The conduction time between the atria and ventricles is represented by the PR interval. A PR interval > 200 milliseconds (0.20 seconds) is considered a first-degree AV block (see Figure 5.4). 

Causes of First-degree AV Blocks

• Structural abnormalities within the AV node
• An increase in vagal tone
• Drugs that slow down conduction, including digoxin, beta-blockers, and calcium channel blockers 

It is important to note that no physical blockage occurs in first-degree AV block.

Related Video – One Quick Question: What are the First-Degree Heart Block Criteria?

Figure 5.4. First-Degree Atrioventricular Block

First-Degree Atrioventricular Block with P Waves Buried Within T Waves

Characteristics of a First-Degree Atrioventricular Block

1. Underlying rhythm determines the regularity
2. Underlying rhythm determines the rate
3. Upright and uniform P waves 
4. Each P wave is followed by a QRS complex
5. Prolonged PR interval ( > 200 milliseconds) 
6. PR interval consistent across the ECG strip
7. QRS complex < 120 milliseconds