The electrocardiograph is a record of the spread of cardiac electrical activity by surface electrodes. The body acts as a volume conductor. The recording electrodes depict the mean electrical vector, which is a sum of all individual vectors at a given instant in time. The magnitude of amplitude of the recorded potential is

Conduction velocities

Sino-atrial node: 0.05 m/s Atrial muscle: 0.8-1.0 m/s Atrio-ventricular node: 0.03-0.05m/s Bundle of His: 0.8-1.0 m/s

e ta dependent on the mass of tissue involved and on the direction of the vector relative to the electrical axis (the angle between the lead and the cardiac dipole). The mean electrical axis is the mean electrical vector determined over time.

The electrocardiograph is essentially a record of impulse formation in the primary pacemaker (sino-atrial node), transmission through specialised conduction tissues, depolarisation of the myocardium and repolarisation of the myocardium.

Components of the electrocardiograph

P wave: atrial depolarisation.

QRS complex: ventricular depolarisation. Ventricular activation involves initially the septal area, followed by the apex and finally the bases. T wave: ventricular repolarisation.

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