# First-degree AV Blocks

by Joseph Alpert, MD

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00:01 Now again, as I mentioned before, sinus node dysfunction is much less common than atrioventricular node dysfunction.

00:08 So, the interruption in the electro conduction system usually occurs in or near the AV node and there are three levels of block.

00:17 One is benign or relatively benign. One is more serious and one is very serious, and we call them first-degree AV block, second-degree AV block and third-degree or complete heart block and we'll talk about each of these now in more detail.

00:32 So, here, we see first-degree AV block and you can see, look on the top strip.

00:40 There is a - you can hardly see the first P-wave, but let's look at the second beat.

00:45 There's a P-wave, then there's a QRS.

00:48 But look at how long the pause is between the P-wave and the QRS and the next beat as well.

00:54 It's taking a long time for the electrical impulse to get through the AV node.

00:59 So, remember that the normal PR interval is 0.2 seconds or five little boxes.

01:05 In this case, the PR interval is actually 9.5 little boxes or 0.38 seconds and we call this first-degree AV block, usually asymptomatic; the patients don't notice it.

01:18 Here is another example of first-degree AV block.

01:21 You'll notice, again, normal PR intervals, are 3 to 5 little boxes, 0.12-0.20 seconds.

01:28 In this case it's a six and a half little boxes or 0.26 seconds, in other words, first-degree AV block.

01:35 Another example, not as severe as the last one.

The lecture First-degree AV Blocks by Joseph Alpert, MD is from the course Electrocardiogram (ECG) Interpretation. It contains the following chapters:

• AV-Blocks - Atrioventricular Blocks
• First Degree AV-Block

### Included Quiz Questions

1. Prolonged (> 200 ms) PR interval
2. Abnormally fast accessory conduction pathway from the atria to ventricles that bypasses the AV node
3. P waves and QRS complexes that are not rhythmically associated
4. Progressive lengthening of the PR interval until a beat is dropped
5. Dropped beats that are not preceded by a change in the length of the PR interval

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