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Basic Pacing Concepts
The Heart Has an Intrinsic Pacemaker
The heart generates electrical impulses that
travel along a specialized conduction pathway
This conduction process makes it possible for
the heart to pump blood efficiently
During Conduction, an Impulse Begins in the
Sinoatrial (SA) Node and Causes the Atria to Contract
Sinoatrial (SA) Node
Atrioventricular (AV) Node
Then, the Impulse Moves to the Atrioventricular (AV) Node and Down
the Bundle Branches, Which Causes the Ventricles to Contract
SA node
AV node
Bundle branches
Diseased Heart Tissue May:
Prevent impulse
generation in the SA
SA node
Inhibit impulse
AV node
Implantable Pacemaker Systems
Contain the Following Components:
Lead wire(s)
Implantable pulse
generator (IPG)
Pacemaker Components Combine with
Body Tissue to Form a Complete Circuit
Pulse generator: power
source or battery
Leads or wires
Cathode (negative
Anode (positive
Body tissue
The Pulse Generator:
Contains a battery
that provides the
energy for sending
electrical impulses to
the heart
Houses the circuitry
that controls
Leads Are Insulated Wires That:
Deliver electrical
impulses from the
pulse generator to
the heart
Sense cardiac
Types of Leads
Endocardial or transvenous leads
Myocardial/Epicardial leads
Transvenous Leads Have Different
“Fixation” Mechanisms
Passive fixation
– The tines become
lodged in the
(fibrous meshwork)
of the heart
Transvenous Leads
Active Fixation
– The helix (or screw)
extends into the
endocardial tissue
– Allows for lead
anywhere in the
heart’s chamber
Myocardial and Epicardial Leads
Leads applied directly to
the heart
– Fixation mechanisms
Epicardial stab-in
Myocardial screw-in
An electrode that is
in contact with the
heart tissue
Negatively charged
when electrical
current is flowing
An electrode that
receives the electrical
impulse after
depolarization of
cardiac tissue
Positively charged
when electrical
current is flowing
Conduction Pathways
Body tissues and
fluids are part of the
conduction pathway
between the anode
and cathode
During Pacing, the Impulse:
Begins in the pulse
Flows through the lead
and the cathode (–)
Stimulates the heart
Returns to the anode (+)
Impulse onset
A Unipolar Pacing System Contains a Lead with Only One
Electrode Within the Heart; In This System, the Impulse:
Flows through the tip
electrode (cathode)
Stimulates the heart
Returns through
body fluid and tissue
to the IPG (anode)
A Bipolar Pacing System Contains a Lead with Two
Electrodes Within the Heart. In This System, the Impulse:
Flows through the
tip electrode located
at the end of the
lead wire
Stimulates the
Returns to the ring
electrode above the
lead tip
Unipolar and Bipolar Leads
Unipolar leads
Unipolar leads may
have a smaller
diameter lead body
than bipolar leads
Unipolar leads
usually exhibit larger
pacing artifacts on
the surface ECG
Bipolar leads
Bipolar leads are less
susceptible to
noncardiac signals
(myopotentials and
Coaxial Lead
A Brief History of Pacemakers
Single-Chamber and Dual-Chamber
Pacing Systems
Single-Chamber System
The pacing lead is
implanted in the
atrium or ventricle,
depending on the
chamber to be paced
and sensed
Advantages and Disadvantages of
Single-Chamber Pacing Systems
Implantation of a
single lead
Single ventricular lead
does not provide AV
Single atrial lead does
not provide ventricular
backup if A-to-V
conduction is lost
Dual-Chamber Systems Have Two Leads:
One implanted in
both the atrium and
the ventricle
Most Pacemakers Perform Four Functions:
Stimulate cardiac depolarization
Sense intrinsic cardiac function
Respond to increased metabolic demand by
providing rate responsive pacing
Provide diagnostic information stored by the
Every Electrical Pacing Circuit Has
the Following Characteristics:
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
Lead Impedance Values Will Change Due to:
Insulation breaks
Wire fractures
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
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
Stimulation Threshold
The minimum electrical stimulus needed to consistently
capture the heart outside of the heart’s refractory period
VVI / 60
Two Settings Are Used to Ensure Capture:
Pulse width
Amplitude is the Amount of Voltage
Delivered to the Heart By the Pacemaker
Amplitude reflects the strength or height of the
– 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
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
0.5 ms
0.25 ms
1.0 ms
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)
The Strength-Duration Curve
Pulse Width (ms)
Adequate safety
margins must be
achieved due to:
– Acute or chronic
pacing system
– Daily fluctuations
in threshold
Stimulation Threshold (Volts)
Clinical Usefulness of the
Strength-Duration Curve
Pulse Width (ms)
Electrode Design May Also Impact
Stimulation Thresholds
Lead maturation process
Lead Maturation Process
Fibrotic “capsule” develops around the
electrode following lead implantation
Sensing is the ability of the pacemaker to “see”
when a natural (intrinsic) depolarization is
– Pacemakers sense cardiac depolarization by
measuring changes in electrical potential of
myocardial cells between the anode and
A Pacemaker Must Be Able to Sense
and Respond to Cardiac Rhythms
Accurate sensing enables the pacemaker to
determine whether or not the heart has created
a beat on its own
The pacemaker is usually programmed to
respond with a pacing impulse only when the
heart fails to produce an intrinsic beat
Accurate Sensing...
Ensures that undersensing will not occur –
the pacemaker will not miss P or R waves that
should have been sensed
Ensures that oversensing will not occur – the
pacemaker will not mistake extra-cardiac activity
for intrinsic cardiac events
Provides for proper timing of the pacing pulse –
an appropriately sensed event resets the timing
sequence of the pacemaker
Accurate Sensing is Dependent on . . .
The electrophysiological properties of the
The characteristics of the electrode and its
placement within the heart
The sensing amplifiers of the pacemaker
Unipolar Sensing
Produces a large
potential difference
due to:
– A cathode and
anode that are
farther apart than
in a bipolar system
Bipolar Sensing
Produces a smaller
potential difference due to
the short interelectrode
– Electrical signals from
outside the heart such
as myopotentials are
less likely to be sensed
Electromagnetic Interference
Electromagnetic Interference (EMI)
Interference is caused by electromagnetic
energy with a source that is outside the body
Electromagnetic fields that may affect
pacemakers are radio-frequency waves
– 50-60 Hz are most frequently associated with
pacemaker interference
Few sources of EMI are found in the home or
office but several exist in hospitals
EMI May Result in the Following Problems:
Transient mode change (noise reversion)
Reprogramming (Power on Reset or “POR”)
Oversensing May Occur When EMI Signals Are
Incorrectly Interpreted as P Waves or R Waves
Pacing rates will vary as a result of EMI:
– Rates will accelerate if sensed as P waves in
dual-chamber systems (P waves are
– Rates will be low or inhibited if sensed in
single-chamber systems, or on ventricular
lead in dual-chamber systems
New technologies will continue to create new,
unanticipated sources of EMI:
 Cellular phones (digital)
Sources of EMI Are Found Most
Commonly in Hospital Environments
Sources of EMI that interfere with pacemaker operation
include surgical/therapeutic equipment such as:
– Electrocautery
– Transthoracic defibrillation
– Extracorporeal shock-wave lithotripsy
– Therapeutic radiation
– RF ablation
– TENS units
Sources of EMI Are Found More Rarely in:
Home, office, and shopping environments
Industrial environments with very high electrical outputs
Transportation systems with high electrical energy
exposure or with high-powered radar and radio
– Engines or subway braking systems
– Airport radar
– Airplane engines
TV and radio transmission sites
Rate Responsive Pacing
Rate Responsive Pacing
When the need for oxygenated blood increases, the
pacemaker ensures that the heart rate increases to
provide additional cardiac output
Adjusting Heart Rate to Activity
Normal Heart Rate
Rate Responsive Pacing
Fixed-Rate Pacing
Daily Activities
Rate Response
Rate responsive (also called rate modulated)
pacemakers provide patients with the ability to vary
heart rate when the sinus node cannot provide the
appropriate rate
Rate responsive pacing is indicated for:
– Patients who are chronotropically incompetent
(heart rate cannot reach appropriate levels during
exercise or to meet other metabolic demands)
– Patients in chronic atrial fibrillation with slow
ventricular response
Rate Responsive Pacing
Cardiac output (CO) is determined by the
combination of stroke volume (SV) and heart
rate (HR)
Changes in cardiac output depend on the
ability of the HR and SV to respond to
metabolic requirements
Rate Responsive Pacing
SV reserves can account for increases in
cardiac output of up to 50%
HR reserves can nearly triple total cardiac
output in response to metabolic demands
A Variety of Rate Response Sensors Exist
Those most accepted in the market place are:
– Activity sensors that detect physical movement
and increase the rate according to the level of
– Minute ventilation sensors that measure the
change in respiration rate and tidal volume via
transthoracic impedance readings
Rate Responsive Pacing
Activity sensors employ
a piezoelectric crystal
that detects mechanical
signals produced by
The crystal translates
the mechanical signals
into electrical signals
that in turn increase the
rate of the pacemaker
Rate Responsive Pacing
Minute Ventilation (MV) is the volume of air
introduced into the lungs per unit of time
MV has two components:
– Tidal volume–the volume of air introduced
into the lungs in a single respiration cycle
– Respiration rate–the number of respiration
cycles per minute
Rate Responsive Pacing
Minute ventilation can be measured by
measuring the changes in electrical impedance
across the chest cavity to calculate changes in
lung volume over time
Rate Responsive Pacing
Adjusting Heart Rate to Activity
Normal Heart Rate
Rate Responsive Pacing
Fixed-Rate Pacing
Daily Activities