Electrical system of heart

Normal heart beats in a normal (sinus) rhythm. In cardiac arrhythmia this normal rhythm of heart beat is lost.
Human heart consists of 
(A) two upper chambers named right atrium and left atrium separated by a septum which is called interatrial septum .This interatrial septum prevents mixing of blood between left and right atrium, 
(B) two lower chambers named right ventricle and left ventricle separated by a septum which is called interventricular septum. This interventricular septum prevents mixing of blood between the left and right ventricle.
A small mass of special heart tissue called SA (sinoatrial) node situated in the upper part of right atrium is called natural pacemaker of heart because normal cardiac impulse starts from this area. From SA node impulse goes to AV ( Atrio ventricular) node via three internodal tracts named Anterior, Middle and Posterior internodal tracts (AV node is located in the triangle of koch an anatomical area within the superficial paraseptal endocardium of the right atrium). From the Anterior internodal tract one extension goes to left atrium called Bachmann’s Bundle. Conduction of impulses from SA node to these tracts cause contraction of both atrium and expel blood to respective ventricle.
Normally heart beats at a rate of 60 to 100 beats / min. At the AV node these impulses slows down briefly giving the ventricles required time to receive blood from respective atrium. From AV node impulse goes to Bundle of His which bifurcates into right and left Bundle branch. Right bundle branch located in the interventricular septum goes to right ventricle and lead to a network of thin conducting fibres called Purkinje Fibres.
Left Bundle branch situated in interventricular septum divides into two branches Left anterior fascicle and left posterior fascicle, both these fascicles goes to the left ventricle and lead to a network of thin conducting fibres called Purkinje Fibres. When impulses spread in left and right ventricles through these conducting networks, ventricles contract.