Basic Pacing Concepts. zThe heart generates electrical impulses that travel along a specialized conduction pathway zThis conduction process makes it possible.
Post on 24-Dec-2015
212 Views
Preview:
Transcript
Basic Pacing ConceptsBasic Pacing Concepts
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
The Heart Has an Intrinsic PacemakerThe Heart Has an Intrinsic Pacemaker
Ventricles
Sinoatrial (SA) Node
Atrioventricular (AV) Node
Atria
During Conduction, an Impulse Begins in the During Conduction, an Impulse Begins in the Sinoatrial (SA) Node and Causes the Atria to ContractSinoatrial (SA) Node and Causes the Atria to Contract
Atria
Ventricles
Bundle branches
AV node
SA node
Then, the Impulse Moves to the Atrioventricular (AV) Node and Down Then, the Impulse Moves to the Atrioventricular (AV) Node and Down the Bundle Branches, Which Causes the Ventricles to Contractthe Bundle Branches, Which Causes the Ventricles to Contract
SA node
Prevent impulse generation in the SA node
Inhibit impulse conduction
AV node
Diseased Heart Tissue May:Diseased Heart Tissue May:
Implantable pulse generator (IPG)
Lead wire(s)
Implantable Pacemaker Systems Implantable Pacemaker Systems Contain the Following Components:Contain the Following Components:
Pulse generator: power source or battery
Leads or wires
Cathode (negative electrode)
Anode (positive electrode)
Body tissue
IPG
Lead
Anode
Cathode
Pacemaker Components Combine with Pacemaker Components Combine with Body Tissue to Form a Complete CircuitBody Tissue to Form a Complete Circuit
Contains a battery that provides the energy for sending electrical impulses to the heart
Houses the circuitry that controls pacemaker operations
Circuitry
Battery
The Pulse Generator:The Pulse Generator:
Deliver electrical impulses from the pulse generator to the heart
Sense cardiac depolarization
Lead
Leads Are Insulated Wires That:Leads Are Insulated Wires That:
Endocardial or transvenous leads
Myocardial/Epicardial leads
Types of LeadsTypes of Leads
Transvenous Leads Have Transvenous Leads Have Different “Fixation” MechanismsDifferent “Fixation” Mechanisms
Passive fixation
– The tines become lodged in the trabeculae(fibrous meshwork) of the heart
Transvenous LeadsTransvenous Leads
Active Fixation
– The helix (or screw) extends into the endocardial tissue
– Allows for lead positioning anywhere in the heart’s chamber
Myocardial and Epicardial LeadsMyocardial and Epicardial Leads
Leads applied directly to the heart
– Fixation mechanisms include:
Epicardial stab-in
Myocardial screw-in
Suture-on
CathodeCathode
An electrode that is in contact with the heart tissue
Negatively charged when electrical current is flowing
Cathode
AnodeAnode
An electrode that receives the electrical impulse after depolarization of cardiac tissue
Positively charged when electrical current is flowing
Anode
Conduction PathwaysConduction Pathways
Body tissues and fluids are part of the conduction pathway between the anode and cathode
Tissue
Cathode
Anode
Begins in the pulse generator
Flows through the lead and the cathode (–)
Stimulates the heart
Returns to the anode (+)
During Pacing, the Impulse:During Pacing, the Impulse:
Impulse onset
*
Flows through the tip electrode (cathode)
Stimulates the heart
Returns through body fluid and tissue to the IPG (anode)
A Unipolar Pacing System Contains a Lead with Only One A Unipolar Pacing System Contains a Lead with Only One Electrode Within the Heart; In This System, the Impulse:Electrode Within the Heart; In This System, the Impulse:
Cathode
Anode
-
+
Anode
Flows through the tip electrode located at the end of the lead wire
Stimulates the heart
Returns to the ring electrode above the lead tip
A Bipolar Pacing System Contains a Lead with Two A Bipolar Pacing System Contains a Lead with Two Electrodes Within the Heart. In This System, the Impulse:Electrodes Within the Heart. In This System, the Impulse:
Cathode
Unipolar and Bipolar LeadsUnipolar and Bipolar Leads
Unipolar leadsUnipolar 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 leadsBipolar leads
Bipolar leads are less susceptible to oversensing noncardiac signals (myopotentials and EMI)
Coaxial Lead Design
A Brief History of PacemakersA Brief History of Pacemakers
Single-Chamber and Dual-Chamber Single-Chamber and Dual-Chamber Pacing SystemsPacing Systems
Single-Chamber SystemSingle-Chamber System
The pacing lead is implanted in the atrium or ventricle, depending on the chamber to be paced and sensed
DisadvantagesDisadvantagesAdvantagesAdvantages
Advantages and Disadvantages of Advantages and Disadvantages of Single-Chamber Pacing SystemsSingle-Chamber Pacing Systems
Implantation of a single lead
Single ventricular lead does not provide AV synchrony
Single atrial lead does not provide ventricular backup if A-to-V conduction is lost
One implanted in both the atrium and the ventricle
Dual-Chamber Systems Have Two Leads:Dual-Chamber Systems Have Two Leads:
Stimulate cardiac depolarization
Sense intrinsic cardiac function
Respond to increased metabolic demand by providing rate responsive pacing
Provide diagnostic information stored by the pacemaker
Most Pacemakers Perform Four Functions:Most Pacemakers Perform Four Functions:
Voltage
Current
Impedance
Every Electrical Pacing Circuit Has Every Electrical Pacing Circuit Has the Following Characteristics:the Following Characteristics:
Impedance Changes Affect Pacemaker Impedance Changes Affect Pacemaker Function and Battery LongevityFunction 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
– High impedance leads will show impedance reading values greater than 1,000 ohms
Lead Impedance Values Will Change Due to:Lead Impedance Values Will Change Due to:
Insulation breaks
Wire fractures
An Insulation Break Around the Lead Wire An Insulation Break Around the Lead Wire Can Cause Impedance Values to FallCan 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
Insulation break
Decreased resistance
A Wire Fracture Within the Insulating Sheath A Wire Fracture Within the Insulating Sheath May Cause Impedance Values to RiseMay 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
Lead wire fracture
Increased resistance
StimulationStimulation
Stimulation ThresholdStimulation Threshold
The minimum electrical stimulus needed to consistently capture the heart outside of the heart’s refractory period
VVI / 60
Capture Non-Capture
Amplitude
Pulse width
Two Settings Are Used to Ensure Capture:Two Settings Are Used to Ensure Capture:
Amplitude is the Amount of Voltage Amplitude is the Amount of Voltage Delivered to the Heart By the PacemakerDelivered 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
Pulse Width Is the Time (Duration) Pulse Width Is the Time (Duration) of the Pacing Pulseof 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
0.5 ms 0.25 ms 1.0 ms
The Strength-Duration CurveThe 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
DurationPulse Width (ms)
.50
1.0
1.5
2.0
.25S
tim
ula
tio
n T
hre
sho
ld (
Vo
lts)
0.5 1.0 1.5
Capture
Clinical Usefulness of the Clinical Usefulness of the Strength-Duration CurveStrength-Duration Curve
Adequate safety margins must be achieved due to:
– Acute or chronic pacing system
– Daily fluctuations in threshold
Capture
0.5 1.0 1.5Duration
Pulse Width (ms)
.50
1.0
1.5
2.0
.25
Sti
mu
lati
on
Th
resh
old
(V
olt
s)
Electrode Design May Also Impact Electrode Design May Also Impact Stimulation ThresholdsStimulation Thresholds
Lead maturation process
Lead Maturation ProcessLead Maturation Process
Fibrotic “capsule” develops around the electrode following lead implantation
SensingSensing
SensingSensing
Sensing is the ability of the pacemaker to “see” when a natural (intrinsic) depolarization is occurring
– Pacemakers sense cardiac depolarization by measuring changes in electrical potential of myocardial cells between the anode and cathode
A Pacemaker Must Be Able to Sense A Pacemaker Must Be Able to Sense and Respond to Cardiac Rhythmsand 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...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 . . .Accurate Sensing is Dependent on . . .
The electrophysiological properties of the myocardium
The characteristics of the electrode and its placement within the heart
The sensing amplifiers of the pacemaker
Unipolar SensingUnipolar Sensing
Produces a large potential difference due to:
– A cathode and anode that are farther apart than in a bipolar system
_
Bipolar SensingBipolar Sensing
Produces a smaller potential difference due to the short interelectrode distance
– Electrical signals from outside the heart such as myopotentials are less likely to be sensed
Electromagnetic InterferenceElectromagnetic Interference
Electromagnetic Interference (EMI)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:EMI May Result in the Following Problems:
Oversensing
Transient mode change (noise reversion)
Reprogramming (Power on Reset or “POR”)
Oversensing May Occur When EMI Signals Are Oversensing May Occur When EMI Signals Are Incorrectly Interpreted as P Waves or R WavesIncorrectly 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 “tracked”)
– 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 Sources of EMI Are Found Most Commonly in Hospital EnvironmentsCommonly 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
– MRI
Sources of EMI Are Found More Rarely in: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 transmission
– Engines or subway braking systems
– Airport radar
– Airplane engines
TV and radio transmission sites
Rate Responsive PacingRate Responsive Pacing
Rate Responsive PacingRate 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 PacingFixed-Rate Pacing
Daily Activities
Rate ResponseRate 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 PacingRate Responsive Pacing
Cardiac output (CO) is determined by the combination of stroke volume (SV) and heart rate (HR)
SV X HR = CO
Changes in cardiac output depend on the ability of the HR and SV to respond to metabolic requirements
Rate Responsive PacingRate 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 ExistA 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 activity
– Minute ventilation sensors that measure the change in respiration rate and tidal volume via transthoracic impedance readings
Rate Responsive PacingRate Responsive Pacing
Activity sensors employ a piezoelectric crystal that detects mechanical signals produced by movement
The crystal translates the mechanical signals into electrical signals that in turn increase the rate of the pacemaker
Piezoelectric crystal
Rate Responsive PacingRate 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 PacingRate 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 PacingRate Responsive Pacing
Adjusting Heart Rate to Activity
Normal Heart Rate
Rate Responsive PacingFixed-Rate Pacing
Daily Activities
PPACINGACING
FUTURE !!??
top related