Basic Pacing ConceptsBasic Pacing Concepts

The heart generates electrical impulses that travel along a specialized conduction pathway

This conduction process makes it possible for the heart to pump blood efficiently

The Heart Has an Intrinsic PacemakerThe Heart Has an Intrinsic Pacemaker

Ventricles

Sinoatrial (SA) Node

Atrioventricular (AV) Node

Atria

During Conduction, an Impulse Begins in the During Conduction, an Impulse Begins in the Sinoatrial (SA) Node and Causes the Atria to ContractSinoatrial (SA) Node and Causes the Atria to Contract

Atria

Ventricles

Bundle branches

AV node

SA node

Then, the Impulse Moves to the Atrioventricular (AV) Node and Down Then, the Impulse Moves to the Atrioventricular (AV) Node and Down the Bundle Branches, Which Causes the Ventricles to Contractthe Bundle Branches, Which Causes the Ventricles to Contract

SA node

Prevent impulse generation in the SA node

Inhibit impulse conduction

AV node

Diseased Heart Tissue May:Diseased Heart Tissue May:

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:

Endocardial or transvenous leads

Myocardial/Epicardial leads

Types of LeadsTypes of Leads

Transvenous Leads Have Transvenous Leads Have Different “Fixation” MechanismsDifferent “Fixation” Mechanisms

Passive fixation

– The tines become lodged in the trabeculae(fibrous meshwork) of the heart

Transvenous LeadsTransvenous Leads

Active Fixation

– The helix (or screw) extends into the endocardial tissue

– Allows for lead positioning anywhere in the heart’s chamber

Myocardial and Epicardial LeadsMyocardial and Epicardial Leads

Leads applied directly to the heart

– Fixation mechanisms include:

Epicardial stab-in

Myocardial screw-in

Suture-on

CathodeCathode

An electrode that is in contact with the heart tissue

Negatively charged when electrical current is flowing

Cathode

AnodeAnode

An electrode that receives the electrical impulse after depolarization of cardiac tissue

Positively charged when electrical current is flowing

Anode

Conduction PathwaysConduction Pathways

Body tissues and fluids are part of the conduction pathway between the anode and cathode

Tissue

Cathode

Anode

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)

Stimulates the heart

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

Stimulates the heart

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

Unipolar and Bipolar LeadsUnipolar and Bipolar Leads

Unipolar leadsUnipolar leads

Unipolar leads may have a smaller diameter lead body than bipolar leads

Unipolar leads usually exhibit larger pacing artifacts on the surface ECG

Bipolar leadsBipolar leads

Bipolar leads are less susceptible to oversensing noncardiac signals (myopotentials and EMI)

Coaxial Lead Design

A Brief History of PacemakersA Brief History of Pacemakers

Single-Chamber and Dual-Chamber Single-Chamber and Dual-Chamber Pacing SystemsPacing Systems

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 implanted in both the atrium and the ventricle

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

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:

Voltage

Current

Impedance

Every Electrical Pacing Circuit Has Every Electrical Pacing Circuit Has the Following Characteristics:the Following Characteristics:

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 1,000

– High impedance leads will show impedance reading values greater than 1,000 ohms

Lead Impedance Values Will Change Due to:Lead Impedance Values Will Change Due to:

Insulation breaks

Wire fractures

An Insulation Break Around the Lead Wire An 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

Insulation break

Decreased resistance

A Wire Fracture Within the Insulating Sheath A 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

Lead wire fracture

Increased resistance

StimulationStimulation

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 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

.25S

tim

ula

tio

n T

hre

sho

ld (

Vo

lts)

0.5 1.0 1.5

Capture

Clinical Usefulness of the Clinical Usefulness of the Strength-Duration CurveStrength-Duration Curve

Adequate safety margins must be achieved due to:

– Acute or chronic pacing system

– Daily fluctuations in threshold

Capture

0.5 1.0 1.5Duration

Pulse Width (ms)

.50

1.0

1.5

2.0

.25

Sti

mu

lati

on

Th

resh

old

(V

olt

s)

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

SensingSensing

SensingSensing

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

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

Accurate Sensing is Dependent on . . .Accurate Sensing is Dependent on . . .

The electrophysiological properties of the myocardium

The characteristics of the electrode and its placement within the heart

The sensing amplifiers of the pacemaker

Unipolar SensingUnipolar Sensing

Produces a large potential difference due to:

– A cathode and anode that are farther apart than in a bipolar system

_

Bipolar SensingBipolar 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

Electromagnetic InterferenceElectromagnetic Interference

Electromagnetic Interference (EMI)Electromagnetic Interference (EMI)

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

EMI May Result in the Following Problems:EMI May Result in the Following Problems:

Oversensing

Transient mode change (noise reversion)

Reprogramming (Power on Reset or “POR”)

Oversensing May Occur When EMI Signals Are Oversensing May Occur When EMI Signals Are Incorrectly Interpreted as P Waves or R WavesIncorrectly 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

New technologies will continue to create new, unanticipated sources of EMI:

Cellular phones (digital)

Sources of EMI Are Found Most Sources of EMI Are Found Most Commonly in Hospital EnvironmentsCommonly in Hospital Environments

Sources of EMI that interfere with pacemaker operation include surgical/therapeutic equipment such as:

– Electrocautery

– Transthoracic defibrillation

– Extracorporeal shock-wave lithotripsy

– Therapeutic radiation

– RF ablation

– TENS units

– MRI

Sources of EMI Are Found More Rarely in:Sources of EMI Are Found More Rarely in:

Home, office, and shopping environments

Industrial environments with very high electrical outputs

Transportation systems with high electrical energy exposure or with high-powered radar and radio transmission

– Engines or subway braking systems

– Airport radar

– Airplane engines

TV and radio transmission sites

Rate Responsive PacingRate Responsive Pacing

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

Rate ResponseRate Response

Rate responsive (also called rate modulated) pacemakers provide patients with the ability to vary heart rate when the sinus node cannot provide the appropriate rate

Rate responsive pacing is indicated for:

– Patients who are chronotropically incompetent (heart rate cannot reach appropriate levels during exercise or to meet other metabolic demands)

– Patients in chronic atrial fibrillation with slow ventricular response

Rate Responsive PacingRate Responsive Pacing

Cardiac output (CO) is determined by the combination of stroke volume (SV) and heart rate (HR)

SV X HR = CO

Changes in cardiac output depend on the ability of the HR and SV to respond to metabolic requirements

Rate Responsive PacingRate Responsive Pacing

SV reserves can account for increases in cardiac output of up to 50%

HR reserves can nearly triple total cardiac output in response to metabolic demands

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

Rate Responsive PacingRate Responsive Pacing

Activity sensors employ a piezoelectric crystal that detects mechanical signals produced by movement

The crystal translates the mechanical signals into electrical signals that in turn increase the rate of the pacemaker

Piezoelectric crystal

Rate Responsive PacingRate Responsive Pacing

Minute Ventilation (MV) is the volume of air introduced into the lungs per unit of time

MV has two components:

– Tidal volume–the volume of air introduced into the lungs in a single respiration cycle

– Respiration rate–the number of respiration cycles per minute

Rate Responsive PacingRate Responsive Pacing

Minute ventilation can be measured by measuring the changes in electrical impedance across the chest cavity to calculate changes in lung volume over time

Rate Responsive PacingRate Responsive Pacing

Adjusting Heart Rate to Activity

Normal Heart Rate

Rate Responsive PacingFixed-Rate Pacing

Daily Activities

PPACINGACING

FUTURE !!??