Heart blocks are disorders of impulse conduction through the atrioventricular (AV) node or infranodal conduction system (Bundle of His, bundle branches, Purkinje fibers). The conduction pathway anatomy is critical: the SA node generates the impulse → atrial depolarization (P wave) → impulse reaches the AV node (physiologic delay, PR interval) → Bundle of His → right and left bundle branches → Purkinje fibers → ventricular depolarization (QRS). Heart blocks are classified by severity: FIRST-DEGREE AV BLOCK — delayed conduction through the AV node; every impulse is conducted but with prolonged PR interval >0.20 sec. All P waves are followed by QRS complexes. Usually benign. SECOND-DEGREE TYPE I (Mobitz I / Wenckebach) — progressive PR prolongation until a P wave is not conducted (dropped QRS), then the cycle repeats. The block is typically at the AV node level. Usually transient, often from increased vagal tone, inferior MI, or medication effects. Rarely progresses to complete block. SECOND-DEGREE TYPE II (Mobitz II) — intermittent failure of conduction WITHOUT progressive PR prolongation. The PR interval is constant for conducted beats, then suddenly a P...