1. Pacemaker II
Lecture (6)

2. Lead wires and electrodes:
Lead wires in addition to being good electrical conductors, it must be mechanically strong.
It should maintain good electrical insulation.

3. The lead wires consists of helical coils of spring-wire alloy molded in a silicon-rubber or polyurethane cylinder.
The helical coiling of the wire minimizes stresses applied to it, and the multiple strands serve as insurance against failure of the pacemaker

4. The soft compliant silicon-rubber encapsulation both maintains flexibility of the lead-wire and provides electrical insulation and biological compatibility.

5. Cardiac pacemaker are either of unipolar or bipolar type.
Bipolar and unipolar pacing have different appearances on surface ECGs.
If the current flows between the two electrodes on the pacemaker lead (the tip and the ring), this is referred to as bipolar pacing.

6. If the current flows between the tip of the lead and the pacemaker generator, this is referred to as unipolar pacing.
In unipolar pacing, the current travels through a large area of the body between the tip of the lead and the pulse generator.
Unipolar pacing, therefore, creates a large stimulus artifact on the surface ECG.
It may stimulate electrically excitable tissue, other than the heart, which lies in the path of the current.

7. The bipolar pacing stimulus may be very difficult to see on the surface ECG, because in bipolar pacing the distance between the two poles that deliver current (i.e. the tip and the ring of the pacemaker lead) is very small (about a centimeter).
It will also be noted that leads that are used for bipolar pacing must have two insulated wires within its outer insulation: one wire for the negative pole (the tip), and one for the positive pole (the ring).

8. Single and dual chamber:
The word chamber in dual or single chamber pacing refers to a chamber of the heart in which a lead is placed.
A single chamber pacemaker usually has a pacing lead in either the right atrium, or the right ventricle. These would be called, respectively, an atrial single chamber pacemaker and a ventricular single chamber pacemaker.
A standard dual chamber pacemaker has a lead in the right atrium and a lead in the right ventricle.

9. Synchronous pacemaker
Many patients requires cardiac pacing only intermittently, because they can establish a normal cardiac rhythm between periods of block.
For these patients, it is not necessary to stimulate the ventricles continuously.

10. Types of synchronous pacemakers:
Demand pacemaker.
Atrial-synchronous pacemaker.
Rate-responsive pacemaker.

11. A demand pacemaker consists of timing circuit, an output circuit, and electrodes, just like those of the asynchronous pacemaker but it has a feedback loop as well.

12. The timing circuit is set to run at a fixed rate, usually 60 to 80 beats/min.
After each stimulus, the timing circuit resets itself waits the appropriate interval to provide the next stimulus, and then generates the next pulse.
If during this interval a natural beat occurs in the ventricle, the feedback circuit detects the QRS complex of the ECG signal from the electrodes and amplifies it.

13. This signal is then used to reset the timing circuit
This signal is then used to reset the timing circuit. It waits its assigned interval before producing the next stimulus.
If the heart beats again before this stimulus is produced, the timing circuit is again reset and the process repeats itself.
Thus we see that, when the heart conduction system is operating normally and the heart has a natural rate that is greater than the rate set for the timing circuit, the pacemaker remains in a standby mode, and the heart operates under its own pacing control.

14. Atrial synchronous pacemaker:
It is a more complicated circuit, the pacemaker is designed to replace the blocked conduction system of the heart.
The heart’s physiological pacemaker located at the SA node, initiates the cardiac cycle by stimulating the atria to contract and then providing a stimulus to the AV node which after appropriate delay stimulates the ventricle.

15. If the SA node is able to stimulate the atria, the electric signal corresponding to atrial contraction can be detected by an electrode implanted In the atrium and used to trigger the pacemaker in the same way that is triggers the AV node.
The figure below shows the voltage V1 that is detected by the atrial electrodes.

16. The voltage is a pulse that corresponds to each beat.
The atrial signal is then amplified and passed through a gate to a monostable multivibrator giving a pulse V2 of 120-ms duration, the approximate delay of the AV node.
Another monostable multivibrator giving a pulse duration of 500 ms is also triggered by the atrial pulse.
It produces V4, which causes the gate to block any signals from the atrial electrode for a period of 500-ms following contraction.

17. This eliminates any artifact caused by the ventricular contraction from stimulating additional ventricular contraction.
The pulse V2 acts as a delay, allowing the ventricular stimulus pulse V3 to be produced 120-ms following atrial contraction.
Then V3 controls an output circuit that applies the stimulus to appropriate ventricular electrodes.

18. Rate-responsive pacing:
The demands of the body during stressful activities such as exercise cannot be fully met by the previous pacemakers.
A new type of pacemaker system that can overcome these limitations is evolving.

19. This pacemaker includes a control system.
A sensor is used to convert a physiological variable in the patient to an electric signal that serves as an input to the controller circuit.
This type of pacemaker is programmed to control the heart rate on the basis of the physiological variable that is sensed.