The Normal Heart Rhythm

The Anatomy of the Heart's conduction system

Key to an understanding the problem of all palpitations is knowing how the heart rhythm is normally controlled.

The heart beat normally starts in an area of the heart called the Sino-atrial node (S-A node, sinus node). This lies in the wall of the right atrium and is also sometimes referred to as the heart's pacemaker.

Electricity is discharged from this area rhythmically and spreads through the muscle walls of the top two chambers of the heart (the right and left atria). This causes the p-wave on the electrocardiogram (ECG). As the electrical current flows through the muscle it causes it to contract and this pushes the blood through the valves (tricuspid and mitral) and into the ventricles. (see below)

	PA pulmonary artery SVC superior vena cava IVC inferior vena cava RA right atrium LA left atrium RV right ventricle LV left ventricle S-A sino-atrial node A-V atrio-ventricular node

The flow of electricty is briefly held up in an area of specialised tissue that lies between the atria and the venticles called the Atrio-ventricular node (A-V node). It then flows down some specialised fibres within the wall of muscle that separates the two ventricles. These are a little like nerve tissue and are called the His-Purkinje fibres (after the doctors that first described them).

This His-Purkinje system splits up into two bundles, the Left Bundle and the Right Bundle which run round into the walls of the two respective ventricles. The Left Bundle further divides into an Anterior Fascicle and a Posterior Fascicle which run to the front and back of the left ventricle respectively. In this way electricity is delivered quickly and simultaneously to the front and back of the left ventricle and the right ventricle. (The sequence of electrical flow through the heart is illustrated in the animation). It then spreads more slowly through the muscle of the ventricles and in doing so causes the muscle to contract pushing blood out of these chambers into the large blood vessels that carry the blood to the lungs (Pulmonary Artery) and the body (Aorta). The electricity flowing through the muscles of the ventricles produces the QRS complex on the ECG. (see below)

The electrocardiogram (ECG)

Three distinct deflections can be seen on the ECG:

• The p-wave
• The QRS complex
• The T-wave

The p-wave represents the electricity as the muscles of the atria contract.

The QRS complex represents the electricity of the contraction of the two ventricles.

The T-wave represents the electricity returning to baseline in the two ventricles ahead of the next electrical impulses arriving.

During the part of the ECG from the p-wave to the T-wave the heart is contracting (systole) pushing blood out into the circulation. During the part of the ECG from the T-wave until the next p-wave the heart is relaxed (diastole) and filling with blood.