Introduction to Heart Block Rhythms


This page provides an introduction to Heart Block rhythms with links to our lessons and drills.

Heart block rhythms occur when the cardiac electric impulse is delayed or blocked within the AV node, bundle of His or the Purkinje system. Heart block rhythms are classified into categories including these:

  • First Degree Heart Block
  • Second Degree Heart Block Type I
  • Second Degree Heart Block Type II
  • Third Degree Heart Block
  • Bundle Branch Block

Heart Block Categories

First Degree Heart Block

First Degree Heart Block EKG tracing

First degree heart block is actually a delay rather than a block. It is cause by a conduction delay at the AV node or bundle of His. This means than the PR Interval will be longer than normal (over 0.20 sec.).

Second Degree Heart Block Type I

Second Degree Heart Block Type I EKG tracing

With second degree heart block, Type I, some impulses are blocked but not all. More P waves can be observed vs QRS Complexes on a tracing. Each successive impulse undergoes a longer delay. After 3 or 4 beats the next impulse is blocked. On an EKG tracing, PR Intervals will lengthen progressively with each beat until a QRS Complex is missing. After this blocked beat, the cycle of lengthening PR Intervals resumes. This heart block is also called a Wenckebach block.

Second Degree Heart Block Type II

Second Degree Heart Block Type II EKG tracing

With Mobitz Type II blocks, the impulse is blocked in the bundle of His. Every few beats there will be a missing beat but the PR Interval will not lengthen.

Third Degree Heart Block

Third Degree Heart Block EKG tracing

With this block, no atrial impulses are transmitted to the ventricles. As a result, the ventricles generate an escape impulse, which is independent of the atrial beat. In most cases the atria will beat at 60-100 bpm while the ventricles asynchronously beat at 30-45 bpm.

Bundle Branch Block

Bundle Branch Block EKG tracing

With this conduction block, either the left or right bundle branch is blocked intermittently or fixed. The QRS complex is wider than normal (> 0.12 sec.). Using a 12 lead EKG, blocks in either the left or right bundle branch may be diagnosed.

Lessons and Drills

Heart Block Rhythms Training

We recommend starting with our heart block rhythms training module. This module focuses on the morphologic features and qualifying criteria of heart block rhythms.

Heart Block Rhythms

EKG Rhythm Tests

Hundreds of heart rhythms. Tests can be tailored for specific learning needs.

EKG Rhythm Tests

; False
An error has occurred. This application may no longer respond until reloaded. Reload 🗙