Every time the heart beats, a sequence of events takes place known as the cardiac cycle. There are two phases:

• Systole, during which the heart contracts so blood is pumped around the body and to the lungs
• Diastole, which is the time between contractions when the atria fill with blood, which then flows into the ventricles.

The cardiac cycle happens spontaneously, but can be altered by the nervous system or chemicals in the body, the latter occurring either naturally or as a result of exposure to drugs. For example, adrenaline speeds up the heart while beta-blockers slow it down. The automatic nature of the cardiac cycle occurs as a result of the heart providing its own electrical stimulation. The sinoatrial (SA) node in the wall of the right atrium emits the impulse that starts the cardiac cycle, by making the atria contract. For this reason, the SA node is sometimes referred to as the body’s natural pacemaker. This electrical activity is then picked up by the atrioventricular (AV) node, which is found between the atria and the ventricles, and spreads along the walls of the ventricles, causing them to contract.

An electrocardiogram (ECG) detects the signals conducted through the heart during the cardiac cycle and allows abnormal heart rhythms – called arrythmias – to be identified.