Pacemaker

Question 1

Pacing indications

Answer 1

A) Rate-related (bradycardia):
* Symptomatic bradycardia
* CHB (complete heart block)
* Trifascicular block
* HOCM
* prolonged QT

B) Rhythm-related (tachyarrhythmia):
1). Refractory VT (overdrive pacing)
2). AF or junctional rhythm in diastolic failure
3). Atrial flutter
4). Recalcitrant SVT

Question 2

Contraindications (all relative)

Answer 2

1). Bleeding
2). Infection
3). Large area of recent infarction (risk ↑)
4). Intracardiac thrombus (might get dislodged)

Question 3

Complications

Answer 3

=>Early
1). Complications of vascular access
2). Damage to tricuspid valve
3). Arrhythmias (VT/VF) due to leads irritating the tissue
4)Ventricular wall perforation
5)Tamponade
6)Stimulation of diaphragm & phrenic nerve contraction

=>Late:
6).Lead thrombosis → PE
7). Lead infection → IE

Question 4

Defintions:
->Sensitivity
->Output
->Capture threshold

Answer 4

Sensitivity (myocardial)
* Minimum voltage that is required to be detected by the pacemaker as a P-wave or R-wave.

Normal settings:
* 0.4–1.0 mV for atria
* 0.8–2.0 mV for ventricles

⸻

Output
* Amount of current produced by the pacemaker in order to pace.
* Measured in mA.
* Results in depolarisation (effective stimulation of the myocardium by the pacemaker).

⸻

Capture Threshold
* Minimum amount of current in mA required to initiate a depolarisation of the paced chamber.

Question 5

What is the mechanism of
Pacemaker-Mediated Tachycardia, aka
Endless Loop Tachycardia?

Answer 5

->Problem with the Dual chamber pacemakers

=>Mechanism:
Trigger:
->A premature ventricular contraction (PVC) or a pacemaker-induced ventricular beat can trigger the process.

->Retrograde Conduction: This ventricular beat travels backward (retrogradely) through the patient’s AV node and conduction system, activating the atria.

->Sensing by the Pacemaker: The atrial lead senses this retrograde atrial activation (P wave) as a normal signal, provided the atrial refractory period (PVARP) has passed.

->AV Delay and Ventricular Pacing: The pacemaker then initiates its programmed AV delay and paces the ventricle.

->Endless Loop: This paced ventricular beat can again travel retrogradely to the atrium, and the cycle repeats, forming an “endless loop” of tachycardia.

Question 6

Key characteristics of
Endless Loop Tachycardia

Answer 6

->Reentrant tachycardia: The electrical signal travels in a continuous loop.
->Pacemaker involvement: The pacemaker plays a critical role by providing the antegrade (forward) limb of the circuit.
->AV node or accessory pathway: The retrograde limb of the circuit is the AV node or, less commonly, an accessory pathway.
->Rate limit: The heart rate is typically limited by the programmed upper rate limit of the pacemaker.
->Termination: It is often temporary and can be stopped by algorithms within the pacemaker or by applying a magnet to the device.

Question 7

Consequences of PMT
&
Treatment

Answer 7

Symptomatic tachycardia, dizziness, fatigue, and even heart failure.
Can be terminated by placing a magnet over the pacemaker.
Pacemaker programming: Pacemakers can be programmed with features like an adaptive PVARP to prevent or terminate PMT by blocking the retrograde atrial signal

Question 8

How do you set the sensitivity of a pacemaker?

Answer 8

To Set Sensitivity
* Pt must have an intrinsic rhythm.
* Change the mode to VVI / AAI / DDD mode (endogenous cardiac activity inhibits the pacemaker).
* Change the output to minimum (to avoid R on T).
* Capture not required, only pacing spikes as indicator.
* Change the rate to lower than pt’s native rhythm.
* Now, keep reducing sensitivity (↑ mV).
* When pacemaker is blind to all intrinsic activity, pacing spikes regularly appear at the baseline rate you have set.
* Now, gradually ↑ the pacemaker sensitivity.
* There will be a value at which the pacemaker senses every P-wave or QRS = sensitivity threshold.
* Most pacemakers are set at ½ the sensitivity threshold, so that even if some tissue grows over the lead, cardiac activity will be sensed.
* * If sensitivity value is turned down any further, there is a risk of oversensing.

Question 9

Sensitivity

Answer 9

• Sensitivity of a freshly inserted pacemaker is higher than older (very subtle endogenous electrical activity).
• Low sensitivity settings are used if myocardium is irritable → may confuse normal fine-wall movement with depolarisation—->
  Pacemaker may fire constantly, mistaking the noise for atrial activity.

Question 10

How do you check the pacing threshold?

Answer 10

• Set pacemaker well above native rate.
• Start reducing the output till paced spike is no longer followed by QRS.
• Now start increasing the output → the value where capture returns is current capture threshold.
• Typically, one sets output to 2× capture threshold, to allow safety margin for small debris to grow around electrode surface.

Question 11

Output

Answer 11

• Delivered in short bursts (≈0.5 ms).
  
  • Narrower pulse width → requires higher voltage.
  • Wider pulse width → doesn’t give added benefit beyond 0.8 ms.
  • Beyond 0.8 ms, the capture threshold curve flattens out.

Question 12

Pacemaker Syndrome

Answer 12

• Caused by improper timing of atrial & ventricular contraction → AV dyssynchrony.
• Loss of atrial kick.
• Clinical: fatigue, dizziness, palpitations, pre-syncope.
• Marked by ↓ in SBP ≥ 20 mmHg on change from native → paced rhythm.
  =>Tx:
  
  • Change to DDD mode if available.
  • If patient has severe LV dysfunction + EF < 35% → CRT will be preferred device.

Question 13

Are there any Reversible Factors in ↑ Pacing Threshold?

Answer 13

1. Acid–base abnormalities
2. Electrolyte abnormalities (↑ K, ↓ K, ↑ Ca, ↓ Mg)
3. Hypglycemia
4. Drugs – β-blockers, CCBs
5. Lead displacement