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Third-Degree AV Block


Complete Heart Block

Third-degree AV block or complete heart block is the most clinically significant AV block associated with ACLS. Complete heart block occurs when the electrical impulse generated in the SA node in the atrium is not conducted to the ventricles.

When the atrial impulse is blocked, an accessory pacemaker in the ventricles will typically activate a ventricular contraction. This accessory pacemaker impulse is called an escape rhythm.

Because two independent electrical impulses occur (SA node impulse & accessory pacemaker impulse), there is no apparent relationship between the P waves and QRS complexes on an ECG.

Characteristics that can be seen on an ECG include:

  1. P waves with a regular P to P interval
  2. QRS complexes with a regular R to R interval
  3. The PR interval will appear variable because there is no relationship between the P waves and the QRS Complexes

3rd Degree Block Diagram ECG Rhythm Strip

In the image above note that the p-waves are independent of the QRS complexes. Also note the 4th QRS complex (impulse) looks different from the others. This is because it is from a different accessory pacemaker in the ventricle than the other QRS complexes.

Common Causes

The most common cause of complete block is coronary ischemia and myocardial infarction. Reduced blood flow or complete loss of blood flow to the myocardium damages the conduction system of the heart, and this results in an inability to conduct impulses from the atrium to the ventricles.

Do you know the difference between myocardial infarction and myocardial ischemia? See answer here.

Those with third-degree AV block typically experience bradycardia, hypotension, and in some cases hemodynamic instability.

The treatment for unstable third-degree AV block in ACLS is transcutaneous pacing.

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

Myocardial infarction is a permanent blockage of an artery of the heart that can rapidly lead to tissue death (heart attack). Myocardial ischemia is a temporary blockage or partial blockage that results in reduced oxygen supply to the heart. This reduced oxygen supply to the heart causes chest pain (angina).

  49 Responses to “Third-Degree AV Block”

  1. I would like add that in some cases you can see block A/V with nodal response and the complex QRS y narrow, means nodal escape, th rhythm is from the A/V NODE, it is another type and some authors named third block. And make the difference with named COMPLETE block when you see wide complex of QRS, means that rhythm is taken by Purkinje cells. the second one it is worst. Always the treatment must be definitively pacemaker

  2. Can you please explain transcutaneous pacing?

    • Transcutaneous pacing is performed by placing pads on top of the skin (transcutaneous). These pads conduct an electrical current that stimulate the heart to contract.
      Kind regards,

  3. I would have expected the QRS complexes to be wider in 3rd degree heartblock since they are generated below the atrium.

  4. I am a new nurse and am struggling with looking at rhythm strips. I watched the video on this site about recognizing 3rd degree heart block on a monitor. On some of the examples it looked like the P-P interval was not regular but the video said they were. Sometimes the monitor labeled a P wave before a T wave, or the P wave must have been buried in the T wave, so I was looking at the wrong distance and the P-P looked irregular. Is there an easy way to tell the difference b/w a T wave a P wave when looking at a strip? I understand the facts about what each one is, I just sometimes have a hard time actually trying to distinguish them when I am looking at a strip. I hope I made sense. Thanks for your help!

    • I just saw your comment from 2012 regarding this subject and it was helpful. However, I still am a little confused. Can your P wave be hidden in a T wave or a QRS? If you are measuring the distance b/w P waves and so far it’s regular, then you come across a QRS and don’t see a P wave, is it safe to assume that the P wave is hidden in the QRS and that your P-P distance is in fact regular? (The same with P waves being hidden in a T wave?) Thanks!

      • 3rd degree block is probably the most difficult rhythm to distinguish. One thing that may make it easier is that a patient in 3rd degree block for long will most likely have symptoms. If you see p-waves that do not match up with QRS complexes and the patient is symptomatic, suspect 3rd degree block until proven otherwise.

        Also with 3rd degree block, the QRS will be wide. This is because the impulse is being generated in the ventricle.

        If the t-wave is visible, it will always directly follow the QRS. When p-waves are disassociated they can be buried within the QRST complex.

        If the p-waves are disassociated and seem to be marching along and then you don’t see one because of the QRST complex, you can assume that it is buried within the complex.

        Kind regards,

  5. I am having a hard time with second degree block type ll and third degree blocks can you give me some pointers on how to tell them apart.?

    • 2nd degree block has the following characteristics:

      1. non-conducted p-waves (electrical impulse conducts through the AV node but complete conduction through the ventricles is blocked, thus no QRS)
      2. P-waves are not preceded by PR prolongation as with second-degree AV block (Type 1)
      3. When there is a p-wave and a QRS, they are coordinated.
      4. fixed PR interval

      3nd degree block has the following characteristics:

      1. P waves with a regular P to P interval
      2. QRS complexes with a regular R to R interval
      3. The PR interval will appear variable because there is no relationship between the P waves and the QRS Complexes

      Kind regards,

  6. Stable complete heart block, is it indicated for transcutaneous pacing? Pt had no complaint of anything but hr in high 20s and mid 30s, freq pvcs, st depression on 12lead but vitals remain stable and as symptomatic. Can we start pacing? Or wait till pt is symptomatic?

    • I have the TCP at the bedside hooked up and ready to start. Also, I would consult a cardiologist and get them to see the patient stat. It is very unlikely that this situation will remain stable for a long time.
      I would be ready for TCP.

      Kind regards,

  7. Is it correct to assume that only Morbitz 11 and 3rd degree block are the only HB that require treatment,and not to worry about the other 2 blocks ?

    • That is correct. Mobitz II and 3rd degree block are usually the only rhythms that are of concern because they can lead quite rapidly to cardiac arrest.
      Other rhythms may need to be worked up, but they are usually not of emergent concern.

      Kind regards,

  8. I am having difficulty rapidly differentiating between 2nd degree type I heart block and 3rd degree heart block. Any quick tips? Also what is the difference hemodynamically and treatment choice?

    • Quick tip for differentiating 2nd degree block type 1 and 3rd degree block:
      It can be a little tricky. With 2nd degree block type 1 the R to R interval will not be the same because of the prolonged conduction time. With 3rd degree block, the P to P interval and the R to R interval will be the same as a rule.

      Most likely the best way is that 3rd degree block will be symptomatic and the patient will become unstable.
      2nd degree block type 1 is usually benign and stable.

      Usually no treatment is required for 2nd degree block type 1. If for some reason 2nd degree block type 1 is symptomatic, treat using the bradycardia algorithm as you would for any symptomatic bradycardia.

      Treatment of choice for symptomatic 3rd degree block is TCP.

      Kind regards,

      • Sir jeff if its 3rd degree HB and symptomatic which one should be given first? atropine or trancutaneous pacing? i encountered this question in a pretest and i answered transcutaneous because at this rate atropine wld be ineefective but it says my answer is wrong and the answer is atropine, it says in the book atropine is given in all symptomatic case of bradycardia… w/c is w/c?

      • I think the key word is symptomatic. Symptomatic does not mean unstable. If a patient with 3rd degree block is symptomatic but stable then the use of atropine would be the right choice. In the case of a patient who is unstable with 3rd degree block, transcutaneous pacing (TCP) should not be delayed. Atropine may be used if it does not delay TCP.
        I hope this makes sense.

        Kind regards,

  9. what is ment by “p waves not part of QRS complexes”

    • This means that the atria and the ventricles are not working in a coordinated fashion as they should. Usually the firing impulses are transferred from the atria to the ventricles and the heart coordinates so that the blood is pumped efficiently.
      With 3rd degree heart block, the electrical impulses and subsequently, the muscle contractions are not coordinated so this can lead to a life threatening decrease in cardiac output.

      Kind regards,

  10. I am getting conflicting info regarding whether atropine is indicated for a 3rd degree block. Since atropine is nodal, wouldn’t it be ineffective in complete heart block?



    • You are correct about atropine being ineffective for 3rd degree block. In most cases it will have no effect. It most likely will not produce any negative effects as well so giving it would not hurt unless giving it slows down the process of implementing TCP. Do not delay TCP in unstable 3rd degree block.

      Kind regards,

  11. I’m an old (61 years) ‘babe in the woods’ with this stuff. I went through several ER admissions to the telemetry ward for tests and evaluations to determine the causes(s) of syncope incidents. On the 3rd occasion I underwent an EP study and the implantation of a Medtronic ECG device. 10 days later (another trip to the ER) a Medtronic Tech printed out the EKG strips wherein it was determined that I had complete heart block. Lo & behold; I went into complete heart-block again that same evening and had the lovely experience provided by an external pacemaker. I thought sure I was about to jump out of my skin when the 1st jolts arrived. I was prepped for emergency surgery and thought I had arrived at the morgue until a kind Nurse pulled back a blanket from my foot and said: “Nope! No toe tag”! I admired her sense of humor. I don’t remember anything else till I awoke several hours after the surgery to implant a pacemaker and discovered my left arm in a sling. I thank ALL of you for the efforts you make to allow folks like me to survive. THANK YOU ALL KINDLY!

    • Thank you for contributing to our education and for allowing people like us to participate in your care.

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