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.
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’).
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
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