1. Basic Pacing Concepts
Welcome to Basic Pacing Concepts, a course module in CorePace. The Basic Pacing module addresses concepts such as pacing system components, stimulation, sensing, EMI, and rate response.

2. 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

3. Atrioventricular (AV) Node
During Conduction, an Impulse Begins in the Sinoatrial (SA) Node and Causes the Atria to Contract
Atria
Sinoatrial (SA) Node
Ventricles
Initiation of the cardiac cycle normally begins with at the SA node. A resulting wave of depolarization passes through the right and left atria, which stimulates atrial contraction.
Atrioventricular (AV) Node

4. Then, the Impulse Moves to the Atrioventricular (AV) Node and Down the Bundle Branches, Which Causes the Ventricles to Contract
Atria
SA node
Ventricles
Following contraction of the atria, the impulse proceeds to the AV node. The impulse slows at the AV node, which allows time for contraction of the atria. Just below the AV node, the impulse passes quickly through the bundle of His, the right and left bundle branches and the Purkinje fibers and lead to contraction of the ventricles.
AV node
Bundle branches

5. Diseased Heart Tissue May:
Prevent impulse generation in the SA node
Inhibit impulse conduction
SA node
AV node
Impulses in a patient with diseased heart tissue may be:
Intermittent
Irregular
Not generated at all
At an inappropriate rate for the patient’s metabolic demand.
Block can occur at any point–within the SA node, AV node, His bundle or distal conduction system.

6. Implantable Pacemaker Systems Contain the Following Components:
Lead wire(s)
Implantable pulse generator (IPG)
A basic pacing system is made up of:
Implantable pulse generator that contains:
A power source—the battery within the pulse generator that generates the impulse
Circuitry—controls pacemaker operations
Leads—Insulated wires that deliver electrical impulses from the pulse generator to the heart. Leads also transmit electrical signals from the heart to the pulse generator.
Electrode—a conductor located at the end of the lead; delivers the impulse to the heart.

7. Pacemaker Components Combine with Body Tissue to Form a Complete Circuit
Pulse generator: power source or battery
Leads or wires
Cathode (negative electrode)
Anode (positive electrode)
Body tissue
Lead
IPG
In a bipolar system, body tissue is part of the circuit only in the sense that it affects impedance (at the electrode-tissue interface). In a unipolar system, contact with body tissue is essential to ground the IPG and allow pacing to occur.
Anode
Cathode

8. The Pulse Generator:
Contains a battery that provides the energy for sending electrical impulses to the heart
Houses the circuitry that controls pacemaker operations
Circuitry
Lithium-iodine is the most commonly used power source for today’s pacemakers. Microprocessors (both ROM and RAM) control sensing, output, telemetry, and diagnostic circuits.
Battery

9. Leads Are Insulated Wires That:
Deliver electrical impulses from the pulse generator to the heart
Sense cardiac depolarization
Lead

10. Types of Leads Endocardial or transvenous leads
Myocardial/Epicardial leads

11. Transvenous Leads Have Different “Fixation” Mechanisms
Passive fixation
The tines become lodged in the trabeculae (fibrous meshwork) of the heart

12. Transvenous Leads Active Fixation
The helix (or screw) extends into the endocardial tissue
Allows for lead positioning anywhere in the heart’s chamber
For smooth-walled hearts or those that lack trabeculation, or in patients that have had a previous CABG procedure, active fixation leads may be a better choice to prevent lead dislodgment.
The lead pictured on top is a fixed screw design. Those pictured at the bottom have an extendable/retractable helix.

13. Myocardial and Epicardial Leads
Leads applied directly to the heart
Fixation mechanisms include:
Epicardial stab-in
Myocardial screw-in
Suture-on
Epicardial or myocardial leads are implanted to the outside of the heart. These implants represent less than 5% of leads implanted, and are used primarily in pediatric cases or for patients in whom transvenous lead implant is contraindicated.

14. Cathode An electrode that is in contact with the heart tissue
Negatively charged when electrical current is flowing
Presenter Note:
Explain that the system presented here is a bipolar system: the anode for a unipolar system is actually the IPG itself. Note that this topic will be covered in more detail within the next few minutes.
Cathode

15. Anode
An electrode that receives the electrical impulse after depolarization of cardiac tissue
Positively charged when electrical current is flowing
On this slide, the anode is labeled for a a bipolar system.
Anode

16. Conduction Pathways
Body tissues and fluids are part of the conduction pathway between the anode and cathode
Anode
Tissue
Cathode

17. During Pacing, the Impulse:
Impulse onset
Begins in the pulse generator
Flows through the lead and the cathode (–)
Stimulates the heart
Returns to the anode (+) *
During pacing, the electrical impulse:
Begins in the pulse generator
Flows through the cathode (negative electrode)
Stimulates the heart tissue
Returns through the body tissue to the anode (positive electrode)
This pathway forms a complete pacing circuit.

18. + - Flows through the tip electrode (cathode) Stimulates the heart
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) +
Anode
In the unipolar system, the impulse:
Travels down the lead wire to stimulate the heart at the tip electrode also referred to as the cathode (–)
Returns to the metal casing of the impulse generator or the anode (+) by way of body fluids
The flow of the impulse makes a complete circuit. -
Cathode

19. Flows through the tip electrode located at the end of the lead wire
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 heart
Returns to the ring electrode above the lead tip
The impulse:
Travels down the lead wire to stimulate the heart at the tip electrode, which is the cathode (–)
Travels to the ring electrode, which is the anode (+), located several inches above the lead tip
Returns to the pulse generator by way of the lead wire
Anode
Cathode

20. Unipolar and Bipolar Leads

21. 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
Lead technology is advancing such that unipolar and bipolar leads will have smaller French sizes than those currently available.
Depending on the monitoring equipment, unipolar pacing usually exhibits a larger pacing spike on some surface ECGs.

22. Bipolar leads
Bipolar leads are less susceptible to oversensing noncardiac signals (myopotentials and EMI)
While unipolar and bipolar leads look similar (both have the appearance of one wire), most bipolar leads have a coaxial design, meaning an inner wire is insulated and wrapped with an outer wire, giving the lead the appearance of having only one wire.
Bipolar leads are less susceptible to oversensing noncardiac signals as the spacing of the two electrodes (located in close proximity to one another) accounts for a much lower incidence of sensing extra-cardiac signals.
Coaxial Lead Design

23. A Brief History of Pacemakers
The first implantable pacemakers, developed in 1960, were asynchronous pacemakers, i.e., pacing without regard to the heart’s intrinsic action (VOO).
Single-chamber “demand” pacemakers were introduced in the late 1960s.
In 1979, the first dual chamber pacemaker (DVI) was introduced, followed closely by the 1981 release of the first DDD pacemaker, the Versatrax.
The first single chamber, rate responsive pacemaker, Activitrax, was released in 1985.
Today, dual-chamber pacemakers use rate responsive pacing to mimic the heart’s rate response to provide/meet metabolic needs, most recently using a combination of sensors to best accomplish this task…
Pictured above:
(upper left) One of the first implantable devices. The device is coated with epoxy.
(upper right) Chardack Greatbatch device, late 1960’s.
(lower left) Model 5943, a VVI device with titanium case (1974).
(Middle) One of the first DDD devices, model number 7004.
(lower right) Early 1998: Kappa 400!

24. Single-Chamber and Dual-Chamber Pacing Systems

25. Single-Chamber System
The pacing lead is implanted in the atrium or ventricle, depending on the chamber to be paced and sensed

26. Advantages and Disadvantages of Single-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
Pacing in the VVI/R mode and loss of AV synchrony can lead to pacemaker syndrome. Pacemaker syndrome can be defined as “an assortment of symptoms related to the adverse hemodynamic impact from the loss of AV synchrony.”
Atrial pacemakers should only be used with patients who have proven AV conduction and regular follow-up testing available.

27. Dual-Chamber Systems Have Two Leads:
One implanted in both the atrium and the ventricle

28. 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 pacemaker
The modern pacemaker supports heart function in the following ways:
Provides effective and consistent cardiac depolarization
Prevents unnecessary pacing by sensing cardiac activity
Increases rate to meet increased metabolic demand
Provides information about how the patient’s heart and the implanted pacemaker are functioning

29. Every Electrical Pacing Circuit Has the Following Characteristics:
Voltage
Current
Impedance
The terms resistance and impedance are used interchangeably in pacing (unless engineers are talking!).

30. 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

31. Lead Impedance Values Will Change Due to:
Insulation breaks
Wire fractures

32. 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
Insulation around the lead wire prevents current loss from the lead wire.
Electrical current seeks the path of least resistance.
An insulation break that exposes wire to body fluids which have low resistance causes:
Lead impedance to fall
Current to drain into the body
Battery depletion
Impedance values below 300 W.
Insulation breaks are often marked by a trend of falling impedance values.
An impedance reading that changes suddenly or one that is >30% is considered significant and should be watched closely.

33. Impedance values across a break in the wire will increase
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
Insulation may remain intact but the wire may break within the insulating sheath.
Impedance may exceed 3,000 W.
Current flow may be too low to be effective.
If a complete fracture of the wire occurs:
No current will flow
Impedance number will be “infinite”
When suspecting a wire break, look for a trend in an increase in impedance values rather than a single lead impedance value.

34. Stimulation

35. Stimulation Threshold
The minimum electrical stimulus needed to consistently capture the heart outside of the heart’s refractory period
Capture
Non-Capture
VVI / 60

36. Two Settings Are Used to Ensure Capture:
Amplitude
Pulse width
Remind audience that amplitude had been discussed when voltage was described.

37. 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

38. 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
The greater the pulse width, the shorter the battery longevity. An increase in pulse width leads to an increase in total energy delivered.
The longer the duration of the stimulus, the lower the amplitude required to capture the heart
0.5 ms
0.25 ms
1.0 ms

39. 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
.50
1.0
1.5
2.0
.25
Stimulation Threshold (Volts)
Capture
The strength-duration curve illustrates the tradeoff of amplitude (intensity of stimulation, measured in volts) and duration (the length of time the stimulation is applied, measured in milliseconds).
Capture occurs when the stimulus causes the tissue to react. On the graph, capture occurs on or above the curve. Anything below the curve will not capture.
The lowest voltage on a stimulation curve which still results in capture at an infinitely long pulse duration is called rheobase. A voltage programmed below is inefficient and will lead to non-capture.
The pulse duration at twice the rheobase value is defined as the chronaxie. Chronaxie time is typically considered most efficient in terms of battery consumption.
0.5
1.0
1.5
Duration
Pulse Width (ms)

40. 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
Safety margins have traditionally been selected as follows, when a threshold is determined by decrementing the pulse width at a fixed voltage:
At a given voltage where the pulse width value is < .30 ms: Tripling the pulse width will provide a two-time voltage safety margin.
Pulse widths at a given voltage value which are >.30 ms are not typically selected, because they are less efficient (expend more energy), while not providing further safety. In this case, the voltage should be doubled to provide a two-time safety margin.
Adequate safety margins must be selected due to daily fluctuations in threshold that can occur due to eating, sleeping, exercise, or other factors that affect thresholds.
Also, a patient with an acute pacing system will typically be programmed to a setting allowing a higher safety margin, due to the lead maturation process, which can occur within the first 6-8 weeks following implant.
.25
0.5
1.0
1.5
Duration
Pulse Width (ms)

41. Electrode Design May Also Impact Stimulation Thresholds
Lead maturation process

42. Lead Maturation Process
Fibrotic “capsule” develops around the electrode following lead implantation
There are three phases that make up the lead maturation process:
The acute phase, where thresholds following implant are low
The peaking phase, where thresholds rise and reach their highest point, usually around one to two weeks following implant; followed by a decline in the threshold as the tissue reaction subsides
The chronic phase, where thresholds assume a stable reading, usually eight to twelve weeks post-implant.
The lead maturation process occurs due to the trauma to cells surrounding the electrode, which causes edema and subsequent development of a fibrotic capsule. The inexcitable capsule reduces the current at the electrode interface, requiring more energy to capture the heart.

43. Sensing

44. Sensing
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

45. 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
When the heart functions normally, there is no need for the pacemaker to deliver artificial pacing impulses.
A pacemaker must be able to sense and respond to normal and abnormal cardiac rhythms.

46. 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

47. 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

48. Unipolar Sensing Produces a large potential difference due to:
A cathode and anode that are farther apart than in a bipolar system
Unipolar sensing produces a large potential difference due to a cathode and anode that are farther apart than a bipolar system. Because both electrodes may contribute to the electrical signal that is sensed, the unipolar electrode configuration may detect electrical signals that occur near the pacemaker pocket as well as those inside the heart. _

49. Bipolar 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
Produces a small potential difference:
Electrodes are close to one another
Intracardiac signal arrives at each electrode at almost the same time
Less likely to sense:
Myopotentials: depolarization of muscles near the heart or the anode (the wave produced after the action potential wave passes along a nerve).
Afterpotentials
Far-field intracardiac signals
Noise
EMI

50. Electromagnetic Interference

51. 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
Electromagnetic interference enters a pacemaker by conduction if the patient is in direct contact with the source or by way of radiation if the patient is in an electromagnetic filed with the pacemaker lead acting as an antenna.
Ellenbogen KA, et al. Clinical Cardiac Pacing London: WB Saunders Company; page 770.

52. EMI May Result in the Following Problems:
Oversensing
Transient mode change (noise reversion)
Reprogramming (Power on Reset or “POR”)

53. 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 “tracked”)
Rates will be low or inhibited if sensed in single-chamber systems, or on ventricular lead in dual-chamber systems

54. Cellular phones (digital)
New technologies will continue to create new, unanticipated sources of EMI:
Cellular phones (digital)
Many patients have heard about possible interaction between cellular phones and devices as they have become commonplace in recent years. When the antenna of a cellular phone is too close to the implant site, radio frequency transmission signals can inhibit pacemaker therapy. The effects are temporary, and moving the antenna signal away from the implant site returns the device to normal operation.

55. 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
MRI

56. 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
Examples of EMI in the home and in shopping environments are rare. Ham radios, etc. have documented instances where pacing is inhibited. In most instances, the interference occurs in unipolar pacing systems and does not involve prolonged inhibition. Some antitheft devices in stores have interfered with patient’s devices seen again most often with unipolar systems.
In patients who work in environments with equipment capable of causing significant EMI, i.e., heavy motors or Arc welders, inhibition may occur. Again, use of bipolar leads can minimize or eliminate most problems.
Furman S, et al. A Practice of Cardiac Pacing 3rd edition, Mount Kisco, NY: Futura Publishing, Inc Page 673.

57. Rate Responsive Pacing

58. 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

59. 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

60. Rate 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

61. 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
Most of the pacing population relies heavily on rate reserves to increase cardiac output because stroke volume reserves are diminished.

62. 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 activity
Minute ventilation sensors that measure the change in respiration rate and tidal volume via transthoracic impedance readings
Other sensors that measure QT interval, central venous temperature, stroke volume, etc., are largely investigational devices or have gained limited acceptance.

63. Rate 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

64. 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

65. 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
Increased tidal volume and rate increase transthoracic impedance, which increases the pacing rate.

66. Rate Responsive Pacing
Adjusting Heart Rate to Activity
Normal Heart Rate
Rate Responsive Pacing
Fixed-Rate Pacing
Daily Activities

67. PACING
FUTURE !!??
Most of the pacing population relies heavily on rate reserves to increase cardiac output because stroke volume reserves are diminished.