Atrial pacing


In patients with sick sinus syndrome and normal atrioventricular conduction, physiological pacing can be accomplished with either a single chamber atrial pacemaker (AOO and AAI). 

In AOO mode, the atrium is paced at a set rate regardless of atrial activity; this is asynchronous atrial pacing. 

In AAI mode, the atrium is sensed and the pacemaker is inhibited if the native atrial rate rises above the pacemaker set rate; this is demand atrial pacing. 

In general AAI is preferred to AOO because with AOO, pacing can occur during atrial repolarization, possibly inducing atrial fibrillation.

Main indications for atrial pacing

  • Symptomatic bradycardia  
  • Brady-tachy syndrome  

ECG features

  • Pacing spike precedes the p wave
  • The morphology of the P wave depends upon the location of the atrial lead; it may be normal, diminutive, biphasic, or negative
  • The PR interval and configuration of the QRS complex are similar to those seen in sinus rhythm. They are independent of the atrial pacemaker; thus, the duration and configuration are determined by the intrinsic characteristics of the patient's conduction system

Figure 1 Atrial pacing spike followed by P wave with spontaneous conduction to ventricles

ECG 1 Atrial pacing spike with spontaneous conduction to ventricles - each atrial spike followed by a P wave.

ECG 2  Each atrial pacing spike followed by a small P wave


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