basic technical concepts in cardiac pacing

Basic Technical Concepts in Basic Technical Concepts in Cardiac PacingCardiac Pacing

Dr D Sunil Reddy

Consultant Cardiologist

KIMS Hospital

TopicTopic

Electrical Stimulation of Cardiac Tissue

Myocardial StimulationMyocardial Stimulation

An artificial electrical stimulus excites cardiac tissue by the creation of an electrical field at the interface of the stimulating electrode and the myocardium

The electric field should be strong enough and should last long enough to initiate action potentials in the cells at the electrode-tissue interface

Myocardial StimulationMyocardial Stimulation

The AP’s at the site of stimulation result in AP’s in the neighbouring areas of the myocardium resulting in a wave of AP’s (depolarization) propagating away from the site of stimulation

Time (Milliseconds)100 200 300 400 500

Phase 2

Phase 1

Phase 3

Phase 4Tran

smem

bran

e Po

tent

ial

(Mill

ivol

ts)

-50

0

50

-100

Phas

e 0

Threshold

An artificial electrical stimulus excites cardiac tissue by the creation of an electrical field at the interface of the stimulating electrode and the myocardium

The electric field should be strong enough and should last long enough to initiate action potentials in the cells at the electrode-tissue interface

Electric Field – Current Density Electric Field – Current Density TheoryTheory

Electric Field Theory

– A Minimum Voltage/cm is required to trigger a self-propagating wave of depolarization

Current Density Theory

– A Minimum Current/cm2 is required to trigger a self-propagating wave of depolarization

Stimulation threshold is a function of Voltage/cm or Current/ cm2 that is induced in the myocardium beneath the stimulating electrode

The two theories are related by Ohm’s Law

The Implantable Pacemaker SystemThe Implantable Pacemaker System The Implantable Pulse Generator (IPG) : metal can

(titanium) containing electronics/battery & an electrode or lead connector header

Lead : Electrical connection between the pacemaker & the heart

The Pacemaker CircuitThe Pacemaker Circuit Stimulation of cardiac tissue using electric pulses

– IPG

– Lead – insulated current conductor (s) & electrodes to transmit pulses to heart tissue and measure or sense electrical activity in the heart

Heart

Lead

IPG

Completion of circuit through

Lead or Body tissue

A Unipolar Pacing System Contains a Lead with A Unipolar Pacing System Contains a Lead with Only One Electrode Within the Heart; In This Only One Electrode Within the Heart; In This

System, the Impulse:System, the Impulse:

Flows through the tip electrode (cathode)

Stimulates the heart

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

Cathode

Anode

-

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

Tip electrode coil

Indifferent electrode coil

Lead ComponentsConnector

Lead body

Electrode(s)

Insulation Conductors

FixationThreshold Sensing

Connector Standards

Electrodes -- Fixation Mechanism

Passive Fixation Mechanism – Endocardial

– Tined – Finned

– Canted/curved

Electrodes – Fixation Mechanism

Active Fixation Mechanism – Endocardial

– Fixed screw

– Extendible/retractable

Electrodes -- Fixation Mechanism

Fixation Mechanism – Myocardial/Epicardial

– Stab-in

– Screw-in

– Suture-on

Single-Chamber SystemSingle-Chamber System

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

One lead implanted in the atrium and one in the ventricle

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

A pacing system can be thought of a standard electrical circuit:

The pacemaker supplies the voltage.

Current (electrons) flow down the

conductor to the lead tip or cathode (-)

Where the lead tip touches the myocardium, electrical resistance is

produced.

The current then flows through the body tissues to the anode (+) and

back to the battery.

All of these things are required for current to flow.

Pacing Circuit ParametersPacing Circuit Parameters

Voltage

Current

Impedance (resistance)

Energy

VoltageVoltage

Pushes electrons through an electric circuit resulting in electric current through the circuit

VoltageVoltage

Voltage is the force or “push” that causes electrons to move through a circuit

In a pacing system, voltage is:

– Measured in volts

– Represented by the letter “V”

– Provided by the pacemaker battery

– Often referred to as amplitude

CurrentCurrent

Current is the flow of charge or electrons through a circuit

CurrentCurrent

The flow of electrons in a completed circuit

In a pacing system, current is:

– Measured in mA (milliamps)

– Represented by the letter “I”

– Determined by the amount of electrons that move through a circuit

Resistance or ImpedanceResistance or Impedance

Opposition offered by a circuit to the flow of current

ImpedanceImpedance

The opposition to current flow in a circuit

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

Ohm’s LawOhm’s Law

Voltage = Current X Resistance

Current = Voltage Resistance

Resistance = Voltage Current

The Pacemaker StimulusThe Pacemaker Stimulus

Time

5 Volts 5 Volts 0.5 ms

1 sec

Pacing Stimulus Voltage or Amplitude – 5 Volts

Pulse Width – 0.0005 seconds or 0.5 milliseconds

Pacing Rate – One stimulus per second or 60 stimuli (beats) per minute

Vol

tage

The Pacing PulseThe Pacing Pulse

t

Pacing Pulse

Pulse Duration (Width)

Out

put V

olta

ge

V = Pulse Amplitude in Volts (V) (say 2.5 V)

t = Pulse Duration or Width in milliseconds (ms) (say 0.5 ms)

R = Impedance of Pacing Circuit (ohms) (say 500 ohms)

I = V/R = Current through pacing circuit (mA) = 2.5 V/ 500 ohms = 0.005 A = 5 mA

E = Energy supplied by Pulse to the Pacing Circuit and Cardiac Tissue = V . I . t = I2Rt = V2t/R = 2.5 V . 5 mA . 0.5 ms = 6.25 micro Joules

V

t

Stimulation ThresholdStimulation Threshold

Pacing Voltage Threshold – The minimum pacing voltage at any given pulse width required to consistently achieve myocardial depolarization outside the heart’s refractory period

Loss of Capture

The Strength-Duration RelationThe Strength-Duration Relation

The intensity of an electrical stimulus (Energy, Voltage, Current, Charge) required to capture (non-refractory) cardiac tissue is dependent on the duration for which the electrical stimulus is applied (i.e. pulse width)

The Voltage-Strength Duration The Voltage-Strength Duration CurveCurve

Stimulus Voltage & Pulse Width have an exponential relationship

At short pulse widths (<0.25 ms)the curve rises sharply (i.e. small reductions in pulse width result in large increases in the voltage threshold)

At long pulse widths (>1.0 ms)the curve is flat (i.e. the voltage threshold does not reduce with increasing pulse width) Duration

Pulse Width (ms)

.50

1.0

1.5

2.0

.25St

imul

atio

n Th

resh

old

(Vol

ts)

0.25 1.0 1.5

Capture

Rheobase & ChronaxieRheobase & ChronaxieRheobase Voltage = Voltage Threshold at infinite Pulse widths (e.g. 2 ms) – also known as fundamental threshold

Chronaxie Point = Pulse Width threshold at twice the rheobase voltage (i.e. 1 V & 0.3 ms)

Energy of Pacing PulseAt rheobase = 0.5V. 2ms. 1mA = 1 MicroJoule

At Chronaxie = 1V. 0.3 ms. 2mA = 0.6 MicroJoules

At PW=0.25 ms, = 1.5V. 0.25ms. 3mA = 1.125 MicroJoules

The chronaxie point approximates the point of minimum threshold energy on the Strength-Duration Curve

DurationPulse Width (ms)

Stim

ulat

ion

Thre

shol

d (V

olts

)

0.2 0.6 1.0

Energy Strength Duration Curve

Capture

0.4

0.5

1.0

2.0

0.8 1.2 1.4

Rheobase Voltage

Chronaxie Point

Goals of Pacemaker Output Goals of Pacemaker Output ProgrammingProgramming

Consistent capture & Patient Safety is ensured

Battery drain minimized, Pacemaker longevity maximized

Programming Chronic Pacemaker Programming Chronic Pacemaker OutputOutput

DurationPulse Width (ms)

Stim

ulat

ion

Thre

shol

d (V

olts

)

0.2 0.6 1.0

Energy Strength Duration Curve

• Capture

Adequate Safety Margin while minimizing Pulse Energy

Operate around the chronaxie point

Programming long pulse widths (& low Voltages) increases Pulse energy but hardly increases safety margin

Programming high voltages (with short Pulse widths) increases Pulse energy but hardly increases safety margin

For a 2 times safety margin : Output = 2 x chronaxie voltage at chronaxie pulse width (2V, 0.35 ms)

Output = 3 x pulse duration threshold at twice chronaxie voltage (2V, 0.51 ms)

0.4

0.5

1.0

2.0

0.8 1.2 1.4

Rheobase

Chronaxie

Determination of Stimulation Determination of Stimulation Threshold during Implant or Follow-upThreshold during Implant or Follow-up

Pace the heart at

– Rate higher than sinus or intrinsic rate

– Stimulus amplitude and pulse width that ensure capture (usually 5 V and 0.5 ms)

Gradually reduce stimulus amplitude while maintaining pulse width constant till capture is lost

The Stimulation threshold is specified by the minimum stimulus amplitude at which capture consistently occurs at a given stimulus pulse width

Programming Pacemaker OutputProgramming Pacemaker Output

Acute – Immediately post-implant

Chronic – Approx. 6 to 8 weeks post implant

Goal : To ensure consistent capture despite potential changes in the SD curve while minimizing the energy delivered by the pulse

Evolution of Pacing ThresholdEvolution of Pacing ThresholdV

olta

geTh

resh

old

(V)

Observation Time (weeks)

Acute Phase

Chronic Phase

Safety Margin

6

5

4

3

2

1

0 4 8 12 16

Programmable ParametersProgrammable Parameters

Pacemaker Pulse Output or Amplitude

– 0.5 V, 1.0 V, 1.5 V…., 7.5 V

Pacemaker Pulse Width

– 0.03, 0.06, …., 0.25, 0.5,…, 1.0 milliseconds

Pacing Impedance - ConsiderationsPacing Impedance - Considerations

Maximize Pacemaker Longevity

– Reduce current drain

– Maintain relatively high impedance

Ensure consistent capture

– Appropriate voltage & current are available at the electrode-tissue interface required to stimulate tissue

I pacing

Components of Pacing ImpedanceComponents of Pacing Impedance

Rcoil

Zpolarisation -electrode tissue

interface

Rtissue

Electrode

Tissue

Interface

Pulse GeneratorV pulse

I pacing

RRcoil - coil - Lead Conductor ResistanceLead Conductor Resistance

– Lead Conductor Impedance

• Reduces the voltage & energy available at the tissue for pacing

• Generates waste heat

• Designed to be low

conductorresistance

Conductor ResistanceConductor Resistance

Total Coil Voltage Voltageimpedance resistance at tip loss

()() (V) %

600 50 2.3 8.3

750 200 1.8 26.7

1200 650 1.1 54.2

1200 50 2.4 4.2

Output Voltage = 2.5 V

Impact of conductor coil resistance upon available tip electrode voltage

Conductor Resistance

V mV2.5 2500

Total imp 600

Current I =V/R 4.166666667 mAConductor imp 50

Voltage drop V = IR 208.3333333 mV0.208333333 V

What is the Polarization Effect – Polarization What is the Polarization Effect – Polarization Impedance ?Impedance ?

As the pacing pulse begins, electrons from the pacemaker battery flow to the lead tip and positively charged ions from the tissue are attracted to the lead tip.

Initially, the movement of these negatively charged ions results in the flow of current from the electrode into the myocardium.

As the pacing pulse continues, positively charged ions surround the electrode tip. This produces a positively charged layer on the electrode called polarization. Polarization can impede current flow from the electrode into the tissue.

Polarization Impedance & Polarization Impedance & Electrode Surface AreaElectrode Surface Area

As electrode surface area goes up

– Polarization impedance decreases

As electrode surface area goes down

– Polarization impedance increases

Polarization ImpedancePolarization Impedance

Pacing Pulse

Polarization

Factors affecting Polarization Factors affecting Polarization ImpedanceImpedance

Pulse Width

Surface area of the electrode tip

– Larger the surface area – Lower the Polarization

Electrode – Tissue Interface Electrode – Tissue Interface ImpedanceImpedance

Resistance to current flow from body tissue at the electrode-tissue interface

Electrode – Tissue Interface Electrode – Tissue Interface ImpedanceImpedance

Smaller the surface area of the tip electrode, the higher the current density (or electric field strength) at the electrode-tissue interface

The higher the field strength, the greater the pacing efficiency & lower the threshold

Electrode – Tissue Interface Electrode – Tissue Interface ImpedanceImpedance

Smaller the geometric size of the tip electrode, the higher the resistance between electrode and tissue

Paci

ng Im

peda

nce

(Ohm

s)

0

500

1000

1500

0 1 2 3 4 5.5 6

Geometric Tip Electrode Surface Area (mm2)

Size = Impedance

The Ideal Stimulating ElectrodeThe Ideal Stimulating Electrode

Needs large surface area to reduce Polarization Impedance

Needs small size to maximize Electric Field Strength and Stimulation Efficacy

Needs small size to maximize Electrode-tissue resistance and minimize pacemaker current drain

Electrodes -- Surface StructureElectrodes -- Surface Structure

Porous Electrode Surface

CapSure® 8.0 mm2

Porous Electrode

CapSure® SP Novus5.8 mm2 Platinized Porous Electrode

CapSure® Z Novus1.2 mm2 Platinized Porous Electrode

15KV x2500 12.0V MDT

Analogy:Lead with low impedance and poor efficiency

Lead Impedance I

Lead Impedance II

(Conductor-) Resistance

Analogy:Lead with high impedance in the conductor and poor efficiency

Lead Impedance III

Analogy:Lead with big tip-tissue impedance low conductor impedance and increased efficiency

The Chronic Virtual ElectrodeThe Chronic Virtual Electrode

IMPLANT CHRONIC(8 weeks or longer)

ExcitableCardiacTissue

Non-ExcitableFibroticTissue

ExcitableCardiacTissue

Evolution of Pacing ThresholdEvolution of Pacing ThresholdV

olta

geTh

resh

old

(V)

Observation Time (weeks)

Acute Phase

Chronic Phase

Safety Margin

6

5

4

3

2

1

0 4 8 12 16

Electrodes -- Steroid ElutionElectrodes -- Steroid Elution

Tines forStable Fixation

Silicone Rubber PlugContaining Steroid

Porous, Platinized Tipfor Steroid Elution

Type - Steroid in matrix

Electrodes -- Steroid ElutionElectrodes -- Steroid Elution Effect 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

Factors that affect Stimulation Factors that affect Stimulation ThresholdThreshold

Eating, Sleeping, Exercise, Medications, Changes in Cardiac condition – 30 to 50 % during the day

Drugs e.g. Steroids reduce stimulation threshold by reducing inflammation

Sympathomimetic drugs decrease threshold

Amiodarone, Class I A (quinidine, procainamide), Class I B (Mexilitene)

Hypokalemia increases threshold (diuretics)

Hypocalcemia increases threshold

Hypoxia & Hypercapnia – Increase threshold

Electrolyte imbalance & pH e.g. acidosis and alkalosis increase threshold

Typical Pacing Circuit Typical Pacing Circuit ImpedancesImpedances

300 to 1200 ohms

May be measured at implant time with a Pacing System Analyzer (PSA) at implant time

May be measured through telemetry with a pacemaker programmer

Pacing Impedance Values Will Change Due Pacing Impedance Values Will Change Due to: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

Unipolar &Bipolar StimulationUnipolar &Bipolar Stimulation

+

-

+ -

-

+ Anode is IPG Case

Cathode is lead tip electrode

-

+ Anode is lead proximal electrode

Cathode is lead tip electrode

Unipolar & Bipolar StimulationUnipolar & Bipolar Stimulation

Unipolar Stimulation– Large Stimulus Artifact

Bipolar Stimulation – Small Stimulus Artifact

There is a greater chance of pacemaker pocket muscle stimulation with Unipolar Stimulation

The Pacemaker BatteryThe Pacemaker Battery

Pacemakers typically use Lithium-Iodide batteries

Most LiI batteries have a Beginning of Life (BOL) value of 2.8 volts

Recommended Replacement Time (ERI/RRT) value of 2.6 V

End of Service (EOL/EOS) value of 2.5 V

As battery depletes its internal resistance goes up – (BOL = 100 ohms , EOL > 5000 ohms )

Recommended Replacement Time RRT (ERI)

2.8V-

2.6V-

2.5V-

BOL

RRT 2.6V

EOS 2.5V

Time

Lithium Iodine Battery depletion

BOL – Beginning of LifeRRT – Recommended Replacement TimeEOS - End of Service

Battery LifeBattery Life Battery Life

– Battery Life = Battery Capacity/Current Drain

– 2.0 Ah/25microamps = 80,000 hours = 9.3 years

Battery properties

– Reliability – no premature failure

– High volumetric energy density – Small battery volume with high storage capacity

– Low self-discharge rate

– High hermiticity – no gas generation during operation

Pacemaker LongevityPacemaker Longevity

Energy = (V.I.t) = (V2/R) . t Joules

Reducing V by a factor of 2 reduces E by a factor of 4

Increasing Z by a factor by 2 reduces E by a factor of 2

Pacemaker LongevityPacemaker Longevity

High Outputs and pulse widths are the primary cause for reduced pacemaker longevity

• Output Voltages higher than the Battery Voltage (2.8 V) require the use of voltage doublers that use high battery energy

• Long Pulse Widths Reduce pacing efficiency due to increased polarization impedance

Sensing Intracardiac Electrical Sensing Intracardiac Electrical ActivityActivity

A Pacemaker Must Be Able to Sense and Respond to Cardiac Rhythms

Accurate sensing enables the pacemaker to determine whether or not the heart has created a beat on its own

The pacemaker is usually programmed to respond with a pacing impulse only when the heart fails to produce an intrinsic beat

Intracardiac Electrical SignalsIntracardiac Electrical Signals

Electrical currents that arise in the myocardium during depolarization and repolarization

A myocardial electrode - records voltage difference wrt reference electrode when the myocardium under the electrode undergoes depolarization or repolarization

The electrical activity measured by such an electrode (which is in direct contact with cardiac tissue) – local tissue electrical activity- Intracardiac Electrogram or EGM

Sensing the EGMSensing the EGM

Depolarization Wave

Processed by Processed by DeviceDevice

The EGM Signal

The signal from a depolarization wave passing between two electrodes

Intracardiac ElectrogramIntracardiac Electrogram

R wave of the EGM indicating depolarisation of cardiac ventricular tissue at lead tip – Roughly corresponds to R wave of the ECG that represents depolarisation of the ventricles

T wave of the EGM indicating repolarisation of cardiac ventricular tissue at lead tip – Roughly corresponds to T wave of the ECG that represents repolarisation of the ventricles

Voltage Deflections of the Sensed Voltage Deflections of the Sensed EGM in a PacemakerEGM in a Pacemaker

Pacemaker

Stimulus

Paced R wave

Post-pace T wave

Intrinsic R wave

T wave corresponding to intrinsic R wave

Undersensing . . .

Pacemaker does not “see” the intrinsic beat, and therefore does not respond appropriately

Intrinsic beat not sensed

Scheduled pace delivered

VVI / 60

Oversensing

An electrical signal other than the intended P or R wave is detected

Marker channel shows

intrinsic activity...

...though no activity is present

VVI / 60

Measured by:

– Amplitude

• Peak-to-peak measurement (height)

of deflection

• Measured in Millivolts (mV)– Slew Rate

• Speed of deflection change over time

• Measured in volts per second (V/s)

EGM amplitude

EGM Amplitude & Slew RateEGM Amplitude & Slew Rate

Amplitude of the EGM R wave = dV

Slew Rate of the EGM R wave= dV/dt

EGM amplitude & slew rate – ideal values

Atrium Ventricle

Slew Rate 0.5 volts/sec 0.75 volts/secAmplitude >1.5 mV >5.0 mV

Which of the these complexes shows the highest slew rate?

a)1 b)2 c)3 d)4

Frequency (Hz)

Accurate Sensing Requires That Extraneous Signals Be Filtered Out

Effect of Filtering on EGMEffect of Filtering on EGM

FILTER

•Filters out frequencies below 5 Hz (T waves) and above 50 Hz (myopotentials)

•Enhances frequencies between 10 Hz and 50 Hz (R waves) with maximum enhancement at around 30 Hz frequency

• Low Slew Rate implies Low Frequency implies Less enhancement due to filtering

LevelDetector

Level Detector or Sensitivity Level Detector or Sensitivity SettingSetting

This is a value specified to the pacemaker in millivolts through programming.

All ventricular EGM deflections AFTER FILTERING that exceed the sensitivity setting will be identified by the pacemaker as intrinsic R waves

The typical sensitivity setting that is programmed for Ventricular sensing is 2.5 mV

Sensitivity The Greater the Number, the Less Sensitive the Device to Intracardiac Events

SensitivityAm

plit

ude

(mV)

Time

5.0

2.5

1.25

Sensitivity SettingSensitivity Setting

Sensitivity settings less than 2.5 mv – High sensitivity – can lead to oversensing

Sensitivity settings greater than 2.5 mV – Low sensitivity – can lead to undersensing

Am

plitu

de (m

V)

Am

plitu

de (m

V)

Time Time

5.0

2.5

1.25

5.0

2.5

1.25

Factors That May Affect Sensing Are:

Lead polarity (unipolar vs. bipolar)

Lead integrity

– Insulation break

– Wire fracture EMI – Electromagnetic Interference

Unipolar Sensing

Produces a large potential difference due to:

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

_

Bipolar Sensing

Produces a smaller potential difference due to the short interelectrode distance

– Electrical signals from outside the heart such as myopotentials are less likely to be sensed

An Insulation Break May Cause Both Undersensing or Oversensing

Undersensing occurs when inner and outer conductor coils are in continuous contact

– Signals from intrinsic beats are reduced at the sense amplifier and amplitude no longer meets the programmed sensing value

–

Oversensing occurs when inner and outer conductor coils make intermittent contact

– Signals are incorrectly interpreted as P or R waves

Wire Fracture Can Cause Both Undersensing and Oversensing

Undersensing occurs when the cardiac signal is unable to get back to the pacemaker – intrinsic signals cannot cross the wire fracture

Oversensing occurs when the severed ends of the wire intermittently make contact, which creates potentials interpreted by the pacemaker as P or R waves

Fracture in one filament leads to an increase in resistance

Electromagnetic Interference

Interference is caused by electromagnetic energy with a source that is outside the body

Electromagnetic fields that may affect pacemakers are radio-frequency waves

– 50-60 Hz are most frequently associated with pacemaker interference

Few sources of EMI are found in the home or office but several exist in hospitals

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

Pacing rates will vary as a result of EMI:

– Rates will accelerate if sensed as P waves in dual-chamber systems (P waves are “tracked”)

– Rates will be low or inhibited if sensed in single-chamber systems, or on ventricular lead in dual-chamber systems

Electrocautery is the Most Common Hospital Source of Pacemaker EMI

Outcomes

– Oversensing–inhibition

– Undersensing (noise reversion)

– Power on Reset

– Permanent loss of pacemaker output

(if battery

voltage is low)

Precautions

– Reprogram mode to VOO/DOO, or place a magnet over device

– Strategically place the grounding plate

– Limit electrocautery bursts to 1-second burst every 10 seconds

– Use bipolar electrocautery forceps

Transthoracic Defibrillation Outcome

– Inappropriate reprogramming

of the pulse

generator (POR)

– Damage to

pacemaker circuitry Precautions

– Position defibrillation paddles apex-posterior (AP) and as far from the pacemaker and leads as possible

Magnetic Resonance Imaging (MRI) is Generally Contraindicated in Patients with Pacemakers

Outcomes

– Extremely high pacing rate

– Reversion to asynchronous pacing

Precautions

– Program pacemaker output low enough to create persistentnon-capture, ODO or OVO mode

Radiation Energy May Cause Permanent Damage

Certain kinds of radiation energy may cause damage to the semi-conductor circuitry

– Ionizing radiation used for breast or

lung cancer therapy Damage can be permanent and requires

replacement of the pacemaker

Therapeutic Radiation May Cause Severe Damage

Outcomes:

– Pacemaker circuit damage

– Loss of output

– “Runaway”

Precautions:

– Keep cumulative radiation absorbed by the pacemaker to less than 500 rads; shielding may be required

– Check pacemaker after radiation sessions for changes in pacemaker function (can be done transtelephonically)

Refractory & Blanking PeriodsRefractory & Blanking Periods

Voltage Deflections of the Sensed Voltage Deflections of the Sensed EGM in a PacemakerEGM in a Pacemaker

Pacemaker

Stimulus

Paced R wave

Post-pace T wave

Intrinsic R wave

T wave corresponding to intrinsic R wave

2.5 mV

Refractory PeriodRefractory Period

Refractory Period Refractory Period Refractory Period

NO SENSING NO SENSING NO SENSING

Refractory PeriodRefractory Period

A programmable period immediately following a pacemaker stimulus or a sensed intrinsic R wave during which the pacemaker does not react to sensed events

To prevent repeated sensing of the same intrinsic R wave

To prevent misidentification of T waves as intrinsic R waves

To prevent misidentification of effects of pacemaker stimulus/evoked R wave

Usually programmed to 325 ms

To Prevent Oversensing

Afterpotential due to PolarizationAfterpotential due to Polarization

Afterpotential

Blanking PeriodBlanking Period

The first portion of every refractory period

Pacemaker is “blind” to any activity and no events can be sensed

Designed to prevent oversensing of pacing stimulus & after-potential

Blanking PeriodRefractory Period

Conductor Tip Electrode Insulation Connector Pin

Pacing Lead ComponentsPacing Lead Components

Conductor Connector Pin Insulation Electrode

ConnectorConnector

Purpose

– Connects lead to IPG, and provides a conduit to:• Deliver current from IPG to lead • Return sensed cardiac signals to IPG

Connector

Connector -- IS-1 StandardConnector -- IS-1 Standard

IS-1 Standard Connectors

Sizes Prior to IS-1 Standard

– 3.2 mm low-profile connectors

– 5/6 mm connectors

Insulation -- TypeInsulation -- Type

Insulation Types

– Silicone

– Polyurethane

– Fluoropolymers (PTFE, ETFE)

Electrodes -- Fixation MechanismElectrodes -- Fixation Mechanism

Passive Fixation Mechanism – Endocardial

– Tined – Finned

– Canted/curved

Electrodes – Fixation MechanismElectrodes – Fixation Mechanism

Active Fixation Mechanism – Endocardial

– Fixed screw

– Extendible/retractable

Electrodes -- Fixation/VisualizationElectrodes -- Fixation/Visualization

Fluoroscopic Visual Quality of Active Fixation Leads

SureFixCapSureFix®

Extended Retracted Fixed Screw

space

Electrodes -- Fixation MechanismElectrodes -- Fixation Mechanism

Fixation Mechanism – Myocardial/Epicardial

– Stab-in

– Screw-in

– Suture-on

Battery CapacityBattery Capacity

A battery is a reservoir of electrical charge measured in Coulombs

Current is the amount of charge delivered per unit time – 1 Ampere = 1 Coulomb per second

– 1 Coulomb = 1 Ampere x 1 second

Qc = Battery Capacity is specified as the quantity of charge it can deliver in AmpereHours (0.5 to 3 Amperehours)

Battery LifeBattery Life Battery Life

– Battery Life = Battery Capacity/Current Drain

– 2.0 Ah/25microamps = 80,000 hours = 9.3 years

Battery properties

– Reliability – no premature failure

– High volumetric energy density – Small battery volume with high storage capacity

– Low self-discharge rate

– High hermiticity – no gas generation during operation

On the figure, the zone of non capture is indicated by which number?

a)1b)2c)3d)4

Which of the following output settings best represents the Chronaxie point on the strength-duration curve when the Rheobase is 0.5V @ 1.5ms?

a) 0.5V @ 1.5ms

b) 1.0V @ 0.5ms

c) 1.5V @ 0.1 ms

d) 2.0V @ 0.05ms

Sensing

Sensing is the ability of the pacemaker to “see” when a natural (intrinsic) depolarization is occurring

– Pacemakers sense cardiac depolarization by measuring changes in electrical potential of myocardial cells between the anode and cathode

Intrinsic deflection on an EGM occurs when a depolarization wave passes directly under the electrodes

Two characteristics of the EGM are:

– Signal amplitude

– Slew rate