Second Degree or Third Degree Atrioventricular Block?

Take another look at the rhythm strip from the current case study, “54 year old female CC: BLS Intercept”:

Initial Strip

As many readers have pointed out, there exists some level of atrioventricular block. The question then is, which one is it: is it a second degree AV block or a third degree AV block?

In order to answer this question, we should start with the basics.

Conduction System of the Heart

The conduction system of a normal heart begins in the right atrium at the sinoatrial, or SA, node. From here, the impulse travels through the internodal pathways (and to the left atrium through Bachmann’s Bundle) before reaching the atrioventricular, or AV, junction located inside Koch’s Triangle.

It is at this point that our case picks up.

Inside the AV junction is the AV node, a compact, specialized area of myocardium responsible for delaying impulses traveling from the atria to the ventricles. This ensures the atria and ventricles operate in a coordinated fashion. On a surface ECG this delay is realized, in part, through the length of the PR interval illustrated below:

Components of the Surface ECG

After an impulse leaves the AV node, it travels through the Bundle of His and on through to the Right and Left Bundle Branches, finally terminating in the ventricles with the Purkinje Fibers.

When the AV node becomes ischemic, injured, diseased, or otherwise compromised, the conduction system becomes impaired. Collectively, these impairments of the AV node are referred to as Atrioventricular Blocks. There are three degrees of atrioventricular blocks, each representing an increasing amount of AV node impairment.

Before we return to the case study, we will cover the rules for differentiation of atrioventricular blocks on the ECG:

A 1° Atrioventricular Block is not a block, per se, but rather an increased delay in the AV node. On the surface ECG this is realized through a prolonged PR interval. Otherwise this block has no other effect on the ECG.

1° Atrioventricular Block
Rate: Unaffected
Regularity: Unaffected
P-Waves: Unaffected
PR Interval: Prolonged, >0.2 s
QRS Duration: Unaffected

2° Atrioventricular Blocks are broken into two types, each representing an increasing level of impairment in the AV node. In a Mobitz Type I, or Wenkebach, block the delay at the AV node gradually increases until conduction to the ventricles is blocked, resulting in a missing QRS complex. Recognition of Wenkebach conduction requires at least 3 P-waves and 2 QRS complexes in order to identify the progressive lengthening of AV nodal delay.

2° Atrioventricular Block: Mobitz Type I (Wenkebach)
Rate: Depends on the underlying atrial rate
Regularity: Regularly irregular
P-Waves: More P-waves than QRS complexes, associated with each conducted QRS complex
PR Interval: Progressively lengthens until a QRS complex is dropped
QRS Duration: Unaffected

In a Mobitz Type II block, the AV node fails to conduct impulses through to the ventricles, resulting in missing QRS complexes. However, the AV nodal delay is not impacted, resulting in a stable PR interval for all associated P-waves.

2° Atrioventricular Block: Mobitz Type II
Rate: Depends on the underlying atrial rate and the ratio of conducted complexes
Regularity: Regular, regularly irregular, or irregular (based on the ratio of conducted complexes)
P-Waves: More P-waves than QRS complexes, associated with each conducted QRS complex
PR Interval: Unaffected
QRS Duration: Unaffected

Finally, a 3° Atrioventricular Block is a complete heart block. The AV node no longer allows atrial impulses to conduct through to the ventricles. Atrial activity proceeds independent of ventricular activity, known as AV disassociation. The rate of ventricular activity depends on the escape rhythm present. If the block is high enough in the AV node, a junctional rhythm may exist with narrow QRS complexes. If the block is lower in the AV node, a ventricular rhythm may exist, with wide QRS complexes.

3° Atrioventricular Block: Complete Heart Block
Rate: Atrial rate will be independent of ventricular rate. V-rate depends on escape. Junctional rhythms 40-60, ventricular rhythms 20-40.
Regularity: Atrial rate will be regular, ventricular rate will be regular.
P-Waves: Present and disassociated from the ventricular activity
PR Interval: Non-existent
QRS Duration: Narrow (≤0.11s) with junctional escape rhythms, wide (>0.11s) with ventricular escape rhythms

With these rules in mind we can analyze the rhythm strip from the case study, marching out the P-waves and the QRS complexes:

Atrial and Ventricular Activity Highlighted

Rate: Bradycardic ventricular rate of ~45 bpm
Regularity: Regular P-P, Regular R-R
P-Waves: Rounded, upright, symmetrical; rate of ~100 bpm; no apparent association with QRS complexes
PR Interval: Non-existent (appears to lengthen and shorten erratically)
QRS Duration: Narrow, 0.1s

Given these findings and the rules we covered before, this rhythm is a sinus tachycardia with a 3° atrioventricular block and a junctional escape rhythm.

When working to differentiate between the atrioventricular blocks, it is important to follow your rules and march out the atrial and ventricular impulses in order to make an accurate determination of the rhythm.

See also: Rhythm Challenge #3


  • I was instructed, when noting a block, to also classify the two rhythms as you did, but AFTER the notation of the block. For example: Monitor shows a third degree heart block, sinus tach 120 and Junctional escape at 30.

    Is this still practiced or is it as long as we mention both rates/rhythms instead of just “3rd AV block.”

    Thanks for the refresher!

  • I am not aware of any preferred order or practice, however, from a Human Performance standpoint you’re spot on!

    The most important qualifier should come first, in order to get your point across, especially over the radio.

    I wouldn’t want to say, “sinus tach <garbled> ETA of 5, do you have any questions?” The receiving facility may get the wrong impression.

    My classification of the rhythm, “sinus tach with 3AVB and junctional escape,” is merely a natural progression of the findings on the surface ECG. I’m an engineer, I like order to things 🙂

  • Baron says:

    My only quibble is not so much the terminology order as it is the terms themselves.

    Tachycardia, by definition, is a rapid HEART rate. I take that to mean the rate controlling the pulse.

    So to me, sinus tachycardia implies a fast pulse. And thus terming sinus tachycardia for an actually bradycardic pt just seems counter productive. I think I used atrial paces, but after consideration I believe “sino-atrial rate” is most accurate.


  • Baron,

    Reminds me of nomenclature for “ejection fraction” and “stroke volume”, where unless otherwise stated they intend that these are for the left ventricle.

    I know when I write down the details of my interpretation I will separate the A-rate and V-rate whenever I have an AVB. However, when I list the written interpretation I use the general terms for each active foci (hence, “sinus tachycardia with 3AVB and a junctional escape at 45”).

    The nurse likely wants to know the pulse rate and if they need a monitored bed. The ED doc might want a little bit deeper. The cardiologist certainly will want a full interpretation. I don’t think there is a right answer, but rather one of preference. Whichever helps you be a better patient advocate!

  • VinceD says:

    Christopher, nice summary, and it seems your case has garnered a boatload of comments; well done!

  • RN says:

    I prefer referring to it as “the atrial rate”, or as Baron said “the sino-atrial rate”.

  • Chris T says:

    I almost feel like this lesson was for me! very helpful everyone thank you!

  • Chris,

    This was as much for you as it was for all of us (I obviously include myself in this, found a great article on Koch’s Triangle)! Any paramedic who would pass up an opportunity to review a topic in medicine will likely fall into a special category of paramedics. Kelly Grayson refers to them as having, “1 year of experience repeated 20 times.”

    Keep up the good work!

  • Chris T says:

    Thank You Christopher, Look forward to picking your brain 🙂 I will always take every opportunity to improve. Thank you for your help, time, and interest in everyones success!

  • Brian L says:

    Christopher,I have found over the years that providing too much information can sometimes be worse than providing too little information. Having worked from Cardiac nursing units to Paramedic, the atrial vs. ventricular rate has only been used on the yearly ECG tests. When communicating heart rhythms, I have found that as soon as a person hears “Sinus” in the rhythm interpretation, the perceived urgency of the situation drops drastically. Even when communicating critical rhythms in a CCU, the addition of the escape rhythm in a third degree block usually slowed the initial treatment of the patient.
    While knowing that the SA node is functioning normally and knowledge of the origin of the escape rhythm is a good indicator of potential stroke volume and cardiac output is intellectually and academically important, the important information to convey would be degree of heart block, ventricular rate and pulse.

  • Brian,

    Agreed. My radio call-in would consist of, “No palpable radials, monitor shows 3rd degree block with junctional escape at 45.” Although I find it interesting that the CCU would fumble with in-depth rhythms, but c’est la vie. Thanks!

  • Melissa Parker says:

    Despite the physiological explanation behind 3rd AVB, I was taught two things… “The SA and AV nodes beat to a different drum” and, the P wave tends to fall on the R in just about every 3rd degree. Of the ones I have seen in the field, it has shown that every time. For example on this strip, look at the second complex in the second strip, the single lead. Now, I realize that you can have a 2nd type two and have similiar display, however you notice in the 2nd type one, there is no correlation of the p waves. They are “going going gone.” I find this an easy way of determining the 3rd AVB. With the above considering. Immediate TCP.

  • Melissa Parker says:

    Also, as mentioned, marching out the p waves and complexes… I forgot to confirm that.

  • Lieantan says:

    Everyone who know rhythum interpretation will know what is 1st degree AV Block, 2nd degree AV Block type I and type II as well as 3rd degree AV Block or complete heart block. One mention of complete heart block, means there is an urgency for pacing. But if this terminology changed to sinus tachycardia with escape junctional rhythum, the urgency might not be there! Too detail it made things complicated. Why not make life simple and easy way to interpret this life threatening rhythum! Don’t confused the new comer who just try to learn how to interpret ECG!

  • Kaden says:

    My hat is off to your asttue command over this topic—bravo!

  • Jason Grady says:

    The other thing to look at is that from leads II and aVF it is an inferior STEMI. An inferior STEMI is caused by an occlusion to the posterior descending artery. In most instances this artery is a branch of the right coronary artery (RCA). The RCA is also responsible for supplying a large amount of blood flow to the SA and AV node. With a proximal occlusion to the RCA you can anticipate bradycardia and high degree heart blocks. 

  • Nancy says:

    Third degree block.

2 Trackbacks

Leave a Reply

Your email address will not be published. Required fields are marked *