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Now let’s look at atrioventricular blocks. The first one is called 1st degree. An atrioventricular or AV heart block, usually caused by a delay, sometimes absent and inconsistent electrical conduction pathway traveling through the AV node. These AV blocks are described as a first degree which has a prolonged PR interval beyond .20 seconds, the rhythm is regular with a normal or slow rate. P-waves are present and upright with a PR interval greater than .20 seconds. The QRS complex is between .06 and .11 seconds and the P-wave to QRS ratio is 1 to 1. There is usually a minimal clinical significance with this form of heart block. A second degree, otherwise called Mobitz type 1 block which is characteristic of having a progressively widening PR interval which is then followed by a QRS complex that is progressively delayed at the AV node until completely is absent altogether which then only shows a P-wave but no QRS following it. So, in interpreting the ECG we see that the rhythm with this type of second-degree block is regularly-irregular. The rate is normal or slow and P-waves are present and upright. The PR interval is progressively lengthening until a QRS complex is dropped altogether. The QRS complex is between .06 and .11 seconds and the P-wave to QRS ratio is 1 to 1 until a P-wave is blocked. It is commonly caused by a heart disease affecting the AV node, vagal stimulation oftentimes associated with a difficult bowel movement or coughing fits or in certain cases medications. Now the third type is called Second Degree Mobitz Type 2 and usually occurs when the heart block is below the AV node. Characteristically the ECG appears to have intermittent blocks where some P-waves do not have QRS complexes following. There is no elongation of the PR interval. In this case of the Type 2 heart block, the rhythm is variable depending on the P to QRS ratio. The rate is variable but will usually be slow, while P-waves are present and upright the PR interval will be between .12-.20 seconds of the normally conducted complexes. QRS complex is .06-.11 seconds while the P-wave to QRS is variable as well and can be seen as 2 to 1, 3 to 1 or even as much as 4 to 1 and beyond. This Type 2 heart block is usually from a more advanced and severe heart disease process and it can originate from damage that is below the Bundle of His. Because of this, Mobitz Type 2 can deteriorate more quickly into a symptomatic dysrhythmia and even become a third degree heart block. And now lastly, the grand-daddy of all AV blocks, the Third degree complete AV heart block. This happens when electrical conduction is completely blocked between the atria and the ventricles. The exact location of the block can vary but it’s usually somewhere around the AV node or lower but will disassociate the SA pacemaker from the AV or Bundle of His pacemakers thereby creating an ECG that has a regular P-wave, regular QRS waves but they will be at different rates and completely disassociated altogether. In this case the ECG is as follows. Rhythm is regular but the rate is bradycardic between 20-40 beats per minute. P-waves are present and upright while the PR interval is variable with no real pattern. QRS complex is greater than .11 seconds and the P-wave to QRS ratio is variable. Now the clinical significance of this dysrhythmia is serious. The patient will be usually symptomatic and unstable due to the very slow bradycardic rhythm. Remember, the block is stopping any pace that would be originated from the SA node, therefore, the ventricular pacemaker will stimulate a pulse rate closer to 20-40 beats per minute which is usually not enough to keep a stable blood pressure. This is why the ECG will usually have wide QRS complexes. Studies have shown that Third Degree heart blocks may be transient or permanent depending on the underlying cause.
In this lesson, we're going to look at the four types of atrioventricular blocks, usually called AV heart blocks or AV blocks for short. The four types are:
We'll include an example ECG for each, so you can see the differences, while also reading about those differences.
First-degree heart blocks are usually caused by a delayed, inconsistent, and sometimes absent electrical conduction pathway traveling through the AV node and can exhibit the following signs on an ECG readout.
*1st Degree AV Heart Block ECG for Patient
There is usually little to no clinical significance with this type of heart block.
Second-degree heart blocks, also known as Mobitz type 1 AV blocks, is commonly caused by:
An ECG for a patient with Mobitz type 1 will exhibit the following signs.
*2nd Degree (Mobitz type 1) AV Heart Block ECG for Patient
Pro Tip #1: The QRS complex will become progressively delayed at the AV node until it completely disappears. When this happens, the ECG will only show a P-wave but no QRS following it.
The third type of heart block is regularly known as a Mobitz type 2 block. It usually occurs when the heart block is below the AV node. A Mobitz type 2 block is usually caused by more advanced heart disease and can also originate from damage below the bundle of His.
Because of this, Mobitz type 2 can deteriorate more quickly into a symptomatic dysrhythmia and could eventually become a 3rd-degree heart block.
An ECG for a patient with Mobitz type 2 will appear to have intermittent blocks where some P-waves do not have a QRS complex following, and there's typically no elongation of the PR interval.
*2nd Degree (Mobitz type 2) AV Heart Block ECG for Patient
The fourth and last type of heart block is called a 3rd degree complete AV heart block and is the most serious of the four.
A 3rd-degree heart block occurs when the electrical conduction is completely blocked between the atria and the ventricles. The exact location of the block can vary, however it's usually around the AV node or lower but will disassociate the SA pacemaker from the AV or bundle of His pacemakers.
Pro Tip #2: When this happens, a 3rd degree AV heart block will create an ECG readout that shows regular P-waves, regular QRS waves, but they'll be at different rates that are completely disassociated altogether.
An ECG for a patient with a 3rd-degree heart block will exhibit the following signs.
*3rd Degree AV Heart Block ECG for Patient
The clinical significance of this type of dysrhythmia is serious. The patient will usually be symptomatic and unstable due to their very slow bradycardic heart rhythm and rate.
Pro Tip #3: This type of heart block is preventing any pace that originates from the SA node. Therefore, the ventricular pacemaker will stimulate a pulse rate closer to 20 to 40 beats per minute, which is usually not enough to maintain a stable blood pressure. This is why the ECG readout will usually display wide QRS complexes.
Studies have shown that 3rd degree AV heart blocks may be transient or permanent, depending on underlying causes.