Introduction to Heart Block Rhythms


This page provides an introduction to Heart Block rhythms and links to training materials on this website.

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:

  • Bundle Branch Block
  • First Degree Heart Block
  • Second Degree Heart Block Type I
  • Second Degree Heart Block Type II
  • Third Degree Heart 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 imulses are transmitted to the ventricles. As a result, the ventricules 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.

Training Resources

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 heart rhythms in this practice test. Test can be tailored for specific learning needs.

EKG Rhythm Tests

Heart Rhythm Quiz also has a heart rhythm quiz that is similar to a written exam. Twenty tracings are presented. Feedback as you progress. Score at the end.

ECG Quiz

ECG Monitor Challenge

Another type of quiz uses a simulated patient monitor. You must evaluate a scrolling waveform rather than a paper tracing.

ECG Monitor Challenge

External References

Free Dictionary