Strength Duration & Ohm's Law

what is stimulation threshold

minimal amount of electrical stimulation that consistently produces a cardiac depolarization (capture)

how is stimulation threshold expressed

• amplitude (miliamps or volts)

• pulse duration (ms)

• charge (microcoulombs)

• energy (microjoules)

permanent pacemaker

amplitude (volts) and pulse duration (ms)

what is strength duration curve

graphic representation of the pulse amplitude (volts) required to produce an action potential in cell.

what are the 2 points of SDC

rheobase and chronaxie

as the pulse duration _____, the voltage requirements _____ (to a point)

increases, decline

what is rheobase

smallest amplitude (voltage) that stimulates myocardium at an infinitely long pulse duration (ms)

____ ms typically flattens the curve (rheobase)

1-2 ms

what is chronaxie

the threshold pulse duration at twice the rheobase voltage

what may you see used on a temp pacemaker

miliamps

if your doctor tells you to measure rheobase, whats the first thing you do?

program pulse duration to an infinitely long number

what’s the infinite pulse width on big blue

1.5 ms

rule of thumb for output

double the voltage or triple pulse width

• ex: threshold is 1.0V @ 0.4ms → output is 2.0V @ 0.4ms OR 1.0V @ 1.2ms

wedensky effect

phenomenon of overcoming A physiological phenomenon in which a nerve or muscle that has been partially fatigued or blocked responds more strongly than expected when stimulated, essentially “overcoming” reduced excitability

Class 1A

Na channel blockers - treat VT, SVT, WPW, AF, Aflutter

Class 1A drug examples

Quinidine, Disopyramide, Procainamide

Class 1B

Na channel blockers

Class 1B drug examples

Lidocaine, Mexiletine, Ethmozine

Class 1C

Na channel blockers - VT, AF concern, proarrhythmic

Class 1C drug examples

Encainide, Lorcainide, Flecanide, Propafenone

Class 2

Beta blockers

Class 2drug examples

Propranolol, Esmolol, Metoprolol

Class 3

K channel blocker

Class 3 drug examples

amiodarone, ibutilide, d-sotalol

what does sotalol do

decreases ICD thresholds

what does amiodarone do

increases ICD thresholds

Class 4

Ca channel blocker

electrolyte abnormalities can do what?

increase thresholds

temporal conditions that increase pacing thresholds

• sleeping

• acute viral illness

• lead maturation (steroid elution)

• excessive alcohol (incr. DFTs)

factors that decrease pacing thresholds

• cortcosteroids - dexamethasone

• exercise

• catecholamines (epi, ne)

• hypocapnia (reduced CO2 in blood)

• sympathomimetic agents (isuprel, epi, ephedrine, atropine, dopamine)

• hyperoxia (incr. O2)

what can isuprel be used for

establish an underlying rhythm for a heart block patient during procedure

3 variables of ohm’s law

voltage, current and resistance (impedance)

Ohm’s law equation

V = I x R

what is voltage (V)

force moving current; electromagnetic push

what is current (I)

flow of electrons through a circut, measures in mA

what is resistance (R)

opposition to current flow, measured in ohms

types of system impedance

• cathode electrode

• anode electrode

• polarization

• tissue

typical range for a pacing lead impedance

300-1200+ ohms BUT relative to pt normal values

Insulation failure will have a _____ change in impedance

gradual

lead fracture will have a ______ change in impedance

abrupt

low resistance = ___ current flow

high

high resistance = ___ current flow

low

T/F: low impedance will drain battery faster

true

lead fracture results in

decrease in current drain (infinite impedance)

energy equation

E = V² x t / R

doubling the voltage is ___ times the energy

4

doubling the pulse width is __ times the energy

2

what is charge in pacing

the amount of electricity delivered to the myocardium during an atrial or ventricular output

Charge equation

Charge (Q) = pulse duration (t) + output + current (I)

charge is expressed in

microcoulombs

what is power

rate of doing work or transmitting energy

power equation

P = I x V

what is energy

the ability to do work (batteries = watt-hour)

what is energy density

the nerrgy contact of a battery or capacitor based on volume or mass (batteries = watt-hrs per cubic cm OR watt-hrs per gram)

what is a watt

international unit of electrical power (equivalent to joules per second)

what is current drain

average amount of current drawn from a battery by the external load (pacemakers draw from 10-30 microamps)

what is Quiescent current

houskeeping current utilized when not pacing (used for monitoring intrinsic rhythm & data collection)

what is a way you would utilize quiescent current?

pt in chronic afib, atrial lead is using current all the time, so program VVIR to save battery

factors affecting current drain

• lead/tissue interface

• stimulation threshold and safety margin

• quiescent current

• impedance

• electrode design and material

• pacemaker programming

• % pacing

factors affecting pacemaker longevity

• battery capacity

• rate of drain

• circuit efficiency

• pacing rate

• parameters: programmed output & pulse width

• % pacing

what factors can you control

• CSIT during implant

• parameters: output, pulse width, rate, diagnostics

A 75-year-old patient presents after a viral illness and reports dizziness. Interrogation shows pacing thresholds have doubled. Which of the following explains the threshold rise?
A. Catecholamine surge
B. Hypocapnia
C. Viral illness
D. Exercise

C. Viral illness

A patient is in chronic AF with no atrial activity. The atrial lead is still programmed to pace. How can you extend battery life?
A. Increase pulse width
B. Program rate-responsive mode
C. Program VVIR mode
D. Add more diagnostics

c: program VVIR mode

After starting amiodarone, an ICD patient requires a higher defibrillation threshold (DFT). Which statement is true?
A. Amiodarone decreases ICD thresholds
B. Amiodarone increases ICD thresholds
C. Amiodarone has no effect
D. Only sotalol affects thresholds

B: Amiodarone increases ICD thresholds

Using Ohm’s law, explain what happens to current flow when resistance increases while voltage stays the same

current flow decreases

Give one example of a temporary condition that can increase pacing thresholds and one that can decrease pacing thresholds

What happens to energy consumption if you double the voltage?

increase energy by 4x