Paper speed: 25 mm/s → 1 small box = 0.04 s (40 ms), 1 large box = 0.20 s.

PR interval: Normal is 0.12–0.20 s (3–5 small boxes).

• 1° AV block: PR > 0.20 s and constant.
• 2° Type I (Wenckebach): PR progressively lengthens, then dropped QRS.
• 2° Type II: PR constant on conducted beats with intermittent dropped QRS.
• 3° (complete) block: AV dissociation — P waves and QRS march independently.

Tip: Use RR box count to estimate rate and compare atrial vs ventricular regularity in blocks.