This page provides an introduction to Heart Block rhythms with links to our free 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 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
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
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
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
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.