Analyzing the P-R Interval (PRI)

■ PRI is consistent, and normal, between 0.12 and 0.20 sec (3–5 small boxes): This is considered a normal PRI.
■ PRI is < 0.12 sec (3 small boxes): consider Junctional rhythm.
■ PRI is longer than 0.20 sec (5 small boxes), it remains consistent in length from PRI to PRI: Consider 1 AV block.
■ Progressive lengthening of PRI until a QRS is dropped: Consider 2 AV block type-I (Mobitz I or Wenckebach).
■ Consistent PRI, however, there are additional P waves that do not precede a QRS complex: Consider 2 AV block type-II (Mobitz II).
■ PRI is not consistent, nor is there any correlation between the P wave and the QRS: Consider 3 AV block (complete heart block).