The heart has its own electrical conduction system. The conduction system sends signals throughout the upper chambers (atria) and lower chambers (ventricles) of the heart to make it beat in a regular, coordinated rhythm. The conduction system consists of two nodes that contain conduction cells and special pathways that transmit the impulse.

A normal heartbeat begins when an electrical impulse is fired from the sinus node (also called sino-atrial or SA node), in the right atrium. The sinus node is responsible for setting the rate and rhythm of the heart and is therefore referred to as the heart’s pacemaker.

The electrical impulse fired from the SA node spreads throughout the atria, causing them to contract and squeeze blood into the ventricles. The electrical impulse then reaches the atrioventricular node (AV node), which acts as a gateway, slowing and regulating the impulses travelling between the atria and the ventricles. As the impulse travels down the pathways into the ventricles the heart contracts and pumps blood around the body. The cycle then begins again.

A normal adult heart beats in a regular pattern 60 to 100 times a minute; this is called sinus rhythm.


Sometimes if the conduction pathway is damaged, blocked, or an extra pathway exists the heart’s rhythm changes. The heart may beat too quickly (tachycardia), too slowly (bradycardia) or irregularly.  This may affect the heart’s ability to pump blood around the body. These abnormal heartbeats are known as arrhythmias. Arrhythmias can occur in the atria or in the ventricles.

Cardiac resynchronisation therapy (CRT)

CRT is a therapy that is sometimes required to maximise the pumping function of the heart in patients with underlying poor ventricular function, for example, patients with heart failure. The two lower chambers of the heart, the ventricles, often become uncoordinated, making the heart less efficient.

A normal pacemaker only has wires (leads) in the right side of the heart. CRT requires an extra wire to be placed into the left ventricle to enable both sides to beat together (resynchronisation).  Making the lower chambers of the heart work together can improve the overall function of the heart as a pump and reduce the symptoms of breathlessness and fatigue. Your doctor has decided that you may benefit from CRT as you have been noted to have reduced ventricular pump function.