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:

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:

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.



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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4 Responses

  1. I thought there would be verbal explanations of the rhythms. I would have preferred it to be so.

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