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Basic Pacing Concepts

Basic Pacing Concepts. The Heart Has an Intrinsic Pacemaker. The heart generates electrical impulses that travel along a specialized conduction pathway This conduction process makes it possible for the heart to pump blood efficiently.

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Basic Pacing Concepts

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  1. Basic Pacing Concepts

  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. During Conduction, an Impulse Begins in the Sinoatrial (SA) Node and Causes the Atria to Contract Atria Sinoatrial (SA) Node Ventricles 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 AV node Bundle branches

  5. Diseased Heart Tissue May: • Prevent impulse generation in the SA node • Inhibit impulse conduction SA node AV node

  6. Implantable Pacemaker Systems Contain the Following Components: Lead wire(s) Implantable pulse generator (IPG)

  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 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 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

  13. Myocardial and Epicardial Leads • Leads applied directly to the heart • Fixation mechanisms include: • Epicardial stab-in • Myocardial screw-in • Suture-on

  14. Cathode • An electrode that is in contact with the heart tissue • Negatively charged when electrical current is flowing Cathode

  15. Anode • An electrode that receives the electrical impulse after depolarization of cardiac tissue • Positively charged when electrical current is flowing 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 (+) *

  18. 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 - Cathode

  19. 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 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

  22. Bipolar leads • Bipolar leads are less susceptible to oversensing noncardiac signals (myopotentials and EMI) Coaxial Lead Design

  23. A Brief History of Pacemakers

  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. 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 Advantages and Disadvantages of Single-Chamber Pacing Systems Advantages Disadvantages

  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

  29. Every Electrical Pacing Circuit Has the Following Characteristics: • Voltage • Current • Impedance

  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

  33. 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

  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

  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 1.0 ms 0.5 ms 0.25 ms

  39. 2.0 1.5 1.0 .50 .25 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 Stimulation Threshold (Volts) Capture 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 .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

  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

  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 _

  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

  50. Electromagnetic Interference

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