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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.

Rate50-2-degree-block-type-1

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.

  55 Responses to “Second Degree Heart Block-Type 1”

  1. 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????

    • 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,
      Jeff

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

  3. 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.

    • 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,
      Jeff

  4. GOOD TO BE ABLE TO REPLAY THIS OVER AND OVER UNTIL YOU GET IT

  5. Love this site..it is very helpful!

  6. 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.

    • 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!

    • Yes, Brenda that is helpful!

    • I teach Going, Going, Gone. It’s a phrase we all know and grew up with.

      • 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.

  7. This makes total sense..It gave me a clear understanding of my strips..thanks so much

  8. This so ROCKS!!!! I can actually make sense of these strips! Big ThanK YOU

  9. (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.

    • Jeff, Thank you for posting this amazing site! It is awesome!!!

      Thank you so much, Jack, for this explanation of where the block can occur! I didn’t realize that the QRS widening or narrowing would be recognized by location in the heart. i really appreciate this explanation! It helps. This is the best ECG Strip site ever!!!

    • Hey jack make it short & sweet. ACLS is already complicated enough. You gave TMI (to much information). Have a great day!

      RC RN, BSN

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