PACEMAKER BASICS

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• Cathode: negative– Electrode on the tip of a

pacing lead• Anode: positive– The “ring” electrode on

a bipolar lead– The PG case on a

unipolar systemAnode

Cathode

Implantable Pacemaker Circuit

• Pulse generator (PG): – Battery– Circuitry– Connector(s)

• Leads– Cathode– Anode

• Body tissueIPG

Lead

Anode

Cathode

The Pulse Generator

• Battery - provide energy• Circuitry - controls

pacemaker operations• Connector- join the PG to

the lead(s)Circuitry

Battery

Connector Block

Lead Characterization

• Position within the heart– Endocardial/transvenous leads– Epicardial leads

• Fixation mechanism– Active/Screw-in– Passive/Tined

• Shape– Straight– J-shaped used in the atrium

• Polarity– Unipolar– Bipolar

• Insulator– Silicone– Polyurethane

Endocardial Passive Fixation Leads• The tines become lodged

in the trabeculae,a fibrous meshwork, of the heart

Tines

Transvenous Active Fixation Leads• The helix, or screw,

extends into the endocardial tissue– Allows for lead positioning

anywhere in the heart’s chamber

– The helix is extended using an included tool

Epicardial Leads

• Leads applied directly to the surface of the heart– Fixation mechanisms include:

• Epicardial stab-in• Myocardial screw-in• Suture-on

– Applied via sternotomy or laproscopy

Lead Polarity

• Unipolar leads – Smaller diameter lead body

than bipolar leads– Usually larger pacing artifacts

on ECG

• Bipolar leads – Usually less susceptible to

oversensing of non-cardiac signals

Unipolar lead

Bipolar coaxial lead

To tip (cathode)

Unipolar Pacing System• Lead has only one electrode –

cathode – at the tip• PG can - anode• When pacing, the impulse:

– Flows through the tip electrode (cathode)

– Stimulates the heart– Returns through body fluid and

tissue to PG can (anode)

Cathode -

Anode +

•more interference (myopotentials)

• Big spike on ECG

• Pectoral (pocket) stimulation possible

Anode

Bipolar Pacing System

• The lead has both an anode and cathode

• The pacing impulse: – Flows through the tip

electrode located at the end of the lead wire

– Stimulates the heart– Returns to the ring electrode,

the anode, above the lead tip

Cathode

Anode +

Cathode -•less interference

• Spike difficult to see on ECG

• No pectoral (pocket) stimulation

STIMULATION THRESHOLD

• The minimum stimulus intensity & duration necessary to reliably initiate a propagated depolarising wavefront from an electrode

• For a pacing stimulus to “capture”, the stimulus must exceed a critical amplitude & must be applied for a sufficient duration

• Stimulus amplitude & duration interact- minimal amplitude required to capture depends on the pulse duration

Strength–Duration Relation

Stimulus amplitude for endocardial stimulation has an exponential relation to duration of the pulse- rapidly rising strength–duration curve at pulse durations <0.25 ms and a relatively flat curve at pulse durations > 1.0 ms

• Chronaxie pulse duration ~ point of min threshold energy on strength–duration curve

• With pulse durations > chronaxie- relatively little ↓ in threshold V

• Wider pulse dur→wasting of energy without providing a substantial ↑ in safety margin

• When threshold determined by ↓ amplitude, stimulus V set twice the threshold value

• PG that determine threshold by automatically ↓pulse duration-at least 3times threshold

• Wedensky effect

Hyperacute phase of threshold evolution- active fixation electrodes may produce,immediately following implantation,an ↑ stimulation threshold that ↓over the next 20-30 min- transient high threshold – a/c injury at myocardial–electrode interface

Proper programming of the stimulus amplitude

• The strength–duration relation must be appreciated• The safety margin that is chosen for a particular pt

must be based on the degree of pacemaker dependency

• An appreciation of the effect of stimulus amplitude & duration on battery longevity

• The overall metabolic & pharmacologic history of the pt must be considered

• Pacing threshold varies inversely with surface area of the stimulating electrode

Strength–Interval Relation

• The stimulation threshold is influenced by the coupling interval of electrical stimuli and frequency of stimulation

• ICD- greater stimulus intensity during ATP than during antibradycardia pacing

(Ztotal) = Zc Ze ZpZp - related to movmnt

of charged ionmyo toward the s in cathode

Zp -directly related to pulse duration & can be ↓ by use of relatively ↓ pulse durations

Polarization is inversely related to SA of electrode- to ↓ Zp but ↑ Ze, SA of electrode can be made large but radius small by use of a porous coating

Afterpotential of opposite chargeis induced in the myocardium at the interface of

the stimulating electrode

The slope of intrinsic deflectn (dV/dt) expressed in V/s -referred to as slew rate

For an EKG to be sensed, the amplitude & slew rate must exceed the sensing threshold

• For a battery,the decay characteristics should be predictable. The ideal battery should have a predictable fall in V near the end of life, yet provide sufficient service life after initial voltage decay to allow time for the elective replacement indicator to be detected and for replacement to be performed

RATE-ADAPTIVE SENSORS

• Activity Sensors and Accelerometers• Minute Ventilation Sensors

• P- Native atrial depolarization• A- Atrial paced event• R- Native ventricular depolarization• V- Ventricular paced event• AV- Sequential pacing in the atrium and ventricle• AVI- Programmed AV pacing interval• AR- Atrial paced event foll by intrinsic ventricular• ARP- Atrial refractory period• PV- Native atrial foll by paced ventricular, P-

synchronous• AEI- Interval from a ventricular sensed/paced to atrial

paced event, the VA interval

NASPE/ BPEG Generic (NBG) Pacemaker Code

I. Chamber II. Chamber III. Response to IV. Programmability V. Antitachy Paced Sensed Sensing Rate Modulation arrhythmia funct.

O= none O= none O= none O= none O= noneA=atrium A= atrium T= triggered P= simple P= pacingV= ventricle V= ventricle I= inhibited M= multi S= shockD= dual D= dual D= dual C= communication D= dual(A+V) (A+V) (T+I) R= Rate Modulation

Manufacturers’ Designation only:

S= single S= single(A or V) (A or V)

PACING MODES

AOO & VOO

• By application of magnet• Useful in diagnosing pacemaker dysfunction• During surgery to prevent interference from

electrocautery

Ventricular asynchronous (VOO) pacing

• Neither sensing nor mode of response• Irrespective of any other events, V pacing

artifacts occur at programmed rate. Timing cycle cannot be reset by any intrinsic event

Atrial asynchronous (AOO)AV sequential asynchronous (DOO)

• AVI & VAI or AEI are both fixed- intervals never change, as is insensitive to any atrial or ventricular activity, and timers never reset

Ventricular demand inhibited (VVI)

• Sensing on V channel- output inhibited by sensed V event

• Refractory after V/R- VRP- any V event in VRP-not sensed & doesn’t reset V timer

• LRL

VVI MODE

Automatic Interval

• Automatic interval starts from a paced complex (to the next paced complex)

• Escape interval starts from a sensed complex (to the next paced complex)

Escape Interval

If the intervals are equal:

• No hysteresis

If the escape interval > automatic interval:

• Hysteresis

VVI MODE (with hysteresis)

1000 ms

850 ms

Escape interval = 1000 ms (60 ppm)

Automatic interval = 850 ms (70 ppm)

AAI• Useful for SSS with N- AV conduction• Should be capable of 1:1 AV to rates 120-140 b/m • Atrial tachyarrhythmias should not be present• Atria should not be “silent”• If no A activity, atria paced at LOWER RATE limit (LR)• If A activity occurs before LR,- “resetting”• An A activity too early may not cause depolarisation & better

not sensed- ARP • Caution- far-field sensing of V activity

Atrial inhibited (AAI) pacing

Single-Chamber Triggered-Mode

• Output pulse every time a native event sensed• ↑current drain• Deforms native signal• Prevent inappropriate inhibition from

oversensing when pt does not have a stable native escape rhythm

• Can be used for noninvasive EPS,with already implanted PPI tracking chest wall stimuli created by a programmable stimulator

Single-Chamber Rate-Modulated Pacing

AV sequential-V Inhibited Pacing (DVI)

• PPI is inhibited & reset by sensed V activity but ignores all intrinsic A complexes

• Native R during AVI sensed – V output inhibited & AEI reset

• For both A&V stimuli to be inhibited, sensed R must occur during AEI

• Modified or partially committed version-physiologic AVI

AV Sequential, Non–P-Synchronous Pacing with D-Chamber Sensing (DDI)

• AAI + VVI• Difference btw DVI & DDI- DDI incorporates A

sensing as well as V sensing- prevents competitive A pacing

• Mode of response is inhibition only- no tracking of P waves - paced V rate cannot be > programmed LRL

• Goes by LR for each• Timing cycles- LRL, AVI, PVARP & VRP

A Synchronous (P-Tracking) Pacing (VDD)

• VAT + VVI • Timing cycle- LRL,AVI,PVARP,VRP,& URL• Concept of AV interval (AVI)• A sensed atrial event initiates AVI • Goes by LR• If no A event occurs, PPI escapes with a V pace

at LRL- VVI • AV block with intact sinus node function (esp

useful in congenital AV block)

DDD• VAT + AAI + VVI• AV interval & VA interval• VA interval replaces the LR• Concept of PVARP (to prevent endless loop or PMT)

• Indications• 1. The combination of AV block and SSS• 2. Patients with LV dysfunction & LV hypertrophy who

need coordination of atrial & ventricular contractions to maintain adequate CO

DDD

DDD ExamplesThe Four Faces of DDD

• Atrial and ventricular pacing

– Atrial pace re-starts the lower rate timer and triggers an AV delay timer (PAV)• The PAV expires without being inhibited by a ventricular sense, resulting in

a ventricular pace

AP

AP

VP

VP

DDD ExamplesThe Four Faces of DDD

• Atrial pacing and ventricular sensing

– Atrial pace restarts the lower rate timer and triggers an AV delay timer (PAV)• Before the PAV can expire, it is inhibited by an intrinsic ventricular event

(R-wave)

AP

AP

VS

VS

DDD ExamplesThe Four Faces of DDD

• Atrial sensing, ventricular pacing

– The intrinsic atrial event (P-wave) inhibits the lower rate timer and triggers an AV delay timer (SAV)• The SAV expires without being inhibited by an intrinsic ventricular event,

resulting in a ventricular pace

AS

AS

VP

VP

DDD ExamplesThe Four Faces of DDD

• Atrial and ventricular sensing

– The intrinsic atrial event (P-wave) inhibits the lower rate timer and triggers an AV delay timer (SAV)• Before the SAV can expire, it is inhibited by an intrinsic ventricular event (R-

wave)

AS

AS

VS

VS

Dual Response to SensingDDD

• The pacemaker can:– Inhibit and trigger– A P-wave inhibits atrial pacing and triggers an SAV

interval– An atrial pace triggers a PAV interval– An R-wave inhibits ventricular pacing

Single-Chamber Timing

Single Chamber Timing Terminology• Lower rate• Refractory period• Blanking period• Upper rate

Lower Rate Interval

• Defines the lowest rate the pacemaker will pace

Lower Rate Interval

VP VP VVI / 60

Refractory Period

• Interval initiated by a paced or sensed event

• Designed to prevent inhibition by cardiac or non-cardiac events

Lower Rate Interval

VP VP VVI / 60Refractory Period

Blanking Period

• The first portion of the refractory period• Pacemaker is “blind” to any activity• Designed to prevent oversensing pacing stimulus

Lower Rate Interval

VP VP VVI / 60Blanking PeriodRefractory Period

Upper Sensor Rate Interval

• Defines the shortest interval (highest rate) the pacemaker can pace as dictated by the sensor (AAIR, VVIR modes)

Lower Rate Interval

VP VP VVIR / 60 / 120Blanking PeriodRefractory Period

Upper Sensor Rate Interval

VP VP VS VP

Lower Rate Interval-60 ppm

Hysteresis

• Allows the rate to fall below the programmed lower rate following an intrinsic beat

Hysteresis Rate-50 ppm

Dual Chamber Timing Parameters

• Lower rate• AV and VA intervals• Upper rate intervals• Refractory periods• Blanking periods

Lower Rate Interval

APVP

APVP

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 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)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

DDDR 60 / 120A-A = 500 ms

APVP

APVP

Upper Activity Rate Limit

Lower Rate Limit

V-APAV V-APAV

Upper 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 Rate

• The maximum rate the ventricle can be paced in response to sensed atrial events

SAV SAVVA VA

Post Ventricular Atrial Refractory Period (PVARP)

Refractory 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)

Blanking 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)

Dual Chamber Timing

• Atrial Pace (AP) - Ventricular Pace (VP) example

DDD 60

A-A interval A-A interval

VRP

ARP PVARP

PVAB

VRP

ARP PVARP

PVAB

PAVV-A interval V-A interval

PAV

Atrioventricular Interval

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