<unk>  is prolongation of the time required by impulse generated at the normal atrial pacemaker (SA node) to conduct via the normal conduction pathway and activate the ventricle.  Normally the conduction time is 0.12 to 0.21 seconds (normal PR<unk> .  A PR<unk>  above 0.21 seconds is called as<unk> .  PR<unk>  is usually<unk> er when the conduction of atrial impulse through AV node is slowed either as a consequence of withdrawal of sympathetic tone or due to an increase in vagal inputs.  It is also important to emphasize that<unk> s can exist in subjects who are otherwise normal.  In atrioventricular dissociation where both atria and ventricle beat independently without conducting each others impulse either antegradely or retrogradely, PR<unk>  may vary from beat to beat.  In some beats, the PR<unk>  may be<unk> .
Long PR interval
Short PR interval
Low QRS voltage
PR interval alternans
Inverted P wave
T wave alternans
U wave alternans
Peaked T waves