AV blocks happen when the electrical current that is being transmitted from the atrium to the ventricles is held in the AV node for a little too long or so long that it’s not even allowed to leave at all (i.e. it’s completely blocked).
1st Degree AV block
In first degree AV block, the electrical activity from the atria that is being transmitted through the AV node is delayed longer than it should but all of the electrical activity is delayed for the same amount of time and all are allowed to flow down to the ventricles. On the ECG, this is shown as a prolonged PR segment or prolonged PR interval.
2nd Degree AV blocks (Type I and Type II)
2nd Degree AV blocks, here.
3rd Degree (Complete) AV Block
Also called Complete heart block
The 3rd degree AV blocks are different from the 2nd degree because the third degree AV block represents a situation where the AV node completely blocks every single electrical activity from the atria from passing through the AV Node. That results in a complete dissociation or complete separation of the electrical activity from the atria from that of the ventricles. When the AV node completely blocks every wave from the atria from being conducted to the ventricles, then a junctional automaticity focus becomes the pacemaker for the ventricles.
3rd degree AV block is characterized by a regular rhythm with complete AV dissociation. Impulses generated by the SA node do not propagate to the ventricles. Two independent rhythms can be noted on the ECG.
*** Again, note that in order to have a 3rd-degree block, none of the p waves should be getting through (i.e. none of the p waves should be conducted). If you have even one p-wave that is conducted, then it can’t be a 3rd-degree block. If none of the P waves get through, then the ventricular rhythm would be some kind of escape rhythm. And the QRS complexes will be regular. If the QRS complexes are not regular, it’s not a 3rd degree AV block.
Advanced or high-degree AV block
High-grade AV block, also called advanced heart block, is a form of second-degree heart block. It is:
- 2nd-degree heart block with a P:QRS ratio of 3:1 or higher, producing an extremely slow ventricular rate.
- Unlike 3rd-degree heart block there is still some relationship between the P waves and the QRS complexes.
- High-grade AV block may result from either Mobitz I or Mobitz II AV block. It’s hard to tell the difference.
- Broad QRS complexes suggest that the cause may be due to Mobitz II block
This is different than “complete heart block, where no sinus node activity conducts through the AV node to the ventricles at all.
Treatment of high-grade AV block is permanent pacemaker implantation.
Educational Videos
PR Interval. It is increased consistently in first degree AV block; Progressively increases in each series of cycles with Wenckebach; totally variable in 3rd degree AV block, and decreased in WPW and LGL
P without QRS response. Wenckebach and Mobitz 2nd degree AV blocks; 3rd degree AV block has independent atrial and ventricular rates.
Reference
Click to access ManagingDysrhythmias.pdf