Pacemakers deliver controlled electrical impulses to specific areas of the heart to restore the normal electrical activity and improve the heart function. They consist of a pulse generator (the little pacemaker box) and pacing leads that carry electrical impulses to the relevant part of the heart. The box is implanted under the skin (most commonly in the left anterior chest wall or axilla) and the wires are implanted into the relevant chambers of the heart.
Modern pacemakers have a computer that monitors the natural electrical activity and tailors its function to that. Basically if it is already working perfectly, no intervention is provided by the pacemaker. The batteries last around 5 years. They do not interact with day to day electrical activities but may be a contraindication for MRI scans (due to powerful magnets) and electrical interventions such as TENS machines and diathermy in surgery. Many modern pacemakers are “MRI compatible”.
It is worth noting that it is essential that pacemakers are removed prior to cremation in deceased patients. On the “cremation form” one of the most important tasks is to confirm whether the deceased patient has a pacemaker and whether it has been removed. You will hear stories about pacemakers “blowing up” crematoriums.
Indications for a Pacemaker
- Symptomatic bradycardias
- Mobitz Type 2 AV block
- Third degree heart block
- Severe heart failure (biventricular pacemakers)
- Hypertrophic obstructive cardiomyopathy (ICDs)
Single-chamber pacemakers have leads in a single chamber, either in the right atrium or the right ventricle. They are placed in the right atrium if the AV conduction in the patient is normal and the issue is with the SA node. This way they stimulate depolarisation in the right atrium and this electrical activity then passes to the left atrium and through the AV node to the ventricles in the normal way. They are placed in the right ventricle if the AV conduction in the patient is abnormal and they stimulate the ventricles directly.
Dual-chamber pacemakers have leads in both the right atrium and right ventricle. This allows the pacemaker to synchronise the contractions of both atria and ventricles.
Biventricular (triple-chamber) Pacemaker
Biventricular pacemakers have leads in right atrium, right ventricle and left ventricle.
These are usually in patients with heart failure. The objective is to synchronise the contractions in these chambers to try to optimise the heart function. They are also called cardiac resynchronisation therapy (CRT) pacemakers.
Implantable Cardioverter Defibrillators (ICDs)
Implantable Cardioverter Defibrillators continually monitor the heart and apply a defibrillator shock to cardiovert the patient back in to sinus rhythm if they identify a shockable arrhythmia.
ECG Changes with Pacemakers
It is quite common to be asked to state pacemaker type based on an ECG in exams. The pacemaker intervention can be seen as a sharp vertical line on all leads on the ECG trace. A line before each p-wave indicates a lead in the atria. A line before each QRS complex indicates a lead in the ventricles. Therefore:
- A line before either the P or QRS but not the other indicates a single-chamber pacemaker
- A line before both the P and QRS indicates a dual-chamber pacemaker
Last updated November 2018