Electrical conduction in the normal heart

In sinus rhythm, the SA node generates APs automatically that conduct:

• Rapidly through the atria, causing atrial contraction
• Relatively slowly through the AV node
• Very rapidly through the bundle of His and left and right bundle branches
• Rapidly through the Purkinje fibres causing ventricular contraction

What is an ECG?

• ECG: a recording of potential changes, detected by electrodes positioned on the body surface, that allows the electrical activity of the heart to be monitored in a manner that is simple to perform and non-invasive

How does it work?

• The action potential, propagating, in sequence, through the conducting system and muscle of the heart causes separation of charge, or differences in potential between cardiac regions
• Charges that are separated constitute an electrical dipole which is a vector with components of magnitude and direction
• Potential differences between different sites on the body surface are detected by electrodes placed on the skin coupled to a sensitive recording device
• A recording of such potential differences between time is the ECG

• Potentials at the body surface arise from currents that flow when the membrane potential of the myocardial tissue is changing (depolarisation, or repolarisation)
• Only large masses of cardiac tissue (i.e. atrial and ventricular muscle) generate sufficient current to be detected at the body surface as potential changes

Placement of electrodes

Precordial electrodes

• V1: 4th intercostal space at the right sternal edge
• V2: 4th intercostal space at the left sternal edge