1) Happen when
a) Atrial depolarization fail to reach the ventricles
b) Atrial depolarization is conducted with a delay
2) Type
a) First degree heart block (AV node level)
- Prolongation of the PR interval on the electrocardiogram (ECG) (>0.20 s in adults and >0.16 s in young children
- All atrial impulses reach the ventricles in first-degree AV block
- Conduction is delayed within the AV node
** can be found in healthy adults, and its incidence increases with age.
b) Second degree heart block
I) Mobitz I second-degree AV block (Wenckebach block)( AV node level)
- Progressive prolongation of the PR interval
- Subsequent occurrence of a single nonconducted P wave that results in a pause
- The pause is shorter than the sum of any 2 consecutive conducted beats (R-R interval)
- An episode of Mobitz I AV block usually consists of 3-5 beats
- Ratio of nonconducted to conducted beats of 4:3, 3:2
- Block is generally in the AV node but can occasionally occur in the His-Purkinje system and is termed infrahisian Wenckebach
**is observed in 1-2% of healthy young people, especially during sleep
II) Mobitz II second-degree AV block (His bundle or lower)
- Constant PR interval followed by sudden failure of a P wave to be conducted to the ventricles,
- Either an occasional dropped P wave or a regular conduction pattern of 2:1 (2 conducted and 1 blocked), 3:1 (3 conducted and 1 blocked)
** rare in healthy individuals
c) Third degree Heart block (V node or the His-Purkinje system)
- No supraventricular impulses are conducted to the ventricles
- P waves on the rhythm strip reflect a sinus node rhythm independent from QRS electrocardiographic wave complexes
- QRS complexes represent an escape rhythm, either junctional or ventricular
- Escape rhythm originating from the junctional or high septal region is characterized by narrow QRS complexes at a rate of 40-50 beats per minute,
- Escape rhythm from low ventricular sites is characterized by broad QRS complexes at a rate of 30-40 beats per minute
- Frequency of P waves (atrial rate) is higher than the frequency of QRS complexes (ventricular rate)
