Analysis of Rhythm
After establishing the rate the next determination should be
the rhythm. Normally, the beginning of the P-wave should precede
the beginning of the R-wave by less than 0.2 seconds or 1 large
box.
Prolongation
over 0.2 seconds suggests a delay in the
conduction system between the SA node and the AV node indicating
a first degree heart block. When it takes two or three P-waves to
initiate a QRS complex this is termed a 
2:1 or 3:1 type second
degree heart block. When the P-R interval becomes progressively
longer until a QRS complex is dropped and then the process
repeats, this is called a 
Wenckebach phenomenon, (a type of
second degree Mobitz I block). If the QRS complex is periodically
blocked without lengthening of the P-R interval this is called a
Mobitz II block.
A 
third degree
block exists when the P and the QRS waves are
entirely disassociated. These blocks often result from
interference along some part of the His-Purkinje system which can
usually be located by examining the chest leads such as Vl-V6 to
determine if it is a right or left bundle branch block as well as
its type.
Prolongation
over 0.2 seconds suggests a delay in the
conduction system between the SA node and the AV node indicating
a first degree heart block. When it takes two or three P-waves to
initiate a QRS complex this is termed a 
2:1 or 3:1 type second
degree heart block. When the P-R interval becomes progressively
longer until a QRS complex is dropped and then the process
repeats, this is called a 
Wenckebach phenomenon, (a type of
second degree Mobitz I block). If the QRS complex is periodically
blocked without lengthening of the P-R interval this is called a
Mobitz II block. 
third degree
block