Rhythms Review: Is this Second Degree AV Block 2:1 Mobitz I or Mobitz II?

Quick Quiz

Q1: Mobitz II is distinguished by a ____________ PR interval when the P-wave is conducted.

Q2: Mobitz I is distinguished by a _____________ PR interval until a P-wave is dropped.

Q3: True or False: In the presence of a 2nd Degree AV Block 2:1, it is possible to definitively diagnosis Mobitz I versus Mobitz II.  

(See answers at end of post.)

OVERVIEW: 2° AV BLOCK 2:1 —— Is this Mobitz I OR Mobitz II ?

We have previously outlined the diagnostic criteria of Mobitz I and Mobitz II and determined that the key to the diagnosis is the ratio of Ps:QRSs.  Specifically these ratio arrays are demonstrated  in  Figure 1 below.


Figure 1 Ratios of Ps:QRSs 

From Figure 1 it is interesting to note that the special case of P:QRS  =  2:1 is an indeterminate one and the diagnosis could go either way (either Mobitz I or Mobitz II);  you cannot be certain of the diagnosis, but you can at least suggest the probability of the diagnosis.  This special case will be the focus of this lesson.


This lesson will discuss in detail the special case of the 2° AV block with a ratio of 2:1 block.  We will concede that this is an indeterminate case but will conclude with a list of findings suggestive of Mobitz I or Mobitz II.


Second-degree AV block is rarely symptomatic and by itself is not dangerous.  The question is whether second-degree block indicates increased risk for complete AV block and if so whether asystole is likely to occur.

AV node block (localized to the AV node-junction area) rarely progresses to third-degree block and often has a reliable escape rhythm if it does.  If the block is localized to the HP (HIS bundle-Purkinje) level it is more likely to progress to complete block and is more likely to result in asystole.  AV junction-node suppression typically results in Mobitz I block, whereas HP block leads to Mobitz II block.  Therefore Mobitz II block places the patient at high risk for complete heart block and sudden death.  In the presence of 2:1 AV block Mobitz I and II cannot be distinguished; widened QRS and normal PR interval suggest that the block is at the HP level.

PEARL:  Atropine improves AV node conduction.  It has no direct effect on HP conduction but by causing increased sinus rate can paradoxically increase HP block and therefore should be avoided.

Regarding the special case of 2° AV Block with 2:1 conduction, please be aware of the following clinical pearls:

  • In the setting of the acute Inferior MI→→→→→ the 2° AV block 2:1 conduction is likely to be Mobitz I (this most often is a benign self-limiting development and usually requires no treatment).
  • In the setting of the acute Anterior MI→→→→→ the 2° AV block 2:1 conduction is likely to be Mobitz II (this is a life-threatening development and places the patient at risk for complete heart block.  You should therefore be prepared for transthoracic cardiac pacing).
  • If  PR-interval normal (≤ 0.20 sec) →→→→ the 2° AV block 2:1  conduction is likely Mobitz II.
  • If | QRS | > 0.10 sec (i.e. is widened) →→→→→ the 2° AV block 2:1 conduction is highly probable Mobitz II.
  • If Mobitz I →→→ Atropine will increase sinus rate.  The conduction defect might improve and actually resolve.
  • If Mobitz II →→→ Atropine might worsen the block and precipitate complete heart block.  It is therefore a relative contraindication to administer Atropine in the presence of 2° AV block Mobitz II.
  • In the setting of the acute Anterior MI and  2° AV block 2:1 conduction →→→→ the development of a new LAFB is suggestive of Mobitz II.  We will revisit this topic in a future post.

Let us begin this presentation by reviewing the characteristic of the two different 2° AV blocks – Mobitz I and Mobitz II.  First let me remind you of the characteristic rhythm pattern of the P-waves.  This is best illustrated in the form of a rhythm strip as shown in Figure 2 (a & b)

When analyzing a given rhythm strip, I advise you to do the following:

  • Photocopy the original tracing and use this as your working copy.  You will feel free to mark, label, fold, erase, experiment with this copy since it does not become a part of the permanent record.
  • With this copy, I recommend  that you identify all the P-waves as shown in Figure 3.

Now measure all the PR-intervals and determine their relationships with the P-Waves (if there is one), as shown in Figure 4 below.


Now let us consider the special case of 2° AV Block 2:1 (Mobitz I or Mobitz II).  This is depicted in Figure 5 below.  We observe the cyclic pattern of two P-waves for every one QRS complex.  It is interesting to note that if the rhythm were Mobitz I (Wenckebach) you would need at least a P:QRS = 3:2 conduction in order to appreciate the progressive PR-interval phenomena.  But alas the ratio of Figure 5 is observed to be only 2:1.  We must conclude that the interpretation of Mobitz I vs Mobitz II is indeterminate.  In the clinical environment, I ask for a long rhythm strip hoping to find an area of 3:2 or 3:1 conduction indicating a diagnosis of Mobitz I or Mobitz II respectively.  If this is not discovered, you can make the following likelihood of Mobitz I vs Mobitz II:

a…..if PR-Interval  > 0.20 sec→→→→→→ likely Mobitz I.

b…..if |QRS| > 0.10 sec →→→→→→→→ likely Mobitz II.

Now let us consider the rhythm strip below:

Image result for mobitz I,II 2:1

Figure 5  2° AV Block 2 : 1

It is clearly true that the ratio of P-wave to QRS complex is 2 to 1.  This places this rhythm strip in the indeterminate category and the best you can do is to

suggest the probability of either Mobitz I or Mobitz II.


  • If you are unsure of the Rhythm diagnosis then ask for a 2-minute rhythm strip.  This is approximately a 6 foot strip.
  • Unscroll the strip and lay it on the countertop.
  • Inspect the full length of the strip with a magnifying glass (after all this IS a “Pink Panther” job) looking for a pattern OR lack of.
  • Don’t forget to choose the lead that shows the P-wave most clearly.  You should get in the habit of looking at ALL 12-leads, Right precordial and posterior leads. 
  • With both Mobitz I and Mobitz II, the P-waves will march through at a regular cadence (please note there may be very slight variability in the P-wave cadence).
  •  You will be surprised at the enhanced insight achieved by choosing the lead which best shows the P and QRS complexes and printing off a long strip from this lead’s “viewpoint”.


When your analysis of a given rhythm strip reveals 2° AV Block with 2:1 conduction you must then concede that the further categorization of Mobitz I vs Mobitz II is indeterminate.  The next step would be to analyze a long rhythm strip with the goal to discover a P to QRS ratio of 3:2, 3:1, 4:3. – thus making the diagnosis of either Mobitz I or II.

If not found, you are limited to the following conclusion:

a.  If the PR-interval >  0.20 sec. →→→→→ probable Mobitz I

b.  If the |QRS| > 0.10 sec. →→→→→ probable Mobitz II (highly probable)

c.  In the setting of the Acute Inferior MI →→→→→ probable Mobitz I

d.  In the setting of the Acute Anterior MI →→→→→ probable Mobitz II (and is at risk for complete heart block)

Answers: Q1: constant  |  Q2: progressively lengthening  |  Q3:  False

Article:  ECGcourse.com  |  Vernon R Stanley, MD, PhD © 2019 All rights reserved.

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