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

Permanent Pacemaker Device

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.1 1Ellenbogen KA, et al. Clinical Cardiac Pacing. London: WB Sanders Company; 1995. Page 71.

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, 1996. PP. 77-79

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. 1995. PP. 83-84

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

Paced Rhythm Recognition 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.