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Basic electrocardiography

Heart beat originates in the SA node.
Impulse spreads to all parts of the atria via internodal pathways .
ATRIAL contraction occurs.
Impulse reaches the AV node where it is delayed by 0.1second.
Impulse is conducted rapidly down the Bundle of His and Purkinje Fibres.
VENTRICULAR contraction occurs.

ECG Waveforms
Normal cardiac axis is downward and to the left.
ie the wave of depolarisation travels from the right atria towards the left ventricle.
When an electrical impulse travels towards a positive electrode, there will be a positive deflection on the ECG.
if the impulse travels away from the positive electrode, a negative deflection will be seen.

The P wave represents atrial depolarisation.
The PR interval is the time from onset of atrial activation to onset of ventricular activation.
The QRS complex represents ventricular depolarisation.
The S-T segment should be iso-electric, representing the ventricles before repolarisation.
The T-wave represents ventricular repolarisation.
The QT interval is the duration of ventricular activation and recovery.