Four-lettered code

  The four-lettered pacemaker code defines the working principle (pacing mode) of the pacemaker.

  These letters respectively correspond to the following:    PACE ,   SENSE,    INHIBIT / TRIGGER,    RATE



The first letter in the code shows the cardiac chamber from which the pacemaker gives stimulus (PACEs):

  A: gives stimulus to the Atrium (paces the atrium).

  V: gives stimulus to the Ventricle (paces the ventricle).

  D: gives stimulus to both the atrium and the ventricle (D=Dual).

  O: No stimulus.



The second letter shows the cardiac chamber which the pacemaker SENSEs:

  A: senses the Atrium.

  V: senses the Ventricle.

  D: senses both the atrium and the ventricle (D=Dual)

  O: does not sense any chamber (no sense).



The third letter shows how the pacemaker responses to the sense:

  I ( inhibit ): inhibits giving stimulus (inhibits pacing). When pacemaker senses the spontaneous

      depolarization in a cardiac chamber it inhibits pacing (does not give stimulus to) that cardiac chamber

      for that cardiac cycle. By this way, pacemaker does not interfere with spontaneous cardiac depolarizations.

  T (
trigger ): triggers giving stimulus (triggers pacing). If the pacemaker senses no spontaneous

      depolarization of the related cardiac chamber, it will trigger (pace / give stimulus to) that chamber.

  D (
Dual ): May both inhibit or trigger giving stimulus.

  O: No response to sense.





The fourth letter shows if there is rate modulation (rate response) of the pacemaker or not:

  R: rate modulation is present.

  O: no rate modulation.

  If there is a need to increase heart rate (such as during exercise or stress),

      the rate modulation provides that compensatory increase.

  In pacemakers with rate modulation, lower and upper limits are defined.

      When patient starts exercising, pacemaker can increase the pacing frequency (heart rate)

      up to this predefined upper limit.





Some commonly used pacemaker modes:

VVI

  First letter V: paces the Ventricle.

  Second letter V:
senses the Ventricle.

  Third letter I: If the pacemaker senses spontaneous depolarization in the ventricle,

      it will
inhibit pacing the ventricle for a certain period of time.

  VVI is the preferred method of pacemaker mode in patients with atrial fibrillation.




AAI

  First letter A: paces the Atrium.

  Second letter A:
senses the Atrium.

  Third letter I: If the pacemaker senses the spontaneous depolarization in the atrium,

      then it will
inhibit pacing (giving stimulus to) the atrium.

  In patients with normal AV conduction, this mode of pacing may be preferred in order to have

      physiological cardiac stimulation. In patients with sick sinus syndome, if the AV nodal conduction

      is normal, then AAI mode can be chosen for symptomatic bradycardia.

  AAI mode is not preferred if there is atrial fibrillation or atrioventricular block.

  If a
magnet is placed over a pacemaker in the AAI mode, the pacemaker turns to AOO mode:

      now the pacemaker starts to give asynchronized stimulus to the atrium.

      If the underlying native atrial rate is below the pacemaker lower limit,

      then the ECG will show atrial pacing spikes.



DDD

  First letter D: Paces (gives stimulus) both the atrium and the ventricle (D=dual).

  Second letter D:
Senses from both the atrium and the ventricle (D=dual).

  Third letter D:

      If it senses atrial depolarization, the pacemaker inhibits atrial pacing,
triggers ventricular pacing.

      If it senses ventricular depolarization, it
inhibits both atrial and ventricular pacing.

  If a
magnet is placed over a pacemaker in the DDD mode, the pacemaker turns to DOO mode:

      without taking into account the underlying rhythm, the pacemaker will now start to give asynchronized

      stimulus to the atrium and the ventricle.



VDD

  First letter V: Paces the Ventricle.

  Second letter D:
Senses both the atrium and right ventricle.

  Third letter D:

      If it senses the depolarization of atrium, the pacemaker will give stimulus from the ventricle.

      If it senses ventricular depolarization, it will inhibit ventricular pacing.

  If a
magnet is placed over a pacemaker in the VDD mode, the pacemaker turns to VOO mode:

      without sensing the underlying rhythm, the pacemaker will now start to give asynchronized

      stimulus to the ventricle.