Basic Pacing Concepts Welcome to Basic Pacing Concepts, a course module in CorePace. The Basic Pacing module addresses concepts such as pacing system.

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.

Objectives Identify the components of pacing systems and their respective functions Define basic electrical terminology Describe the relationship of amplitude and pulse width defined in the strength duration curve Explain the importance of sensing Discuss sources of electromagnetic interference (EMI) and patient/clinician guidelines related to these sources Understand the need for and types of sensors used in rate responsive pacing

Pacing Systems

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

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

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

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.

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.

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

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

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

Types of Leads Endocardial or transvenous leads
Myocardial/Epicardial leads

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

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.

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.

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

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

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

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.

+ - 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

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

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 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.

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

Lead Insulation May Be Silicone or Polyurethane

Advantages of Silicone-Insulated Leads
Inert Biocompatible Biostable Repairable with medical adhesive Historically very reliable Silicone has proven to be a reliable insulating material for more than three decades of clinical experience. However, silicone is a relatively fragile material which can tear easily. Therefore, the silicone layer must be relatively thick to resist nicks at implant. Silicone also has a high coefficient of friction and the moving of two leads through a single vein may be difficult. Platinum-cured silicone rubber, characterized by improved mechanical strength, has partially alleviated the issues of low tear strength and friction (Silicure). From Cardiac Pacing, K. Ellenbogen, ed., 2nd edition, PP

Advantages of Polyurethane-Insulated Leads
Biocompatible High tear strength Low friction coefficient Smaller lead diameter Current polyurethane lead performance is excellent. Historically, polyurethane leads have not performed as well as silicone due to lead degradation, the causes of which fall into two categories: Environmental Stress Cracking (ESC) Metal-induced Oxidation (also referred to as metal ion oxidation) Environmental stress cracking is the result of the lead being exposed to a “hostile” biological environment (both at and after implant). This exposure leads to eventual breakdown of the insulation to the conductor. Metal-induced oxidation is an oxidative degradation of the polyurethane introduced by the bodies own defense mechanisms responding to the foreign body (lead). From Clinical Cardiac Pacing, K. Ellenbogen et al PP

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!

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

Paced Rhythm Recognition
AAI / 60

VVI / 60

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.

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

The notation at the top refers to mode, lower rate, and upper rate parameters. This mode of operation can be described as atrial synchronous pacing or atrial tracking. DDD / 60 / 120

DDD / 60 / 120

Pacing in the atrium and ventricle is often described as AV sequential pacing. DDD / 60 / 120

DDD / 60 / 120

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

Electrical Concepts

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!).

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 The terms “amplitude” and “voltage” are often used interchangeably, undoubtedly to express “Voltage Amplitude” which refers to the voltage output.

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 Current: Measured in amperes (I) 1 Ampere =1000 milliamps Movement of electricity or free electrons through a circuit One ampere is a unit of electrical current produced by 1 volt acting through a resistance of 1 ohm

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 Impedance is the sum of all resistance to the flow of current. The resistive factors to a pacing system include: Lead conductor resistance The resistance to current flow from the electrode to the myocardium Polarization impedance, which is the accumulation of charges of opposite polarity in the myocardium at the electrode-tissue interface. Resistance is a term used to refer to simple electric circuits without capacitors and with constant voltage and current. Impedance is a term used to describe more complex circuits with capacitors and with varying voltage and current. Therefore, the use of the term impedance is more appropriate than resistance when discussing pacing circuits.

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

Voltage and Current Flow
Spigot (voltage) turned up (high current drain) Using the garden hose as an analogy, the higher the voltage, the greater the push, or “flow” of electrons (and the greater the current drain). Spigot (voltage) turned low (low current drain)

Resistance and Current Flow
“Normal” resistance “Low” resistance Resistance affects current flow. Leads with an insulation breach, such as the garden hose pictured in the middle, will measure a low resistance reading with a resultant high current flow, and possible premature battery depletion. Conversely, if there is a high resistance, such as a lead conductor break, the current flow will be low or non-existent. High current flow “High” resistance Low current flow

Ohm’s Law is a Fundamental Principle of Pacing That:
Describes the relationship between voltage, current, and resistance V I R V = I X R I = V / R R = V / I Can be expressed in three ways: V = I x R R = V ÷ I I = V÷ R If any two values are known, the third may be calculated (cover the value you are seeking and the others appear in the appropriate format to calculate the unknown value). x

When Using Ohm’s 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

Ohm’s 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 = Amperes 0.010 x 1000 = 10 mA If: Voltage = 5 V Impedance = 500 ohms 1 Ampere = 1000 milliamps (mA) Then use the formula I = V ÷ R to find I (the current). The amount of current that flows through a pacemaker system is very small: Use milliamps as a unit of measurement rather than Amps. Convert amps to milliamps by multiplying amps by 1000 or move the decimal three places to the right.

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 = Amperes 0.005 x 1000 = 5 mA If: Voltage = 2.5 V Impedance = 500 ohms 1 Ampere = 1000 milliamps Then use the formula I = V ÷ R to find I (the current). The amount of current that flows through a pacemaker system is very small: Use milliamps as a unit of measurement rather than amps. Convert Amps to milliamps by multiplying Amps by 1000 or move the decimal three places to the right.

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 = Amperes 0.020 x 1000 = 20 mA If: Voltage = 5 V Impedance = 250 ohms 1 Ampere = 1000 milliamps Then use the formula I = V ÷ R to find I (the current). The amount of current that flows through a pacemaker system is very small: Use milliamps as a unit of measurement rather than Amps. Convert Amps to milliamps by multiplying Amps by 1000 or move the decimal three places to the right.

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 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.

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.

Stimulation

Transmembrane Potential
Stimulation Process -50 50 -100 Phase 1 Phase 2 Phase 0 Transmembrane Potential (Millivolts) Phase 3 Threshold The stimulation process can be described in “phases:” The output voltage produces an electrical field at the electrode-tissue interface. The electrical field permeates cardiac cells via ionic movement and changes voltage on the cell membrane, which brings the cell membrane “above threshold” and alters its permeability. Phase 0 is the result of this part of the process. During Phase 0, sodium rushes in, which results in depolarization followed by cellular repolarization via sodium/potassium infusion. NOTE: The electrical field generated by the stimulation pulse must last long enough to excite the tissue. To effectively raise the membrane potential, the intensity of the stimulation must be balanced with the length of time it is applied. Phase 4 Time (Milliseconds) 100 200 300 400 500

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

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

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

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

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)

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)

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 Most lithium-iodine batteries (those used most often in pacemakers today), the battery voltage is 2.8 V at the beginning of battery life. If an output is programmed above this setting (for example, to 5 V), a voltage multiplier must be used to achieve the higher amplitude. The impact on battery longevity is significant. Using two capacitors can reduce longevity by as much as half .

Factors That Affect Battery Longevity Include:
Lead impedance Amplitude and pulse width setting Percentage paced vs. intrinsic events Rate responsive modes programmed “ON”

Electrode Design May Also Impact Stimulation Thresholds
Lead maturation process

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.

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 Steroid eluting leads reduce inflammation by employing a capsule of dexamethasone sodium phosphate, which gradually emits steroid over time, nearly eliminating the peaking phenomenon of the lead maturation process. Silicone rubber plug containing steroid Tines for stable fixation

Lead Maturation Process
Effect of Steroid on Stimulation Thresholds Implant Time (Weeks) Textured Metal Electrode Smooth Metal Electrode 1 2 3 4 5 Steroid-Eluting Electrode 7 8 9 10 11 12 Volts This graph compares the stimulation thresholds of contemporary pacing leads. Older electrodes exhibited higher threshold peaking than that of steroid leads shown on the slide. The different types of electrodes exhibit a wide range of threshold peaking. Steroid-eluting electrodes continue to show lower chronic stimulation thresholds and no significant peaking. Threshold changes are shown here over a 12-week period post-implant, where a comparison is made between: • smooth metal electrode • textured metal electrode • steroid-eluting electrode Traditionally, implant stimulation thresholds are relatively low. Non-steroid-eluting electrodes exhibit a peaking phase from week 1 to approximately week 6, due to the maturation process at the electrode-tissue interface. Steroid-eluting electrodes exhibit virtually no peaking. The chronic phase of stimulation threshold occurs 8-12 weeks post-implant which is characterized by a plateau. This plateau is higher than the acute phase, due to fibrotic encapsulation of the electrode. Steroid-eluting lead chronic thresholds remain close to implant values. 3 6 Pulse Width = 0.5 msec

Sensing

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

An Electrogram (EGM) is the Recording of Cardiac Waveforms Taken From Within the Heart
Intrinsic deflection on an EGM occurs when a depolarization wave passes directly under the electrodes Two characteristics of the EGM are: Signal amplitude Slew rate Electrograms are generated by the difference in electrical potential between the two electrodes. The intracardiac EGM is characterized in clinical practice in terms of its amplitude (measured in millivolts), and slew rate (measured in volts per second). Ellenbogen, KA, et al. Clinical Cardiac Pacing London: WB Saunders Company; Page 41.

The longer the signal takes to move from peak to peak:
Slew Rate of the EGM Signal Measures the Change in Voltage with Respect to the Change in Time The longer the signal takes to move from peak to peak: The lower the slew rate The flatter the signal Higher slew rates (number in mV) translate to greater sensing Measured in volts per second Change in voltage Slew rate= Time duration of voltage change Voltage Slope Typically, slew rate measurements at implant should exceed .5 volts per second for P waves; .75 volts per second for R wave measurements. Time

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.

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

Scheduled pace delivered Intrinsic beat not sensed
Undersensing . . . Pacemaker does not “see” the intrinsic beat, and therefore does not respond appropriately Scheduled pace delivered Intrinsic beat not sensed VVI / 60

Oversensing VVI / 60 ...though no activity is present Marker channel shows intrinsic activity... An electrical signal other than the intended P or R wave is detected Oversensing will exhibit pauses in single chamber systems. In dual chamber systems, atrial oversensing may cause fast ventricular pacing without P waves preceding the paced ventricular events.

Sensitivity – The Greater the Number, the Less Sensitive the Device to Intracardiac Events
Pacemakers have programmable sensitivity settings that can be thought of like a fence: with a lower fence more of the signal is seen; with a higher fence less of the signal is seen.

Sensitivity 5.0 Amplitude (mV) 2.5 1.25 Time
If the system is sensing myopotentials, then raise the fence or increase the number of the sensitivity setting. The pacemaker will "see less" of the incoming signal. If the pacing system is not “seeing” intrinsic cardiac events, set the fence lower or decrease the number of the sensitivity setting. The pacemaker will then "see” more of the incoming signal. Time

In this example, the sensitivity number is set higher than the signal. The pacemaker is unable to see any activity and undersensing will result. Time

In this example, the sensitivity setting is set such that the pacemaker will likely sense the T wave. Oversensing will occur. Time

Accurate Sensing Requires That Extraneous Signals Be Filtered Out
Sensing amplifiers use filters that allow appropriate sensing of P waves and R waves and reject inappropriate signals Unwanted signals most commonly sensed are: T waves Far-field events (R waves sensed by the atrial channel) Skeletal myopotentials (e.g., pectoral muscle myopotentials)

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

Factors That May Affect Sensing Are:
Lead polarity (unipolar vs. bipolar) Lead integrity Insulation break Wire fracture EMI – Electromagnetic Interference

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. _

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

An Insulation Break May Cause Both Undersensing or Oversensing
Undersensing occurs when inner and outer conductor coils are in continuos contact Signals from intrinsic beats are reduced at the sense amplifier and amplitude no longer meets the programmed sensing value Oversensing occurs when inner and outer conductor coils make intermittent contact Signals are incorrectly interpreted as P or R waves

A Wire Fracture Can Cause Both Undersensing and Oversensing
Undersensing occurs when the cardiac signal is unable to get back to the pacemaker – intrinsic signals cannot cross the wire fracture Oversensing occurs when the severed ends of the wire intermittently make contact, which creates potentials interpreted by the pacemaker as P or R waves Lead fractures that are intermittent are also referred to as “make and break” fractures, due to the artifacts in the electrogram that are produced as the conductor wires make and break contact.

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 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.

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 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

“Noise” sensed by the pacemaker
EMI “Noise” sensed by the pacemaker Should have paced In this example, the first complex is paced and starts a timing cycle. The pacemaker detects noise and interprets it as intrinsic activity. The pacing output should have occurred earlier but the timer was reset due to the “noise”, hence the pause.

Noise Reversion Continuous refractory sensing will cause pacing at the lower or sensor driven rate Lower Rate Interval Noise Sensed The portion of the Refractory Period after the Blanking Period ends is commonly called the "noise sampling period." This is because a sensed event in the noise sampling period will initiate a new Refractory Period and Blanking Period. If events continue to be sensed within the noise sampling period causing a new Refractory Period each time, the pacemaker will eventually pace at the Lower Rate since the Lower Rate timer is not reset by events sensed during the Refractory Period. This behavior is known as "noise reversion." Note: In rate-responsive modes, noise reversion will cause pacing to occur at the sensor-driven rate. SR SR SR SR VP VP VVI/60

EMI May Lead to Inadvertent Reprogramming of the Pacing Parameters
Device will revert to Power on Reset (POR or “backup” mode) Power on Reset may exhibit a mode and rate change which are often the same as ERI In some cases, reprogrammed parameters may be permanent POR parameters for Medtronic devices: Partial reset maintains polarity, pacing mode, and other parameters but the rate will change to 65. A full reset will resume pacing in the VVI mode at a rate of 65. POR can be distinguished from true ERI by checking the battery voltage and the ability to reprogram the device.

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.

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

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.

Electrocautery is the Most Common Hospital Source of Pacemaker EMI
Outcomes Oversensing–inhibition Undersensing (noise reversion) Power on Reset Permanent loss of pacemaker output (if battery voltage is low) Precautions Reprogram mode to VOO/DOO, or place a magnet over device Strategically place the grounding plate Limit electrocautery bursts to 1-second burst every 10 seconds Use bipolar electrocautery forceps Electrosurgery used within six inches of an implanted pacemaker/lead system has the potential to cause permanent loss of pacemaker output. Earlier designed pacemakers are more susceptible to loss of output as the battery voltage decreases. Precautions: Monitor the patient’s pulse during application of the cautery. Program the pacemaker to VOO/DOO if the patient is pacemaker dependent, or secure a magnet over the device. Place the grounding plate as close to the operative site as possible—usually under buttocks or thighs—and as far from pacemaker as possible (a minimum of 15 cm from pacemaker). Limit electrocautery to 1-second bursts every 10 seconds. Use bipolar electrocautery forceps where practical.

Transthoracic Defibrillation
Outcome Inappropriate reprogramming of the pulse generator (POR) Damage to pacemaker circuitry Precautions Position defibrillation paddles apex-posterior (AP) and as far from the pacemaker and leads as possible Defibrillation concerns for IPGs are similar to those mentioned for use with electrocautery. If possible, position the electrodes so that currents are not passing through the pacing system. Place the defibrillator electrodes at least thirteen centimeters or five inches from the IPG. Use the least amount of energy to satisfactorily revert the patient. Medtronic IPGs are designed to withstand 400 watt-seconds of defibrillation energy. Always check the operation of the IPG following defibrillatory discharges. Damage may be to various components of the circuitry.

Magnetic Resonance Imaging (MRI) is Generally Contraindicated in Patients with Pacemakers
Outcomes Extremely high pacing rate Reversion to asynchronous pacing Precautions Program pacemaker output low enough to create persistent non-capture, ODO or OVO mode

Lithotripsy Shock Waves May Have an Effect on Pacemaker Systems
Outcomes in dual-chamber modes: Inhibition of ventricular pacing Outcomes in rate adaptive pacemakers High pacing rates Piezoelectric crystal damage Precautions: Program pacemaker to VVI or VOO mode Lithotriptor focal point should be greater than 6 inches from the pacemaker Carefully monitor heart function throughout procedure Extracorporeal shock-wave lithotripsy is a non-invasive treatment for renal tract calculi. The shock wave can produce ventricular extrasystoles, so it is synchronized to the R wave. Pacemakers could be subject to electrical interference from the spark gap and mechanical damage from the hydraulic shock wave. In VVI pacing, the shock waves do not affect pacemaker performance. In dual-chamber mode, the waves may synchronize with atrial output and cause inhibition of ventricular pacing output. In rate-adaptive pacemakers: High pacing rates may result from shock wave sensing. Piezoelectric crystal may be damaged. Ellenbogen KA, et al. Clinical Cardiac Pacing London: WB Saunders Company; Page 776.

Radiation Energy May Cause Permanent Damage
Certain kinds of radiation energy may cause damage to the semi-conductor circuitry Ionizing radiation used for breast or lung cancer therapy Damage can be permanent and requires replacement of the pacemaker

Therapeutic Radiation May Cause Severe Damage
Outcomes: Pacemaker circuit damage Loss of output “Runaway” Precautions: Keep cumulative radiation absorbed by the pacemaker to less than 500 rads; shielding may be required Check pacemaker after radiation sessions for changes in pacemaker function (can be done transtelephonically) Diagnostic x-ray exposure poses no risk. Therapeutic radiation may cause severe damage. Patients receiving therapeutic radiation in treatment of malignant thoracic disease are particularly at risk. Appropriate shielding of the pulse generator is essential. If adequate shielding is not possible, repositioning of the IPG may have to be carried out. Patients should be checked after each session.

Pacemaker Features That Address Interference
Pacemaker sensing circuits amplify, filter and either process or reject incoming signals Input Bandpass filter Absolute value Reversion circuit Level detector Pacemaker logic The intracardiac electrogram is conducted from the electrodes to the sensing circuit of the pulse generator, where it is amplified and filtered. A bandpass filter selectively attenuates unwanted components of the electrogram. The absolute value assesses signals in such a way that positive and negative deflections are treated equally. The reversion circuit adjusts the baseline to eliminate noise. The processed signal is compared with a reference voltage (the level detector) to determine if the signal exceeds the programmed sensing level. Signals lower than the sensing level are discarded as noise. Finally, signals with amplitudes greater than the sensitivity threshold are passed along to the pacemaker logic where timing intervals and marker channels, among other operations, are initiated. Sensitivity adjustment

Rate Responsive Pacing

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

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

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.

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

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.

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

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

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.

Summary of Basic Pacing Concepts Module
Pacing systems Electrical concepts Stimulation thresholds Sensing Electromagnetic Interference (EMI) Rate response

Basic Pacing Concepts Welcome to Basic Pacing Concepts, a course module in CorePace. The Basic Pacing module addresses concepts such as pacing system.

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Basic Pacing Concepts Welcome to Basic Pacing Concepts, a course module in CorePace. The Basic Pacing module addresses concepts such as pacing system.

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Presentation on theme: "Basic Pacing Concepts Welcome to Basic Pacing Concepts, a course module in CorePace. The Basic Pacing module addresses concepts such as pacing system."— Presentation transcript:

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