When the AV node completely fails to conduct any impulses from the atria to the ventricles, the patient has a third-degree AV block (see Figure 5.7). During a third-degree AV block, there is no association between P waves and QRS complexes. 

A pacemaker is necessary for a patient with complete heart block.

Causes of Third-Degree AV Block

• Coronary artery disease
• Enhanced vagal tone
• Congenital disorders
• Underlying structural heart disease, such as myocardial infarction, hypertrophy, inflammation, or infiltration
• Lyme disease
• Post-cardiac surgery
• Cardiomyopathies
• Rheumatologic diseases
• Autoimmune diseases
• Amyloidosis
• Sarcoidosis
• Muscular dystrophy

Figure 5.7. Third-Degree AV Block (Complete Heart Block)

P waves occur every 920 milliseconds, and R-R interval exhibits a dissociation in impulse rates every 1,240 milliseconds.

Characteristics of Third-Degree Atrioventricular Block 

1. Irregular rhythm
2. Atrioventricular rate normal: 60–100 bpm 
3. Ventricular rate: 40–60 bpm if the focus is junctional and 20–40 bpm if the focus is ventricular 
4. Upright and uniform P waves
5. More P waves than QRS complexes
6. No relationship between P waves and QRS complexes 
7. P waves occasionally found superimposed on the QRS complex
8. QRS complex: < 120 milliseconds if the focus is junctional and ≥ 120 milliseconds if the focus is ventricular