Interpret the rhythm strip shown in Figure 1. The widened beats on the tracing are not ventricular. What else might they be?
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| Figure 1 – Right-sided MCL-1 monitoring lead rhythm strip. Why might beats #4-thru-7 be wide? |
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.
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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).
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