Heart blocks: first, second and third degree

These are atrioventricular blocks, arising from some abnormality of conduction between the atria and the ventricle. The PR interval describes this conduction.

The PR interval is not just the conduction through the atria- it is also the AV node, the bundle of His and the Purkinje fibers. But, most of the time when this is prolonged, it's the AV node ( in most cases with a PR of 200-300 msec). First degree and Mobitz type 1 are usually AV nodal processes; Mobitz type 2 and third degree heart block are typically due to disease below the AV node.

Previous SAQs involving these phemomena include the following:

• Question 30.3 from the first paper of 2015 (2nd degree heart block, Mobitz type I)
• Question 30.4 from the second paper of 2012 (complete heart block)

Causes of atrioventricular block:

• Increased vagal tone, eg in athletes, or during sleep
• Idiopathic progressive conduction system disease: 50% of cases. One example Is Lenegres disease
• Ischaemic heart disease: 40% of cases
• Cardiac surgery
• Inherited familial AV block
• Drugs
  • Verapamil
  • Digoxin
  • Amiodarone
  • Adenosine
  • Beta-blockers
  • Quinidine
  • Procainamide

First-degree Heart Block:

-  The PR interval is prolonged : more than 200msec ( 1 large square)
-  This means conduction through the AV node is slowed.
-  This is a benign entity and may not need any treatment.
-  Atropine may resolve this: blocking the vagal input into the AV node will likely speed conduction through it.
-  It is frequently due to drugs, like calcium channel blockers and cholinergic drugs. 

Second-degree Heart Block  … Mobitz Type 1 (Wenckebach)

- The PR interval is prolonged progressively over several beats, until the beat is missed. 
-  ie. A P wave occurs which is not followed by a QRS
-  This means conduction through the AV node is slowed.
-  This is a benign entity and may not need any treatment.
-  Atropine may help 

Second-degree Heart Block  … Mobitz Type 2 

-  The PR interval is NOT prolonged progressively;
-  A beat is missed every few beats with a normal PR interval.
-  This means conduction through the Bundle of His or the Purkinje fibers is slowed.
-  It is typically a prelude to either Third Degree Heart Block, or asystole.
-  Atropine probably wont do anything.
-  It requires a pacemaker. 

Third-degree Heart Block  … complete heart block

-  There is no relationship between the Pwaves and the QRS complexes.
-  This is called “AV dissociation”. The AV node is not conducting anything.
-  Atropine will not accomplish anything here.
-  It requires a dual-chamber pacemaker.
(image stolen from LITFL)