• URL (Upper rate limit)
• PVARP (Post ventricular atrial refractory period)
• TARP (Total atrial refractory period)
• Upper rate behavior
• Pacemaker 2:1 Block
• Frequency Before 2:1 Block
• Pacemaker Wenckebach
• SA Node Frequency and DDD Pacemaker

URL (Upper rate limit)

• Is the maximum frequency at which the pacemaker can track P waves to the ventricles (P wave tracking)
• Sometimes referred to as maximum tracking rate (MTR)
• URL protects the ventricles from tachycardia
• For example, during atrial fibrillation, impulses from the atria could be transmitted to the ventricles at a rate of 350-600/min.

DDD pacemaker (ApVp)

• The dual-chamber pacemaker has a DDD mode (ApVp)
• URL (Upper rate limit)
• Is the maximum frequency at which the pacemaker can track P waves to the ventricles (P wave tracking)
• If the P waves have a higher frequency than the URL, upper rate behavior is activated

TARP (Total atrial refractory period)

• TARP = AV interval + PVARP
• TARP is the total refractory period for atrial sensing
• A P wave that occurs during TARP will not be transmitted to the ventricles (no QRS complex will occur)
• TARP determines the highest frequency of P waves that can be transmitted to the ventricles before a 2:1 block occurs

DDD pacemaker (AsVp)

• The dual-chamber pacemaker has a DDD mode (AsVp)
• AV delay (AV interval)
• It is the interval between atrial pacing (sensing) and ventricular pacing (sensing)
• PVARP (Post ventricular atrial refractory period)
• It is the period after ventricular pacing (sensing) during which an atrial signal cannot trigger AV conduction
• Prevents sensing of retrograde P waves or atrial extrasystoles
• TARP (Total atrial refractory period)
• TARP = AV interval + PVARP
• It is the refractory period for atrial sensing
• The atrial electrode ignores sensing during TARP
• A P wave that occurs during TARP will not be transmitted to the ventricles
• A 2:1 AV block occurs
• Every second P wave is blocked due to the TARP period

Pacemaker 2:1 Block

Pacemaker 2:1 block, DDD pacemaker (AsVp)

• A dual-chamber pacemaker in DDD mode (AsVp)
• AV delay (AV interval)
• This is the interval between atrial pacing (sensing) and ventricular pacing (sensing)
• PVARP (Post ventricular atrial refractory period)
• This is the period after ventricular pacing (sensing) when the atrial signal cannot trigger AV conduction
• It serves to prevent sensing of retrograde P waves or atrial extrasystoles
• TARP (Total atrial refractory period)
• TARP = AV interval + PVARP
• This is the refractory period of atrial sensing
• The atrial electrode ignores sensing during TARP
• A P wave occurring during TARP will not be transmitted to the ventricles
• Resulting in a 2:1 AV block
• Every second P wave is blocked due to the TARP period
• MTR (Maximum tracking rate) = URL (Upper rate limit)
• This is the maximum frequency at which the pacemaker can transmit P waves to the ventricles

DDD Pacemaker

• Dual-chamber pacemaker in DDD mode
• SA node generates impulses regularly
• Initially, there is sensing of intrinsic P waves (S)
• Which are transmitted to the ventricles through the AV interval
• Every second P wave occurs during the PVARP (asterisk) and is blocked
• This results in a 2:1 block on the ECG
• At the end, the SA node slows down and atrial pacing is activated
• Because no intrinsic P wave occurs during the escape interval
• The pacemaker ignores the P wave during the PVARP
• AP is a stimulated P wave, thus it has a different shape than intrinsic P waves
• When the SA node frequency slows down, the 2:1 block disappears

Pacemaker Wenckebach (4:3), DDD mode: AsVp

• The dual-chamber pacemaker operates in DDD mode (AsVp)
• The SA node is intact
• It generates impulses regularly (the PP interval does not change)
• First P wave
• It is transmitted to the ventricles, causing ventricular pacing at the end of the AV interval
• Second P wave
• It occurs during the MTR interval
• The AV interval extends, and ventricular pacing occurs exactly at the end of the MTR interval
• Third P wave
• It occurs even earlier during the MTR interval
• The AV interval extends even more, and ventricular pacing occurs exactly at the end of the MTR interval
• Fourth P wave
• It occurs during the TARP interval (black arrow)
• This P wave is completely blocked
• After the blocked P wave, the cycle repeats
• The PQ interval gradually lengthens until a P wave is blocked
• The ECG shows a pattern of second-degree AV block (Wenckebach) with 4:3 conduction to the ventricles
• Out of 4 P waves, 3 QRS complexes are produced

SA Node Frequency and DDD Pacemaker

• With a rising frequency of intrinsic P waves
• The DDD (AsVp) pacemaker response gradually changes
  
  
• If the pacemaker has the programmed parameters:
• AV interval: 120ms
• PVARP: 280ms
• MTR: 140/min.
• We calculate:
• TARP = 400ms (AV interval + PVARP)
• Threshold frequency for 2:1 block = 150/min. (60,000/400)