Thursday, March 7, 2013

ECG Interpretation Review #62 (AV Block - Mobitz I - AV Wenckebach - Mobitz II - 2-to-1 AV Block)

Imagine the ECG tracings shown below were obtained from a patient with syncope.
  • How would you interpret the rhythm in Tracings A and B. Is AV block present? If so - what kind?
  • Has the degree of AV block worsened between Tracing A and Tracing B?
Figure 1: Sequential tracings obtained from a patient with syncope. (Figure reproduced from ACLS-2013-ePub). NOTEEnlarge by clicking on FiguresRight-Click to open in a separate window (See text).
Interpretation of Tracing A:
There is group beating. The QRS complex is narrow. Similar-looking P waves occur at a slightly irregular rate of between ~ 60-70/minute (ventriculophasic sinus arrhythmia). The PR interval gradually increases within groups until a beat is nonconducted (the P waves that occur after beats #1 and #3 are not conducted). This is 2nd degree AV block, Mobitz Type I (AV Wenckebach). Of interest – Note the following:
  • The 2 pauses that contain dropped beats are of equal duration (1.78 second for the interval between beats #1-2 and between #3-4).
  • Each pause ends with a P wave that conducts with the shortest PR interval (0.23 second for the PR interval preceding beats #2 and #4 in Tracing A). Progressive PR lengthening then begins anew.
Interpretation of Tracing B:
The QRS complex is again narrow. The ventricular rhythm is fairly regular at a rate just under 40/minute. A slightly irregular sinus rhythm is again present – but this time at a faster rate (range between 80-85/minute). Every-other-P wave conducts – as determined by the presence of a fixed (albeit prolonged) PR interval preceding each QRS complex on the tracing. This is 2nd degree AV block with 2:1 AV conduction.
  • Although fewer beats are conducted and the overall ventricular rate in Tracing B is slower – there has not necessarily been any “worsening” in the degree of AV block between Tracing A and Tracing B.
  • Instead – it may simply be that at the slightly faster atrial rate (of 80-85/minute) – fewer impulses are able to penetrate the AV node.
Brief Review of the 2nd Degree AV Blocks:
It is important to remember that there are 3 types of 2nd degree AV block: i) Mobitz I (AV Wenckebach); ii) Mobitz II; and iii) 2nd degree AV block with 2:1 AV conduction. This distinction is important because the clinical course and recommended management for the various types of 2nd degree AV block is quite different.
  • HINT: Use of a pair of calipers is invaluable for understanding and facilitating diagnosis of the AV blocks. Your ability to recognize the various AV blocks (and distinguish them from “mimics” of AV block) will be instantly enhanced the moment you use calipers ...
  • Mobitz I is recognized by: i) Progressive lengthening of the PR interval until a beat is dropped; ii) Group beating; iii) A regular (or almost regular) atrial rate; and iv) The pause that contains the dropped beat is less than twice the shortest R-R interval. These characteristics are evident in Tracing A. Note that the pause containing the dropped beat (the pauses between beats #1-2 and 3-4) is less than twice the shortest R-R interval (which is the R-R interval between beats #2-3 and #4-5).
  • Mobitz II is recognized by QRS widening and a constant PR interval for consecutively conducted beats – until one or more beats are dropped.
  • By far - the most common form of 2nd degree AV block is Mobitz I. Because Mobitz I 2nd degree AV block usually occurs at the level of the AV node - the QRS complex is typically narrow - the conduction defect may respond to Atropine (especially if due to acute inferior infarction and treated within the first few hours of onset) - and a permanent pacemaker is usually not needed.
  • In contrast - the less common Mobitz II form is more likely to occur at a lower level in the conduction system (below the AV node) - often in association with large anterior infarction. As a result - the QRS complex is likely to be wide - Atropine will usually not work - and permanent pacing is much more likely to be needed.
  • The reason for addition of a 3rd category (= 2:1 AV conduction) – is to acknowledge that because you never see 2 consecutively conducted beats in a row, you cannot tell IF the PR interval would progressively lengthen if given a chance to do so. As a result – one cannot be certain if 2nd degree AV block with 2:1 AV conduction represents Mobitz I or Mobitz II simply from looking at a rhythm strip such as Tracing B.
Clinical PEARLS regarding the 2nd Degree ABlocks:
The 2 sequential tracings in Figure 1 serve to highlight the following important points regarding the 2nd degree AV blocks:
  • The atrial rate should be regular (or almost regular) when there is AV block. It is common to see slight variation (known as ventriculophasic sinus arrhythmia) in the setting of 2nd or 3rd degree AV blocks (as is evident in Tracings A and B). However – marked P wave irregularity – change in P wave morphology – or prolonged sinus pauses all suggest some phenomenon other than AV block is operative (blocked PACs; escape rhythms without AV block; sick sinus syndrome; sinus arrest).
  • The KEY for diagnosing Mobitz II as the type of 2nd degree AV block is that the PR interval remains constant for consecutively conducted P waves. Because you never see 2 consecutively conducted complexes in Tracing B – you cannot tell if the PR interval would increase if given a chance to do so. Therefore – you cannot diagnose Mobitz II for Tracing B – but instead should classify this rhythm as 2nd degree AV block with 2:1 AV conduction.
  • That said – we can make an educated guess that Tracing B is almost certain to represent Mobitz I 2nd degree AV block because: i) Mobitz I is so much more common than Mobitz II; ii) The QRS is narrow; iii) The PR interval of conducting beats is prolonged (far more common in Mobitz I); and iv) Tracing A is definitely Mobitz I – and it is rare to go back-and-forth between Mobitz I and Mobitz II in the same patient.
  • Knowing that Tracing B is almost certain to be Mobitz I would support a trial of Atropine as an initial intervention IF this patient was symptomatic. Whether or not pacing would eventually be needed would be determined by other factors including the cause of the conduction disturbance – how slow the ventricular response remained – degree of symptoms.
  • Although Atropine may be highly effective during the early hours of vagally induced Mobitz I 2nd degree AV block – this treatment is not bengin. In addition to improving AV conduction, Atropine may speed up the sinus rate. This in fact could be what happened between Tracing A and Tracing B – in which case this sequence of tracings would illustrate a potential paradoxical response from appropriate use of atropine. That is, by increasing the atrial rate – use of Atropine may sometimes result in slowing of the overall ventricular response.
  • Remember to assess the atrial rate whenever evaluating patients with AV block. Forgetting to do so may result in overlooking the reason for a change in AV conduction ratios (as occurs here between Tracing A and Tracing B).
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