Now, third-degree heart block is called complete heart block.
Complete heart block means the P waves are not ever followed by a QRS that relates to it.
In other words, the ventricle has a subsidiary pacemaker that takes over it at a very slow rate,
and often, the patients are very symptomatic.
In third-degree AV block, the wave of depolarization from the atria as I said is completely blocked
and there's a subordinate pacemaker in the ventricle taking over.
There's a markedly slow heart rate.
There's a wide QRS unlike type I and type II which have narrow QRSs.
And they have wide QRSs cuz the impulse is generated in the ventricle.
And abnormally transmits through the ventricular myocardium
and there's no relationship between the P waves and the QRS waves.
So, here's an example of third-degree AV block.
Notice, we start off with a QRS, then there's a P wave and there's another P wave.
And here's a QRS. Oh, and look, there's another P wave afterwards.
You'll notice these P waves are not in any relationship to the QRS.
So, in the second complex, you have what could be a normal PR interval.
But wait a minute, the next one is long and then the next one is short.
In other words, the P waves are moving at their own rhythm and the QRS is moving.
One way to tell is to determine the heartrate of the P waves.
In this case, the heartrate of the P wave is 75; that's four big boxes
whereas the rate of the QRS is much slower, it's about 35 to 40.