Dual Brady Therapy week 5

Goal of dual chamber device

fill in the missing beats and maintain AV synchrony

States of DDD pacing

A sense / V sense

A pace / V sense

A pace / V pace

A sense / V pace

Atrial vs Ventricular Channel: Atrial

atrial channel: based on sinus rate

paces when sinus rate slows below programmed base rate

paces at base rate

Atrial vs Ventricular Channel: Ventricular

ventricular channel: base on AV conduction

paces when PR interval is longer than AV delay

paces at the end of AV delay

A sense / V sense: Total inhibition

sinus rate is faster than base rate: inhibits in atrium

PR interval shorter than AV delay: inhibits in ventricle

A pace / V sense

sinus rate is slower than base rate: paces in atrium

AV conduction faster than AV delay: inhibits in ventricle

pacing atrium at base rate

A pace / V pace: AV sequential pacing

sinus rate is slower than base rate: paces in atrium

AV conduction slower than AV delay: pace in ventricle

pacing both chambers at base rate

A sense / V pace: Atrial tracking

sinus rate is faster than base rate: inhibits in atrium

AV conduction slower than AV delay: paces in ventricle

pacing ventricle at intrinsic atrial rate

Base Rate

programmed rate, pacing rate, lower rate limit

dual chamber: base rate controls atrial pacing

intrinsic atrial rate cannot go below base rate

AV Delay

controls ventricular pacing

also an atrial refractory period

SAV

sensed AV delay

typically 20-30 ms less than PAV

PAV

paced AV delay

typically 200 ms

electronic PR interval

Atrial Latency

delay between intrinsic depolarization and sensing

placement of atrial lead: right atrial appendage

signal originates in SA node

25-50 ms conduction delay

PVARP

post-ventricular atrial refractory period

follows every paced or sensed ventricular event

device ignores atrial signals until PVARP expires

nominal 250-275 ms

TARP

total atrial refractory period

AV delay + PVARP

point of 2:1 block

Blanking Periods

same effect as absolute refractory period: device completely ignores signals

often programmable

self-blanking periods and cross-chamber blanking periods

Atrial Blanking Period

self-blanking period

blanks the atrial channel after atrial pacing

prevents double counting of paced atrial signals

PVAB

post-ventricular atrial blanking period

cross-chamber blanking period

blanks atrial channel after ventricular pacing and sensing

typically 100 ms

shorter than PVARP

PVARP: Retrograde P Waves and PMT

retrograde p waves: conduction from ventricles to atria

can trigger inappropriate ventricular pacing

PMT: pacemaker mediated tachycardia

PVAB: Far-field R wave Oversensing

far-field R wave oversensing: atrial lead senses ventricular signals

atrial rate appears faster than it is

can inappropriately trigger algorithms (eg. mode switching)

Ventricular Refractory Period (VREF)

programmable refractory period in ventricle

triggered by paced and sensed ventricular beats

typically 250 ms

prevents T wave oversensing

Ventricular Blanking Period

self-blanking period

blanks the ventricular channel after ventricular pacing

prevents double counting of paced ventricular signals

PAVB

post-atrial ventricular blanking period

cross-chamber blanking period

blanks ventricular channel after atrial pacing

no need to blank after intrinsic P waves

Cross-Talk

pacing in one chamber leads to sensing in the other

Cross-Talk Inhibition

pacing in one chamber leads to sensing and inhibition in the other chamber

atrial pacing can cause ventricular inhibition

ventricles unsupported

prevented by PAVB

PAVB can only extend so long: use safety pacing

SAV vs PAV

SAV and PAV: atrial refractory periods, ventricular alert periods

SAV in the ventricle: only an alert period

PAV in the ventricle: three periods

PAVB

Cross-talk detection window

PAV Response to Sensing

PAVB: ignores signals

Alert period: inhibits in response to sensing

Cross-talk detection window: triggers early ventricular pacing

typically 100-120 ms after A pacing

Safety Pacing

cross-talk: paces and protects the ventricles

intrinsic conduction: paces into QRS to be safe

early pacing signals device specialist to investigate

Ventricular Alert Periods

AV delay: after atrial pacing

entire SAV

last third of PAV

inhibits ventricle

Second ventricular alert period: after VREF

inhibits atrium and ventricle

Max Tracking Rate

upper rate limit of ventricles

fastest device will pace the ventricle when atrial tracking

protects ventricles from rapid rates

Step by Step Paced ECG Analysis

1.identify chambers paced

single or dual?

atrium or ventricle?

2.measure pacing interval rate

3.measure AV delay (dual chamber only)

4.assess capture

5.assess sensing

6.assess underlying rhythm

7.final assessment of pacemaker function

normal or abnormal?

Measuring Pacing Interval (dual chamber)

measure between consecutive atrial paced beats

Measuring SAV and PAV

SAV: P wave to ventricular pacing

PAV: atrial pacing to ventricular pacing

Assessing Capture

move across entire strip, check each pacing spike

Assessing Atrial Capture

AV sequential pacing

Paced A waves can be hard to see

Delayed P wave: underlying rhythm, indicates loss of capture

Fusion and Pseudofusion

Single chamber: competition between underlying rate and pacing rate

Dual chamber: competition between AV conduction and AV delay

Functional Non-capture

should the beat be captured?

pacing during intrinsic refractory period

may be due to undersensing

Atrial Undersensing

ensure every intrinsic signal is sensed

atrial channel:

sensing inhibits atrial pacing

sensing triggers new pacing interval

sensing triggers SAV

Ventricular Undersensing

ventricular channel:

sensing is first alert period: inhibits ventricular pacing

sensing in second alert period: inhibits atrial pacing

Oversensing: Atrial Channel

unexplained deviation from base rate

lone ventricular paced beat (tracking)

Oversensing: Ventricular Channel

atrial beats with no ventricular beats

atrial inhibition during second alert period

When in Doubt, Test it Out

find the underlying cause

ignore secondary effects: run tests first

sensing test

capture threshold test

Underlying Rhythm

is the sinus node healthy?

sensing in the atrium: yes

pacing in the atrium: no

is the AV conduction healthy?

sensing in the ventricle: yes

pacing in the ventricle: no

Upper Rate Behavior

requires specific circumstances

Atrial tracking: A sense / V pace

fast underlying atrial rate

no ventricular conduction: complete heart block

Pacemaker Wenckebach

AKA Pseudo Wenckebach

progressive lengthening before dropped ventricular beat

regular atrial rhythm

paced ventricular beats later and later

controlled by MTR and PVARP

Pacemaker Wenckebach Timing Cycles

first atrial beat triggers pacing interval and SAV

SAV expires, triggering ventricular pacing

ventricular pacing triggers PVARP and MTR

MTR = 120 ms: device will only pace every 500 ms

atrial rate faster than MTR

next SAV expires before MTR times out

delay longer than SAV

underlying atrial rate continues

next atrial beat closer to last ventricular beat

SAV expires earlier: even longer delay to ventricular pacing

pattern continues until atrial beat falls inside PVARP

refractory period: P wave ignored

no ventricular pacing at all

atrial rate continues: pattern starts over next beat

2:1 Block

every other atrial beat is ignored

2:1 pattern from A to V

controlled by TARP

2:1 Block Calculation

every other beat falls in the previous TARP

2:1 block rate = 60000/TARP

eg. TARP = 400 ms

Block rate = 60000/400 = 150 ms

Atrial rate = 187 bpm: faster than block rate, causes 2:1

2:1 Block Timing Cycles

MTR has no effect

TARP = SAV + PVARP

atrial tracking: only SAV is relevant

Fixed Ratio Block

2:1 block, 3:1 block, 4:1 block, etc.

Wenckebach Window

fixed ratio block: worst upper rate behavior

1:1 tracking: best upper rate behavior

wenckebach: in the middle, acts as a buffer

Dynamic AV Delay

shortens at faster rates

mirror PR interval shortening

higher 2:1 block point

Dynamic PVARP

longer at lower rates to prevent PVARP

shorter at higher rates to increase 2:1 block point

TARP = shortest SAV + shortest PVARP

Retrograde P waves

signal originates in the ventricles

VA conduction backwards through AV node

atria depolarize after ventricles: loss of atrial kick

Pacemaker Mediated Tachycardia

pacemaker participates in maintaining tachycardia

1.dual chamber pacemaker in tracking mode

2.patient has retrograde conduction

3.retrograde conduction time longer than PVARP

4.triggering event that breaks AV synchrony

PMT Initiation: AV Synchrony

normal: atrial depolarization followed by ventricular depolarization

AV synchrony maintained

Atria refractory: no retrograde conduction

PMT Initiation: Triggering Event

triggering event breaks AV synchrony: PVC

atria no longer refractory retrograde conduction

PMT Initiation: PVARP

retrograde P wave inside PVARP: ignored

retrograde P wave after PVARP: sensed

triggers SAV

SAV expires: V pacing

PMT Initiation: Paced Ventricular Beat

paced ventricular beat conducts retrograde too

second retrograde P wave also sensed

device paces ventricle again

PMT Initiation: Endless Loop

paced ventricular beat conducts retrograde too

second retrograde P wave also sensed

device paces ventricle again: endless loop tachycardia

Recognizing PMT

initiation: loss of AV synchrony followed by rapid atrial tacking

rate: typically at max tracking rate

PMT Timing Cycles

PVC: initiates PVARP, MTR

PVARP shorter than retrograde conduction

retrograde P wave: initiates SAV

usually SAV expires before MTR: V pacing at URL

PMT vs Pacemaker Wenckebach

pacemaker tachycardia: intrinsic atrial rate driving rhythms

PMT: paced ventricular rate driving rhythm

rhythm never faster than MTR

no dropped ventricular beats

Unusual Presentations

PMT slower than MTR

extra long retrograde conduction

SAV expires after MTR

normal atrial tracking at MTR

sinus rate exactly MTR

PMT Prevention

extend PVARP

run retrograde conduction test

set PVARP to PVAC + 50 ms

beware of 2:1 block

PMT prevention algorithms: focus on PVCs

PMT Prevention Algorithms

PVC detected by pacemaker

two consecutive ventricular events

second event is sensed

no intermediate P wave

PVC triggers PMT prevention algorithm

extends PVARP for one beat

pace in the atrium

PMT Triggers

PVCs: used to be most common

Loss of atrial capture: currently most common

Magnet application and removal

PACs

Atrial undersensing

Long AV delays

Myopotential oversensing

Device algorithms

PMT Termination

identify PMT

atrial tracking at MTR for certain number of beats

Terminate PMT

extend PVARP for one beat

Chronotropic Incompetence

Inability of the heart rate to appropriately respond to metabolic demand or physiologic stress

• Physical stress (exercise)

• Mental stress

• Emotional stress

Causes of Chronotropic Imcompetence

• Sinus node dysfunction

• Autonomic dysfunction

• Myocardial ischemia

• Prior myocardial infarction
  Medications (beta blockers, calcium channel blockers, digoxin, amiodarone)

• Aging

Maximum Heart Rate

Ideal heart rate to meet demand of aerobic exercise

Aerobic: muscles operating off of oxygen alone

• AKA cardio

• Increases heart rate and breathing

• Can sustain for extended periods

Anaerobic: muscles produce lactic acid

• Less energy efficient

• Only possible in short bursts

MHR Formula

Chronotropic incompetence: inability to reach 80% of MHR

during exercise

Traditional formula: Framingham heart study

• MHR = 220 - Age (years)

• 90 year old MHR = 220 - 90 = 130 bpm

• Cl if unable to reach 104 bpm

Updated formulas: eg. Tanka formula

• MHR = 208 - (70% of age)

• 90 year old MHR = 208 - 63 = 145 bpm

• Cl if unable to reach 116 bpm

Wilkoff Model

• Bruce Wilkoff

• Heart rate vs level of exercise / oxygen consumption

• Should be 1:1 ratio

• MET: metabolic equivalent of a task

Ask Patient Questions

Be specific:

"How far can you walk?"

"How do you feel climbing stairs?"

Listen for common responses:

"I'm just getting old."

"I get tired easily."

"I can't do the things I used to."

Types of Chronotropic Incompetence

• 4 general patterns

• Healthy heart:

• Gradual increase in heart rate to MHR when exercising

• Gradual decline in heart rate when exercise ends

Inability to Reach MHR

• Healthy heart:

• Gradual increase in heart rate to MHR when exercising

• Gradual decline in heart rate when exercise ends

Inability to reach MHR

• HR increases and decreases but never reaches MHR

Delay in Reaching MHR

• Healthy heart:

• Gradual increase in heart rate to MHR when exercising

• Gradual decline in heart rate when exercise ends

Delay in reaching MHR

• HR reaches MHR but only after a significant warm-up period

Inappropriate Recovery

• Healthy heart:

• Gradual increase in heart rate to MHR when exercising

• Gradual decline in heart rate when exercise ends

Inappropriate Recovery from Exercise

• Sudden drop in heart rate when exercise ends

• Highest mortality

Rate Instability

Healthy heart:

• Gradual increase in heart rate to MHR when exercising

• Gradual decline in heart rate when exercise ends

Rate instability

• Fluctuations in rate throughout exercise

Treatment MHR

Class Ila indication for pacing

• Rate response on

• Programmed for specific patient

• Drug induced: modify or withhold medications

• Address underlying causes: hyperthyroidism, ischemia

Rate Responsive Pacing

• Rate adaptive pacing, rate modulation, sensor-driven pacing

• 4th position of NBG code: eg. DDDR

• Adjust atrial rate (dual chamber) to meet activity level

Wilkoff Model: Chronotropic Index

• Should have 1:1 relationship between heart rate and oxygen demand of activity

Chronotropic index:

• A HR / A demand

• 1:1 ratio: Cl = 1.0

• Slow HR response: CI < 1.0

• Fast HR response: CI > 1.0

• Normal range: 0.8-1.3

Sensor-Driven Pacing

• Built-in sensors detect patient's activity level

• Activity level determines atrial pacing rate

  • Increased activity: increased rate

  • Decreased activity: rate returns to normal

Sensors By Companies

Accelerometer - All Companies

Minute Ventilation - Boston Scientific, MicroPort

Closed Loop Stimulation - Biotronik

Accelerometers

• Developed in 1980s

• Piezoelectricity: mechanical stress creates electrical charge

Original accelerometers: piezoelectric crystal

• Mounted to back of device

• Up and down vibrations

• Vibration frequency threshold

Current accelerometers: piezoelectric mass

• Inside hybrid circuitry

• Back and forth movement

• More specific

Minute Ventilation

Minute ventilation = respiratory rate x tidal volume

Respiratory rate: breaths per minute

• Rest: 12-20 breaths per min

• Exercise: 20-60 breaths per min

Tidal volume: volume inhaled and exhaled in one breath

• Rest: 1 L

• Exercise: 2-5 L

Transthoracic Impedance

• Resistance in chest cavity

• Low level current sent between device and ring electrode

• Measures voltage drop with indifferent electrode and tip electrode

• Tracks respiratory rate and tidal volume

Sensors Chart

Sensor	Companies	Mechanism	Scope of Response	Speed of Response
Accelerometer	All	Piezoelectricity	Activities of Daily Living (ADLs)	Fast
Minute Ventilation	Boston Scientific, MicroPort	Transthoracic Impedance	Extended exercise and endurance	Moderate
Closed Loop Stimulation	Biotronik	Intracardiac Impedance	Mental and emotional stress	Slow

Blended Sensors

• Combine accelerometer with minute ventilation

• Accelerometer for beginning of exercise

• Minute ventilation for sustained exercise

• Chronotropic index of 0.92

Closed Loop Stimulation

• Auto-adjusting: very little intervention

Intracardiac impedance in the right ventricle

• Measure of contractility

• Indirect measure of sympathetic tone

• Responds to mental and emotional stress

Passive Rate Response

• Sensors on, no change in rate

• Stores rates device would pace at if programmed on

• Test rate response before using

Timing Cycles in Rate Response

Sensor-indicated rate (SIR): rate controlled by rate response sensor

Max sensor rate (MSR): highest rate device will pace atrium (dual chamber)

• Often set same as MTR

• Set for patient's activity level

• Other parameters (eg. slope of response)

Who Needs Rate Response?

• Patients with chronotropic incompetence

• Patients who can tolerate fast rate

• Active patients

  • Even moderate activity requires increased heart rates

• Ensure correct programming

Mode Switching

• Ventricular rate control algorithms

• Changes the mode of the device from tracking mode to non tracking mode

• Prevents tracking of fast atrial arrhythmias

Mode Switching Example

• Patient with paroxysmal AF

• DDD device

• Normal: sinus node drives atrial rate, ventricular tracking

• Patient goes into atrial fibrillation

  • RVR: no pacing

  • CVR: very little pacing

  • SVR: device still pacing in ventricular channel

• Tracking of AF: very fast ventricular pacing

• Patient may be at rest, don't need fast rate

• Tracking serves no purpose: already lost AV synchrony

• Mode switch activated: tracking turned off

Mode Switching Algorithm

All companies: nominally on

Common algorithm features:

• Detect/trigger rate

• Entry/onset criteria

• Exit criteria

• Fallback mode

• Fallback rate

Detect Rate

• Atrial rate that triggers mode switch

• Nominal 150-170 bpm

• Counts signals in relative refractory period (not blanking period)
  

Atrial tachycardia begins (above detect rate)

Entry Criteria

• Entry count: number of atrial beats above detect rate in a row

• Duration: time that atrial arrhythmia is sustained

Exit Criteria

• Exit count: number of beats at normal rate

• Active as soon as entry count satisfied

• Allows device to return to original mode

Fallback Mode

• Non-tracking mode

• DDI, DDIR, VDI, VDIR

• VVI mode: actually VDI

  • Device can sense return to rhythm and switch back