Pacemaker basics

PACEMAKER BASICSPACEMAKER BASICS

Dr. R. Praveen Babu

Vijaya Hospital

Chennai

Implantable pulse generator (IPG)

Lead wire(s)

Implantable Pacemaker Systems Implantable Pacemaker Systems Contain the Following Components:Contain the Following Components:

Pulse generator: power source or battery

Leads or wires

Cathode (negative electrode)

Anode (positive electrode)

Body tissue

IPG

Lead

Anode

Cathode

Pacemaker Components Combine with Pacemaker Components Combine with Body Tissue to Form a Complete CircuitBody Tissue to Form a Complete Circuit

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:

Deliver electrical impulses from the pulse generator to the heart

Sense cardiac depolarization

Lead

Leads Are Insulated Wires That:Leads Are Insulated Wires That:

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*

Flows through the tip electrode (cathode)


Returns through body fluid and tissue to the IPG (anode)

A Unipolar Pacing System Contains a Lead with Only One A 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

-

+

Anode

Flows through the tip electrode located at the end of the lead wire


Returns to the ring electrode above the lead tip

A Bipolar Pacing System Contains a Lead with Two A 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

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:

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CaptureDepolarization of atria and/or ventricles in

response to a pacing stimulus

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SensingAbility of device to detect intrinsic cardiac

activityUndersensing: failure to senseOversensing: too sensitive to activity

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 PacingFixed-Rate Pacing

Daily Activities

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

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Types

1. Asynchronous/Fixed Rate

2. Synchronous/Demand

3. Single/Dual ChamberSequential (A & V)

4. Programmable/nonprogrammable

Single-Chamber SystemSingle-Chamber System

The pacing lead is implanted in the atrium or ventricle, depending on the chamber to be paced and sensed

DisadvantagesDisadvantagesAdvantagesAdvantages

Advantages and Disadvantages of Advantages 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

One lead implanted in the atrium

One lead implanted in the ventricle

Dual-Chamber Systems Have Two Leads:Dual-Chamber Systems Have Two Leads:

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

Benefits of Dual-Chamber PacingBenefits of Dual-Chamber Pacing

Study ResultsHigano et al. 1990

Gallik et al. 1994

Santini et al. 1991

Rosenqvist et al. 1991

Sulke et al. 1992

Improved cardiac index during low levelexercise (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, especiallyin patients with poor LV function

Decreased incidence of mitral and tricuspidvalve regurgitation

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Examples

VVIV: Ventricle is the paced chamberV: Ventricle is the sensed chamberI: Inhibited response to a sensed signal

Thus, a synchronous generator that paces and senses in the ventricle

Inhibited if a sinus or escape beat occursCalled a “demand” pacer

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ExamplesDVI

D: Both atrium and ventricle are pacedV: Ventricle is sensedI: Response is inhibited to a sensed

ventricular signal

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)

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Stimulation ProcessStimulation Process

Time (Milliseconds)100 200 300 400 500

Phase 2

Phase 1

Phase 3

Phase 4

Tran

smem

bran

e Po

tent

ial

(Mill

ivol

ts)

-50

0

50

-100

Phas

e 0

Threshold

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

Amplitude

Pulse width

Two Settings Are Used to Ensure Capture:Two Settings Are Used to Ensure Capture:

Amplitude is the Amount of Voltage Amplitude 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

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

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

DurationPulse Width (ms)

.50

1.0

1.5

2.0

.25St

imul

atio

n Th

resh

old

(Vol

ts)

0.5 1.0 1.5

Capture

Strength-Duration Curve Strength-Duration Curve

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

SDCSDC

Lead impedance

Amplitude and pulse width setting

Percentage paced vs. intrinsic events

Rate responsive modes programmed “ON”

Factors That Affect Battery Factors That Affect Battery Longevity Include:Longevity Include:

ImpedanceImpedance

The opposition to current flow

In a pacing system, impedance is:

– Measured in ohms

– Represented by the letter “R” (for numerical values)

– The measurement of the sum of all resistance to the flow of current

Impedance Changes Affect Pacemaker Impedance 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

– High impedance leads will show impedance reading values greater than 1,500 ohms

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

Ohm’s Law is a Fundamental Ohm’s Law is a Fundamental Principle of Pacing That:Principle of Pacing That:

VV

II RRV = I X RV = I X RI = V / RI = V / RR = V / IR = V / I

Describes the relationship between voltage, current, and resistance

xx

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 Law When Using Ohm’s Law You Will Find That:You Will Find That:

Voltage, Current, and Impedance Voltage, 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

Voltage and Current FlowVoltage and Current Flow

Spigot (voltage) turned up(high current drain)

Spigot (voltage) turned low(low current drain)

Resistance and Current FlowResistance and Current Flow

“Normal” resistance

“Low” resistance

“High” resistance Low current flow

High current flow

Electrode Design May Also Impact Electrode Design May Also Impact Stimulation ThresholdsStimulation Thresholds

Lead maturation process

Lead Maturation ProcessLead Maturation Process

Fibrotic “capsule” develops around the electrode following lead implantation

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.

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 tipfor steroid elution

Silicone rubber plugcontaining steroid

Tines forstablefixation

Lead Maturation ProcessLead Maturation ProcessEffect of Steroid on Stimulation Thresholds

Pulse Width = 0.5 msec

03 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

Vol

ts

A Pacemaker Must Be Able to Sense A 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

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

Sensitivity – The Greater the Number, the Sensitivity – The Greater the Number, the LessLess Sensitive the Device to Intracardiac EventsSensitive the Device to Intracardiac Events

Accurate Sensing Requires That Accurate Sensing Requires That Extraneous Signals Be Filtered OutExtraneous Signals Be Filtered OutSensing 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)

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

Single-Chamber TimingSingle-Chamber Timing

Single Chamber Timing TerminologySingle Chamber Timing Terminology

Lower rate

Refractory period

Blanking period

Upper rate

Lower Rate IntervalLower Rate Interval

Lower Rate Interval

VP VP VVI / 60

Defines the lowest rate the pacemaker will pace

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

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 PeriodRefractory Period

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 PeriodRefractory Period

Upper Sensor Rate Interval

Single Chamber Mode ExamplesSingle Chamber Mode Examples

VOO ModeVOO Mode

Blanking Period

VP VP

Lower Rate Interval

VOO / 60

Asynchronous pacing delivers output regardless of intrinsic activity

VVI ModeVVI Mode

Lower Rate Interval

VP VSBlanking/Refractory

VP

{

VVI / 60

Pacing inhibited with intrinsic activity

VVIR VVIR

VP VP

Refractory/Blanking

Lower Rate

Upper Rate Interval(Maximum Sensor Rate)

VVIR / 60/120Rate Responsive Pacing at the Upper Sensor Rate

Pacing at the sensor-indicated rate

Dual-Chamber TimingDual-Chamber Timing

Rate = 60 bpm / 1000 msA-A = 1000 ms

APVP

APVP

V-AAV V-AAV

Atrial Pace, Ventricular Pace (AP/VP)

Four “Faces” of Dual Chamber PacingFour “Faces” of Dual Chamber Pacing

Rate = 60 ppm / 1000 msA-A = 1000 ms

AP VS

AP VS

V-AAV V-AAV

Atrial Pace, Ventricular Sense (AP/VS)


ASVP

ASVP

Rate (sinus driven) = 70 bpm / 857 msA-A = 857 ms

Atrial Sense, Ventricular Pace (AS/ VP)

V-AAV AV V-A


Rate (sinus driven) = 70 bpm / 857 msSpontaneous conduction at 150 msA-A = 857 ms

ASVS

ASVS

V-AAV AV V-A

Atrial Sense, Ventricular Sense (AS/VS)


Dual Chamber Timing ParametersDual Chamber Timing Parameters

Lower rate

AV and VA intervals

Upper rate intervals

Refractory periods

Blanking periods

Lower Rate Interval

APVP

APVP

Lower Rate Lower Rate

The lowest rate the pacemaker will pace the atrium in the absence of intrinsic atrial events

DDD 60 / 120

APVP

ASVP

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

Lower Rate Interval

APVP

APVP

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 / 120PAV 200 ms; V-A 800 ms

200 ms 800 ms

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

DDDR 60 / 120A-A = 500 ms

APVP

APVP

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

ASVP

ASVP

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

Post Ventricular Atrial Refractory Period (PVARP)

Refractory PeriodsRefractory PeriodsVRP 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)

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

Blanking PeriodsBlanking PeriodsFirst portion of the refractory period-sensing is disabled

AP

VPAP

Post Ventricular Atrial Blanking (PVAB)

Post Atrial Ventricular Blanking

Ventricular Blanking (Nonprogrammable)

Atrial Blanking (Nonprogrammable)

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

What is happening here?

DDD / 60 / 120 / 310

PVARP

Wenckebach Operation

Upper Tracking Rate


AS AS AR APVPVP VP

TARPSAV PAV PVARP SAV PVARP

P Wave Blocked (unsensed or unused)

• Prolongs the SAV until upper rate limit expires– Produces gradual change in tracking rate ratio

TARP TARP

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

What is happening here?

DDD / 60 / 120 / 310

• Every other P wave falls into refractory and does not restart the timing interval

Upper Tracking Limit


{P Wave Blocked

AS AS

VPVPARAR

Sinus rate = 133 bpm (450 ms)PVARP = 300 ms SAV = 200 ms

TARP=500 ms

AV PVARP AV PVARPTARP TARP

2:1 Block

PVARP

Upper Tracking Rate

Lower Rate Interval

{No SAV started for events sensed in the TARP

AS AS

VPVP

SAV = 200 msPVARP = 300 ms

Thus TARP = 500 ms (120 ppm)

DDDLR = 60 ppm (1000 ms)

UTR = 100 bpm (600 ms) SAVTARP

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

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

PACEMAKER MODE?PACEMAKER MODE?



WHAT IS HAPPENING HERE?WHAT IS HAPPENING HERE?

WHAT IS HAPPENING HERE?WHAT IS HAPPENING HERE?

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Pacemaker basics