Lecture Notes: Essentials of Cardiac Pacing

Introduction

• Lecturer: Dr. Kumar Narayanan
• Session Topic: Pacemaker Timing Cycles
• Previous Sessions: Basic pacemaker concepts, pacing, sensing, threshold, and impedance

Importance of Timing Cycles

• Understand annotations on a pacemaker programmer
• Troubleshoot pacemaker behaviors
• Governs timing of events (intrinsic or paced)

Key Concepts in Timing Cycles

• Pacemaker Responses:
  • Pace
  • Inhibit pacing
• Interval-based: Measured in milliseconds, not beats per minute
• Conversion: Interval in ms = 60,000 / beats per minute

Periods in Timing Cycles

• Blanking Period: After a sensed event, sense amplifier is turned off to prevent double sensing
• Refractory Period: Sense amplifier is on, but sensed events do not alter timing cycles

Single Chamber Timing Cycles

• VVI Mode:
  • Paces and senses the ventricle
  • Inhibits pacing with intrinsic activity
• Lower Rate Interval:
  • Basic rate programmed
  • Ventricular escape interval
• Intrinsic Event: Can cut short the lower rate interval

Dual Chamber Timing Cycles

• DDD Mode:
  • Both chambers are paced and sensed
  • Dual response (inhibition and tracking)
• Timing Parameters:
  • Lower Rate Interval
  • AV Interval (Delay): Maximum time after atrial event before ventricular pacing
  • VA Interval: Time between ventricular event and next atrial pacing
  • Upper Rate Interval/Maximum Tracking Rate: Fastest rate for 1:1 atrial-ventricular tracking

Blanking and Refractory Periods

• Ventricular Channel: Post-atrial ventricular blanking, crosstalk sensing window
• Atrial Channel: Post-ventricular atrial blanking and refractory period

Concepts and Issues

• Cross-Talk: Inappropriate sensing between chambers
• Pacemaker Mediated Tachycardia (PMT): Caused by retrograde conduction

Upper Rate Behavior

• Pseudo AV Wenckebach: Progressive prolongation of AV delay
• 2:1 AV Block: Occurs when atrial rates exceed TARP

Programming Considerations

• AV Delay and PVARP: Affect TARP
• Dynamic Intervals: Modern pacemakers adapt AV delay and PVARP based on rate

Modes and Usages

• DDI Mode: No atrial tracking, used in atrial arrhythmias
• Asynchronous Modes (VOO): For situations with expected interference

Conclusion

• Timing cycles crucial for differentiating normal and malfunctioning pacemaker behaviors
• Adapt programming based on physiologic conditions and patient needs

Next Sessions

• Upcoming Topics: Common pacemaker algorithms and programming demonstrations

Q&A Highlights

• T wave oversensing addressed by ventricular refractory period
• No pacing below the lower rate interval in VDD; pacemaker ensures lower rate pacing

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This session provided an in-depth understanding of pacemaker timing cycles, emphasizing their role in effective pacemaker function and troubleshooting. The discussion included both single and dual chamber timing mechanisms, critical intervals, and programming considerations for optimal cardiac pacing.