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
I R
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
I I
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
Ohm’s LaOhm’s LaV
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Ohm’s LawOhm’s Law
What is the current in a 2.5 volt system with a500 Ω resistance?
Ohm’s LaOhm’s LaV
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Ohm’s LawOhm’s Law
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
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?
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
^AfA^AfA^AfA
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
^AfA^AfA^AfA
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
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
Ohm’s LaOhm’s LaV
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Ohm’s LawOhm’s Law
What would be the calculated voltage inWhat would be the calculated voltage ina 10 mA system with 500 Ω of impedance?
Ohm’s LaOhm’s LaV
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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.
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
Fundamentals of ElectronicsFundamentals of ElectronicsV
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
V
I R
Voltage: Electromotive pushe‐ Provided by
The batteryPacemaker
Voltage: Electromotive push y
ConductorWire
Heart
Ohm’s LaOhm’s LaV
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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
F ndamentals of ElectronicsF ndamentals of ElectronicsV
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Fundamentals of ElectronicsFundamentals of Electronics
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)
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
Fundamentals of ElectronicsFundamentals of ElectronicsV
I R Fundamentals of ElectronicsFundamentals of Electronics
Low Resistance = high current flowLow Resistance = high current flow
PW
High Resistance = low current flow
F ndamentals of ElectronicsF ndamentals of ElectronicsV
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Fundamentals of ElectronicsFundamentals of Electronics
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