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:
- 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.
- 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
- 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.
- 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)
From "the ECG made easy", by Hampton (2003), and ECGs shamelessly stolen from Life in The Fastlane without any sort of permission, but in the non-commercial spirit of free education
2012 ACCF/AHA/HRS Focused Update of the 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities A Report of the American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines