Friday, July 5, 2013

ECG Interpretation Review #71 (PVC – Aberrant Conduction – Ashman Phenomenon – Coupling Intervals - AFib)

Interpret the lead MCL-1 rhythm strip shown in Figure-1.
  • What is beat #13?   Is the Ashman phenomenon operative in this tracing?
  • Can you also explain the slightly different appearance of beats #4 and #7 compared to most other beats on the tracing?
NOTE: You may want to refer to our ECG Blog #70 in which we discussed the Ashman phenomenon in detail.
Figure-1: What is the underlying rhythm in this lead MCL-1 rhythm strip? Does beat #13 represent the Ashman phenomenon? (Figure reproduced from ACLS-2013-Arrhythmias: Expanded Version- pg 218.) NOTE — Enlarge by clicking on Figures — Right-Click to open in a separate window (See text).
Interpretation of Figure 1:
     The underlying rhythm in Figure-1 is irregularly irregular. The QRS complex for most beats on the tracing is narrow. No P waves are seen. Therefore the underlying rhythm is AFib with a relatively rapid ventricular response.
  • Beat #13 occurs relatively early. QRS morphology manifests a typical RBBB pattern with rSR’ complex showing similar initial deflection (upright) as for normal beats and taller right rabbit ear (Figure-2). This characteristic appearance of beat #13 strongly suggests this beat is not a PVC, but is instead an aberrantly conducted supraventricular impulse.
  • Beats #4 and #7 in this tracing also look different than the normally conducted beats. They both manifest an rSr’ pattern, albeit not quite as pronounced as for beat #13. We strongly suspect the appearance of beats #4 and 7 reflects aberrant conduction with a pattern of incomplete RBBB.
  • Clinically it probably matters little whether beats #4, 7 and 13 represent isolated PVCs vs aberrant conduction of several AFib impulses. In either case the primary problem is rapid AFib in a hemodynamically stable patient. As a result management priorities rest with trying to find and “fix” the precipitating cause of AFib and with controlling the ventricular response. Regardless of the etiology of beats #4, 7 and 13 it is likely that widened complexes will decrease in frequency (or resolve completely) once the ventricular rate of AFib is controlled.
Figure-2: Use of QRS morphology in a right-sided lead (V1 or MCL-1) to distinguish between PVCs vs aberrant conduction. Only a typical RBBB pattern (rsR’ with descent of S wave below the baseline and with terminal taller right rabbit ear) is predictive of aberration (A or B). Any other pattern (C, D, E, F ) predicts ventricular ectopy. (Figure reproduced from ACLS-2013-Arrhythmias: Expanded Version- pg 219.)
The ASHMAN Phenomenon is Not Applicable in AFib
     At first glance, beats #4, 7 and 13 in Figure-1 all appear to manifest the Ashman phenomenon in that these slightly widened and different-looking beats all follow a relatively longer preceding R-R interval (ECG Blog #70). That said the Ashman phenomenon is of uncertain value with AFib (Atrial Fibrillation). Marriott and Conover have emphasized that length of the R-R interval in AFib is continually influenced by the phenomenon of concealed conduction, in which variable penetration of the 400-to-600 atrial impulses that arrive each minute at the AV node with AFib affects conduction in a way that the preceding R-R interval no longer accurately reflects the duration of the subsequent refractory period.
  • Another reason definitive diagnosis of aberrant conduction is more difficult in the setting of AFib is that one loses the diagnostic utility of identifying a premature P wave (since there are no P waves with AFib …).
  • Despite these caveats we estimate a greater than 90% likelihood that beats #4, 7 and 13 in Figure-1 all represent aberrantly conducted AFib impulses because of their highly characteristic appearance. Specifically beat #13 in Figure-1 looks identical to B in Figure-2, in that beat #13 manifests an rsR’ with S wave that descends below the baseline and taller right rabbit ear (R’ ) in a right-sided lead (such as MCL-1).
  • Final Comment: It is good to be aware of the Ashman phenomenon because this concept is often cited by those with an interest in interpreting challenging arrhythmias. That said it will be relatively uncommon (if not rare) that one truly has opportunity to invoke clinical use of the Ashman phenomenon. Despite description of this phenomenon by Gouaux and Ashman in 1947 about a patient with atrial fibrillation enhanced appreciation of the importance of concealed conduction in the setting of AFib reduces the diagnostic utility of the Ashman phenomenon to cases in which a widened early beat follows a long pause in a patient with sinus rhythm (and not AFib).
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