Sinus rhythm is the normal regular heartbeat that most people are in.

Your heart pumps in time based on an electrical conducting system that spans from the top chambers (atria) to the bottom chambers (ventricles). An electrical signal starts spontaneously in the right atrium in a small group of cells called the sinoatrial node. This signal spreads down a special wiring system to the middle of the heart, where it then splits into two signals, one for each ventricle.

When you are in sinus rhythm, the top chambers beat once for every beat of the bottom chambers, and beat speeds up in response to the body needing more blood flow, like with exercise. It will then slow down when you are inactive, like when you are asleep.

There are a couple of important features of sinus rhythm:

• It is regular, meaning that each beat is the same time from the previous beat (actually, to confuse matters just slightly, these beats aren’t perfectly regular. They vary by just a few milliseconds, which is called Heart Rate Variability, and changes here can reflect illness).
• It responds to physical (or even emotional) activity, by speeding up.
• It slows down when you rest.
• The top chambers pump beat once for every beat of the bottom chambers.
• The signals come from a specific part of your heart called the sinus node.

Put simply, sinus rhythm is the normal rhythm of the heart. When you are not in sinus rhythm, you have a type of arrhythmia, of which there are many. The most well known are atrial fibrillation, supraventricular tachycardia (SVT), ventricular tachycardia and ventricular fibrillation.

Arrhythmias aren’t necessarily permanent – you can go back and forth between them and sinus rhythm, either spontaneously or in response to treatment. The aim of a cardioversion is to restore sinus rhythm.

An ECG can very quickly determine the rhythm you are in at the time it is taken.