2. Implantable pulse
generator (IPG)
Lead wire(s)
Implantable Pacemaker SystemsImplantable Pacemaker Systems
Contain the Following Components:Contain the Following Components:
3. Pulse generator: power
source or battery
Leads or wires
Cathode (negative
electrode)
Anode (positive
electrode)
Body tissue
IPG
Lead
Anode
Cathode
Pacemaker Components Combine withPacemaker Components Combine with
Body Tissue to Form a Complete CircuitBody Tissue to Form a Complete Circuit
4. Contains a battery
that provides the
energy for sending
electrical impulses to
the heart
Houses the circuitry
that controls
pacemaker
operations
Circuitry
Battery
The Pulse Generator:The Pulse Generator:
5. Deliver electrical
impulses from the
pulse generator to
the heart
Sense cardiac
depolarization
Lead
Leads Are Insulated Wires That:Leads Are Insulated Wires That:
6. Begins in the pulse
generator
Flows through the lead
and the cathode (–)
Stimulates the heart
Returns to the anode (+)
During Pacing, the Impulse:During Pacing, the Impulse:
Impulse onset
*
7. Flows through the tip
electrode (cathode)
Stimulates the heart
Returns through
body fluid and tissue
to the IPG (anode)
A Unipolar Pacing System Contains a Lead with Only OneA Unipolar Pacing System Contains a Lead with Only One
Electrode Within the Heart; In This System, the Impulse:Electrode Within the Heart; In This System, the Impulse:
Cathode
Anode
-
+
8. 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
9. Stimulate cardiac depolarization
Sense intrinsic cardiac function
Respond to increased metabolic demand by
providing rate responsive pacing
Provide diagnostic information stored by the
pacemaker
Most Pacemakers Perform Four Functions:Most Pacemakers Perform Four Functions:
11. 11
Sensing
Ability of device to detect intrinsic cardiac
activity
Undersensing: failure to sense
Oversensing: too sensitive to activity
12. Rate Responsive PacingRate Responsive Pacing
When the need for oxygenated blood increases, the
pacemaker ensures that the heart rate increases to
provide additional cardiac output
Adjusting Heart Rate to Activity
Normal Heart Rate
Rate Responsive Pacing
Fixed-Rate Pacing
Daily Activities
13. A Variety of Rate Response Sensors ExistA Variety of Rate Response Sensors Exist
Those most accepted in the market place are:
– Activity sensors that detect physical movement
and increase the rate according to the level of
activity
– Minute ventilation sensors that measure the
change in respiration rate and tidal volume via
transthoracic impedance readings
16. DisadvantagesDisadvantagesAdvantagesAdvantages
Advantages and Disadvantages ofAdvantages and Disadvantages of
Single-Chamber Pacing SystemsSingle-Chamber Pacing Systems
Implantation of a
single lead
Single ventricular lead
does not provide AV
synchrony
Single atrial lead does
not provide ventricular
backup if A-to-V
conduction is lost
17. One lead implanted
in the atrium
One lead implanted
in the ventricle
Dual-Chamber Systems Have Two Leads:Dual-Chamber Systems Have Two Leads:
18. Benefits of Dual Chamber PacingBenefits of Dual Chamber Pacing
Provides AV synchrony
Lower incidence of atrial fibrillation
Lower risk of systemic embolism and stroke
Lower incidence of new congestive heart failure
Lower mortality and higher survival rates
19. Benefits of Dual-Chamber PacingBenefits of Dual-Chamber Pacing
Study Results
Higano et al. 1990
Gallik et al. 1994
Santini et al. 1991
Rosenqvist et al. 1991
Sulke et al. 1992
Improved cardiac index during low level
exercise (where most patient activity occurs)
Increase in LV filling
30% increase in resting cardiac output
Decrease in pulmonary wedge pressure
Increase in resting cardiac output
Increase in resting cardiac output, especially
in patients with poor LV function
Decreased incidence of mitral and tricuspid
valve regurgitation
20.
21. 21
Examples
VVI
V: Ventricle is the paced chamber
V: Ventricle is the sensed chamber
I: Inhibited response to a sensed
signal
Thus, a synchronous generator that paces
and senses in the ventricle
Inhibited if a sinus or escape beat occurs
Called a “demand” pacer
22. 22
Examples
DVI
D: Both atrium and ventricle are
paced
V: Ventricle is sensed
I: Response is inhibited to a sensed
ventricular signal
23. Examples
DDDRA
Dual chamber, adaptive-rate pacing with
multisite atrial pacing (i.e., biatrial pacing,
more than one pacing site in one atrium,or
both features)
23
25. Stimulation ThresholdStimulation Threshold
The minimum electrical stimulus needed to consistently
capture the heart outside of the heart’s refractory period
VVI / 60
Capture Non-Capture
27. Amplitude is the Amount of VoltageAmplitude is the Amount of Voltage
Delivered to the Heart By the PacemakerDelivered to the Heart By the Pacemaker
Amplitude reflects the strength or height of the
impulse:
– 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
28. Pulse Width Is the Time (Duration)Pulse Width Is the Time (Duration)
of the Pacing Pulseof the Pacing Pulse
Pulse width is expressed in milliseconds (ms)
The pulse width is just the length of time each pacing pulse is
delivered & must be long enough for depolarization to disperse to
the surrounding tissue
5 V
0.5 ms 0.25 ms 1.0 ms
29. The Strength-Duration CurveThe Strength-Duration Curve
The strength-duration
curve illustrates the
relationship of amplitude
and pulse width
– Values on or above
the curve will result
in capture
Duration
Pulse Width (ms)
.50
1.0
1.5
2.0
.25StimulationThreshold(Volts)
0.5 1.0 1.5
Capture
31. SDCSDC
Rheobase- (the lowest point on the curve) by definition is the
lowest voltage that results in myocardial depolarization at
infinitely long pulse duration
Chronaxie(pulse duration time ) by definition, the chronaxie is
the threshold pulse duration at twice the rheobase voltage
The ideal pulse duration should be greater than the chronaxie
time
Cannot overcome high threshold exit block by increasing the
pulse duration, If the voltage output remains less than the
rheobase
33. Lead impedance
Amplitude and pulse width setting
Percentage paced vs. intrinsic events
Rate responsive modes programmed “ON”
Factors That Affect BatteryFactors That Affect Battery
Longevity Include:Longevity Include:
34. ImpedanceImpedance
The opposition to current flow
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
35. Impedance Changes Affect PacemakerImpedance Changes Affect Pacemaker
Function and Battery LongevityFunction and Battery Longevity
High impedance reading reduces battery
current drain and increases longevity
Low impedance reading increases battery
current drain and decreases longevity
Impedance reading values range from 300 to
1,500 Ω
– High impedance leads will show impedance
reading values greater than 1,500 ohms
36. ImpedanceImpedance
Factors that can influence impedance
– Resistance of the conductor coils
– Tissue between anode and cathode
– The electrode/myocardial interface
– Size of the electrode’s surface area
– Size and shape of the tip electrode
37. Ohm’s Law is a FundamentalOhm’s Law is a Fundamental
Principle of Pacing That:Principle of Pacing That:
VV
II RR
V = I X RV = I X R
I = V / RI = V / R
R = V / IR = V / I
Describes the relationship between voltage,
current, and resistance
xx
38. If you reduce the voltage by half, the current is
also cut in half
If you reduce the impedance by half, the
current doubles
If the impedance increases, the current
decreases
When Using Ohm’s LawWhen Using Ohm’s Law
You Will Find That:You Will Find That:
39. Voltage, Current, and ImpedanceVoltage, Current, and Impedance
Are InterdependentAre Interdependent
The interrelationship of the three components can
be likened to the flow of water through a hose
– Voltage represents the force with which . . .
– Current (water) is delivered through . . .
– A hose, or lead, where each component
represents the total impedance:
The nozzle, representing the electrode
The tubing, representing the lead wire
40. Voltage and Current FlowVoltage and Current Flow
Spigot (voltage) turned up
(high current drain)
Spigot (voltage) turned low
(low current drain)
41. Resistance and Current FlowResistance and Current Flow
“Normal” resistance
“Low” resistance
“High” resistance
Low current flow
High current flow
42. Electrode Design May Also ImpactElectrode Design May Also Impact
Stimulation ThresholdsStimulation Thresholds
Lead maturation process
43. Lead Maturation ProcessLead Maturation Process
Fibrotic “capsule” develops around the
electrode following lead implantation
44. Lead Maturation Process
3 phases
1. A/c phase, where thresholds immediately following implant are low
2. Peaking phase- thresholds rise and reach their highest point(1wk)
,followed by a ↓ in the threshold over the next 6 to 8 wks as the tissue
reaction subsides
3. C/c phase- thresholds at a level higher than that at implantation but
less than the peak threshold
Trauma to cells surrounding the electrode→ edema and subsequent
development of a fibrotic capsule.
Inexcitable capsule ↓ the current at the electrode interface, requiring more
energy to capture the heart.
45. Steroid Eluting LeadsSteroid Eluting Leads
Steroid eluting
leads reduce the
inflammatory
process and thus
exhibit little to no
acute stimulation
threshold peaking
and low chronic
thresholds
Porous, platinized tip
for steroid elution
Silicone rubber plug
containing steroid
Tines for
stable
fixation
46. Lead Maturation ProcessLead Maturation Process
Effect of Steroid on Stimulation Thresholds
Pulse Width = 0.5 msec
0
3 6
Implant Time (Weeks)
Textured Metal Electrode
Smooth Metal Electrode
1
2
3
4
5
Steroid-Eluting Electrode
0 1 2 4 5 7 8 9 10 11 12
Volts
47. A Pacemaker Must Be Able to SenseA Pacemaker Must Be Able to Sense
and Respond to Cardiac Rhythmsand 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
48. Accurate Sensing...Accurate Sensing...
Ensures that undersensing will not occur –
the pacemaker will not miss P or R waves that
should have been sensed
Ensures that oversensing will not occur – the
pacemaker will not mistake extra-cardiac activity
for intrinsic cardiac events
Provides for proper timing of the pacing pulse –
an appropriately sensed event resets the timing
sequence of the pacemaker
49. Sensitivity – The Greater the Number, theSensitivity – The Greater the Number, the LessLess
Sensitive the Device to Intracardiac EventsSensitive the Device to Intracardiac Events
50.
51. Accurate Sensing Requires ThatAccurate Sensing Requires That
Extraneous Signals Be Filtered OutExtraneous Signals Be Filtered Out
Sensing amplifiers use filters that allow appropriate
sensing of P waves and R waves and reject
inappropriate signals
Unwanted signals most commonly sensed are:
– T waves
– Far-field events (R waves sensed by the atrial
channel)
– Skeletal myopotentials (e.g., pectoral muscle
myopotentials)
52. Pacemaker TimingPacemaker Timing
Pacing Cycle : Time between two consecutive
events in the ventricles (ventricular only
pacing) or the atria (dual chamber pacing)
Timing Interval : Any portion of the Pacing
Cycle that is significant to pacemaker operation
e.g. AV Interval, Ventricular Refractory period
55. Single Chamber Timing TerminologySingle Chamber Timing Terminology
Lower rate
Refractory period
Blanking period
Upper rate
56. Lower Rate IntervalLower Rate Interval
Lower Rate Interval
VP VP
VVI / 60
Defines the lowest rate the pacemaker will pace
57. Refractory PeriodRefractory Period
Lower Rate Interval
VP VP
VVI / 60
Interval initiated by a paced or sensed event
Designed to prevent inhibition by cardiac or
non-cardiac events
Refractory Period
58. Blanking PeriodBlanking Period
Lower Rate Interval
VP VP
VVI / 60
The first portion of the refractory period
Pacemaker is “blind” to any activity
Designed to prevent oversensing pacing stimulus
Blanking Period
Refractory Period
59. Upper Sensor Rate IntervalUpper Sensor Rate Interval
Lower Rate Interval
VP VP
VVIR / 60 / 120
Defines the shortest interval (highest rate) the pacemaker
can pace as dictated by the sensor (AAIR, VVIR modes)
Blanking Period
Refractory Period
Upper Sensor Rate
Interval
65. Rate = 60 bpm / 1000 ms
A-A = 1000 ms
AP
VP
AP
VP
V-AAV V-AAV
Atrial Pace, Ventricular Pace (AP/VP)
Four “Faces” of Dual Chamber PacingFour “Faces” of Dual Chamber Pacing
66. Rate = 60 ppm / 1000 ms
A-A = 1000 ms
AP
VS
AP
VS
V-AAV V-AAV
Atrial Pace, Ventricular Sense (AP/VS)
Four “Faces” of Dual Chamber PacingFour “Faces” of Dual Chamber Pacing
67. AS
VP
AS
VP
Rate (sinus driven) = 70 bpm / 857 ms
A-A = 857 ms
Atrial Sense, Ventricular Pace (AS/ VP)
V-AAV AV V-A
Four “Faces” of Dual Chamber PacingFour “Faces” of Dual Chamber Pacing
68. Rate (sinus driven) = 70 bpm / 857 ms
Spontaneous conduction at 150 ms
A-A = 857 ms
AS
VS
AS
VS
V-AAV AV V-A
Atrial Sense, Ventricular Sense (AS/VS)
Four “Faces” of Dual Chamber PacingFour “Faces” of Dual Chamber Pacing
69. Dual Chamber Timing ParametersDual Chamber Timing Parameters
Lower rate
AV and VA intervals
Upper rate intervals
Refractory periods
Blanking periods
70. Lower Rate Interval
AP
VP
AP
VP
Lower RateLower Rate
The lowest rate the pacemaker will pace the atrium in
the absence of intrinsic atrial events
DDD 60 / 120
71. AP
VP
AS
VP
PAV SAV
200 ms 170 ms
Lower Rate Interval
AV IntervalsAV Intervals
Initiated by a paced or non-refractory sensed atrial event
– Separately programmable AV intervals – SAV /PAV
DDD 60 / 120
72. Lower Rate Interval
AP
VP
AP
VP
AV Interval VA Interval
Atrial Escape Interval (V-A Interval)
The interval initiated by a paced or sensed ventricular event
to the next atrial event
DDD 60 / 120
PAV 200 ms; V-A 800 ms
200 ms 800 ms
73. Atrial Escape Interval (V-A Interval)Atrial Escape Interval (V-A Interval)
Lower rate interval- AV interval
=V-A interval
The V-A interval is the longest period that may elapse after a
ventricular event before the atrium must be paced in the absence of
atrial activity.
The V-A interval is also commonly referred to as the atrial escape
interval
74. DDDR 60 / 120
A-A = 500 ms
AP
VP
AP
VP
Upper Activity Rate Limit
Lower Rate Limit
V-APAV V-APAV
Upper Activity (Sensor) RateUpper Activity (Sensor) Rate
In rate responsive modes, the Upper Activity Rate provides
the limit for sensor-indicated pacing
75. AS
VP
AS
VP
DDDR 60 / 100 (upper tracking rate)
Sinus rate: 100 bpm
Lower Rate Interval {
Upper Tracking Rate Limit
Upper Tracking RateUpper Tracking Rate
SAV SAVVA VA
The maximum rate the ventricle can be paced in response to
sensed atrial events
76. Post Ventricular Atrial
Refractory Period (PVARP)
Refractory PeriodsRefractory Periods
VRP and PVARP are initiated by sensed or paced
ventricular events
– The VRP is intended to prevent self-inhibition such
as sensing of T-waves
– The PVARP is intended primarily to prevent sensing
of retrograde P waves
AP
VPVentricular Refractory Period
(VRP)
A-V Interval
(Atrial Refractory)
77. Post-Ventricular Atrial Refractory PeriodPost-Ventricular Atrial Refractory Period
PVARP is initiated by a ventricular
event(sensed/paced), but it makes the atrial channel
refractory
PVARP is programmable (typical settings around 250-
275 ms)
Benefits of PVARP
– Prevents atrial channel from responding to
premature atrial contractions, retrograde P-waves,
and far-field ventricular signals
– Can be programmed to help minimize risk of
pacemaker-mediated tachycardias
78. Blanking PeriodsBlanking Periods
First portion of the refractory period-sensing is disabled
AP
VP
AP
Post Ventricular Atrial
Blanking (PVAB)
Post Atrial Ventricular
Blanking
Ventricular Blanking
(Nonprogrammable)
Atrial Blanking
(Nonprogrammable)
79. PVARP and PVABPVARP and PVAB
The PVAB is the post-ventricular atrial blanking
period during which time no signals are “seen” by the
pacemaker’s atrial channel
It is followed by the PVARP, during which time the
pacemaker might “see” and even count atrial events
but will not respond to them
PVAB-independently programmable
– Typical value around 100 ms
81. PVARP
Wenckebach Operation
Upper Tracking Rate
Lower Rate Interval {
AS AS AR AP
VPVP VP
TARP
SAV PAV PVARPSAV PVARP
P Wave Blocked (unsensed or unused)
• Prolongs the SAV until upper rate limit expires
– Produces gradual change in tracking rate ratio
TARP TARP
82. Wenckebach
• Occurs when the intrinsic atrial rate lies
between the UTR and the TARP rate
• Results in gradual prolonging of the AV
interval until one atrial intrinsic event occurs
during the TARP and is not tracked
84. • Every other P wave falls into refractory and does not restart the
timing interval
Upper Tracking Limit
Lower Rate Interval {
{
P Wave Blocked
AS AS
VPVP
ARAR
Sinus rate = 133 bpm (450 ms)
PVARP = 300 ms SAV = 200 ms
TARP=500 ms
AV PVARP AV PVARP
TARP TARP
2:1 Block
85. PVARP
Upper Tracking Rate
Lower Rate Interval
{
No SAV started for events sensed in the TARP
AS AS
VPVP
SAV = 200 ms
PVARP = 300 ms
Thus TARP = 500 ms (120 ppm)
DDD
LR = 60 ppm (1000 ms)
UTR = 100 bpm (600 ms) SAV
TARP
PVARP
Total Atrial Refractory Period (TARP)
• Sum of the AV Interval and PVARP
• defines the highest rate that the pacemaker will
track atrial events before 2:1 block occurs
SAV
86. Fixed Block or 2:1 Block
• Occurs whenever the intrinsic atrial rate
exceeds the TARP rate
• Every other atrial event falls in the TARP
when the atrial rate exceeds the TARP rate
• Results in block of atrial intrinsic events in
fixed ratios
A basic pacing system is made up of:
Implantable pulse generator that contains:
A power source—the battery within the pulse generator that generates the impulse
Circuitry—controls pacemaker operations
Leads—Insulated wires that deliver electrical impulses from the pulse generator to the heart. Leads also transmit electrical signals from the heart to the pulse generator.
Electrode—a conductor located at the end of the lead; delivers the impulse to the heart.
Lithium-iodine is the most commonly used power source for today’s pacemakers. Microprocessors (both ROM and RAM) control sensing, output, telemetry, and diagnostic circuits.
Other sensors that measure QT interval, central venous temperature, stroke volume, etc., are largely investigational devices or have gained limited acceptance.
Pacing in the VVI/R mode and loss of AV synchrony can lead to pacemaker syndrome. Pacemaker syndrome can be defined as “an assortment of symptoms related to the adverse hemodynamic impact from the loss of AV synchrony.”
Atrial pacemakers should only be used with patients who have proven AV conduction and regular follow-up testing available.
Studies have been done that demonstrate the differences in outcome, hemodynamic improvement, and quality of life assessment by using AV synchronous, or "atrial-based," pacing modes instead of VVI/R. Some of the benefits of using an atrial-based pacing mode include:
AV synchrony–Clinical benefits such as increased cardiac output, augmentation of ventricular filling (especially important for the majority of the pacing population with LVD and reduced compliance from effects of aging). Providing AV synchrony minimizes valvular regurgitation, and preserves atrial electrical stability.
In the Framingham Study, the development of chronic AF was associated with a doubling of overall mortality and of mortality from cardiovascular disease (Kannel, 1982) The following emphasize the importance of preventing atrial fibrillation:
Patients with AF unrelated to rheumatic or prosthetic valvular disease have a risk of ischemic stroke about five times higher than those with normal sinus rhythm.
AF is associated with over 75,000 cases of stroke per year.
See bibliography for listing of studies cited.
Included is a summary of some studies depicting long-term results of AV synchronous (atrial based) and non-synchronous (VVI/R) pacing.
Higano, et al. Hemodynamic importance of atrioventricular synchrony during low levels of exercise. PACE, 1990; 13:509 Abstract.
Gallik DM, et al. Comparison of ventricular function in atrial rate adaptive versus dual chamber rate adaptive pacing during exercise. PACE, 1994; 17(2):179-185
Santini, et al. New Perspectives in Cardiac Pacing. Mount Kisco, NY: Futura Publishing, 1991.
Rosenquist M, et al. Relative importance of activation sequence compared to atrioventricular synchrony during low levels of exercise. AM J Cardiology, 1991;67:148-156.
Sulke N, et al. “Subclinical pacemaker syndrome: A randomized study of symptom free patients with ventricular demand (VVI) pacemakers upgraded to dual chamber devices. Brit Heart J, 1992; 67(1):57-64.
The stimulation process can be described in “phases:”
The output voltage produces an electrical field at the electrode-tissue interface.
The electrical field permeates cardiac cells via ionic movement and changes voltage on the cell membrane, which brings the cell membrane “above threshold” and alters its permeability. Phase 0 is the result of this part of the process.
During Phase 0, sodium rushes in, which results in depolarization followed by cellular repolarization via sodium/potassium infusion.
NOTE: The electrical field generated by the stimulation pulse must last long enough to excite the tissue. To effectively raise the membrane potential, the intensity of the stimulation must be balanced with the length of time it is applied.
Remind audience that amplitude had been discussed when voltage was described.
The greater the pulse width, the shorter the battery longevity. An increase in pulse width leads to an increase in total energy delivered.
The longer the duration of the stimulus, the lower the amplitude required to capture the heart
The strength-duration curve illustrates the tradeoff of amplitude (intensity of stimulation, measured in volts) and duration (the length of time the stimulation is applied, measured in milliseconds).
Capture occurs when the stimulus causes the tissue to react. On the graph, capture occurs on or above the curve. Anything below the curve will not capture.
The lowest voltage on a stimulation curve which still results in capture at an infinitely long pulse duration is called rheobase. A voltage programmed below is inefficient and will lead to non-capture.
The pulse duration at twice the rheobase value is defined as the chronaxie. Chronaxie time is typically considered most efficient in terms of battery consumption.
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.
Impedance in pacing is defined as everything that opposes the flow of current through a circuit. Although in strict engineering terminology, resistance and impedance are different things, in pacing the terms are used interchangeably.
It is important to know a pacing lead’s impedance because it can be an early indicator of possible lead problems. Lead impedance is a measured value; you cannot adjust it. Lead manufacturers state acceptable impedance values in broad ranges rather than as specific values because many things can impact lead impedance.
Can be expressed in three ways:
V = I x R
R = V ÷ I
I = V÷ R
If any two values are known, the third may be calculated (cover the value you are seeking and the others appear in the appropriate format to calculate the unknown value).
Using the garden hose as an analogy, the higher the voltage, the greater the push, or “flow” of electrons (and the greater the current drain).
Resistance affects current flow. Leads with an insulation breach, such as the garden hose pictured in the middle, will measure a low resistance reading with a resultant high current flow, and possible premature battery depletion. Conversely, if there is a high resistance, such as a lead conductor break, the current flow will be low or non-existent.
There are three phases that make up the lead maturation process:
The acute phase, where thresholds immediately following implant are low
The peaking phase, where thresholds rise and reach their highest point, usually around one week post-implant; followed by a decline in the threshold over the next 6 to 8 weeks1 as the tissue reaction subsides.
The chronic phase, where thresholds assume a stable reading to a level somewhat higher than that at implantation but less than the peak threshold.1
The lead maturation process occurs due to the trauma to cells surrounding the electrode, which causes edema and subsequent development of a fibrotic capsule. The inexcitable capsule reduces the current at the electrode interface, requiring more energy to capture the heart.
1Hayes DH et al. Cardiac Pacing and Defibrillation: A Clinical Approach. Armonk, NY: Futura Publishing Company; 2000. Page 7.
There are three phases that make up the lead maturation process:
The acute phase, where thresholds immediately following implant are low
The peaking phase, where thresholds rise and reach their highest point, usually around one week post-implant; followed by a decline in the threshold over the next 6 to 8 weeks1 as the tissue reaction subsides.
The chronic phase, where thresholds assume a stable reading to a level somewhat higher than that at implantation but less than the peak threshold.1
The lead maturation process occurs due to the trauma to cells surrounding the electrode, which causes edema and subsequent development of a fibrotic capsule. The inexcitable capsule reduces the current at the electrode interface, requiring more energy to capture the heart.
1Hayes DH et al. Cardiac Pacing and Defibrillation: A Clinical Approach. Armonk, NY: Futura Publishing Company; 2000. Page 7.
Steroid eluting leads reduce inflammation by employing a capsule of dexamethasone sodium phosphate, which gradually emits steroid over time, nearly eliminating the peaking phenomenon of the lead maturation process.
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.
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.
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.
Single chamber timing has three components:
Lower rate interval
Refractory period
Blanking period
Single chamber devices that are programmed to a rate responsive mode add a fourth component, the upper rate interval.
The lower rate defines the lowest rate that the pacemaker will pace. For example, if the lower rate is programmed to 60 ppm in the VVI mode, the pacemaker is required to pace at a rate of 60 ppm if the patient's intrinsic ventricular rate is less than 60 bpm. A paced or non-refractory sensed event restarts the rate timer at the programmed rate.
During refractory periods, the pacemaker “sees” but is unresponsive to any signals. This is designed to avoid restarting the lower rate interval in the event of oversensing. T-wave oversensing in VVI and AAI modes will occur if refractory periods are too short. In the AAI mode, the pacemaker may even sense the QRS complex (“far-field R wave”) if the refractory period is not long enough.
Events that fall into the refractory period are sensed by the pacemaker (the marker channel will display a “SR” denoting ventricular refractory or atrial refractory in single chamber systems) but the timing interval will remain unaffected by the sensed event.
A refractory period is started by a paced, non-refractory, or refractory sensed event.
A paced or sensed event will initiate a blanking period. Blanking is a method to prevent multiple detection of a single paced or sensed event by the sense amplifier (e.g., the pacemaker detecting its own pacing stimuli or depolarization, either intrinsic or as a result of capture). During this period, the pacemaker is "blind" to any electrical activity. A typical blanking period duration in a single-chamber mode is 100 msec*.
Note: In Thera and Kappa devices, nonprogrammable blanking parameters are dynamic (ranging from 50-100 ms) depending on the strength/duration of the paced or sensed signal.
The upper sensor rate interval in single chamber pacing is available only in rate-responsive modes. The upper rate defines the limit at which sensor-driven pacing can occur.
VOO mode paces in the ventricle but will not sense and, therefore, has no response to cardiac events. Pacemakers programmed to the VVI, VVIR, and VDD modes will revert to VOO mode upon magnet application.
In this example, an intrinsic beat occurs, but it has no effect on the timing interval and another ventricular pace is delivered at the programmed rate. No sensing occurs, thus, the entire lower rate interval is unresponsive to intrinsic activity.
In inhibited modes (VVI/AAI), intrinsic events that occur before the lower rate interval expires will reset the lower rate interval, as shown in the example above. As with paced events, sensed events will also initiate blanking and refractory periods.
Single chamber rate-responsive pacing is identical to non-rate responsive pacing operation, with the exception that the pacing rate is driven by a sensor. The sensor determines whether or not a rate increase is indicated, and adjusts the rate accordingly. The highest rate that the pacemaker is allowed to pace is the upper rate limit or interval.
In this example, the pacemaker is pacing at the maximum sensor indicated rate of 120 ppm.
Knowing the basic A-V and V-A intervals will help in understanding the four modes or “faces” of dual chamber pacing. In the first example, the pacemaker is pacing in both the atrium and the ventricle–most likely a patient with sinus node dysfunction and AV block.
In this example, the atrium is being paced, but AV conduction is intact, so the ventricular output is inhibited by a sensed ventricular event.
In this example, the atrial rate is driving the ventricular rate–also called atrial tracking. This patient has adequate sinus node function with AV block.
In this example, the patient has adequate sinus node function and intact AV conduction, but may experience little to no increase in sinus rate with activity and/or AV block that occurs at increased rates. At appropriate rates, it is best to try and utilize the patient’s intrinsic rhythm when possible.
Dual-chamber pacing requires attention to these parameters:
Lower rate
AV and V-A intervals
Upper rates
Refractory periods
Blanking periods
In order to provide optimal hemodynamic benefit to the patient, dual-chamber pacemakers strive to mimic the normal heart rhythm. In dual-chamber pacemakers, the lower rate is the rate at which the pacemaker will pace the atrium in the absence of intrinsic atrial activity. Similar to single-chamber timing, the lower rate can be converted to a lower rate interval (A-A interval), or the longest period of time allowed between atrial events.
The SAV is usually programmed to a shorter duration than the PAV to allow for the difference in interatrial conduction time between intrinsic and paced atrial events. Think of the difference in the activation sequence between a cycle initiated with an intrinsic atrial event versus a paced atrial event. The cycle starting with the intrinsic atrial event will use the normal conduction pathways between the right atrium and the left atrium. The cycle starting with the paced atrial beat will not use the normal interatrial conduction pathways but will instead use muscle tissue, which takes a little longer to reach the left atrium and causing it to contract.
If the AV interval is timed to allow the appropriate amount of time for left ventricular filling when the cycle is initiated with a sensed atrial event, the same duration for the PAV may not be the appropriate amount of time to allow for left ventricular filling when the cycle is initiated by a paced atrial event. Proper LA-LV timing promotes left ventricular filling ("atrial kick") and prevents regurgitant flow through an open mitral valve. Therefore, it is beneficial to have separately programmable PAV and SAV intervals.
In this example, the lower rate interval is terminated by a sensed atrial event, which initiates a SAV interval (and restarts the the lower rate interval).
Knowing the lower rate interval and the PAV interval (A-V interval after a paced atrial event), the V-A interval can be found:
V-A interval = lower rate interval minus the AV interval.
The V-A interval is the longest period that may elapse after a ventricular event before the atrium must be paced in the absence of atrial activity. The V-A interval is also commonly referred to as the atrial escape interval.
The A-V interval is employed to allow the appropriate amount of time to optimize ventricular filling and mimic the activation sequence of the normal heart. Knowing the lower rate interval and the PAV interval (A-V interval after a paced atrial event), the V-A interval can be found:
V-A interval = lower rate interval minus PAV interval.
The V-A interval is the longest period that may elapse after a ventricular event before the atrium must be paced in the absence of atrial activity. The V-A interval is also commonly referred to as the atrial escape interval.
This upper rate is defined as the upper activity rate, also known as the upper sensor rate or maximum sensor rate.
Before mode switching was available, pacemakers utilized a separate activity/sensor rate and upper tracking rate to limit the rate to which the patient could track (e.g., in the presence of SVTs), but allow the patient to pace to higher rates if they were exercising.
The sequence of an atrial intrinsic event being sensed, starting an SAV interval, timing out the SAV interval, and pacing in the ventricle can be referred to as "tracking." If the atrial rate begins to increase and continues to increase, is it desirable to let the ventricle "track" to extremely high rates? No. It is desirable to limit the rate at which the ventricle can pace in the presence of high atrial rates. This limit is called the upper tracking rate.
The Post-Ventricular Atrial Refractory Period (PVARP) is the period of time after a ventricular pace or sense when the atrial channel is in refractory. In other words, atrial senses outside of blanking that occur during this period are "seen" (and marked “AR) on the marker channel), but do not initiate an AV interval.
The purpose of PVARP is to avoid allowing retrograde P waves, far-field R waves, or premature atrial contractions to start an AV interval which would cause the pacemaker to pace in the ventricle at a high rate.
The refractory period after a ventricular event (paced or sensed) is designed to avoid restarting of the V-A interval due to a T wave. Ventricular sensed events occurring in the noise sampling portion of the ventricular refractory period are "seen" (and marked “VR” on the marker channel) but will not restart the V-A interval.
The atrial channel is refractory following a paced or sensed event during the AV interval. This allows atrial senses occurring in the AV interval to be "seen" but not restart another AV interval .
DDD/R modes have four types of blanking periods:
A non-programmable atrial blanking period (varies from 50-100 msec) is initiated each time the atrium paces or senses. This is to avoid the atrial lead sensing its own pacing pulse or P wave (intrinsic or captured). In Thera and Kappa devices, this blanking period is dynamic, depending on the strength of the paced/sensed signal.
The PVAB-(Post-Ventricular Atrial Blanking Period) is initiated by a ventricular pace or sensed event (nominally set at 220 msec) to avoid the atrial lead sensing the far-field ventricular output pulse or R wave.
In dual-chamber timing, a non-programmable ventricular blanking period occurs after a ventricular paced or sensed event to avoid sensing the ventricular pacing pulse or the R wave (intrinsic or captured). This period is 50-100 msec in duration and is dynamic, based on signal strength.
There also is a ventricular blanking period after an atrial pacing pulse in order to avoid sensing the far-field atrial stimulus (crosstalk). This period is programmable (nominally set at 28 msec). This blanking period is relatively short because it is important not to miss ventricular events (e.g., PVCs) that occur early in the AV interval. Ventricular blanking does not occur coincident with an atrial sensed event. This is because the intrinsic P wave is relatively small and will not be far-field sensed by the ventricular lead.
The issue of ventricular safety pacing and cross-talk will be addressed later on in the presentation.
A note of caution in programming long ventricular blanking periods after an atrial pace should be mentioned. If the ventricular blanking period after an atrial pace is excessively long, conducted ventricular events may go unsensed and cause the pacemaker to pace in the ventricle after the AV interval expires. This pace could occur before the ventricle has recovered from depolarization and may induce a ventricular arrhythmia (R on T phenomena).
This ECG depicts Wenckebach operation.
Pacemaker Wenckebach has the characteristic Wenckebach pattern of the PR (AV) interval gradually extending beat-to-beat until an atrial event falls into the PVARP and cannot restart an AV interval. In effect, a ventricular beat is “dropped”.
In this graphic, starting from the left side of the ECG, the pacemaker senses an atrial beat and starts an SAV. Because no ventricular event occurs by the end of the SAV, a ventricular pace is delivered. Now the pacemaker is looking for a sensed atrial beat. An atrial beat is sensed outside of the PVARP and starts an SAV. This time, when the SAV times out, the upper rate interval has not yet expired. Since the pacemaker can never violate the upper tracking rate, the ventricular pace has to be delayed until the end of the upper rate interval, at which time a ventricular pace is delivered.
This pattern of sensing a P wave, starting an SAV, waiting for the upper rate interval to time out, and pacing in the ventricle repeats until a P wave falls into the PVARP and does not start an SAV. The amount of delay created by the time from the sensed P wave until the upper rate interval expires is a little longer each time, producing the gradually lengthening of the P wave to ventricular pace intervals.
Once a P wave falls into the PVARP and does not initiate an SAV, the pacemaker looks for the next sensed P wave and the pattern starts all over again. This is how the classic Wenckebach pattern develops.
The rate at which the pacemaker will exhibit Wenckebach behavior is at the upper tracking rate (or upper rate if the pacemaker does not have a separate upper tracking rate and upper activity rate).
Pacemaker 2:1 block is characterized by two sensed P waves per paced QRS complex. This pattern develops because every other P wave falls into PVARP.
Starting on the left side of this ECG, the sequence begins with a sensed P wave. This P wave initiates a SAV, followed by a paced ventricular event. The next P wave falls into the PVARP, started by the ventricular pace, so no SAV is initiated. The following P wave is sensed outside of the PVARP, so a SAV is started. Again, no ventricular event occurs during the SAV, so the pacemaker paces in the ventricle. In this manner, a 2:1 block pattern is created.
The rate at which the pacemaker will exhibit a 2:1 block pattern is determined by the SAV and the PVARP (or the TARP). Atrial rates with a P-P coupling interval shorter than the TARP will result in 2:1 block. To determine at what rate the pacemaker will go into 2:1 block, the TARP is simply converted from an interval to a rate. Therefore, the rate the pacemaker will go into 2:1 block is: 60,000/TARP.
The total time that the atrial chamber of the pacemaker is in refractory is during the AV interval and during the PVARP. The Total Atrial Refractory Period (TARP) is equal to the SAV interval plus the PVARP. The TARP is important to understand as it defines the highest rate that the pacemaker will track atrial events before 2:1 block occurs.
