Third-degree AV Blocks

00:01 Now, third-degree heart block is called complete heart block.

00:04 Complete heart block means the P waves are not ever followed by a QRS that relates to it.

00:12 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.

00:21 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.

00:30 There's a markedly slow heart rate.

00:33 There's a wide QRS unlike type I and type II which have narrow QRSs.

00:38 And they have wide QRSs cuz the impulse is generated in the ventricle.

00:42 And abnormally transmits through the ventricular myocardium and there's no relationship between the P waves and the QRS waves.

00:51 So, here's an example of third-degree AV block.

00:54 Notice, we start off with a QRS, then there's a P wave and there's another P wave.

01:01 And here's a QRS. Oh, and look, there's another P wave afterwards.

01:04 You'll notice these P waves are not in any relationship to the QRS.

01:09 So, in the second complex, you have what could be a normal PR interval.

01:14 But wait a minute, the next one is long and then the next one is short.

01:16 In other words, the P waves are moving at their own rhythm and the QRS is moving.

01:21 One way to tell is to determine the heartrate of the P waves.

01:25 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.