An atrioventricular (AV) block occurs when atrial depolarizations fail to reach the ventricles or when there is a delay in atrial depolarization conductions.

First Degree AV Block

• Rate: 60-100 bpm
• Rhythm: Regular
• P Waves: Identical in shape and before each QRS complex
• PR Intervals: > 0.20 seconds and equal
• R-R Intervals: Equal
• QRS Complexes: Normal and after each P wave
• Key Points: The First Degree AV Block is simply a prolonged delay in conduction. The PR interval, which represents the time it takes the heart’s electrical impulse to travel from the atria through the AV node, is normally between 0.12-0.20 seconds. In this case, the PR interval is > 0.20 seconds and remains constant.

Second Degree AV Block Type I (Mobitz Type I – Wenckebach)

• Rate: 60-100 bpm
• Rhythm: Regularly irregular
• P Waves: Identical in shape and before each QRS complex (when present)
• PR Intervals: > 0.20 seconds and get progressively longer until a QRS complex is “dropped”
• R-R Intervals: Equal when QRS complex is present
• QRS Complexes: Normal and after each P wave, but are absent periodically
• Key Points: The Second Degree AV Block Type I is characterized by a progressively prolonged PR interval. Electrical impulses traveling through the AV node take longer and longer until one impulse is blocked completely.

The next QRS complex on this rhythm strip would be "dropped" and it would look like a pause. The cycle would then restart and the PR intervals would continue to get longer and longer before another QRS complex is "dropped".

Second Degree AV Block Type II (Mobitz Type II – Hay)

• Rate: 60-100 bpm
• Rhythm: Regular, regularly irregular or irregular (based on the ratio of QRS complexes)
• P Waves: Identical in shape and before each QRS complex (when present)
• PR Intervals: Normal and equal or > 0.20 seconds and equal
• R-R Intervals: Equal or unequal
• QRS Complexes: Normal and after each P wave, but are absent periodically
• Key Points: The Second Degree AV Block Type II is when a conduction delay occurs below the AV node, either at the bundle of His or bundle branches. The ECG will show a pattern of conducted P waves (constant PR interval), followed by one or more non-conducted P waves.

Third Degree AV Block (Complete Heart Block)

• Rate: 30-60 bpm (ventricular) 60-100 bpm (atrial)
• Rhythm: Regular
• P Waves: Identical in shape and occurring independently of the QRS complexes
• PR Intervals: Unequal (disassociation between P waves and QRS complexes)
• R-R Intervals: Equal
• QRS Complexes: Normal or > 0.12 seconds (ventricular pace site)
• Key Points: The Third Degree AV Block is indicated by a complete disassociation between the atria and ventricles. The SA node generates an electrical impulse that causes the atria to contract, but the impulse is then blocked and never reaches the ventricles. This causes an accessory pacemaker in the ventricles to initiate and contract the ventricles at their own slower rate. Because two independent electrical impulses occur — the SA node and accessory pacemaker impulse — there is no relationship between the P waves and QRS complexes.