Module 3: The pacing system

Question 1

Pacemaker?

Answer 1

• small implantable device that helps control abnormally slow heart rhythms.
• mimics the patient’s natural heart rhythm as much as possible.
• two main functions: sensing the heart and pacing the heart.

Question 2

Pacing system?

Answer 2

• Consists of an implantable pulse generator (IPG) = battery, circuitry, and connectors.
• One or more leads, which are wires that conduct, or carry, the impulses to the heart.
o A cathode = electrode with a negative charge that delivers the electrical impulse to the myocardium.
o An anode = electrode with a positive charge to which the impulse returns after stimulating the heart.

• Body tissue and fluids + IPG + leads + cathode + anode = conduction of electricity

Question 3

Purpose of pacing system

Answer 3

• Form a conduction pathway to pace the heart when necessary

* Form a sensing circuit to sense intrinsic cardiac activity, withholding pacing when appropriate.

Question 4

Purpose of a telemetry coil

Answer 4

• establish a connection with a Medtronic programmer
• Once connected
  o Sends programming instructions down to the device.
  o Sends diagnostic data up to the programmer

Question 5

Purpose of sensors

Answer 5

• Measure the patient’s movement and level of exertion. (Info is used to change the heart rate so that it is appropriate for the patient’s current activity level.)

Question 6

Purpose of Leads, or insulated conductor wires

Answer 6

• Deliver electrical impulses to the heart.
• Sense the electrical signals of the patient’s own heart beat. One end of the lead connects to the pulse generator, and the other is in contact with cardiac tissue

Question 7

Pacemaker component: Circuit board

Answer 7

• Contains microprocessors that allow both sending programmed instructions to, and receiving and analyzing data from, the heart.
  Circuits permit:
• Programming of pacing therapies and diagnostic functions by establishing telemetry or communication with the programmer
• the integrated circuit’s ability to sense the heart’s electrical activity
• Collecting diagnostic data of both pacing system operation and the patient’s intrinsic heart activity
• Components in the circuit board
  o Resistors = control the flow of electricity
  o The defibrillation protection unit = allows the pacemaker to safely accept high voltage external defibrillation without destroying the other components
  o The output capacitors and “voltage doubler,” = allow the pacemaker to deliver varying voltages
  o Timers = keep device operation and function consistent within the pacemaker
  o The reed switch = allows mode and rate changes when a magnet is placed near the device
  o The telemetry antenna = enables communication between the programmer and the pacemaker

Question 8

Pacemaker component: Battery

Answer 8

• Provides energy for sending electrical impulses to the heart. Lithium-iodine is the most commonly used power source for today’s pacemakers (an initial open circuit voltage of 2.8 volts).
• ICDs use Silver-Vanadium-Oxide cells with a higher voltage
• Functions
  o Provides the power source for the device’s multiple functions.
  o Provide the energy needed to send electrical impulses to the heart.

Question 9

Pacemaker component: Header block

Answer 9

• Connects the lead to the pacemaker > the device can sense what is happening in the patient’s heart and pace when necessary.
• In a dual-chamber pacemaker, the atrial and ventricular connectors each have their own housings in the connector block.

Question 10

Leads?

Answer 10

• Insulated wires that deliver electrical impulses from the pulse generator to the heart, and sense cardiac depolarization.
• Lead is subject to mechanical torque, flexing, bending, and the body’s natural defenses, including bio-chemicals produced in response to inflammation.

Question 11

Fixation Mechanism: Active leads

Answer 11

Better choice to prevent lead dislodgment in patients:

• With smooth-walled hearts
• Whose hearts lack trabeculation, such as dilated hearts
• Who have had a previous CABG procedure

Question 12

Where is active lead tip screwed to? Adv?

Answer 12

Lead tip is screwed to the endocardial wall.

• Fixation mechanism secured into heart muscle
• Creates a solid connection between the lead tip and the heart > mitigates the risk of lead dislodgements
• positioned almost anywhere in the heart.
• Makes active fixation leads easier to remove (when necessary) because the lead tip can easily be unscrewed back up into the helix of the lead at any time.
• Steroid eluting > reduce tissue inflammation and heal the cardiac tissue, negating the consequences of inflamed cardiac tissue caused by the sharp screw.

Question 13

Where is passive lead tip screwed to?

Answer 13

Implanted where there is a atrial appendage, or where there is trabeculae in the ventrical. Cannot be implanted on areas where the heart wall is smooth.

Question 14

Implantation of a passive lead tip

Answer 14

During implantation, the lead tip is lodged between the web-like pectinate of the atrium or trebeculae of the ventricle.

Once the lead tip is in place, the tines, made of soft silicone, get “hung up” in the pectinate or trebeculae, helping to keep the lead in place. After roughly 4 to 8 weeks, a fibrotic meshwork of the heart tissue encapsulates the tined lead tip. At this point, the lead is extremely secure and unlikely to dislodge. However, if the lead needs to be removed after this time, it is very difficult to do so, and the patient needs a very specialized procedure to get the lead removed.

Question 15

Adv of passive leads

Answer 15

• less tissue damage to the heart
• Becomes lodged in trabeculae
• Extremely secure once fibrotic tissue locks the lead in place

Question 16

Epicardial leads

Answer 16

implants represent less than 5% of leads implanted
primarily in pediatric cases
applied directly to the surface of the heart

Question 17

Insulation material: Silcone leads

Answer 17

• Reliable
• Relatively fragile, must be thicker
• Higher coefficient of friction
• Medtronic’s silicure has improved mechanical strength
• Inert
• Biocompatible
• Biostable
• Repairable with medical adhesive

Question 18

Insulation material: Polyurethane leads

Answer 18

• Slightly more durable
• Allows thinner applications when compared to silicone
• more lubricious due to a lower friction coefficient. This characteristic can make it easier to implant.
• Early types of urethane were subject to chemical reactions
• Higher tear strength
• slightly smaller in diameter.

Question 19

Unipolar lead

Answer 19

consist of a single conductor coil extending to the tip electrode, or cathode

Question 20

Bipolar lead

Answer 20

two coils—one to the tip cathode, the other to the ring-anode. The coils are separated by an insulating layer.

Question 21

Unipolar Pacing

Answer 21

• Travels down the lead wire to stimulate the heart at the tip electrode, which is also referred to as the cathode;
• Returns to the metal casing of the impulse generator, or the anode, by way of body fluids.
• producing larger spikes on the ECG. Since the electrodes are located at a relatively large distance from each other, detected surface ECG signals are usually larger.
• Another characteristic is producing extracardiac stimulation in the form of muscle stimulation or pacemaker pocket stimulation, which is when pacemaker current flows between the tip and the can.

Question 22

Bipolar Pacing

Answer 22

• the impulse travels down the inner wire of the lead to stimulate the heart at the tip electrode, or the cathode.
• The impulse then travels to the ring electrode, or the anode, which is located several millimeters above the lead tip.
• From the anode, the impulse travels up the outer wire of the lead to the pulse generator and completes the circuit.
• There is less likelihood of extracardiac stimulation, such as pocket stimulation or muscle stimulation. This is because current returns to the ring electrode within the heart, rather than to the can as in unipolar pacing.
• inability to see pacing spikes on an ECG. This is because the current pathway is so short between the electrodes.

Question 23

Bipolar Sensing

Answer 23

• A bipolar system is less susceptible to oversensing non-cardiac signals, such as myopotentials, far-field intercardiac signals, noise, and EMI. This is because the “antenna,” or the distance between the two electrodes, is small and insulated within the chamber of the heart that is being sensed.

Question 24

Unipolar Sensing

Answer 24

• susceptibility to oversensing. This is due to the big distance between electrodes. Extracardiac signals, such as myopotential signals or Electromagnetic Interference (EMI) outside the body, could potentially be sensed. Older IPGs were more susceptible to sensing extracardiac signals, but today’s modern IPGs have better sense filters to avoid this problem.

Question 25

Biopolar system: lead characteristics

Answer 25

• Larger diameters. Over the years, however, bipolar sensing systems have grown smaller due to coaxial conductor designs, advanced thinner insulations, and smaller conductor coils and cables. Note also that the second conductor coil and insulation between the conductor coils make bipolar leads stiffer. A softer tip-to-ring spacer is used in many lead designs to make the distal tip more flexible.
• built-in safety mechanism.” If the lead developed a problem, such as with the outer insulation or outer wire, the lead could be programmed to unipolar and still function for the patient. Thus, bipolar leads provide more alternatives when the lead experiences problems.

Question 26

Unipolar system: lead characteristics

Answer 26

• smaller diameters > one conductor coil, so the insulator between the two conductor coils is eliminated.
• less rigidity > only one conductor coil and one layer of insulation.

Question 27

Adv/Disadv of unipolar system

Answer 27

Advantages of the unipolar system include:
• The sensing of smaller cardiac signals that may not be detected in bipolar;
• The larger pacing spikes on the ECG; and
• The smaller diameter of the leads when compared to bipolar leads.
A disadvantage of the unipolar system
- may over-sense myopotentials
- “far-field” intracardiac signals, noise, and EMI.

Question 28

2 key clinical goals?

Answer 28

• To deliver sufficient energy (including a safety margin) to consistently cause the heart to contract when necessary; and
• Maximizing battery performance so the device lasts as long as possible.

Question 29

Pacing systems focus on 2 key relationships?

Answer 29

1. Voltage and current drain; and

2. Impedance and current drain

Question 30

What is current drain?

Answer 30

The total amount of electrical capacity used in a pacing system over a given period of time.

Question 31

Current? Voltage? Impedance?

Answer 31

Current: Rate at which electricity travels through a circuit
Impedance: Resistance to the flow of electricity
Voltage: The force that pushes on electrons causing them to move through a pacing circuit