Second Degree Heart Block-Type 1

Second-Degree Heart Block (Type 1)

Also called Mobitz 1 or Wenckebach is a disease of the electrical conduction system of the heart in which the PR interval» has progressive prolongation until finally the atrial impulse is completely blocked and does not produce a QRS electrical impulse.

Once the p-wave is blocked and no QRS is generated, the cycle begins again with the prolongation of the PR interval.

One of the main identifying characteristics of second degree heart block type 1 is that the atrial rhythm will be regular.

2nd Degree Block Type 1 Rhythm Strip with dropped QRS

In the above image, notice that the p-waves are regular, the PR-interval progressively gets longer until a QRS is dropped and only the p-wave is present.

Although second degree heart block type-1 is not clinically significant for ACLS, recognition of the major AV blocks is important because treatment decisions are based on the type of block present.

Below is a short video which will help you quickly identify second-degree AV block type 1 on a monitor. Please allow several seconds for the video to load.

The PR interval is the electrical firing of the atria and conduction of that electrical impulse through the AV node to the ventricles.Powered by Hackadelic Sliding Notes 1.6.5


  1. aba says

    Hi Jeff,
    I’m new to this. can you please explain the difference between Mobitz I and II. All I see is dropped QRS in both!

    • says

      With Mobitz I, the PR interval will get longer and longer and then there will be a p-wave with no QRS.

      Mobitz II, the PR interval will be the same, but there will be p-waves without any QRS complexes.

      That is the easiest way to tell the difference. Look at this page and the page below a couple of times and make sure to watch the videos at the bottom of each page. It will get easier as you learn to recognize the PR interval.

      2nd Degree Block Type 2

      Kind regards,

      • Blonde4fun says

        While I know the definition of Type 2 HB and 3 Degree HB, it is still difficult for me to actually recognize (especially if the monitor is moving the rhythm strip across the screen). Any suggestions of quick recognition would be appreciated. Love your site. So helpful!

      • says

        These can be tricky blocks to recognize. Here is what will do the trick. For a month do this: Each day you work, look at 2 different examples of each rhythm that you want to be able to recognize. Go through the entire interpretation process for each rhythm. You might have to spend 5 or 10 minutes per day. After a month you should be able to quickly recognize each. In this case, practice does really make perfect.

        Kind regards,

  2. says

    thanks for the link on Heart Blocks. I am just beginning my review of the material for ACLS and am reviewing general information first to be able to put the specifics of ACLS in context. This brief link is worth a look

  3. says

    Great info. Great for ACLS and just for overall cardiac knowledge. In regards to blocks, hat is a Bundle Branch Block then? Does it fall in one of these types of categories that you have mentioned?

  4. says

    I had a patient in Mobitz 1 rhythm at a rate of 60. She was frightened “to death” about the test anyway and I told her if she refused the test that was her decision. The doctor didn’t really give her an informed consent because I think he thought it would scare her out of the Lexi more. I told her don’t do it. Her symptoms were very atypical and the doctor told her that he didn’t think it was her heart. She could then see her cardiologist and come back as an outpatient. Actually my question is what is the danger of Lexiscan in this rhythm?

    • says

      The danger would be minimal. As with any test utilizing a medication, there are possible side effects that may be experienced.
      “The most common side effects that occurred in clinical trials of Lexiscan were shortness of breath, headache, flushing, chest discomfort or chest pain, dizziness, nausea, abdominal discomfort, a metallic taste in the mouth, and feeling hot. Most common side effects began soon after receiving Lexiscan and went away within 15 minutes except for headache, which resolved in most patients within 30 minutes.” Quote found here.

      Kind regards,

  5. ls1581 says

    Why is TCP the treatment for Mobitz Type II because a dropped QRS = no circulation but on a Mobitz I, the PR lengthens and results in a dropped QRS but there is usually no treatment needed????

    • says

      TCP is the treatment of choice for Mobitz Type II because this rhythm does not usually respond well to atropine. Also, Mobitz II will usually be associated with symptomatic bradycardia. Mobitz I will not usually be associated with symptoms or an unstable condition (it is usually benign).

      Kind regards,

      • Amber says

        I think I need some clarification too.

        The dropped QRS in Mobitz II = no circulation. Does the dropped QRS in Mobitz I also present the same danger?

        Please help me understand. It seems to me that both are concerning.

        By the way, your site has been very helpful for me. Thank you for your work on this website. I have already recommended it to several colleagues.

      • says

        Unlike Mobitz I, which is produced by progressive fatigue of the AV nodal cells, Mobitz II is an “all or nothing” phenomenon whereby the His-Purkinje cells suddenly and unexpectedly fail to conduct a supraventricular impulse. Mobitz II is much more likely than Mobitz I to be associated with hemodynamic compromise, severe bradycardia and progression to 3rd degree heart block.
        With Mobitz I, after the dropped beat, the process of progressive fatigue and dropped beat starts over. This makes Mobitz I a much safer bradyarrhythmia. With Mobitz II, the conduction can fail and it may no long conduct an impulse (Third degree block). This makes Mobitz II a very dangerous rhythm which must be dealt with as soon as possible.

        Kind regards,

  6. aclsnursewh says

    So helpful when I can replay the rhythms. I am learning quite a bit from watching the videos. Looking forward to more.

  7. okeyi2000 says

    I probably don’t see what others see in this diagram ! where is the dropped beat ,how do you distinguish that from the T wave? The prolongation is clear but can someone show me where we are missing a QRS.

    • says

      The dropped beat is the first p-wave in the diagram and the p-wave that is almost in the middle of the image.
      The 2nd non-conducted p-wave occurs on the t-wave after a QRS.
      Kind regards,

  8. Brenda Biancosino says

    Don’t forget the helpful phrase we all learned in nursing school: “Longer, longer, longer, DROP, then we have a Wenckebach”.
    Studying this weekend to recertify for the first time. Love your site. Thank you for all your hard work to make this an enlightening experience as well as reducing my stress level.
    I have already recommended your site and will continue to do so.

    • Dan says

      Thank you Brenda ! I personally love mnemonics. They free me up from that momentary ” pause” ( no pun intended) in the thought process that usually leads to a complete block ( again, no pun intended) in my review. If you have one for the Glascow Coma Scale that would be fantastic!

      • Albert EMT-P says

        And, this is a great site. I just found it today and will make a note to show my EMT-P students at the college. I know this will help them in more ways than one.

  9. Jack says

    (1) The SA Node is the hearts primary pacemaker. (2) The SA Node generates an impulse which propagates across/thru the atria and to the A/V Node in attempt to cause the stimulation to the ventricles (via the Bundle of His and then the Purkinje Fibers. (3) The A/V Nodes primary function is to figuratively act as a gatekeeper as if there was a little man standing there with his hand on the gate allowing signals to pass thru in such a way that optimizes the hearts contracting as a smooth functioning unit (this ensures the greatest movement of blood thru the heart chambers and sets the stage for the greatest EF from the left ventiricle). (4) In the case of heart block the gatekeeper is (for some physiologic reason) restricting the passage of necessary stimulations from above the A/V Node (in the best case scenario the SA Node) from passing thru his gate so that they can transfer their stimulus to the ventricles. (5) If the QRS’s are narrow the block is in the A/V Node (internodal), if the are QRS’s wide (aberrant conduction) then the block is below the A/V Node and somewhere in the Bundle of His or Bundle branches which is much more serious and could ultimately lead to 3rd degree block and will typically create hemodynamic issues for the patient. The MD will most likely end up installing an artificial pacemaker in the patient to safegaurd the possibility of more serious heart failure.

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