Ohm's Law

Ohm’s Law

• voltage, current and resistance

• energy and power

• clinical relevance

strength duration curve

• rheobase

• chronaxie

• clinical relevance

Ohm’s Law Equation

• V= I*R

• I = V/R

• R= V/I

  • V(V), I (A), R (Ω)

Fundamentals of Electronics

• voltage (V): force moving current- electromotive push

• current (I): flow of electrons through a circuit, mA

• resistance (R): Impedance- opposition to current flow: unit of measure (Ω)

System Impedance

• cathode electrode impedance (tip)

• anode electrode impedance (ring)

• polarization impedance

• tissue impedance

typical range for a pacing lead impedance: 300-1200+ ohms

Current Flow

low resistance: high current flow

high resistance: low current flow

Lead fracture results in what?

decrease in current drain

Equation

E= V²*t/R

• doubling the voltage is 4x the energy and doubling the PW is 2x the energy

Charge

• in pacing, the charge describes the amount of electricity delivered to the myocardium during an atrial or ventricular output

• charge (Q) is the product of pulse duration (t) in ms and output current (I) in mA. In pacing it is usually expressed in microcoulombs

Power

• the rate of doing work or transmitting energy

• power (P) = I*V; where P is in watts, I is in amperes and V is in volts

Energy

the ability to do work; for batteris usually expressed in watt-hours

Energy Density

the energy content of a battery or capacitor based on volume or mass. For a battery it is expressed in units of watt-hrs per cubic cm (Wh/cm³) or in watt-hr per gram¹ (Wh/g¹)

Watt

international unit of electrical power, watts are equivalent to joules per second (J/s)

Current Drain

the average amount of current drawn from a battery by the external load; typically, pacemakers draw from 10-30 mA²

Quiescent Current

housekeeping-current utilized when not pacing

• i.e., current used for ongoing tasks like monitoring the intrinsic rhythm and data collection

• Typically, the 8-12 uA range for pacemakers

• markers, intervals, diagnostics, bluetooth, RF

Factors Affecting Current Drain

• lead/tissue interface

• stimulation threshold and safety margin desired

• quiescent current

• impedance

• electrode design and material

• pacemaker programming

• % pacing

Factors Affecting Pacemaker Longevity

• battery capacity

• rate of drain

• circuit efficiency

• pacing rate

• parametier: programmed output and pulse width

• pacing: % of the time

Factors You Can Control

• implant

• parameters

• ethics

Implant CSIT(T)

• current of injury

• sensing

• impedance

• threshold

Parameters

• output (V); pulse width (ms)

• rate

• diagnostics

Ethics

• “right is always right”

Pacing Rate

highest consumption of battery