Pacemaker: Types, How It Works & Why It's Needed

A pacemaker is a small implanted device that keeps the heart from beating too slowly. Here's why people need one, the different types, how the implant works, and what a paced ECG looks like.

What is a pacemaker?

A pacemaker is a small battery-powered device implanted under the skin that delivers electrical impulses to make the heart beat when its own rhythm is too slow. It has two parts: a pulse generator (battery and circuitry) and one or more leads (wires) that carry the impulse to the heart and sense its natural activity.

Why is a pacemaker needed?

Pacemakers treat a heart that beats too slowly or unreliably. Common indications:

• Symptomatic bradycardia — a slow rate causing fatigue, dizziness, or fainting
• High-grade or complete (third-degree) AV block — see heart block
• Sick sinus syndrome — an unreliable SA node
• Certain cases after cardiac surgery or with specific conduction disease

Types of pacemaker

Type	Leads	Used for
Single-chamber	One (RA or RV)	Simple bradycardia
Dual-chamber	Two (RA + RV)	AV block — restores atrial-ventricular timing
Biventricular (CRT)	Three	Heart failure with dyssynchrony
Leadless	None (self-contained in the RV)	Selected single-chamber cases

How the implant works

Most pacemakers are placed transvenously: leads are threaded through a vein under the collarbone into the heart, and the generator sits in a small pocket beneath the skin. The device continuously senses the heart's own beats and only fires when the rate drops below its programmed limit — so it works on demand rather than constantly.

The paced ECG

On the ECG, a pacemaker leaves a tell-tale pacing spike — a sharp vertical line — immediately before the wave it triggers. A spike before the QRS means ventricular pacing; a spike before the P wave means atrial pacing. Recognising pacing spikes is a common exam point; practise on our ECG strip questions.

Living with a pacemaker

Modern devices last many years and are checked remotely or in clinic. Most household electronics are safe; strong magnetic fields are the main caution, and many newer pacemakers are MRI-conditional. Patients carry a device ID card.

Key takeaways

• A pacemaker paces the heart when its own rhythm is too slow.
• Main indications: symptomatic bradycardia, AV block, and sick sinus syndrome.
• Types: single-chamber, dual-chamber, biventricular (CRT), and leadless.
• Leads are usually placed transvenously with the generator under the skin.
• A pacing spike before the P or QRS is the ECG signature.

Frequently asked questions

What does a pacemaker do?

It delivers small electrical impulses to make the heart beat when its natural rhythm is too slow, working on demand only when the rate drops below a set limit.

Why would someone need a pacemaker?

For symptomatic bradycardia, high-grade or complete AV block, or sick sinus syndrome — conditions where the heart beats too slowly or unreliably.

What are the types of pacemaker?

Single-chamber, dual-chamber, biventricular (cardiac resynchronization therapy), and leadless pacemakers.

How is a pacemaker implanted?

Usually transvenously: leads are threaded through a vein under the collarbone into the heart and the generator is placed in a pocket beneath the skin.

What does a paced beat look like on an ECG?

A sharp pacing spike appears immediately before the wave it triggers — before the QRS for ventricular pacing or before the P wave for atrial pacing.

Can you have an MRI with a pacemaker?

Many modern pacemakers are MRI-conditional and allow scanning under specific protocols, but older devices may not; it must be checked first.