Second-degree AV blocks are characterized by occasional non-conducted P waves and prolonged RR intervals. These blocks are classified into two types: Mobitz type I (also known as Wenckebach; see Figure 5.5) and Mobitz type II. 

Mobitz type I occurs when an intermittent conduction block within the AV node prevents an atrial impulse from traveling to the ventricles. In simple terms, the PR interval becomes longer and longer until a QRS complex is dropped. A tip for recognizing Mobitz type 1 is: “Wenckebach WARNS you with a lengthened PR interval.”

Related Video – ECG Rhythm Review – Second Degree Type 1 Heart Block (Wenckebach)

Figure 5.5. Second-Degree Atrioventricular Block, Mobitz Type I

Second-Degree Atrioventricular Block with Four QRS Complexes Followed by Absent Complex

Mobitz type I rarely causes hemodynamic instability, and asymptomatic patients usually require no further treatment. 

Characteristics of a Mobitz Type I Second-Degree AV Block

1. An irregular rhythm with a pattern of grouped beats
2. A slightly lower rate than normal
3. Upright and uniform P waves
4. Some P waves not followed by QRS complexes
5. Progressive lengthening of the PR interval until one P wave is blocked
6. QRS complex < 120 milliseconds

The Mobitz type II AV block is secondary to a disease process involving the His-Purkinje system. Like Mobitz type I, some impulses fail to travel from the atria into the ventricles (see Figure 5.6). A block occurs after the AV node within the bundle of His or both bundle branches. 

There is no lengthening of the PR interval before a QRS complex is dropped. A Mobitz type II block has a higher risk of complete heart block than does Mobitz type I.

Related Video – ECG Rhythm Review – Second-Degree Heart Block (Type 2)

Figure 5.6. Second-Degree AV Block, Mobitz Type II

The P waves are at a constant rate, there’s no PR interval prolongation, and the R-R intervals before the dropped beats are constant.

Characteristics of a Mobitz Type II Second-Degree AV block 

1. Regular or irregular R–R interval 
2. Regular P–P interval
3. Slower than normal rate (< 60 bpm), often one-half to one-third of the normal rate
4. Upright and uniform P waves 
5. More than one P wave for every QRS complex
6. Constant PR interval across the strip until the QRS complex is completely dropped 
7. PR interval > 200 milliseconds
8. QRS complex < 120 milliseconds