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Pacemaker Intervals, Sensing, and Pacing - ECG

Pacemaker timing cycles, Sensing, Pacing

Sensing and VVI Pacemaker

• We will demonstrate sensitivity using the example of a VVI pacemaker
• V - ventricular pacing
• V - ventricular sensing
• I - positive sensing inhibits pacing
• The VVI pacemaker senses intrinsic QRS through the electrode in the ventricle

Undersensing

• Sensing is set to 5mV
• The intrinsic QRS complex did not exceed the sensitivity threshold
• Sensing is negative (which is an error)

Pacing

• It is the ability of a pacemaker to stimulate the myocardium in a given chamber via an electrode
• The electrode stimulates the myocardium with an electrical impulse (similar to the SA node)
• A single electrode can perform both sensing and pacing
• Pacing appears on the ECG as a pacing artifact (a vertical line, also known as a spike)
• Sensing does not appear on the ECG
• The pacing artifact (the electrical impulse from the electrode) can stimulate both the atria and the ventricles
• Placing a magnet on the chest over the implanted pacemaker activates asynchronous pacing

Ventricular Pacing

• The pacemaker stimulates the ventricles
• The artifact is before the QRS complex
• VVI mode

Interval and Pacemaker

• 1 min = 60 s = 60,000 ms
  
  
• Frequency is the number of systoles/minute
• Systole is equivalent to the QRS complex on an ECG
• Commonly used when evaluating an ECG
  
  
• Interval is measured in ms (milliseconds)
• RR interval is the number of ms between RR peaks
• Used when evaluating a pacemaker
  
  
• Frequency = 60,000 / interval (ms)

DDD Pacemaker

• On the ECG, the pacemaker is in DDD mode
• LRI (Lower Rate Interval)
• It is the slowest rate of QRS complexes that the pacemaker allows
• If a natural QRS does not occur during the LRI, ventricular pacing is activated at the end of the LRI
• VRP (Ventricular Refractory Period)
• It is the period after ventricular sensing or pacing
• The pacemaker ignores sensing during the VRP
• This serves to prevent sensing the T wave
• AVI (AV Interval)
• It is the interval between atrial pacing (or sensing) and ventricular pacing
• If a natural QRS does not occur at the end of the AVI, ventricular pacing is activated
• PVARP (Post Ventricular Atrial Refractory Period)
• It is the period after ventricular pacing (or sensing) when an atrial signal cannot trigger AV conduction
• This prevents sensing of the retrograde P wave or atrial extrasystole
• TARP (Total Atrial Refractory Period)
• TARP = AVI + PVARP
• TARP is the total refractory period for atrial sensing
• A P wave that occurs during the TARP will not be conducted to the ventricles (no QRS complex will occur)
• TARP determines the maximum rate of P waves conducted to the ventricles before a 2:1 block occurs
• AEI (Atrial Escape Interval)
• It is the interval between ventricular pacing (or sensing) and atrial pacing
• If a natural P wave does not occur at the end of the AEI, atrial pacing is activated

Automatic Interval and Escape Interval

• Automatic Interval
• It is the interval between ventricular pacings
• Usually equal to the escape interval
• Escape Interval
• It is the interval between sensing the natural QRS and ventricular pacing

VVI Pacemaker

• The pacemaker is in VVI mode
• Automatic Interval and Escape Interval

Pacemaker Refractory Period (ARP, PVARP, VRP)

• It is not the refractory period of the action potential
• The pacemaker has its own defined refractory period in milliseconds
  
  
• Pacemaker Refractory Period
• It is the time period after sensing or pacing
• During which the pacemaker does not respond to positive sensing with pacing
• Without a refractory period, the atrial electrode could incorrectly interpret (QRS, T wave) as sensing P waves
  
  
• The pacemaker has three defined refractory periods:
• VRP (Ventricular Refractory Period)
• Prevents sensing of the T wave
• ARP (Atrial Refractory Period)
• Used in single-chamber atrial pacing
• PVARP (Post Ventricular Atrial Refractory Period)
• Used in dual-chamber pacing

ARP (Atrial Refractory Period)

• It is the period after atrial sensing or pacing
• The atrial pacemaker ignores sensing during ARP
• It serves to prevent sensing of the QRS complex and T wave

AAI Pacemaker and ARP (Atrial Refractory Period)

• The pacemaker is in AAI mode
• ARP (Atrial Refractory Period)
• It is the period after atrial sensing or pacing
• The atrial pacemaker ignores sensing during ARP
• It serves to prevent sensing of the QRS complex and T wave
• The LRL is seen as pacing artifacts
• The 3rd P wave is intrinsic, followed by an escape interval

DDD Pacemaker (ApVp)

• A dual-chamber pacemaker operates in DDD mode (ApVp)
• It performs atrial pacing (Ap) and ventricular pacing (Vp)
• LRL (Lower Rate Limit)
• It is the interval between ventricular pacing events
• AV interval (AV delay)
• It is the interval between atrial and ventricular pacing
• VA interval (Ventricle to Atrium)
• It is the interval between ventricular and atrial pacing
• Sometimes referred to as the atrial escape interval

DDD Pacemaker (ApVs)

• A dual-chamber pacemaker operates in DDD mode (ApVs)
• An paced P wave is followed by an intrinsic QRS complex
• LRL (Lower Rate Limit)
• It is the interval between ventricular pacing events
• Pacing is inhibited as the intrinsic QRS occurs before the LRL interval elapses
• AV interval (AV delay)
• It is the interval between atrial and ventricular pacing
• Pacing is inhibited as the intrinsic QRS occurs before the AV interval elapses
• VA interval (Ventricle to Atrium)
• It is the interval between ventricular and atrial pacing
• Atrial pacing is activated as no intrinsic P wave occurs during the VA interval

Cross Talk

• It is a situation where pacing from one electrode is sensed by another electrode
• Atrial pacing creates a false ventricular sensing
• The result is the inhibition of ventricular pacing

Cross Talk

• A dual-chamber pacemaker operates in DDD mode (ApVs)
• The atrial electrode performs atrial pacing, resulting in a P wave
• The ventricular electrode mistakenly senses atrial pacing as intrinsic QRS complexes
• The result is inhibition of ventricular pacing
• There are no paced or intrinsic QRS complexes present
• The outcome is ventricular asystole

A-blank and V-blank Periods

• Atrial Blanking Period (A-blank)
• It is a blind period after ventricular pacing
• The atrial electrode ignores electrical activity
• Ventricular Blanking Period (V-blank)
• It is a blind period after atrial pacing
• The ventricular electrode ignores electrical activity
  
  
• A-blank and V-blank periods serve as protection against cross talk

Maximum Sensor Rate (MSR)

• MSR is a parameter of pacemakers with rate adaptation (DDDR, VVIR)
• These pacemakers have a built-in sensor that detects vibrations (from breathing and movement)
• Based on the vibrations, the pacemakers adjust the pacing rate
• They are identified by the letter "R" in the fourth position of the NBG code (DDDR, VVIR)

Maximum Sensor Rate (MSR)

• A dual-chamber pacemaker has a DDDR mode
• The sensor detects vibrations (from breathing, movement) and sets the pacing rate to the frequency indicated by the red arrows
• The frequency is controlled by the value of the MSR interval

DDD pacemaker (ApVp)

• The dual-chamber pacemaker has a DDD mode
• AV delay
• Starts with atrial pacing (sensing)
• If no intrinsic P wave occurs during this period, ventricular pacing is activated at the end of the AV delay
• V-Blanking
• Is the blanking period after atrial pacing (sensing)
• The ventricular electrode ignores electrical activity to prevent cross talk
• A-Blanking
• Is the blanking period after ventricular pacing (sensing)
• The atrial electrode ignores electrical activity to prevent cross talk
• PVARP (Post ventricular atrial refractory period)
• Is the period after ventricular pacing (sensing) when the atrial signal cannot initiate AV conduction
• It prevents sensing of retrograde P waves or atrial extrasystoles
• VA interval
• Is the interval between ventricular pacing (sensing) and atrial pacing (sensing)
• If no intrinsic P wave occurs during the VA interval, atrial pacing is activated
• VRP (Ventricular refractory period)
• Is the period after ventricular pacing (sensing)
• The pacemaker ignores sensing during the VRP
• It prevents sensing of T waves
• URL (Upper rate limit)
• Is the maximum rate at which the pacemaker can track P waves to the ventricles (P wave tracking)
• If P waves have a higher frequency than the URL, upper rate behavior is activated
• LRL (Lower rate limit)
• Is the interval between ventricular pacing (sensing)
• If no intrinsic QRS occurs during the LRL, ventricular pacing is activated