ECG Interpretation Review #58 (2nd Degree - 3rd Degree AV Block - AV Dissociation)

The ECGs in Figure 1 and Figure 2 were both interpreted as showing 3rd degree (complete) AV block.

• Do you agree?

Figure 1: Lead II rhythm strip. Is this complete AV block? (Figure reproduced from Case M in ACLS: Practice Code Scenarios-2013-ePub).

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Figure 2: Lead MCL-1 rhythm strip. Is this complete AV block? (Figure reproduced from Case M in ACLS: Practice Code Scenarios-2013-ePub). NOTE - Enlarge by clicking on Figures - Right-Click to open in a separate window.

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INTERPRETATION: We assess both rhythm strips by the Ps,Qs & 3R Approach, looking for: i) Presence (or absence) of P waves (atrial activity?); ii) QRS width (>0.10 sec is wide?); iii) Regularity of the rhythm; iv) Rate; and v) If P waves are present - Are P waves Related to the QRS (Are P waves conducting?).

FIGURE 1: - The QRS complex is narrow. The first 3 beats show sinus bradycadia at ~55/minute. The PR interval then noticeably shortens (ie, the PR preceding beats #4,5, and 6 is clearly too short to conduct!). Thus, there is transient AV Dissociation (since P waves are at least temporarily unrelated to the QRS).

• Beats #7 and 8 represent a junctional rhythm at ~58/minute.
• It is because the sinus rate slows (to 55/minute) that a slightly faster junctional rhythm can take over (beginning with beat #4).
• We see NO evidence of any AV block on Tracing A - because P waves never fail to conduct when given a chance to do so (P waves before #4,5,6 don't have a chance to conduct ...).

FIGURE 2: - shows complete (3rd degree) AV Block. Note that the QRS does not necessarily have to be overly wide for there to be 3rd degree AV block.

• The atrial rate is regular (marched out in Figure 3 with RED arrows).
• There is also a regular ventricular rhythm - but P waves at all points in the cardiac cycle fail to conduct despite having adequate opportunity to do so (P waves "march through" the QRS).

Figure 3: Arrows indicate regular atrial activity that "marches through the QRS" in this patient with 3rd degree AV block (See text).

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AV Dissociation vs Complete AV Block

AV Dissociation is not the same as 3rd degree AV block. The term, "AV dissociation" merely means that one or more P waves is not related to a neighboring QRS. AV dissociation may be transient or permanent. It may be due to pathologic conditions such as 2nd or 3rd degree AV block - or it may be a benign manifestation of the simple fact that the sinus node temporarily slows down and is replaced in its pacemaker function by an appropriate AV nodal escape rate between 40-60/minute.

• Always try to determine the cause of AV dissociation - of which there are three: 1) AV block itself (could be 2nd or 3rd degree AV block); 2) Usurpation - in which P waves transiently do not conduct because an accelerated junctional rhythm takes over ("usurps" - as commonly occurs with Dig toxicity); and 3) Default - in which a junctional escape rhythm takes over by default" (because of SA node slowing).
• Figure 1 is an example of AV dissociation by "default" (slowing of the rate from sinus bradycardia that allows emergence of an appropriate junctional escape rhythm).
• PEARL: In order to confidently diagnose complete AV block - the ventricular rate needs to be slow enough (usually <45-50/minute) in order to guarantee that P waves will occur in all phases of the cardiac cycle, yet still fail to conduct despite having adequate "opportunity" to conduct. This clearly occurs in Figure 3.

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- For more information - GO TO:

• ECG Basics of AV Block (in ECG-2014-ePub) - 

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