A<unk>  is a term in cardiology that connotes a<unk> ened time for the pacemaker in the atrium to conduct an impulse and activate the ventricle (the larger pumping chamber of the heart).  While it normally takes 0.12 to 0.21 seconds for the impulse to pass from the atrium to the ventricle (the normal PR<unk> , a<unk>  is defined as a PR<unk>  of less than 0.12 seconds.  Although a<unk>  may be a normal variant, it is also associated with the presence of an accessory bypass tract (e.g. WPW syndrome and LGL syndrome), and close proximity of the atrial impulse to the AV node such as occurs in a premature atrial beat.  In AV dissociation, the atrial impulse does not conduct to the ventricle, but the atrium and the ventricle may separately beat at similar rates and the PR<unk>  may vary from beat to beat.  In some beats, the PR<unk>  may be quite<unk> .
Short PR interval
Long PR interval
Low QRS voltage
PR interval alternans
Inverted P wave
T wave alternans
Peaked T waves
U wave alternans