1. Basics of Pacemaker Functioning
T K Govindarajan
Medtronic
Title slide for this presentation.
This presentation is an up-to-date overview of cardiac pacing for physicians and clinical staff referring patients for pacemaker therapy.
For more about this presentation, see the companion binder Guidelines for Presenting Programs to Referring Physicians. This binder has guidelines and additional resources for presenting programs about cardiac pacing.

2. Topics Components of a Pacemaker System
Types of Pacemakers & their Operation
Functions of a Pacemaker System
Stimulation of cardiac tissue
Sensing of intrinsic heartbeats
Single chamber timing cycle

3. intrinsic heartbeats causing it to contract
The Pacemaker System
Cardiac Pacing is the artificial electrical stimulation of the heart in the absence of
intrinsic heartbeats causing it to contract

4. A Unipolar Pacing System
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

5. A Bipolar Pacing System
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
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
Tip electrode coil
Indifferent electrode
coil

6. NBG Code Revised - 2002 V: Ventricle V: Ventricle T: TriggeredII
III IV V
Chamber
Chamber
Response
Programmable
Multi Site
Paced
Sensed
to Sensing
Functions/Rate
Pacing
Modulation
V: Ventricle
V: Ventricle
T: Triggered
V: Ventricle
A: Atrium
A: Atrium
I: Inhibited
A: Atrium
D: Dual (A+V)
D: Dual (A+V)
D: Dual (T+I)
D: Dual (A+V)
O: None
O: None
O: None
R: Rate modulating
O: None
S: Single
(A or V)
S: Single
(A or V)
O: None

7. Single Chamber Pacemakers
Ventricular Single Chamber Pacing or VVI pacing
Pacing Rate
Pacing Rate
Pace
Sense
Pace

8. Rate Responsive Pacemakers
Rate-Responsive Pacing
Fixed-Rate Pacing
Normal Heart Rate
Running
150
Walking
100
Heart Rate (bpm)
Wake-up
Heart rate variability is especially important for nearly all elderly patients since they rely on heart rate reserves even during normal daily activities.
Sleeping
Sitting
Resting 50
Daily Activities

9. Piezoelectric crystal Optional Information:
Activity Sensors
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
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10. Pacing system - a standard electrical circuit
The pacemaker provides the voltage. Current (electrons) flow down the conductor to the lead tip or cathode (-) Where the lead tip touches the myocardium, electrical resistance is produced. The current then flows through the body tissues to the anode (+) and back to the battery. All of the above are required for current to flow. I V R
Speakers Notes
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In a basic electrical circuit the voltage drops around the circuit at each resistance. Assuming the heart/electrode interface is the only significant impedance, all voltage should be “spent” there
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11. Voltage, Current, and Impedance
Voltage: The force moving the current (V)
In pacemakers it is a function of the battery chemistry
Current: The actual continuing volume of flow of electricity (I)
This flow of electrons causes the myocardial cells to depolarize
Impedance: The sum of all resistance to current flow (R)
Impedance is a function of the characteristics of the conductor (wire), the electrode (tip), and the myocardium (tissue). 11

12. V I R Ohm’s Law V = I X R I = V / R R = V / I V = I X R V R = I V I =
Describes the relationship between voltage, current, and resistance (impedance)
V = I X R
I = V / R
R = V / I V = I X R V I R V R I =
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). V I = R 12

13. Lead Impedance Values Electrical Analogies
Normal resistance – friction caused by the hose and nozzle
Normal 300 – 1200 Ohms
Low resistance – leaks in the hose reduce the resistance
Similar to a pacemaker lead with an insulation breach which results in low resistance and high current drain; may cause premature battery depletion.
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 drain, and possible premature battery depletion. Conversely, if there is a high resistance, such as a lead conductor break (represented by the knot), the current flow will be low or non-existent.
High resistance – a knot results in low total current flow
Similar to a pacemaker lead with a lead conductor break - impedance will be high with little or no current reaching the myocardium. 13

14. High Impedance Conditions A Fractured Conductor
A fractured wire can cause Impedance values to rise
Current flow from the battery may be too low to be effective
Impedance values may exceed 3,000 W
Lead wire fracture
Increased resistance
An example of a high impedance condition is when the conductor coil fractures with the insulation remaining intact.
Impedance may exceed 3,000 Ohms and the 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,” or 9999 Ohms
When suspecting a wire break, look for a trend of increasing impedance values, rather than a single lead impedance value. The multifilar construction of the leads sometimes means that some of the small conductor filaments fail, while others remain intact. You’ll typically see a trend if increasing impedances over time.
Other reason for high impedance: Lead not seated properly in pacemaker header (usually an acute problem). 14

15. Low Impedance Conditions An Insulation Break
Insulation breaks can cause impedance values to fall
Current drain is high and can lead to more rapid battery depletion
Current can drain through the insulation break into the body or other lead wire, not through myocardium
Impedance values may be less than 300 W
Insulation around the lead wire prevents current loss from the lead wire. Electrical current seeks the path of least resistance, which is normally the conductor coil. Insulation breaks are often marked by a trend of falling impedance values. An impedance reading that changes suddenly, or one that is >30% lower than previously recorded values for a given patient, is considered significant and should be watched closely.
Insulation break that exposes a conductor causes the following
Impedance values to fall
Current to drain through the insulation break into the body, or into the other wire
Potential for loss of capture
More rapid battery depletion 15

16. Stimulation Threshold
Pacing Voltage Threshold
The minimum pacing voltage
at any given pulse width
required to consistently stimulate the heart
outside the myocardial refractory period causing it to contract

17. Capture – Loss of Capture
Non-Capture
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VVI / 60

18. The Pacemaker Stimulus
2.5 Volts
Voltage
0.5 ms
1 sec
Time
Pacing Stimulus Voltage or Amplitude – 2.5 Volts
Pulse Width – seconds or 0.5 milliseconds
Pacing Rate – One stimulus per second or 60 stimuli (beats) per minute

19. The Voltage-Strength Duration Curve
Stimulus Voltage & Pulse Width have an exponential relationship
At short pulse widths (<0.25 ms) the curve rises sharply
At long pulse widths (>1.0 ms) the curve is flat
.50
1.0
1.5
2.0
.25
Stimulation Threshold (Volts)
Capture
0.25
1.0
1.5
Duration
Pulse Width (ms)

20. The Pacing Pulse Pacing Pulse
V = Pulse Amplitude in Volts (V) (say 2.5 V)
t = Pulse Duration or Width in milliseconds (ms)
(say 0.5 ms)
R = Impedance of Pacing Circuit (ohms)
(say 500 ohms)
I = V/R = Current through pacing circuit (mA)
= 2.5 V/ 500 ohms = A = 5 mA
E = Energy delivered by Pulse to the Pacing Circuit
and Cardiac Tissue
= V . I . t = I2Rt = V2t/R
= 2.5 V . 5 mA ms = 6.25 micro Joules
If V = 5 V Energy = 25 micro Joules t
Pacing Pulse V
Output Voltage t
Pulse Duration (Width)

21. Why measure stimulation threshold?
To enable programming stimulus voltage amplitude and pulse width such that
Consistent capture & Patient Safety is ensured
Battery drain minimized, Pacemaker longevity maximized
Good thresholds
Ventricle - 0.5ms
Atrium - 0.5ms

22. Evolution of Pacing Threshold6 s
Safety Margin 4
Voltage Threshold (V) 3 2
Chronic Phase 1
Acute Phase 4 8 12 16
Observation Time (weeks)

23. Sensing of intrinsic heartbeats
Sensing is the ability of the pacemaker to “see” when an intrinsic depolarization is occurring
Pacemakers record the Intracardiac Electrogram (EGM) by constantly recording the potential difference between the cathode and anode
Depolarization Wave
Processed by Device

24. Intrinsic R wave Amplitude
Intrinsic R wave amplitude 5 mV
Intrinsic P wave amplitude 2 mV
Intrinsic R wave in EGM
Amplitude
The Intrinsic R wave amplitude is usually much greater than the T wave amplitude

25. Sensitivity Setting
5.0
5.0
2.5
2.5
Amplitude (mV)
Amplitude (mV)
1.25
1.25
Time
Time
Sensitivity settings less than 2.5 mv – High sensitivity – can lead to oversensing
Sensitivity settings greater than 2.5 mV – Low sensitivity – can lead to undersensing

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

27. Oversensing
Marker channel shows intrinsic activity...
...though no activity is present
An electrical signal other than the intended P or R wave is detected
Pacing is inhibited
Speakers Notes
What to say:
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.
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28. Refractory & Blanking Periods
Refractory period
Prevent lower rate timer reset due to oversensing
Blanking Period
The first portion of every refractory period
Pacemaker is “blind” to any activity and no events can be sensed
Designed to prevent oversensing of pacing stimulus & after-potential
Lower Rate Interval
A paced or sensed event will initiate a blanking period. Blanking is a method to prevent multiple detection of a single paced or sensed event by the sense amplifier (e.g., the pacemaker detecting its own pacing stimuli or depolarization, either intrinsic or as a result of capture). During this period, the pacemaker is "blind" to any electrical activity. A typical blanking period duration in a single-chamber mode is 100 msec*.
Note: In Thera and Kappa devices, nonprogrammable blanking parameters are dynamic (ranging from ms) depending on the strength/duration of the paced or sensed signal. VP
VVI / 60
Blanking Period
Refractory Period

29. Values to remember Pacing Thresholds
Atrium - <1.5V; Ventricle 0.5ms PW
Outputs
2 X threshold Voltage
Doubling Voltage output = 4 X Energy drain from battery
P / R wave amplitudes
P Wave > 2mV
R Wave > 5mV
Sensitivity
R / P wave > 2 X Sensitivity setting
Oversensing – Increase sensitivity ( Reduce value)
Undersensing – Reduce sensitivity ( Increase value)
Lead impedance
300 – 1200 ohms – Normal
> 3000 Ohms - lead fracture / Loose set screw / inadequate lead pin insertion
< 300 Ohms – Insulation break
If impedance is out of range – try unipolar configuration

30. Dual Chamber Pacemakers
Advanced type of pacemakers that
closely mimic the natural heart
Work on both – RA and RV
Usually 2 leads
Maintain AV Synchrony
AV Interval
Provide rate response

31. Dual Chamber Pacemakers
DDD, DDDR – sense & pace both atrium and ventricle
VDD – Sense atrium, pace ventricle

32. The four faces of dual chamber pacing
AV SEQUENTIAL PACING Atrial Pacing Rate – 60, AV Interval – 200 ms
AV Synchronous Pacing :NATURAL ATRIAL CONTRACTION & VENTRICULAR PACING : VDD
AV Interval = 150 ms
Spontaneous Atrial Rate – 55
Spontaneous Atrial Rate – 110

33. The four faces of dual chamber pacing
Atrial Pacing Rate = 70, Natural AV conduction
NATURAL ATRIAL CONTRACTION WITH NORMAL AV CONDUCTION
Spontanoeus Atrial Rate = 65, Spontaneous PR interval = 160 ms

34. Thank You