electrical relationships: ohm’s law, curve2009.cardiorhythm.com/pdf/21/95.pdffundamental...

Fundamental Electrical Relationships Fundamental Electrical Relationships: Ohm’s Law, Strength Duration Curve & Factors Affecting Thresholds

Mark W. Sweesy, FHRS,CCDS,CEPSy, , ,Arrhythmia Technologies Institute

O tlineO tlineOutlineOutline

Ohm’s LawOhm’s LawOhm s LawOhm s LawVoltage, current and resistance (impedance)Voltage, current and resistance (impedance)E d E d Energy and powerEnergy and powerClinical relevanceClinical relevance

Strength duration curveStrength duration curveRheobaseRheobaseRheobaseRheobaseChronaxieChronaxieCli i l lCli i l lClinical relevanceClinical relevance

Ohm’s LaOhm’s LaV

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Ohm’s LawOhm’s Law

V = I x RV = I x R VV = I x RV = I x R

I = V / RI = V / R

R = V / IR = V / I I xR

The equation uses current in the form of AmpsIn pacing current is expressed in mAIn pacing current is expressed in mA

F ndamentals of ElectronicsF ndamentals of ElectronicsV

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Fundamentals of ElectronicsFundamentals of Electronics

Voltage (V): force moving the current: electromotive Voltage (V): force moving the current: electromotive pushpushpushpushCurrent (I): flow of electrons through a circuit Current (I): flow of electrons through a circuit mAmAResistance (R) / Impedance: opposition to current Resistance (R) / Impedance: opposition to current flow: uflow: unit nit of measure: Ohm ( of measure: Ohm ( ΩΩ))flow: uflow: unit nit of measure: Ohm ( of measure: Ohm ( ΩΩ))

Fundamentals of ElectronicsFundamentals of ElectronicsV

I R Fundamentals of ElectronicsFundamentals of ElectronicsLow Resistance = high current flow High Resistance = low current flow

PPP{PPP{g

V VV V

PW PW

R

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Ohm’s La Pacers Ohm’s La Pacers sICD’ssICD’sOhm’s Law: Pacers Ohm’s Law: Pacers vsICD’svsICD’s

Typical range for pacemaker voltage 1.0‐5.0 Vyp g p gTypical range for ICD voltage 50‐800 V

Typical range for pacemaker current 2‐16 mATypical range for ICD current 1 000 75 000 mATypical range for ICD current 1,000‐75,000 mA

T i l f i l d i d OhTypical range for pacing lead impedance: 300‐1200 OhmsTypical impedance range for a high voltage lead 10‐60 Ohms


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What is the current in a 2.5 volt system with a500 Ω resistance?


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What is the current in a 2.5 volt system with a 500 Ω resistance?

I = VR

2.5 V500 Ω

= = .005 A

.005 A x1000 mA1 A = 5 mA1 A 5

When the impedance is 500 ohms the absolute value of the current is 2x’s the voltageWhen the impedance is 500 ohms the absolute value of the current is 2x s the voltage

18 A lead fracture would result in which of the following?18. A lead fracture would result in which of the following?a. decreased battery longevityb decreased conductor impedance valuesb. decreased conductor impedance valuesc. decreased electrode impedance valuesd d d dd. decreased current drain

Energ Form laEnerg Form laV

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Energy FormulaEnergy Formula

E V E V xx I I xxPWPWE = V E = V xx I I xxPWPW

E = E = VV22xx PWPWE = E = VV xx PWPW

E E = I= I22xx R R xx PWPWR

If you leave current in mA and time inyMsec your answer will be in uJ

15. A patient’s stimulation threshold was 1.0 Volt & 0.2 msec pulse duration. The pacemaker is programmed p p p gto 2.0 Volt & 0.4 msec duration. Lead impedance measures 1000 Ohms The output energy for this measures 1000 Ohms. The output energy for this patient would be:a 2X thresholda. 2X thresholdb. 4X threshold

h h ldc. 6X thresholdd. 8X threshold

E = V2 x tE V x tRR

22 x .4 = 1.612 x .2 = .21000 1000

Doubling the voltage is 4 times the energy and d bli th PW i 2 ti th doubling the PW is 2 times the energy

V

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Lead Impedance and EnergLead Impedance and EnergLead Impedance and EnergyLead Impedance and Energy

Energ V I TEnerg V I TEnergy = V x I x TEnergy = V x I x T

Pacemaker Pulse Widths= .1 ‐1.5 ms

4 0 V x 8 0 mAx 5 ms = 20 uJ4.0 V x 8.0 mAx .5 ms 20 uJ

ICD Pulse Widths = 3‐15 ms range1

750 V x 14,000 mAx 3 ms = 31.5 J75 4, 3 3 5or 31,500,000 uJ

1. Thammanomai A, et al. Heart Rhythm 2006; 3:1053-1059

ChargeChargeChargeCharge

In In pacing charge describes the amount of electricity pacing charge describes the amount of electricity In In pacing, charge describes the amount of electricity pacing, charge describes the amount of electricity delivered to the delivered to the myocardium during an atrial or myocardium during an atrial or

t i l t tt i l t tventricular output.ventricular output.

Charge (Q) Charge (Q) is the product of pulse is the product of pulse duration (duration (tt) ) in in Charge (Q) Charge (Q) is the product of pulse is the product of pulse duration (duration (tt) ) in in milliseconds & output milliseconds & output current (I) current (I) in in milliamperesmilliamperes. In . In

i it i ll d i i it i ll d i i l bi l bpacing it is usually expressed in pacing it is usually expressed in microcoulombsmicrocoulombs..

Q = I x TQ = I x TQ = I x TQ = I x T

Po erPo erPowerPower

Power is the rate of doing work or transmitting Power is the rate of doing work or transmitting Power is the rate of doing work or transmitting Power is the rate of doing work or transmitting energy.energy.

Power P = I Power P = I xxV: where P is in watts, I is in amperes V: where P is in watts, I is in amperes and V is in volts and V is in volts and V is in volts. and V is in volts.

22. What is the estimated battery service life of a k ith A h b tt d pacemaker with a 1.0 Amp‐hour battery and a 20

microamp current drain?a. 4.0 yearsb. 5.0 years5 yc. 5.7 yearsd 10 0 yearsd. 10.0 years

1 0 Amp hour battery and a 20 microamp current drain1.0 Amp-hour battery and a 20 microamp current drain

Time = Q battery capacity in AhQ y p yI current drain in amperes

Time = 1 000 000 uA/hr = 50 000 hr = 5 7 yrsTime = 1,000,000 uA/hr = 50,000 hr = 5.7 yrs20 uA 8,760 hr/yr, y

Factors Affecting Current DrainFactors Affecting Current Drainacto s ect g Cu e t aacto s ect g Cu e t a

Lead/tissue interfaceLead/tissue interfaceLead/tissue interfaceLead/tissue interface

Stimulation threshold & safety margins desiredStimulation threshold & safety margins desired

Quiescent currentQuiescent current

ImpedanceImpedance

Electrode design & materialsElectrode design & materialsElectrode design & materialsElectrode design & materials

Pacemakers programmed settingsPacemakers programmed settings

% pacing% pacing

Quiescent CurrentQuiescent CurrentQuiescent CurrentQuiescent Current

H k i H k i t tili d h t i i t tili d h t i i Housekeeping Housekeeping –– current utilized when not pacing; i.e., current utilized when not pacing; i.e., current used for ongoing tasks like monitoring the intrinsic current used for ongoing tasks like monitoring the intrinsic h th & l i d t T i ll i th 8h th & l i d t T i ll i th 8 A f A f rhythm & logging data. Typically in the 8rhythm & logging data. Typically in the 8‐‐12 uA range for 12 uA range for pacemakerspacemakers

Reducing the output from 4.0 to 2.0 V may increase the Reducing the output from 4.0 to 2.0 V may increase the battery longevity from 4 to 6 yrs, still have housekeeping battery longevity from 4 to 6 yrs, still have housekeeping y g y 4 y , p gy g y 4 y , p gneedsneeds

Bipolar UnipolarBipolar UnipolarBipolar UnipolarBipolar Unipolar

F t / F t / C t / C t / U iU i L d L d Fracture / Fracture / Connector / Connector / UniUni Leads Leads

Causes of High & Low ImpedanceCauses of High & Low ImpedanceCauses o g & o peda ceCauses o g & o peda ceHigh ImpedanceHigh Impedance

Conductor Fracture, Loose Connector or Unipolar leads Conductor Fracture, Loose Connector or Unipolar leads programmed to Bipolarprogrammed to Bipolar

Low ImpedanceLow ImpedanceInsulation degradation, YInsulation degradation, Y‐‐adaptor or parallel pathways, adaptor or parallel pathways, loose connection maintainingcontact with body fluidsloose connection maintainingcontact with body fluids11

1. In Ellenbogen K, et al. Clinical Cardiac Pacing, Defibrillation & CRT. 2007; pp 834

Measured DataMeasured DataMeasured DataMeasured Data

If ld l i k l hi h b d iIf you could only pick 3 values on which to base your deviceevaluation; which 3 would you pick?

Stim lation ThresholdStim lation ThresholdStimulation ThresholdStimulation Threshold

The minimal amount of electrical stimulation The minimal amount of electrical stimulation that consistently produces a cardiac that consistently produces a cardiac depolarization The threshold can be depolarization The threshold can be depolarization. The threshold can be depolarization. The threshold can be expressed in terms of amplitude expressed in terms of amplitude ((milliamperesmilliamperes or volts), pulse duration (or volts), pulse duration (msecmsec), ), charge (charge (microcoulombsmicrocoulombs), or energy ), or energy charge (charge (microcoulombsmicrocoulombs), or energy ), or energy ((microjoulesmicrojoules).).

Strength Duration CurveStrength Duration CurveStrength Duration CurveStrength Duration Curve

A graphic representation of the pulse amplitude, A graphic representation of the pulse amplitude, i lt t i d t d i lt t i d t d in voltage or current, required to produce an in voltage or current, required to produce an action potential in the cells. The curve is plotted action potential in the cells. The curve is plotted f ti f l d ti d b d fi d f ti f l d ti d b d fi d as a function of pulse duration and can be defined as a function of pulse duration and can be defined

by 2 points; the by 2 points; the RheobaseRheobase and the and the ChronaxieChronaxie. As . As th l d ti i th lt th l d ti i th lt the pulse duration increases the voltage the pulse duration increases the voltage requirements decline (to a point).requirements decline (to a point).

capture

RheobaseRheobaseRheobaseRheobase

The value derived from a strengthThe value derived from a strength duration curve that duration curve that The value derived from a strengthThe value derived from a strength‐‐duration curve that duration curve that indicates the minimum output that will stimulate the indicates the minimum output that will stimulate the myocardium irrespective of the pulse duration The curve myocardium irrespective of the pulse duration The curve myocardium irrespective of the pulse duration. The curve myocardium irrespective of the pulse duration. The curve typically levels out around a pulse duration of 1 typically levels out around a pulse duration of 1 msecmsec

ChronaxieChronaxieChronaxieChronaxie

The pulse duration value that is twice the rheobaseThe pulse duration value that is twice the rheobaseThe pulse duration value that is twice the rheobase.The pulse duration value that is twice the rheobase.

6 Th fib 26. The term or fibrous capsule surrounding the electrode:a. is a normal foreign body reactionb. is an unexcitable capsule of tissue pc. decreases the current density at the lead

tissue interfacetissue interfaced. all of the above

ÂfAÂfAÂfA

Virtual ElectrodeVirtual ElectrodeVirtual ElectrodeVirtual Electrode

is a normal foreign body reaction which results from acute is a normal foreign body reaction which results from acute injury to the cell membraneinjury to the cell membraneinjury to the cell membraneinjury to the cell membrane

collagen is formed resulting in anucollagen is formed resulting in anunexcitable nexcitable capsule of capsule of g gg g pptissue which decreases the current density at the lead tissue tissue which decreases the current density at the lead tissue interfaceinterface

ÂfAÂfAÂfA

17. Which of the above 3 month stimulation threshold graphs would be most characteristic threshold graphs would be most characteristic of steroid eluting tined atrial lead?a Aa. Ab. Bc. Cd. D

^ f^AfA

Electrode Threshold Electrode Threshold CharacteristicsCharacteristics

1 21.2

1.0

h (m

s)

0.8

0.6

se w

idth

APolished platinum

Porous platinum

0 2

0.4Puls

B

C

Porous platinum

Activated carbon

0.0

0.2DSteroid eluting tined lead

^AfA

1 2 3 64 75 8 9 10 11 12 1413

I l t ti ( k )

^ f

Implant time (weeks)

The difference noted in thresholds when decrementing until loss of capture versus going from sub threshold to until loss of capture versus going from sub‐threshold to capture is known as?

l ta. latencyb.Wedensky effectc. intermittent thresholdd. polarizationp

Wedensk EffectWedensk EffectWedensky EffectWedensky Effect

Also called capture hysteresisAlso called capture hysteresisAlso called capture hysteresisAlso called capture hysteresis

The difference noted in thresholds when decrementing The difference noted in thresholds when decrementing gguntil loss of capture versus going from subuntil loss of capture versus going from sub‐‐threshold to threshold to capturecapturepp

Ellenbogen suggests the phenomena is probably Ellenbogen suggests the phenomena is probably l i d b i t d i i l f t l i d b i t d i i l f t explained by varying rates during gain or loss of capture explained by varying rates during gain or loss of capture

and that effects may be greatest at narrow pulse durationsand that effects may be greatest at narrow pulse durations

Ellenbogen & Woods. Cardiac Pacing &ICD’s, p 53

Output ProgrammingOutput ProgrammingOutput ProgrammingOutput Programming

R l f h bR l f h bRule of thumbRule of thumb

Double Double VoltageVoltage

Triple PWTriple PWpp

Doubling VoltageDoubling VoltageDoubling VoltageDoubling Voltage

Chronic Chronic thresholdthreshold5ms & 2 5V so voltage 5ms & 2 5V so voltage .5ms & 2.5V so voltage .5ms & 2.5V so voltage was doubledwas doubledTripling PW would Tripling PW would give an inadequate give an inadequate g qg qsafety marginsafety margin

63 . If the threshold was determined to be at point C in the above 3 pfigure; which of the following would result in an efficient and appropriate safety margin?pp p y g

a. Double voltageb. Triple the pulse widthc. Triple the voltaged. Double the pulse width

Voltage Strength Duration CurveVoltage Strength Duration Curveg gg g

best programming

O tp t Settings & Longe it O tp t Settings & Longe it Output Settings & Longevity Output Settings & Longevity

Antiarrhythmics That Increase Antiarrhythmics That Increase yyPacing ThresholdsPacing Thresholds

Class IAClass IAQuinidineQuinidineQuinidineQuinidineDisopyramideDisopyramideProcainamideProcainamideProcainamideProcainamide

Class ICClass ICFlecainide Flecainide –– notoriousnotoriousPropafenonePropafenonepp

Class IIIClass IIIAmiodoroneAmiodorone*Sotalol*Sotalol‐‐ decreases ICD thresholdsdecreases ICD thresholdsPrapafenonePrapafenone

Electrolyte Abnormalities That Electrolyte Abnormalities That yyIncrease Pacing ThresholdsIncrease Pacing Thresholds

AcidosisAcidosisAcidosisAcidosis

AlkalosisAlkalosis

HyperkalemiaHyperkalemia

HypocalcemiaHypocalcemia

HyperglycemiaHyperglycemiaHyperglycemiaHyperglycemia

HypercarbiaHypercarbiaypyp

HypoxemiaHypoxemia

Temporal Conditions That Temporal Conditions That ppIncrease Pacing ThresholdsIncrease Pacing Thresholds

SleepingSleepingSleepingSleeping

Acute viral illnessAcute viral illness

EatingEating

Lead maturationLead maturation

Excessive alcohol Excessive alcohol –– increases defibrillation increases defibrillation thresholdsthresholds11

PapaioannouGI, et al. PACE 2002; 25:1144

Factors That Decrease Pacing Factors That Decrease Pacing ggThresholdsThresholds

Corticosteroids Corticosteroids ‐‐ dexamethasonedexamethasoneCorticosteroids Corticosteroids ‐‐ dexamethasonedexamethasone

ExerciseExercise

CatecholaminesCatecholamines

HypocapniaHypocapnia

Sympathomimetic Sympathomimetic agents agents ‐‐ IsuprelIsuprel, epinephrine, ephedrine, , epinephrine, ephedrine, atropineatropine

HyperoxiaHyperoxia

What’s the point?


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What would be the calculated voltage inWhat would be the calculated voltage ina 10 mA system with 500 Ω of impedance?


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Ohm’s LawOhm’s LawWhat would be the calculated voltage in a 10 mA system with 500 Ω of impedance?

V = I x R 10 mA x 500 Ω = 5000

5000 mV x 1 V1000 mV1000 mV

5V5V

Tripling Pulse WidthTripling Pulse WidthTripling Pulse WidthTripling Pulse Width

Chronic threshold Chronic threshold Chronic threshold Chronic threshold 2.5V & .2ms PW, 2.5V & .2ms PW,

ddPW programmed PW programmed to .6ms providing to .6ms providing a 2.1x safety a 2.1x safety margin.margin.margin.margin.

Ohm’s La tattooOhm’s La tattooV

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Ohm’s Law tattooOhm’s Law tattoo

VV

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Polari ationPolari ationV

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PolarizationPolarizationCapacitance effect: positively charged ions resist ion flow

e-

N +Na+

Cl-

e-e- Na+

Na+

ClCl-

Cl Cl-

e-e-

e-e-Na+

Na+ Cl- ClCl-

eElectrode Electrode-Tissue

InterfaceInterface

Polari ation/AfterpotentialPolari ation/AfterpotentialV

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Polarization/AfterpotentialPolarization/Afterpotential

Voltage that persists for a time after a paced impulseVoltage that persists for a time after a paced impulseVoltage that persists for a time after a paced impulseVoltage that persists for a time after a paced impulse

Could cause oversensingCould cause oversensinggg

Polarization can represent 30Polarization can represent 30‐‐40% of total pacing 40% of total pacing i di dimpedanceimpedance

Garden Hose AnalogGarden Hose AnalogV

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Garden Hose AnalogyGarden Hose Analogy

Normal Resistance = normal current flow

Low Resistance = high current flow

High Resistance = low current flow


I RFundamentals of ElectronicsFundamentals of Electronics

T i lT i lTerminologyTerminologyCurrent: flow of electrons through a circuit Current: flow of electrons through a circuit mAmA

P kCurrent Flow

V

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Pacemakere‐

ConductorWiWire

Heart

e-

V

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Voltage: Electromotive pushe‐ Provided by

The batteryPacemaker

Voltage: Electromotive push y

ConductorWire

Heart

V IR Oh ’ LV = IR Ohm’s Law

V 5 0 mA x 500 KohmsV = 5.0 mA x .500 Kohms

V = 2 5 VoltsV = 2.5 Volts


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Ohm’s LawOhm’s LawWhat would be the impedance of a 2.5 volt system with a 10 mAcurrent drain?

R = VI

2.5 V10mA

= = 2.5 V.01 A

= 250 Ω

10 mA x 1 A1000 mA1000 mA

.01 A


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Energy: Voltage Energy: Voltage xx Current Current xxTimeTimeEnergy: Voltage Energy: Voltage xx Current Current xxTimeTime

E = V E = V xx I I xtxt

Current in Current in mAmACurrent in Current in mAmATime in msTime in ms

Unit = Joule (J)Unit = Joule (J)

V

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46. Match the units: watt‐seconda. energyb. powerc. capacitanced d id. current drain

joule: unit of energy or workjoule: unit of energy or workjoule: unit of energy or workjoule: unit of energy or workhenry: unit of inductance (induced by a magnetic field)henry: unit of inductance (induced by a magnetic field)

l b it f hl b it f hcoulomb: unit of chargecoulomb: unit of charge

34. The rate at which work is done is is:34a. joule b. farad

unit of energy or workbasic unit of capacitanceb. farad

c. powerd coulomb

basic unit of capacitance

unit of charged. coulomb g

62. In the above figure line B represents the:a Chronaxiea. Chronaxieb. Pulse width thresholdc Knee of the curvec. Knee of the curved. Rheobase


I R Fundamentals of ElectronicsFundamentals of Electronics

Low Resistance = high current flowLow Resistance = high current flow

PW

High Resistance = low current flow


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System ImpedanceSystem ImpedanceSystem ImpedanceSystem ImpedanceCathode Electrode ImpedanceCathode Electrode ImpedanceAnode Electrode ImpedanceAnode Electrode ImpedanceAnode Electrode ImpedanceAnode Electrode ImpedancePolarization Polarization ImpedanceImpedanceTissue ImpedanceTissue ImpedanceTissue ImpedanceTissue Impedance

Typical range for pacing lead impedance: 300 to over 1200 Typical range for pacing lead impedance: 300 to over 1200 hhOhmsOhms

Typical impedance range for a high voltage lead 10Typical impedance range for a high voltage lead 10‐‐60 60 Typical impedance range for a high voltage lead 10Typical impedance range for a high voltage lead 10 60 60 OhmsOhms

Energy, Power, Watts & Current DrainEnergy, Power, Watts & Current Draingy, ,gy, ,

Energy: the ability to do work; for batteries usually expressed in Energy: the ability to do work; for batteries usually expressed in wattwatt‐‐hourshours11

Energy density: the energy content of a battery or capacitor based Energy density: the energy content of a battery or capacitor based on volume or mass. For a battery is expressed in units of watton volume or mass. For a battery is expressed in units of watt‐‐hrs hrs per cubicper cubic‐‐centimeter or wattcentimeter or watt‐‐hrs per gramhrs per gram11

Power: the rate of doing work or transmitting energy, a product of Power: the rate of doing work or transmitting energy, a product of voltage & current usually expressed in wattsvoltage & current usually expressed in watts22

Watt: international unit of electrical power, watts are equivalent to Watt: international unit of electrical power, watts are equivalent to joules per secondjoules per second22

WattWatt hour the amount of energy at a rate of 1 watt per hour 1 hour the amount of energy at a rate of 1 watt per hour 1 WattWatt‐‐hour: the amount of energy at a rate of 1 watt per hour, 1 hour: the amount of energy at a rate of 1 watt per hour, 1 joule = 1watt delivered for 1 second, 1 wattjoule = 1watt delivered for 1 second, 1 watt‐‐hour = 3,600 jouleshour = 3,600 joules22

Current drain: the average amount of current drawn from a battery Current drain: the average amount of current drawn from a battery Current drain: the average amount of current drawn from a battery Current drain: the average amount of current drawn from a battery by the external load; typically pacemakers draw from 10 by the external load; typically pacemakers draw from 10 –– 30 30 microamperesmicroamperes22

1. In Ellenbogen K, et al. Clinical Cardiac Pacing, Defibrillation & CRT. 2007; pp 2362. In Hayes &Asirvatham. Dictionary of Cardiac Pacing, Resynchronization, & Arrhythmias. 2007

C rrent I V/RC rrent I V/RCurrent I = V/RCurrent I = V/R

P k V (I) hPacemaker 4.0 V = (I) 500 ohms4.0 V/.500 Kohms = 8.0 mA4.0 V/.500 Kohms 8.0 mA

ICD 600 830 V i e 750 V = (I) 50 ohmsICD 600‐830 V i,e. 750 V = (I) 50 ohms750 V/.050 Kohms = 15,000 mA75 / 5 5,

Pacemaker Pacemaker Longe it FactorsLonge it FactorsV

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Pacemaker Pacemaker Longevity FactorsLongevity Factors

Battery CapacityBattery CapacityBattery CapacityBattery Capacity

Rate of DrainRate of Drain

Circuit EfficiencyCircuit Efficiency

Pacing RatePacing Rate

Programmed output & Pulse DurationProgrammed output & Pulse Duration

% f ti i% f ti i% of time pacing% of time pacing

45. The horizontal asymptote of a strength duration curve defines which of the following:a. charge gb. pulse width thresholdc. rheobase c. rheobase d. chronaxie

V

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electrical relationships: ohm’s law, curve2009.cardiorhythm.com/pdf/21/95.pdffundamental...

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