Sunday, November 27, 2011

ECG Interpretation Review #32 (Bundle Branch Block - Rate Related - Aberrant Conduction - LBBB)

Interpret the rhythm strip shown in Figure 1.  The widened beats on the tracing are not ventricular.  What else might they be?
Figure 1 – Right-sided MCL-1 monitoring lead rhythm strip. 
Why might beats #4-thru-7 be wide? (Figure reproduced from Section 19.0 of ACLS-2013-ePub).  
NOTEEnlarge by clicking on FiguresRight-Click to open in a separate window.
INTERPRETATION:  The rhythm strip in Figure 1 shows the rhythm to be irregularly irregular in this right‑sided MCL-1 monitoring lead.  No P waves are seen – so that the underlying rhythm is atrial fibrillation.  Fine undulations in the baseline represent “fib waves”.  The interesting part of the rhythm strip is intermittent widening of the QRS complex.  
  • Although at first glance one might be tempted to interpret the run of widened beats (beats #4-thru-7) as AIVR (Accelerated IdioVentricular Rhythm) — subsequent rhythm strips proved this not to be the case.  AIVR is often an “escape rhythm” that arises when the patient’s underlying rhythm slows.  AIVR is typically (although not always) a regular rhythm.  In contrast to this – the widened beats in Figure 1 do not manifest the delayed timing of escape beats, nor is the run (beats #4-thru-7) regular.  
An alternative explanation for the QRS widening seen in Figure 1 is rate-related BBB (Bundle Branch Block).  We make the following points:
  • It is admittedly difficult to be certain of the diagnosis of rate-related BBB from inspection of the single rhythm strip shown in Figure 1.  That said, the important point is to be aware of this entity – since it helps to explain why the run of widened beats in Figure 1 is not AIVR or NSVT (NonSustained Ventricular Tachycardia). 
  • In support of the premise that the widened run in Figure 1 represents rate-related BBB – QRS morphology of these widened beats 1 is consistent with the predominantly negative QS or rS complex expected in right-sided lead V1 when LBBB (left bundle branch block) is present. 
  • Rather than two competing rhythms – the overall irregular irregularity of the rhythm suggests that all beats seen represent atrial fibrillation.  Rate-related BBB characteristically begins when heart rate speeds up.  In atrial fibrillation – it is typically seen following a longer R‑R interval, since the relative refractory period of the next beat is dependent on the length of the preceding R‑R interval. 
  • Close inspection of Figure 1 reveals that beat #3 is slightly widened.  This beat follows a longer R‑R interval (the R‑R between beats #1-to-2).  Thus, beat #3 actually represents the onset of LBBB-conduction in this tracing (albeit with a less complete form of LBBB given its lesser degree of QRS widening). 
  • The run of rate-related LBBB conduction continues until beat #8 when the rate of atrial fibrillation slows. 
  • Beat #11 at the end of the tracing represents a final widened beat that manifests LBBB‑conduction as a result of its short coupling interval with beat #10. 
FINAL THOUGHTS:  Subsequent rhythm strips on this patient proved beyond doubt that LBBB conduction consistently occurred during periods of more rapid atrial fibrillation – and consistently resolved soon after the rate slowed down.  Of interest (and further complicating diagnostic recognition of this important but uncommon phenomenon) is the fact that: i) the rate of onset of BBB conduction is often not the same as the rate where normal conduction resumes (ie, rate-related BBB may begin when heart rate exceeds 90 or 100/minute – but normal conduction may not resume until heart rate goes back down to 80/minute or less); and ii) AIVR is not always a precisely regular rhythm (making it more difficult to determine when irregular widened beats represent AIVR vs atrial fibrillation with rate-related BBB conduction).
  - See also ECG Blogs #14 and #15.

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