1. Basic Pacing Concepts Part III
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. 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
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).1
1Ellenbogen, KA, et al. Clinical Cardiac Pacing London: WB Saunders Company; Page 41.

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
Typically, slew rate measurements at implant should exceed .5 volts per second for P waves; .75 volts per second for R wave measurements.
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
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.

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. 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

9. 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.

10. 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.

11. 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

12. Sensitivity 5.0 Amplitude (mV) 2.5 1.25 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

13. Sensitivity 5.0 Amplitude (mV) 2.5 1.25 Time
In this example, the sensitivity setting is set such that the pacemaker will likely sense the T wave. Oversensing will occur.
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
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. _

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
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

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
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.

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
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.

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. “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.

26. 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

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
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.

28. 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.

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
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.

31. 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.

32. 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.

33. 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

34. 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.

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. 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.

37. 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

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

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
Other sensors that measure QT interval, central venous temperature, stroke volume, etc., are largely investigational devices or have gained limited acceptance.

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

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.
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.