03.12 2nd Degree AV Heart Block Type 1 (Mobitz I, Wenckebach)

Hey guys, in this lesson we are going to talk about 2nd degree AV heart block type 1. So are 2 types of 2nd degree blocks, there is type 1 and type 2. So in this lesson we are going to cover type 1, type 1 has a couple other names, it is also called Mobitz I or Wenckebach. Named after the people that discovered them and here to make our life hard, but we are going to keep it simple so at the end of these 2 lessons you are going to be able to identity the difference between these 2 blocks. 2nd degree AV heart block type 2 is also called Mobitz II, so let’s talk about 2nd degree AV heart block type 1, also called Mobitz I and also called Wenckebach.

So in 2nd degree AV heart block type 1 the atrial impulses have difficulties reaching the ventricles, this is because the AV node is defective. Sometimes the AV node delays the impulse just like in 1st degree block, the difference is sometimes it gets by regularly and other times it takes longer until it drops a beat. It is measured with the PR interval, remember the PR interval is how long it takes the electricity from getting from the atria to the ventricles. Normal PR interval is 0.12-0.20 seconds, in 2nd degree AV heart block type 1, the AV node allows some impulses to get down at a normal speed and others take longer to get to the ventricles and eventually the signal does not make it down to the ventricle so it does not contract and the QRS is dropped. So here the PR interval starts out normal at 0.20 seconds then it goes to 0.28 seconds and then to 0.40 seconds. So the PR interval progressively gets longer until a QRS is dropped. Then it picks back up again and it repeats the cycle. Let’s break down the characteristics and do the 6-step method.

So in step one we need to look at the rhythm and find out if it is regular or irregular. So if we count the boxes from here to here we have 21- 22, here to here 21-22 and here we have 37. So the rhythm is regular and irregular when the QRS is dropped. Now in step 2 we look at the heart rate and we multiply 6 by 10 and get 60 beats per minute. Since it is irregular we need to do count the apical heart rate for a full minute. The 1500 method would not work. Ok now in step 3 let’s look at the P:QRS ratio. Do we have 1 P wave followed by 1 QRS and for the most part we do, except for when we drop the QRS. So our P:QRS ratio varies when the QRS is dropped. In step 4 we need to look at the PR interval. So let’s count this one first, and we have about 5 boxes so that would be 0.20 seconds. In this PR interval we have about 7 boxes so it would be 0.28 and here to here we have 10 boxes so we have 0.40 seconds. So you can see how the PR interval gets longer with each beat until a QRS is dropped. This is the biggest characteristic of 2nd degree AV heart block type 1. Now in step 5 we need to look at the QRS complex, and we have about one and a half boxes or 0.06 second. In step 6 we identify the rhythm and we have 2nd degree AV heart block type 1. So let’s recap the characteristics, the rhythm is regular and irregular when the QRS is dropped, the rate varies depending on the underlying rhythm but because it is a block it will be normal or bradycardic. The P:QRS ratio varies, we have a 1 to 1 until we drop a QRS. The PR interval gets longer with each QRS until a QRS is dropped. The best way I learned this rhythm in nursing school was to picture a person jumping and singing wencke, wencke, bach, so with each wencke the person was jumping wider and wider and then when the bach came a long, they fall! Another example is longer longer longer, drop, now you have a wenckebach. Hey whatever helps you remember that the PR interval just gets longer and longer until it drops and after it drops a QRS it starts the cycle over again.

So when patients present with 2nd degree AV heart block is relatively a harmless rhythm so when they come in they are usually asymptomatic. The main nursing interventions are to continue to monitor them if they are asymptomatic and stable and know the underlying rhythm. If you can determine the cause and treat it. Some causes can be from an electrolyte abnormality or medications like digoxin toxicity or an MI. So always find out the cause and try to treat it, and lastly if they are symptomatic with bradycardia follow the ACLS protocols.


So the key points from this lesson are to remember the abnormalities of 2nd degree AV heart block type 1 also called Mobitz I or Wenckebach, so the AV node is defective and it delays the impulse from the atria to the ventricles so the PR interval progressively gets longer and longer until a QRS is dropped. Nursing interventions are to continue to monitor the patient since most people are asymptomatic. Treatment measures would be to identify and treat the cause and follow ACLS guidelines if they are symptomatic and bradycardic.

I hope that you guys have enjoyed this lesson and feel more comfortable understanding 2nd degree AV heart block type 1 also known as Mobitz I and Wenckebach. Make sure you check out all the resources attached to this lesson and the cheat sheets to become more familiar and keep looking at different EKG so you feel more comfortable identifying rhythms. Now, go out and be your best self today! And, as always, happy nursing!