Third-degree AV Blocks

by Joseph Alpert, MD

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    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.

    About the Lecture

    The lecture Third-degree AV Blocks by Joseph Alpert, MD is from the course Electrocardiogram (ECG) Interpretation.

    Included Quiz Questions

    1. No relationship between the P waves and QRS waves
    2. Dropped beats that are not preceded by a change in the length of the PR interval
    3. Progressive lengthening of the PR interval until a beat is dropped
    4. Consistently prolonged PR interval (> 200 ms)
    5. Abnormally fast accessory conduction pathway from atria to ventricles that bypasses the AV node

    Author of lecture Third-degree AV Blocks

     Joseph Alpert, MD

    Joseph Alpert, MD

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