Sunday, May 8, 2011

ECG Interpretation Review #19 (2-to-1 AV Block - Mobitz I vs Mobitz II?)

The Lead II rhythm strip shown below was obtained from a patient during cardiopulmonary resuscitation. Systolic BP was 90 mmHg at the time this tracing was recorded.  Does this represent 2nd degree AV Block, Mobitz Type II?  How to proceed clinically?
Figure 1 - Lead II rhythm strip from a patient in cardiac arrest.
Is this 2nd degree AV block, Mobitz Type II?
(Figure reproduced from ACLS-2013-ePub ).
- Note - Enlarge by clicking on Figures - Right-Click to open in a separate window.
INTERPRETATION:  The rhythm in the Figure 1 is slow but regular. The QRS complex is narrow, and a regular atrial rate is seen at 100/minute (arrows). Every other P wave conducts (as evidenced by the fact that a P wave does precede each QRS complex with a fixed PR interval! ).  Traditionally – the AV blocks are divided into 3 degrees based on severity of the conduction disturbance:
  • 1st degree AV block – in which all atrial impulses are conducted to the ventricles, albeit with delay (so that the PR interval exceeds 0.20 second).
  • 2nd degree AV block – in which some (but not all) atrial impulses are conducted to the ventricles.
  • 3rd degree (or “complete”) AV block – in which none of the atrial impulses are conducted to the ventricles, despite having more than adequate opportunity for conduction to occur.
Second degree AV blocks are further classified into 3 types (Figure 2):
Figure 2 - Types of 2nd Degree AV Block:
A) Mobitz I (AV Wenckebach); B) Mobitz II; and C) 2-to-1 AV Block.
(Figure reproduced from ACLS-2013-ePub ).
- Note - Enlarge by clicking on Figures - Right-Click to open in a separate window.
  • Panel AMobitz I (AV Wenckebach) – in which the PR interval progressively lengthens until a beat is dropped.  This is by far the most common form of 2nd degree AV block. Mobitz I usually occurs at the level of the AV node.  As a result – the QRS complex is typically narrow. Mobitz I is generally associated with inferior infarction; it often spontaneously resolves, and typically responds to atropine (which works on the AV node).
  • Panel B = Mobitz II – in which there is a constant PR interval for consecutively conducted beats until one or more beats are dropped. Because Mobitz II typically occurs low down in the conduction system – the QRS complex is generally wide. This less common form of 2nd degree AV block is generally associated with anterior infarction; it usually does not respond to atropine – and is important to recognize because pacing will probably be needed.
  • Panel C = 2-to-1 AV Block – in which one never sees two consecutively conducted beats, so that you can not tell if the PR interval is lengthening or not.  As a result – it is impossible to know for sure whether this form of 2nd degree AV block represents Mobitz I or Mobitz II.
Returning to this Case: The rhythm in Figure 1 is the same as the one shown in Panel C of Figure 2. Because two conducted beats never occur in a row - we are unable to be certain of the type of 2nd degree AV block. That said  We strongly suspect this rhythm represents 2nd degree AV block, Mobitz Type I (AV Wenckebach) because: i) Mobitz I is so much more common than Mobitz II; and ii) the QRS complex is narrow, as it almost always is with Mobitz I.  Finding additional rhythm strips on this patient that clearly showed progressive lengthening of consecutively conducted QRS complexes before dropping a beat would strongly support our suspicion.  Clinically the distinction is important because no treatment (other than perhaps atropine) is likely to be needed for Mobitz I (especially given that the ventricular rate in Figure 1 is not overly slow at 50/minute). In contrast – pacing would probably be needed if the rhythm was Mobitz II.


   See also this pdf from Section 20.0 of our ACLS-2013-ePub 


  1. Can vagal maneuvers or atropin help to distinguish, in 2-to-1 AV Block, between Mobitz I and Mobitz II?

    1. Thank you Stillcho for your comment. I would not try a vagal maneuver with 2:1 AV block, for fear of further reducing the ventricular response. However Atropine may help both diagnostically and therapeutically. On occasion (especially during the early hours of acute inferior infarction) - Atropine may succeed in resolving the AV block. Atropine is far less likely to work with Mobitz II ... That said - one cannot exclude Mobitz I if there is no response to Atropine, though depending on the setting - lack of response is a factor in favor of Mobitz II.

      That said - QRS width (usually narrow with Mobitz I - wide with Mobitz II) - and the clinical setting (often inferior MI with Mobitz I; anterior MI with Mobitz II) - typically go a long way toward distinguishing between Mobitz I and Mobitz II.

      FINALLY - Please check out my ECG video on AV Block - GO TO -