) integrity
• Insulation break
• Wire fracture
– The characteristics of the electrode
– Electrode placement within the heart
– The sensing amplifiers of the pacemaker
– Lead polarity (unipolar vs. bipolar)
– The electrophysiological properties of the myocardium
– EMI – Electromagnetic Interference
Lead Conductor Coil Integrity
Affect on Sensing
• 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
– Creates signals interpreted by the pacemaker as P- or R-waves
Lead Insulation Integrity
Affect on Sensing
• 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
Unipolar Pacemaker
• Where is the sensing circuit?
Anode
Click for Answer
Lead tip to can
This can produce a large potential
difference (signal) because the cathode
and anode are far apart
_
Cathode
Bipolar Pacemaker
• Where is the sensing
circuit?
Click for Answer
Lead tip to ring on the lead
This usually produces a smaller potential
difference due to the short inter-electrode
distance
• But, electrical signals from outside the
heart (such as myopotentials) are less
likely to be sensed
Anode and
Cathode
Cardiac Conduction and Device Sensing
By now we should be
familiar with the surface
ECG and its relationship to
cardiac conduction. But,
how does this relate to
pacemaker sensing?
28
Vectors and Gradients
Sense
2.5 mV
Click for More
The wave of depolarization produced by
normal conduction creates a gradient
across the cathode and anode. This
changing polarity creates the signal.
Once this signal exceeds the
programmed sensitivity – it is
sensed by the device.
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Changing the Vector
Sense
2.5 mV
Click for More
A PVC occurs, which is conducted
abnormally. Since the vector relative
to the lead has changed, what effect
might this have on sensing?
In this case, the wave of
depolarization strikes the anode
and cathode almost simultaneously.
This will create a smaller gradient
and thus, a smaller signal.
30
Putting It All Together
• Appropriate output programming can improve device longevity
– But, do not compromise patient safety!
• Lead design can improve device longevity via
– Steroid eluting leads
• Can help keep chronic pacing thresholds low by reducing inflammation
and scarring
– High Impedance leads
• Medtronic CapSure Z and Medtronic CapSure Z Novus
• Designed so electrode W is high, but V low so current (I) is low as well,
reducing battery drain
• Control of manufacturing
– Batteries, circuit boards, capacitors, etc., specific to needs, can lead to
improved efficiencies and lowered static current drain
– Highly reliable lead design
31
Putting It All Together
• Pacemaker Longevity is:
– A function of programmed parameters (rate, output, % time pacing)
– A function of useful battery capacity
– A function of
• Static current drain
• Circuit efficiency
• Output Impedance
• The lower the programmed sensitivity the MORE sensitive
the device
– Lead integrity also affects sensing
32
Status Check
• Determine the threshold amplitude
1.25 V
1.00 V
0.75 V
0.05 V
Click for Answer
Capture threshold = lowest value with consist capture
This is at 1.25 V
33
Status Check
• Which of these pacemakers is more sensitive?
Pacemaker
A
OR
Pacemaker
B
Programmed
Sensitivity 2.5 mV
Programmed
Sensitivity 0.5 mV
Click for Answer
Pacemaker A is able to “see” signals as small as 0.5 mV. Thus, it is
more sensitive.
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Status Check
• A pacemaker lead must flex and move as the heart beats.
On average, how many times does a heart beat in 1 year?
Click for Answer
35 MILLION times. It is not a simple task to design a
lead that is small, reliable, and lasts a lifetime.
35
Status Check
Do you notice anything on this x-ray?
Click for Answer
Lead Fracture:
• High Impedance
• Possible failure to
capture myocardium
36
Status Check
What would you expect?
• Which value is out of range?
• What could have caused this?
Pacemaker Interrogation Report
Mode: DDDR
Click for Answer
Insulation failure
Lower: Rate 60 ppm
UTR: 130 ppm
USR: 130 ppm
Atrial Lead Impedance: 475 Ohms
Ventricular Lead Impedance: 195 Ohms
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Brief Statements
Indications
•
Implantable Pulse Generators (IPGs) are indicated for rate adaptive pacing in patients who ay benefit from increased
pacing rates concurrent with increases in activity and increases in activity and/or minute ventilation. Pacemakers are
also indicated for dual chamber and atrial tracking modes in patients who may benefit from maintenance of AV
synchrony. Dual chamber modes are specifically indicated for treatment of conduction disorders that require restoration
of both rate and AV synchrony, which include various degrees of AV block to maintain the atrial contribution to cardiac
output and VVI intolerance (e.g. pacemaker syndrome) in the presence of persistent sinus rhythm.
•
Implantable cardioverter defibrillators (ICDs) are indicated for ventricular antitachycardia pacing and ventricular
defibrillation for automated treatment of life-threatening ventricular arrhythmias.
•
Cardiac Resynchronization Therapy (CRT) ICDs are indicated for ventricular antitachycardia pacing and ventricular
defibrillation for automated treatment of life-threatening ventricular arrhythmias and for the reduction of the symptoms of
moderate to severe heart failure (NYHA Functional Class III or IV) in those patients who remain symptomatic despite
stable, optimal medical therapy and have a left ventricular ejection fraction less than or equal to 35% and a QRS
duration of ≥130 ms.
•
CRT IPGs are indicated for the reduction of the symptoms of moderate to severe heart failure (NYHA Functional Class
III or IV) in those patients who remain symptomatic despite stable, optimal medical therapy, and have a left ventricular
ejection fraction less than or equal to 35% and a QRS duration of ≥130 ms.
Contraindications
•
IPGs and CRT IPGs are contraindicated for dual chamber atrial pacing in patients with chronic refractory atrial
tachyarrhythmias; asynchronous pacing in the presence (or likelihood) of competitive paced and intrinsic rhythms;
unipolar pacing for patients with an implanted cardioverter defibrillator because it may cause unwanted delivery or
inhibition of ICD therapy; and certain IPGs are contraindicated for use with epicardial leads and with abdominal
implantation.
•
ICDs and CRT ICDs are contraindicated in patients whose ventricular tachyarrhythmias may have transient or
reversible causes, patients with incessant VT or VF, and for patients who have a unipolar pacemaker. ICDs are also
contraindicated for patients whose primary disorder is bradyarrhythmia.
38
Brief Statements (continued)
Warnings/Precautions
• Changes in a patient’s disease and/or medications may alter the efficacy of the device’s programmed
parameters. Patients should avoid sources of magnetic and electromagnetic radiation to avoid
possible underdetection, inappropriate sensing and/or therapy delivery, tissue damage, induction of an
arrhythmia, device electrical reset or device damage. Do not place transthoracic defibrillation paddles
directly over the device. Additionally, for CRT ICDs and CRT IPGs, certain programming and device
operations may not provide cardiac resynchronization. Also for CRT IPGs, Elective Replacement
Indicator (ERI) results in the device switching to VVI pacing at 65 ppm. In this mode, patients may
experience loss of cardiac resynchronization therapy and / or loss of AV synchrony. For this reason,
the device should be replaced prior to ERI being set.
Potential complications
• Potential complications include, but are not limited to, rejection phenomena, erosion through the skin,
muscle or nerve stimulation, oversensing, failure to detect and/or terminate arrhythmia episodes, and
surgical complications such as hematoma, infection, inflammation, and thrombosis. An additional
complication for ICDs and CRT ICDs is the acceleration of ventricular tachycardia.
• See the device manual for detailed information regarding the implant procedure, indications,
contraindications, warnings, precautions, and potential complications/adverse events. For further
information, please call Medtronic at 1-800-328-2518 and/or consult Medtronic’s website at
www.medtronic.com.
Caution: Federal law (USA) restricts these devices to sale by or on the order of a physician.
39
Brief Statement: Medtronic Leads
Indications
• Medtronic leads are used as part of a cardiac rhythm disease management system. Leads are
intended for pacing and sensing and/or defibrillation. Defibrillation leads have application for patients
for whom implantable cardioverter defibrillation is indicated
Contraindications
• Medtronic leads are contraindicated for the following:
• ventricular use in patients with tricuspid valvular disease or a tricuspid mechanical heart valve.
• patients for whom a single dose of 1.0 mg of dexamethasone sodium phosphate or dexamethasone
acetate may be contraindicated. (includes all leads which contain these steroids)
• Epicardial leads should not be used on patients with a heavily infracted or fibrotic myocardium.
• The SelectSecure Model 3830 Lead is also contraindicated for the following:
• patients for whom a single dose of 40.µg of beclomethasone dipropionate may be contraindicated.
• patients with obstructed or inadequate vasculature for intravenous catheterization.
40
Brief Statement: Medtronic Leads (continued)
Warnings/Precautions
• People with metal implants such as pacemakers, implantable cardioverter defibrillators (ICDs), and
accompanying leads should not receive diathermy treatment. The interaction between the implant and
diathermy can cause tissue damage, fibrillation, or damage to the device components, which could
result in serious injury, loss of therapy, or the need to reprogram or replace the device.
• For the SelectSecure Model 3830 lead, total patient exposure to beclomethasone 17,21-dipropionate
should be considered when implanting multiple leads. No drug interactions with inhaled
beclomethasone 17,21-dipropionate have been described. Drug interactions of beclomethasone
17,21-dipropionate with the Model 3830 lead have not been studied.
Potential Complications
• Potential complications include, but are not limited to, valve damage, fibrillation and other arrhythmias,
thrombosis, thrombotic and air embolism, cardiac perforation, heart wall rupture, cardiac tamponade,
muscle or nerve stimulation, pericardial rub, infection, myocardial irritability, and pneumothorax.
Other potential complications related to the lead may include lead dislodgement, lead conductor
fracture, insulation failure, threshold elevation or exit block.
• See specific device manual for detailed information regarding the implant procedure, indications,
contraindications, warnings, precautions, and potential complications/adverse events. For further
information, please call Medtronic at 1-800-328-2518 and/or consult Medtronic’s website at
www.medtronic.com.
Caution: Federal law (USA) restricts this device to sale by or on the order of a physician.
41
Disclosure
NOTE:
This presentation is provided for general educational purposes
only and should not be considered the exclusive source for this
type of information. At all times, it is the professional
responsibility of the practitioner to exercise independent
clinical judgment in a particular situation.
42