pacing, capture, thresholds

polarization

• layers of negative and positive ions that surround the electrode during the pulse stimulus

• it can slow the movement of charge

capture

successful depolarization of cardiac tissue from a pacing stimulus

stimulation threshold

minimum amount of energy required to cause depolarization

conduction

movement of a charge under the influence of an electrical field

voltage

• force/push that drives a current

• amplitude

• variable

pulse width

duration of time in a pacing impulse (ms)

ex. 2.0V @ 0.4ms

current

• flow of electrons measure in mAs

• not programmable

• calculated

• constant

output capacitor

stores and delivers pacing impulses

impedance

resistance to the flow of current within an electrical circuit (ohms, Ω)

true

T/F: thresholds can change

true

T/F: auto capture algorithms can be tricked

true

T/F: a threshold can be checked on a pacemaker dependent patient

false, don’t want to pace faster than A-fib rhythm

T/F: A-fib always needs a threshold check

beginning of life

BOL

elective replacement interval

ERI

end of life

EOL

phase 0

rapid depolarization, Na+ rushes in

phase 1

initial repolarization

phase 2

plateau phase

phase 3

final repolarization

phase 4

alert period/resting phase

amplitude

how many volts are delivered per pacing impulse

safety margin

2:1

rheobase

• smallest amplitude that stimulates the myocardium at an infinitely long pulse duration

• used to calculate chronaxie

chronaxie

• threshold pulse duration at 2x the rheobase voltage

• approximates the point of minimum threshold energy

battery longevity

energy consumption determines what?

strength-duration curve

defines lowest amount of energy to reliably capture the heart

Ohm’s law

V = I x R

I = V/R

R = V/I

V → voltage

I → impedance

R → resistance

energy equation

E = (V²/R) x t

E → energy

V → voltage

R → impedance

t → pulse width

• lead integrity

• lead location

• medications

• electrolyte imbalance

• arrhythmias → A-fib, A-flutter, sinus tach, slow VT

factors affecting threshold

appropriate atrial capture

appropriate ventricular capture

appropriate dual chamber capture

atrial failure to capture

ventricular failure to capture

• lead dislodgment

• poor connection at connector block

• lead maturation (exit block)

• lead failure

• low output (inappropriate programming)

failure to capture causes