Basic Pacing Concepts Part III

1. Basic Pacing ConceptsPart III

2. Sensing

3. 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

4. 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

5. 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 Time

6. 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

7. 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

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

9. VVI / 60 Oversensing • An electrical signal other than the intended P or R wave is detected ...though no activity is present Marker channel shows intrinsic activity...

10. Sensitivity – The Greater the Number, the Less Sensitive the Device to Intracardiac Events

11. Sensitivity 5.0 2.5 Amplitude (mV) 1.25 Time

12. Sensitivity 5.0 2.5 Amplitude (mV) 1.25 Time

13. Sensitivity 5.0 2.5 Amplitude (mV) 1.25 Time

14. 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)

15. 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

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

17. Unipolar Sensing • Produces a large potential difference due to: • A cathode and anode that are farther apart than in a bipolar system _

18. 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

19. 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

20. 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

21. Electromagnetic Interference

22. 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

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

24. 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

25. EMI “Noise” sensed by the pacemaker Should have paced

26. Lower Rate Interval Noise Reversion • Continuous refractory sensing will cause pacing at the lower or sensor driven rate Noise Sensed SR SR SR SR VP VP VVI/60

27. 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

28. New technologies will continue to create new, unanticipated sources of EMI: • Cellular phones (digital)

29. 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

30. 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

31. 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 Electrocautery is the Most Common Hospital Source of Pacemaker EMI

32. 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 Transthoracic Defibrillation

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

34. Outcomes indual-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 Lithotripsy Shock Waves May Have an Effect on Pacemaker Systems

35. 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

36. 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) Therapeutic Radiation May Cause Severe Damage

37. Input Bandpass filter Absolute value Reversion circuit Level detector Pacemaker logic Sensitivity adjustment Pacemaker Features That Address Interference • Pacemaker sensing circuits amplify, filter and either process or reject incoming signals

38. Rate Responsive Pacing

39. 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

40. 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

41. 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

42. 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

43. 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

44. 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

45. 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

46. 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

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

48. General Medtronic Pacemaker Disclaimer INDICATIONS Medtronic pacemakers are indicated for rate adaptive pacing in patients who may benefit from increased pacing rates concurrent with increases in activity (Thera, Thera-i, Prodigy, Preva and Medtronic.Kappa 700 Series) or increases in activity and/or minute ventilation (Medtronic.Kappa 400 Series). Medtronic pacemakers are also indicated for dual chamber and atrial tracking modes in patients who may benefit from maintenance of AV synchrony. Dual chamber modes are specifically indicated for treatment of conduction disorders that require restoration of both rate and AV synchrony, which include various degrees of AV block to maintain the atrial contribution to cardiac output and VVI intolerance (e.g., pacemaker syndrome) in the presence of persistent sinus rhythm. 9790 Programmer The Medtronic 9790 Programmers are portable, microprocessor based instruments used to program Medtronic implantable devices. 9462 The Model 9462 Remote Assistant™ is intended for use in combination with a Medtronic implantable pacemaker with Remote Assistant diagnostic capabilities. CONTRAINDICATIONS Medtronic pacemakers are contraindicated for the following applications: ·       Dual chamber atrial pacing in patients with chronic refractory atrial tachyarrhythmias. ·       Asynchronous pacing in the presence (or likelihood) of competitive paced and intrinsic rhythms. ·       Unipolar pacing for patients with an implanted cardioverter-defibrillator because it may cause unwanted delivery or inhibition of ICD therapy. ·       Medtronic.Kappa 400 Series pacemakers are contraindicated for use with epicardial leads and with abdominal implantation. WARNINGS/PRECAUTIONS Pacemaker patients should avoid sources of magnetic resonance imaging, diathermy, high sources of radiation, electrosurgical cautery, external defibrillation, lithotripsy, and radiofrequency ablation to avoid electrical reset of the device, inappropriate sensing and/or therapy. 9462 Operation of the Model 9462 Remote Assistant™ Cardiac Monitor near sources of electromagnetic interference, such as cellular phones, computer monitors, etc. may adversely affect the performance of this device. See the appropriate technical manual for detailed information regarding indications, contraindications, warnings, and precautions.  Caution: Federal law (U.S.A.) restricts this device to sale by or on the order of a physician.

49. Medtronic Leads For Indications, Contraindications, Warnings, and Precautions for Medtronic Leads, please refer to the appropriate Leads Technical Manual or call your local Medtronic Representative. Caution: Federal law restricts this device to sale by or on the order of a Physician. Note: This presentation is provided for general educational purposes only and should not be considered the exclusive source for this type of information. At all times, it is the professional responsibility of the practitioner to exercise independent clinical judgment in a particular situation.