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 often times
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.