Conducting system

• Electrical activity begins with pacemaker cells of the sinoatrial (SA) node. Here, cells are able to generate a spontaneous impulse without nervous innervation and normally determine the heart rate.

• Once the SA node depolarises, waves of electrical activity travel across the right and left atria, causing these chambers to contract.

• The impulses then reach the atrioventricular (AV) node, where there is a brief period of delayed conduction.

• Impulses are then propagated downwards in a co-ordinated fashion, initially along the atrioventricular bundle of His.

• The bundle consists of specialised conducting tissue that divides into the right and left bundle branches.

• The bundle of His and bundle branches are central to the propagation of impulses, which eventually terminate in fine branches or Purkinje fibres.

• The Purkinje fibres are embedded in ventricular myocardium and these will transmit impulses to the ventricular muscle mass, resulting in contraction. Ventricles comprise a large muscle mass and are able to generate spontaneous impulses but at a slower rate than pacemaker cells, namely at a rate of 20-40 beats per minute (bpm) (Meek & Morris 2000a).

The spread of impulses from cell to cell and across the entire heart is known as 'depolarisation', whereas the stage of 'repolarisation' relates to myocardial recovery. In practice, the 'electrical' cycle of the heart can be fully assessed by a 12-lead ECG from the patient. Specifically, an ECG machine measures and records, on graph-like paper, voltage variations (of the atria and ventricles) plotted against time. These waves of electrical activity have three distinctive features.

Duration

The time element that is measured along the horizontal axis and in fractions of a second.

• 1 small square equals 0.04sec or 1mm in width.

• 1 large square equals 0.2 sec or 5 mm in width.

• 300 large squares represent one minute.

Amplitude

A voltage component that is measured along the vertical axis in millivolts.

Composition

This relates to specific arrangements in the waves in terms of shape and appearance.

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