PACEMAKER

Typical Number in Hospital: 50

Cost Bands: 3

References: 3,6

Heart block is the incorrect generation or transmission of the natural pacing pulses arising from the sinoatrial node. The problem may be congenital or may result from cardiac surgery or heart disease. In such cases a common treatment is to use an electronic pacemaker to present pacing pulses to the atrium or ventricles, depending upon the site of the disease or malfunction.

Pacemakers may be external devices connected to the heart via long catheters passed through a vein or even by electrodes placed in the oesophagus, but more commonly they are implanted devices operating from internal batteries or via an inductive link to an external power source. There are two basic types of device, the fixed rate pacemakers, and those which provide pacing pulses only when required. These latter types are called non- competitive or demand pacemakers because they pulse only when intrinsic activity fails.

There are many variations on these themes, summarised by the following abbreviations:

AAT Triggered by atrial depolarization giving reinforcing stimulus to the atrium.

VOO Asynchronous ventricular pacemaker.

VVI Demand pacemaker inhibited by ventricular depolarization.

VVT Triggered by ventricular depolarization giving a reinforcing stimulus to the ventricles.

DVI Stimulating both chambers sequentially but inhibited by ventricular polarization.

The pacing may be unipolar or bipolar. In the bipolar case the two electrodes are mounted on the tip of the pacing catheter. In unipolar stimulation a single electrode is in the heart muscle while the other is elsewhere, usually mounted on the pacemaker itself.

The power source for implanted pacemakers may be batteries which require replacement every few years, of which the lithium- iodine cells are now the most attractive (up to ten years, life). Alternatively, a nuclear-powered pacemaker may last longer than this, using plutonium-238 or promethium-147. The life of plutonium cells may exceed ten years.

Pacing leads and electrodes must produce minimal electrochemical reaction and also withstand flexing and corrosion. They are normally helical coils of platinum-irridium alloys encapsulated in silicon rubber. The electrodes are often helical coils which screw into the heart muscle, thus simplifying the implantation procedures. If the pacemaker must be changed for any reason it is common for the same electrodes to be used.

The term pacemaker is sometimes used incorrectly to describe other forms of electrical stimulator such as those used for bladder or urethral muscle stimulation.