How do pacemakers work?

A pacemaking system has a pulse generator (the actual pacemaker) and either one or two

electrode leads. Pacemakers with one lead are called 'single chamber pacemakers'. Pacemakers with two leads are 'dual chamber pacemakers'.

The pacemaker has two parts: the power supply, or batteries, and the electronic circuitry. It is completely covered in metal and sealed to prevent body fluids leaking into the unit. The whole pacemaker weighs only about 20-50g (1-2 ounces) and is smaller than a matchbox. Most pacemakers are powered by lithium batteries. Pacemakers last on average between six and ten years before they need to be replaced.

Actual size of a pacemaker is approximately 4cm by 8mm.

The electronic circuit in the pacemaker draws energy from the batteries and transforms this into a series of electrical impulses. These are conducted down the electrode lead to the heart. Each electrical impulse discharged by the pacemaker stimulates the heart to contract and produce a heartbeat. The rate at which these electrical impulses are sent out is called the 'discharge rate'.

Single chamber pacemaker

right ventricle

Single chamber pacemakers have one lead which is connected either to the right atrium of the heart, or to the right ventricle (as shown)

Some pacemakers discharge electrical impulses at a fixed rate but almost all work `on demand'. `On demand' means that, if the pacemaker senses that the heart has missed a beat or is beating too slowly, or if there is no natural heartbeat, it will discharge electrical impulses at the fixed rate. If there is no natural heartbeat, it will discharge impulses continually. However, if the pacemaker senses that the heart is beating naturally on its own, it will not discharge any electrical impulse. Some types of pacemaker can speed up the discharge rate when necessary, for example when you are exercising.

Most pacemakers can be programmed to deliver electrical impulses to the heart at a rate that suits your particular needs. Even after the pacemaker has been implanted, it can be re-programmed if necessary by electromagnetic signals from an external programmer.

Some pacemakers have a sensing device which can recognise the rhythm of the heart and respond by automatically changing the discharge rate, without any external programming.

Some pacemakers can also analyse and store information about your natural heart rhythms.

This information can be retrieved when you attend your follow—up appointments at the pacemaker clinic.

Dual chamber pacemakcr

right ventricle

Dual chamber pacemakers have two leads. One is connected to the right atrium and the other to the right ventricle.