Third-Degree (Complete) Heart Block

Third-degree atrioventricular (AV) block, also called complete heart block, occurs when NO atrial impulses are conducted to the ventricles. There is complete electrical dissociation between the atria and ventricles: the atria continue to fire at their intrinsic rate (SA node, typically 60-100 bpm) while the ventricles are driven by an escape pacemaker below the block. The location of the block determines the escape rate and QRS width. If the block is at the AV node, a junctional escape rhythm takes over (rate 40-60 bpm, narrow QRS). If the block is below the AV node in the His-Purkinje system (infranodal), a ventricular escape rhythm takes over (rate 20-40 bpm, wide QRS). Infranodal block is more dangerous because the ventricular escape rate is slower, less reliable, and the wide QRS indicates abnormal ventricular activation. Causes include degenerative fibrosis of the conduction system (most common in elderly), MI (inferior MI affects AV node, anterior MI affects His-Purkinje), medications (beta-blockers, CCBs, digoxin), post-cardiac surgery, myocarditis, and congenital complete heart block. The hallmark ECG finding is AV dissociation: P waves and QRS complexes are completely...