Basic Pacing Concepts Part II

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Basic Pacing ConceptsPart II
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Electrical Concepts
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Every Electrical Pacing Circuit Has the
Following Characteristics

• Voltage
• Current
• Impedance

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Voltage

• Voltage is the force or push that causes
  electrons to move through a circuit
• In a pacing system, voltage is
• Measured in volts
• Represented by the letter V
• Provided by the pacemaker battery
• Often referred to as amplitude

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Current

• The flow of electrons in a completed circuit
• In a pacing system, current is
• Measured in mA (milliamps)
• Represented by the letter I
• Determined by the amount of electrons that move
  through a circuit

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Impedance

• The opposition to current flow
• In a pacing system, impedance is
• Measured in ohms
• Represented by the letter R (W for numerical
  values)
• The measurement of the sum of all resistance to
  the flow of current

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Voltage, Current, and Impedance Are Interdependent

• The interrelationship of the three components can
  be likened to the flow of water through a hose
• Voltage represents the force with which . . .
• Current (water) is delivered through . . .
• A hose, or lead, where each component represents
  the total impedance
• The nozzle, representing the electrode
• The tubing, representing the lead wire

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Voltage and Current Flow

Spigot (voltage) turned up(high current drain)
Spigot (voltage) turned low(low current drain)
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Resistance and Current Flow
Normal resistance

Low resistance
High current flow
High resistance
Low current flow
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Ohms Law is a Fundamental Principle of Pacing
That

• Describes the relationship between voltage,
  current, and resistance

V I X R I V / R R V / I
x
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When Using Ohms Law You Will Find That

• If you reduce the voltage by half, the current is
  also cut in half
• If you reduce the impedance by half, the current
  doubles
• If the impedance increases, the current decreases

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Ohms Law Can Be Used to Find Amounts of Current
Passing Through Pacemaker Circuitry

• If Voltage 5 V
• Impedance 500 W
• What will the current be?
• I V/R
• I 5 V 500 W 0.010 Amperes
• 0.010 x 1000 10 mA

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In This Example, the Voltage is Halved

• If Voltage 2.5 V
• Impedance 500 W
• Current ?
• I V/R
• V 2.5 V 500 W 0.005 Amperes
• 0.005 x 1000 5 mA

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In This Example, the Impedance is Reduced By Half

• If Voltage 5 V
• Impedance 250 W
• Current ?
• I V/R
• I 5 V 250 W 0.020 Amperes
• 0.020 x 1000 20 mA

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Impedance Changes Affect Pacemaker Function and
Battery Longevity

• High impedance reading reduces battery current
  drain and increases longevity
• Low impedance reading increases battery current
  drain and decreases longevity
• Impedance reading values range from 300 to 1,000
  W
• High impedance leads will show impedance reading
  values greater than 1,000 ohms

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Lead Impedance Values Will Change Due to

• Insulation breaks
• Wire fractures

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An Insulation Break Around the Lead Wire Can
Cause Impedance Values to Fall

• Insulation breaks expose the wire to body fluids
  which have a low resistance and cause impedance
  values to fall
• Current drains through the insulation break into
  the body which depletes the battery
• An insulation break can cause impedance values to
  fall below 300 W

Insulation break
Decreased resistance
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A Wire Fracture Within the Insulating Sheath May
Cause Impedance Values to Rise

• Impedance values across a break in the wire will
  increase
• Current flow may be too low to be effective
• Impedance values may exceed 3,000 W

Lead wire fracture
Increased resistance
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Stimulation
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Stimulation Process
Phase 1
Phase 2
Phase 0
Transmembrane Potential (Millivolts)
Phase 3
Threshold
Phase 4
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Stimulation Threshold

• The minimum electrical stimulus needed to
  consistently capture the heart outside of the
  hearts refractory period

Capture
Non-Capture
VVI / 60
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Two Settings Are Used to Ensure Capture

• Amplitude
• Pulse width

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Amplitude is the Amount of Voltage Delivered to
the Heart By the Pacemaker

• Amplitude reflects the strength or height of the
  impulse
• The amplitude of the impulse must be large enough
  to cause depolarization ( i.e., to capture the
  heart)
• The amplitude of the impulse must be sufficient
  to provide an appropriate pacing safety margin

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Pulse Width Is the Time (Duration) of the Pacing
Pulse

• Pulse width is expressed in milliseconds (ms)
• The pulse width must be long enough for
  depolarization to disperse to the surrounding
  tissue

5 V
1.0 ms
0.5 ms
0.25 ms
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The Strength-Duration Curve

• The strength-duration curve illustrates the
  relationship of amplitude and pulse width
• Values on or above the curve will result in
  capture

Stimulation Threshold (Volts)
Capture
0.5
1.0
1.5
Duration Pulse Width (ms)
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Clinical Usefulness of the Strength-Duration Curve

• Adequate safety margins must be achieved due to
• Acute or chronic pacing system
• Daily fluctuations in threshold

2.0
1.5
Stimulation Threshold (Volts)
1.0
Capture
.50
.25
0.5
1.0
1.5
Duration Pulse Width (ms)
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After Patient Safety, the Second Most Important
Goal in Programming is to Extend Battery Life

• The best way to extend the service life of a
  battery is to lower voltage settings while
  maintaining adequate safety margins
• Amplitude values greater than the cell capacity
  of the pacemaker battery require a voltage
  multiplier, resulting in decreased battery
  longevity

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Factors That Affect Battery Longevity Include

• Lead impedance
• Amplitude and pulse width setting
• Percentage paced vs. intrinsic events
• Rate responsive modes programmed ON

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Electrode Design May Also Impact Stimulation
Thresholds

• Lead maturation process

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Lead Maturation Process

• Fibrotic capsule develops around the electrode
  following lead implantation

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Steroid Eluting Leads

• Steroid eluting leads reduce the inflammatory
  process and thus exhibit little to no acute
  stimulation threshold peaking and low chronic
  thresholds

Porous, platinized tip for steroid elution
Silicone rubber plugcontaining steroid
Tines forstablefixation
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Lead Maturation Process

• Effect of Steroid on Stimulation Thresholds

0
3
6
Pulse Width 0.5 msec
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General Medtronic Pacemaker Disclaimer INDICATIONS
Medtronic pacemakers are indicated for rate
adaptive pacing in patients who may benefit from
increased pacing rates concurrent with increases
in activity (Thera, Thera-i, Prodigy, Preva and
Medtronic.Kappa 700 Series) or increases in
activity and/or minute ventilation
(Medtronic.Kappa 400 Series). Medtronic
pacemakers are also indicated for dual chamber
and atrial tracking modes in patients who may
benefit from maintenance of AV synchrony. Dual
chamber modes are specifically indicated for
treatment of conduction disorders that require
restoration of both rate and AV synchrony, which
include various degrees of AV block to maintain
the atrial contribution to cardiac output and VVI
intolerance (e.g., pacemaker syndrome) in the
presence of persistent sinus rhythm. 9790
Programmer The Medtronic 9790 Programmers are
portable, microprocessor based instruments used
to program Medtronic implantable
devices. 9462 The Model 9462 Remote Assistant is
intended for use in combination with a Medtronic
implantable pacemaker with Remote Assistant
diagnostic capabilities. CONTRAINDICATIONS Medtro
nic pacemakers are contraindicated for the
following applications        Dual chamber
atrial pacing in patients with chronic refractory
atrial tachyarrhythmias.        Asynchronous
pacing in the presence (or likelihood) of
competitive paced and intrinsic rhythms.       
Unipolar pacing for patients with an implanted
cardioverter-defibrillator because it may cause
unwanted delivery or inhibition of ICD
therapy.        Medtronic.Kappa 400 Series
pacemakers are contraindicated for use with
epicardial leads and with abdominal
implantation. WARNINGS/PRECAUTIONS Pacemaker
patients should avoid sources of magnetic
resonance imaging, diathermy, high sources of
radiation, electrosurgical cautery, external
defibrillation, lithotripsy, and radiofrequency
ablation to avoid electrical reset of the device,
inappropriate sensing and/or therapy. 9462 Operati
on of the Model 9462 Remote Assistant Cardiac
Monitor near sources of electromagnetic
interference, such as cellular phones, computer
monitors, etc. may adversely affect the
performance of this device. See the appropriate
technical manual for detailed information
regarding indications, contraindications,
warnings, and precautions.  Caution Federal law
(U.S.A.) restricts this device to sale by or on
the order of a physician.
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Medtronic Leads For Indications,
Contraindications, Warnings, and Precautions for
Medtronic Leads, please refer to the appropriate
Leads Technical Manual or call your local
Medtronic Representative.   Caution Federal law
restricts this device to sale by or on the order
of a Physician. Note This presentation is
provided for general educational purposes only
and should not be considered the exclusive source
for this type of information. At all times, it
is the professional responsibility of the
practitioner to exercise independent clinical
judgment in a particular situation.
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Continued inBasic Pacing ConceptsPart III