The heart beats through a progression of electrical impulses known as conduction. Heart block occurs when these impulses fail to transit from the atria to the ventricles or there is a delay in conduction.
There are four main types of heart blocks, also known as conduction disorders: first degree; second degree, type I; second degree, type II; and third degree.
Recognizing the nuances of each block is essential for correctly identifying the disorder and initiating the correct treatment modality.
Here is a simple visualization technique to help you picture the conduction pathway in your mind and identify the mechanics of the blocked impulse.
The Journey Begins
Imagine that you are an electrical impulse in the heart.
You leave the sinoatrial (SA) node, travel down a path, reach the atrioventricular (AV) node and head on to the bundle branch system. During the course of your stroll along the path, electrical conduction causes the atria and the ventricles to contract.
This normal conduction appears on an EKG tracing as a P wave with a normal PR interval (PRI) of less than or equal to 0.20 seconds, followed by a QRS complex.
First Degree: The Gated Pathway
Now imagine that you leave the SA node and walk along that same path except now there is a gate blocking the way.
Every time you leave the SA node, you must pause to open the gate, walk through it then close it behind you before continuing down the path. You still reach your destination; it just consistently takes you a little longer to get there.
On an EKG strip, your pause to open and close the gate would appear as a lengthened PRI (greater than 0.20 seconds).
This is first degree AV block.
Second Degree, Type I: The Swinging Gate
You, the electrical impulse, leave the SA node and travel along the path. The gate is there but it is wide open and you easily pass through.
The next time you leave the SA node, however, the gate has swung shut a bit, causing you to pause and walk around it in order to continue down the path. Every time you leave the SA node, you find that the gate has swung further and further shut, slowing you down.
With each trip down the path, it takes you longer and longer to pass through the gate and reach the AV node until, finally, the gate is closed and locked. You are unable to reach your destination. The pattern repeats itself and the next time you leave the SA node, the gate is wide open again.
On an EKG tracing, your encounters with the swinging gate appear as a progressively widening PRI that ultimately ends with a missing QRS complex. Remember, the PRI starts out normal but gets longer and longer; the missing or dropped QRS complex represents your inability to pass through the closed and locked gate. The pattern is cyclical and the same each time.
This repeating pattern is second degree, type I block, also called Mobitz I or Wenckebach.
Second Degree, Type II: The Broken Gate
As you, the electrical impulse, leave the SA node, you find that the latch on the gate is broken.
Sometimes the gate is completely open and allows you to pass through, and other times it is completely closed and locked, blocking your journey down the path. There is no consistency. Every time you leave the SA node, you are uncertain as to whether or not the gate will be open or closed.
So what does this look like on an EKG strip?
When the gate is open and conduction occurs, the tracing will show a P wave with a normal PRI, followed by a normal QRS complex. However, when the gate is closed and locked, blocking the pathway, the tracing will reveal a P wave that is not followed by a QRS complex.
Remember, without this complex you cannot measure the distance from the P wave to the QRS, so there is no PRI. The missing or dropped QRS represents the closed and locked gate that prevented you from traveling down the path.
This is a second degree, type II block, also called Mobitz II (but not Wenckebach II).
Third Degree: One Path, Two Impulses
This time when you leave the SA node, things are a little different. You travel along the path and notice that there is another impulse on the path below you. This second impulse enters the path from the AV node or the bundle branch system.
You advance down the path until you bump into the other impulse. Deciding not to speak to or even acknowledge each other, you each independently continue on your way.
On an EKG tracing, you appear as a P wave (atrial contraction) and the second impulse is represented by the QRS complex (ventricular contraction). All of the P waves are consistent in size, shape and location and all of the QRS complexes are consistent in size, shape and location.
However, since there is no relationship between you and the second impulse, there is no relationship between the P waves and the QRS complexes. You do your thing and the second impulse does its thing, meaning that the atria and the ventricles contract independently of one another.
This is third degree heart block, also called complete heart block (CHB) or AV dissociation.
Walk in the Park
There you have it! Keep these visual references in mind while reviewing your EKG strips and you will soon find that recognizing heart blocks is a walk in the park.
Denise Faraone Diaz is a clinical nurse education specialist at Pennsylvania Hospital in Philadelphia.
