pacing, capture, and threshold

pacemaker battery

lithium iodine, first implanted in 1972, unsurpassed reliability and predictability

why do polarities reverse between battery and tissue

tissue has electrical charge (+anode and -cathode)

battery anode

donates or losses electrons through oxidation

battery cathode

receives or gains electrons through reduction

depolarization

electrons sent from pulse generator to -cathode tip; attract +Na

phase 0

rapid depolarization

phase 1

initial repolarization

phase 2

plateau

phase 3

final repolarization

phase 4

alert period or resting phase

depolarization action potential for someone with CHB

isoelectric line because nothing to depolarize

action potential x and y axis

time (msec) and transmembrane potential (mV)

threshold

have enough energy to trigger depolarization; without, give impulse with no reaction

time zero

phase 4: resting phase pacing impulse begins

0.3 mSec

impulse delivered: electrons sent to lead tip (cathode)

0.6msec

phase 0: capture/depolarization

0.7 msec (after potential)

phase 1-3: ions return to repolarization

what drives polarization to depolarization

battery

amplitude

how many volts are delivered per pacing impulse; how much power

pulse width

duration of the impulse; how long amplitude delivered; pacing output setting

goal

effective and safe output setting while maximizing battery longevity

safety margin

2:1

why do you want to pace with minimum power

to save battery

pulse width

mseconds; how long impulse is delivered usually 0.4 ms or 0.5

strength-duration curve

amplitude (V) and pulsewidth (ms); lowest threshold for best battery

rheobase

smallest amplitude that stimulates myocardium at infinitely long pulse duration; double to get reference

chronaxie

threshold pulse duration at 2x rheobase voltage; approximates point of minimum threshold energy

3 lines on strength duration curve

1. rheobase, 2. chronaxie, 3. energy

center of strength-duration curve

knee of the curve; defines lowest amount of energy to reliably capture the heart

Ohm’s law

Voltage = I (current-amps) x R (resistance/impedence in Ohm’s)

units of Ohm’s law

amps but deliver in milliamps

energy equation

E=V²/R x t; E=energy in microjoules, V=voltage, R=impedance, t= Pulse Width

when threshold elevates

increase output/volts, or increase/extend pulse width

doubling voltage

quadruple energy

doubling pulse width

double energy

energy equation units

want microjoule, convert from millijouleto

to depolarize

run a threshold test; control the heart rate

when to not run threshold test

AFib or VT

mode for PPM dependent patient

DDD never AAI

Dual chamber device testing

DDD only one chamber decremented at a time

ventricular threshold test mode

VVI

overdrive pace

power (adequate starting amplitude) and speed (faster than underlying rhythm)

decrement power

decrease amplitude incrementaly and watch the rhythm

when to stop threshold test

loss of capture, verify rhythm again

important when running a threshold test

know the presenting rhythm and know the underlying rhythm

final result of threshold test

previous output with successful depolarization, not loss of capture

factors affecting threshold

lead integrity/location, medications, electrolyte imbalance, arrhythmia (flutter, fib, tach, slow VT)

atrial capture

AAI

ventricular capture

VVI

dual chamber capture

DDD

atrial failure to capture

ventricular failure to capture

failure to capture- short term

physician fix: lead dislodgment or poor connection at connector block

failure to capture- long term

hyprid fix; lead maturation (exit block)or lead failure

failure to capture you can fix

low output; pulse width and amplitude

intrinsic r-wave

fusion beat

paced ventricular beat

pseudo fusion beat

pseudo pseudofusion beat

why use automatic capture algorithms

scheduled frequency and prolong battery life

ACA scheduled frequency

•More frequent testing compared to in-office visits

•Every 8hrs, 24hrs, etc

•Some beat-to-beat

•Typically set to 1:00/2:00 am (programmable)

ACA safety margin

•Pacing Amplitude reprogramming (if necessary)

•By not applying a 2:1 safety margin and instead use 1.0V safety margin,

you prolong battery life

ACA temporary programming and evoked response

•Ventricular morphology discriminator – creates a higher output template for a captured beat. When LOC occurs, the LOC morphology is confirmed against the reference template

ACA backup pulse

protects patient and minimizes symptoms at LOC