Atrial pacing, the forgotten pacing mode.

Created by on Oct. 23, 2020

Brief history

1966: The idea to pace the atrium in patients with sick sinus syndrome and intact AV conduction was first realised in 1966 by implantation of electrodes in the wall of either atrium by thoracotomy, and subsequently by the transvenous route.

1969: Kastor et al. reported on long-term pervenous atrial pacing in five patients with intact AV conduction; this was for the treatment of refractory ventricular arrhythmias in two patients and marked sinus bradycardia in three, two of whom also had paroxysmal supraventricular arrhythmias. The pervenous method was used to avoid a thoracotomy, and atrial pacing was chosen over ventricular pacing to preserve the normal AV contraction sequence.

1978Moss et al. reported their ten-year experience with atrial pacing from the coronary vein in 50 patients with implanted pervenous pacemakers. The indications for atrial pacing were symptomatic sinus bradycardia (72%), atrial bradycardia-tachycardia (brady-tachy) syndrome (20%) and recurrent tachyarrhythmias (8%). Follow-up averaged 31 months per patient, with a median of 26 months, range 3 to 9 months. Eleven patients died in the series during the ten years, but pacemaker malfunction could not be implicated as the cause of death in any of these patients. In each case effective pacing was documented during the terminal episode.

Nowadays: Single chamber pacemker (atrial lead) implant is not very common. 


Pacemakers are electric activity generating devices which are used for the treatment of patients with slow heart rate, or symptomatic heart blocks and in patients with heart failure. All cardiac pacemakers are generally composed of a pulse generator that generates the electrical current required for stimulation of heart musculature and one or two leads, which are responsible for transmitting the electrical activity generated by the pulse generator to the heart musculature.

Pacemaker systems

  • Trans venous systems: Most of the cardiac pacing systems make use of the transvenous electrodes for transmitting electrical impulses from the pulse generator to the heart musculature.
  • Epicardial systems: These work by the direct stimulation through the pulse generator by attaching directly to the surface of the heart. They are less common use nowadays but still used after open heart surgery. 
  • Leadless systems: There have been some newer innovations to develop leadless systems due to some limitations with transvenous and epicardial pacing systems.

Types of pacemakers

    There are three basic kinds of pacemakers:

  • Single chamber: One lead attaches to the upper or lower heart chamber.
  • Dual-chamber: Uses two leads, one for the upper and one for the lower chamber.
  • Biventricular pacemakers: Used in cardiac resynchronization therapy.

Modes of Cardiac Pacing:

   The modes of pacemakers typically consist of 5 letters.

  • Letter 1. The area being paced, A stands for atria, V stands for Ventricle, D stands for Dual, O stands for none.
  • Letter 2. The area which is sensed, A stands for atria, V stands for Ventricle, D stands for Dual, O stands for none.
  • Letter 3. The response of the pacemaker to sensing: O stands for none, I stands for inhibiting, T stands for triggering, D stands for dual.
  • Letter 4. Rate adaptiveness. O stands for none, R stands for rate adaptiveness.


Single Chamber Modes

VOO (In this mode, pacemaker paces at a programmed rate regardless of the intrinsic electrical activity of the heart.)

V- Pacing in the ventricle
O- Sensing is OFF
O- Response to sensing is OFF

VVI (In this mode, the pacemaker can sense the electrical activity and withhold pacing when not required.)

V- Pacing in the ventricle
V- Sensing in the ventricle

AOO (In this mode, pacemaker paces at a programmed rate regardless of the intrinsic electrical activity of the heart.)

A- Pacing in the atrium
O- Sensing is OFF
O- Response to sensing is OFF

AAI (In this mode, the pacemaker can adapt to the intrinsic atrial rate and should be able to pace when needed and inhibit when not required).

A- Pacing in the atrium
A- Sensing in the atrium
 I- Inhibit

*If the patient is chronotropically incompetent, the mode of choice will be DDDR.
*If the patient is chronotropically competent, the mode of choice will be DDD.

NOTE: In patients with sick sinus syndrome and normal atrioventricular conduction, physiological pacing can be accomplished with either a single chamber atrial pacemaker AAI/R or a dual chamber pacemaker DDD/R.


atrial lead


Pacemaker spike

The pacing spikes are visible on the EKG as a sharp vertical line and can appear above or below the isoelectric line.

atrial pacing


This EKG was recorded in a 67-year-old male with history of symptomatic sinus node dysfunction without AV conduction abnormalities after single chamber pacemaker (one lead in the atrium, see video below) implant. Notice the pacing spikes before each P waves. In addition, sensing of intrinsic atrial activity inhibits release of the pacing stimulus. 

Atrial pacing

Single chamber pacemaker


Indications for pacemaker implantation

The American College of Cardiology (ACC), the American Heart Association (AHA), and the Heart Rhythm Society (HRS) have jointly established national guidelines to direct the implantation of artificial cardiac pacemakers. A detailed discussion of these guidelines is beyond the scope of this article. The main indications for pacemaker implantation include:

  • Symptomatic bradycardia from sinus node disease
  • Symptomatic bradycardia from atrioventricular node disease
  • Long QT syndrome
  • Hypertrophic obstructive cardiomyopathy
  • Dilated cardiomyopathy
  • During AV node ablation
  • Cardiac resynchronization therapy with biventricular pacing
  • Advanced 2nd or third-degree heart block
  • Recurrent syncope


  • Pneumothorax
  • Wound infection
  • Pericarditis
  • Skin erosion
  • Lead dislodgment
  • Hematoma
  • Failure to sense, capture or output
  • Pacemaker mediated tachycardia
  • Twiddler syndrome
  • Pacemaker syndrome
  • Pacemaker pseudomalfunction

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AAI atrial pacing pacemaker single chamber dr roig