The Basics of Paced Rhythms

Objectives

• Review the indications for permanent pacing
• Appreciate pacemaker timing cycles
• Develop awareness of the revised NASPE/BPEG Generic (NGB) Pacemaker Code
• Understand the basics of single chamber and dual chamber pacing modes
• Become familiar with the “four faces” of DDD pacing
• Identify loss of ventricular capture

Introduction

A basic knowledge of how pacemakers function can be useful when interpreting paced rhythms.

It is helpful to know:

• The type of pacemaker
• The programmed parameters
• The diagnosis that necessitated a pacemaker

However, this information is often not available at the time of medical evaluation (although you can ask to see the patient’s device I.D. card).

The clinician should ask the following questions:

• What is the reason the patient is undergoing medical evaluation?
• Could the pacemaker be part of the problem?
• Is there a paced rhythm on the monitor?
• Is capture present?
• Could there be intermittent loss of capture?
• Is the rate appropriate for the clinical condition?
• Does the device need to be interrogated?
• What is the estimated battery life?
• Is there cardiac ischemia or injury?

Indications for a Permanent Pacemaker

• Symptomatic bradycardia
  • Sinus node dysfunction
  • AV node dysfunction
• Pacing for specific conditions
  • Cardiac transplantation
  • Neuromuscular diseases
  • Sleep apnea syndrome
  • Cardiac sarcoidosis
• Prevention and termination of arrhythmias
  • Antitachycardia pacing (ATP)
  • Long QT syndrome
• Pacing for hemodynamic indications
  • Cardiac resynchronization therapy

Timing Cycles

Pacemakers function based on timing cycles. These timers can function in two ways:

• A timer can complete its cycle and release a pacing stimulus (or initiate another timing cycle)
• A timer can reset which starts the timing cycle over again from the beginning

Revised NASPE/BPEG Generic (NGB) Pacemaker Code

For example DDDRO would mean:

• D: The pacemaker is capable of pacing both the atria and ventricles
• D: The pacemaker is capable of sensing intrinsic cardiac activity in both the atria and ventricles
• D: The pacemaker is capable of triggered pacing (rarely used) and inhibiting itself in response to sensed events
• R: The pacemaker is capable of rate modulation (increasing rate during periods of increased metabolic demand)
• O: It is not a multi-site pacemaker (it is not used for re-synchronization therapy)

In practice it is common for only the first 3 letters to be used (VVI, DDD).

Single Chamber Pacing Modes

• VOO: Ventricular asynchronous pacing
  • Continuous ventricular pacing at a prescribed rate
  • No alert period or sensing capability (intrinsic R-waves are ignored)

• VVI: Ventricular inhibited pacing
  • Demand ventricular pacing (ventricles are paced when needed)
  • Intrinsic R-waves are sensed and inhibit ventricular output

• AOO: Atrial asynchronous pacing
  • Continuous atrial pacing at a prescribed rate
  • No alert period or sensing capability (intrinsic P-waves are ignored)
  • Assumes AV conduction is intact (there is no heart block)

• AAI: Atrial inhibited pacing
  • Demand atrial pacing (atria are paced when needed)
  • Intrinsic P-waves are sensed and inhibit atrial output
  • Assumes that AV conduction is intact (there is no heart block)

VVI Pacing with VA Conduction (Retrograde P-Waves)

Retrograde P-waves best seen in lead V1

When VVI pacing with retrograde P-waves is accompanied by symptoms like fatigue, weakness, headache, or syncope it is sometimes referred to as Pacemaker Syndrome.

Dual Chamber Pacing Modes

• DOO: Dual chambered asynchronous pacing
  • Continuous dual chambered pacing at prescribed rate
  • Intrinsic P-waves and R-waves are ignored

• DVI: Dual chambered inhibited pacing with ventricular sensing
  • Intrinsic P-waves are ignored
  • Intrinsic R-waves inhibit atrial and ventricular outputs

• DDI: Dual chambered inhibited pacing with atrial and ventricular sensing
  • Intrinsic P-waves are sensed and inhibit atrial output but P-waves are not “tracked”
  • Intrinsic R-waves inhibit ventricular output

• DDD: Dual chambered pacing with atrial tracking
  • Intrinsic P-waves are sensed, inhibit atrial output, and “tracked” (trigger ventricular pacing after a prescribed PR interval)
  • Intrinsic R-waves inhibit ventricular output

The “Four Faces” of DDD Pacing

The “four faces” of DDD pacing.

DDDR Pacemaker Showing Dual Chambered Pacing

Dual chambered (AV sequantial) pacing. Note that the pacing detector is turned on (block arrows along the bottom of the ECG).

Capture and Loss of Capture

Intermittent loss of ventricular capture

See also: Stroke-Like Symptoms Caused by Pacemaker Malfunction

Further Reading

Basic Principles of Pacing by Kirk M. (PDF)

Pacemaker Timing Cycles by Hayes and Levine (PDF)

A beer inspired mnemonic to remember the NBG pacemaker code from CanadiEM can be found here.

References

Barold S, Stroobandt R, Sinnaeve A. Cardiac Pacemakers Step By Step. Elmsford, N.Y.: Futura; 2004.

Epstein A, DiMarco J, Ellenbogen K et al. ACC/AHA/HRS 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities. Journal of the American College of Cardiology. 2008;51(21):e1-e62. doi:10.1016/j.jacc.2008.02.032.

Hayes D, Lloyd M, Friedman P. Cardiac Pacing And Defibrillation. Armonk, NY: Futura Pub. Co.; 2000.

Sweesy M, Holland J, Smith K, Irwin M. Heart Rhythm Device Follow-Up Vol. 1. Cardiac Device Consultants, Inc.; 2005.

Tracy C, Epstein A, Darbar D et al. 2012 ACCF/AHA/HRS Focused Update of the 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities. Heart Rhythm. 2012;9(10):1737-1753. doi:10.1016/j.hrthm.2012.08.021.