Third-degree atrioventricular nodal block, also known as third-degree heart block or complete heart block, occurs when no action potentials conduct through the AV node. This results in the P waves (atrial depolarizations) being completely unrelated to the QRS complexes (ventricular depolarizations) ― meaning the P waves occur at one rate and the QRS complexes at another. This is termed “AV dissociation.”
In this situation, the ventricles never see action potentials originating from the atria and compensate by making action potentials of their own. However, the ventricles are unable to create action potentials at a fast rate. This means the ventricular rate (QRS complexes) is slow (around 30 to 40 bpm), and the atrial rate (P waves) is faster than the ventricular rate (around 60 to 100 bpm).
“High grade AV nodal block,” a type of 3rd degree heart block, occurs when there is AV dissociation similar to complete heart block, but occasional P waves do conduct through the AV node to produce a QRS complex.
Complete heart block is usually symptomatic from the slow ventricular rates. These symptoms include fatigue, dyspnea, dizziness and syncope. Because intrinsic conduction disease of the His-Purkinje system is the cause of 3rd degree AV block (not autonomic tone or AV blocking medications), the rhythm is usually irreversible and a permanent pacemaker is indicated.
1. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
2. Surawicz B, et al. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram. Circulation. 2009; doi:10.1161/CIRCULATIONAHA.108.191095.