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Pacemakers are used to treat bradyarrhythmias. They are usually inserted subcutaneously below the left clavicle. A wire connects the pacemaker to the myocardium of the right atrium and/or right ventricle. This supplies electrical stimulation which initiates myocardial depolarisation and subsequent contraction. It can be programmed externally. The battery lasts 5-10 years.

Common indications 

For permanent pacing

  • Mobitz type 2 second degree heart block
  • Complete heart block
  • Symptomatic bradycardias (e.g. sick sinus syndrome)
  • Symptomatic pauses (>3 seconds)
  • Trifascicular block with syncope/pre-syncope

For temporary pacing

  • Haemodynamically unstable bradycardia unresponsive to atropine
  • Haemodynamically unstable heart block post-myocardial infarction (rare)

NB: temporary pacing may also be used to supress drug-resistant tachyarrhythmias (e.g. VT storm) by pacing at a higher rate than the native heart rate (‘overdrive pacing’).

Basic types

  • Dual-chamber pacemaker (two leads, one in right atrium and one in right ventricle): paces both chambers; used for most patients requiring a pacemaker unless they meet the criteria below for a single chamber pacemaker
  • Single-chamber pacemaker 
    • One lead in right ventricle: used in patients with permanent AF because there is no point pacing a fibrillating atrium     
    • One lead in right atrium: sometimes used for sick sinus syndrome with normal AV conduction because the pacemaker only needs to replace the SA node when the rest of the heart functions normally – although generally a dual-chamber pacemaker would still be used in this situation because there is an increased risk of AV problems in the future

NB: implantable cardioverter defibrillators look like pacemakers but have a different function – they are used for automatic defibrillation in patients who are at risk of VF or VT and sudden cardiac death (e.g. patients with previous episodes of ventricular arrhythmias and haemodynamic compromise or poor ejection fraction; repaired congenital heart disease; or familial cardiac conditions). Some implantable cardioverter defibrillators may also function as pacemakers.  


3 letter codes

The pacemaker can pace the right ventricle, the right atrium or both. The pacemaker can also sense spontaneous heart depolarisations through the same lead(s), and pacing can either be triggered by that spontaneous heart depolarisation or inhibited by it (most). 

  • Letter 1: indicates which chamber is paced (Atria, Ventricles, Dual chamber)
  • Letter 2: indicates which chamber is sensed (Atria, Ventricles, Dual chamber)
  • Letter 3: indicates pacemaker response (Triggered, Inhibited, Dual)

Further letters which may be used

  • Letter 4: if rate responsive features present (Rate responsive), e.g. rate can increase during exercise
  • Letter 5: anti-tachycardia features
    • P: in tachycardia, it will pace
    • S: in tachycardia, it will shock 
    • D: dual ability to pace and shock

NB: O can mean none for any letter.


  • VVI: ventricles are paced, but pacing is inhibited when spontaneous ventricular depolarisations are sensed
  • AAI: as above but for atria – rarely used for reasons above
  • DDD: both chambers are paced, but atrial pacing is inhibited when spontaneous atrial depolarisation is sensed (within a predetermined maximum RP interval), and ventricular pacing is inhibited when spontaneous ventricular depolarisation is sensed (within a predetermined maximum PR interval).

ECG of paced rhythm

  • The ECG of a paced rhythm has vertical pacing spikes when it is pacing (but they may be difficult to see) – you cannot interpret a paced ECG for other abnormalities 
    • If the atrium is paced, a pacing spike is seen immediately before a P wave
    • If the ventricle is paced, a pacing spike is seen immediately before a broad QRS complex
    • In dual chamber pacing, both of these pacing spikes are seen
Atrial and ventricular pacing spikes on ECG