Friday, July 5, 2013

ECG Interpretation Review #70 (PVC – Aberrant Conduction – Ashman Phenomenon – Coupling Intervals)

Interpret the lead MCL-1 rhythm strip shown in Figure-1.
  • What is the underlying rhythm?
  • Is widened beat #12 a PVC or aberrantly conducted PAC? How certain are you of your answer?
  • What is the Ashman phenomenon?
Figure-1: What is the underlying rhythm in this lead MCL-1 rhythm strip? What is beat #12? (Figures reproduced from ACLS-2013-Arrhythmias: Expanded Version- pg 215-217.) NOTE — Enlarge by clicking on Figures — Right-Click to open in a separate window (See text).
Interpretation of Figure 1:
     The underlying rhythm is appears to be sinus although it is admittedly difficult to determine the underlying sinus rate due to continual irregularity of this tracing.
  • The reason we interpret the underlying rhythm as sinus is the similar P wave morphology and PR interval for beats #4,6,11 and 14.
  • There are multiple PACs on this tracing. Although P wave morphology seems to change slightly for many of these PACs it is admittedly difficult to be certain if some of the earlier-than-anticipated beats are PACs vs rhythm regularity from sinus arrhythmia. Clinically it doesn’t matter, since “the theme” of this rhythm is sinus with multiple PACs.
  • Beyond-the-Core: Technically we can’t rule out the possibility of MAT for this rhythm. Our tendency is to favor sinus rhythm with multiple PACs as the diagnosis because of similar P wave shape and PR interval for beats #4,6,11 and 14 (vs continually varying P wave morphology from beat-to-beat that is typical for MAT). That said sinus rhythm with multiple PACs and MAT are essentially two points on different ends of the same spectrum. Practically speaking, distinction between these 2 entities does not matter since clinical implications of these two rhythms are virtually the same (See also ECG Blog #65).
  • Beat #12 is an aberrantly conducted PAC. We are able to confidently make this diagnosis because: i) Beat #12 is preceded by a premature P wave (arrow in Figure-2); ii) The “theme” of this rhythm is sinus with multiple PACs so it is more likely than not that the cause of the single widened beat in Figure-1 will also be a PAC; iii) Beat #12 manifests typical RBBB morphology (rsR’ with taller right rabbit ear in right-sided MCL-1); and iv) Beat #12 manifests the Ashman phenomenon.
Figure-2: Arrows have been added to Figure-1 to highlight the relative relationship between the coupling interval of the premature P waves preceding beats #5, 7 and 12  and the preceding R-R interval. Beat #12 is an aberrantly conducted PAC that manifests the Ashman phenomenon (See text).
The ASHMAN Phenomenon:
     The Ashman phenomenon reflects the effect of ectopic coupling interval and preceding R-R interval on the likelihood that a PAC or PJC will be conducted with aberration. Simply stated:
  •  “The funniest-looking (ie, most aberrant) beat is most likely to follow the longest pause”. The rationale for the Ashman phenomenon is explained by Figure-3:
Figure-3: Illustration of the effect that the preceding R-R interval exerts on duration of the subsequent refractory period (See text).
  • Panel A in Figure-3 schematically illustrates that a premature impulse (PAC or PJC) occurring during the ARP (Absolute Refractory Period corresponding to Point X) will be blocked. In contrast a PAC (or PJC) occurring after repolarization is complete (corresponding to Z in Panel A) will be conducted normally. Aberrant conduction will only occur IF a premature impulse occurs during the RRP (Relative Refractory Period corresponding to Point Y in Panel A).
  • Events in Panel B suggest a different clinical situation. Once again points X, Y and Z represent theoretical timing for 3 PACs. Premature impulse X will again be blocked (since it occurs within the ARP). This time both Y and Z fall beyond the RRP, so both of these premature impulses will be conducted normally to the ventricles.
KEY Point: Whether a premature impulse will fall within the RRP (and conduct with aberration) will also be determined by length of the R-R interval immediately preceding the anomalous (widened) beat. This is because duration of the refractory period is directly proportional to the length of the preceding R-R interval. When heart rate slows (as it does in Panel C of Figure-3) the subsequent ARP and RRP will both be prolonged.
  • Panel C shows the effect of rate slowing on conduction of the 3 PACs from Panel B. Premature impulse X will again be blocked (it occurs within the ARP). Premature impulse Z will again be conducted normally (it occurs after the refractory period is over). However, premature impulse Y (which in Panel B had occurred after repolarization was complete) will now be conducted with aberrancy (since the preceding longer R-R interval has now prolonged the RRP ).
Synthesis of the Ashman Phenomenon
     Our favorite way to remember the Ashman phenomenon is as follows:
  • “The funniest-looking (ie, most aberrant) beat is most likely to follow the longest pause”.
How Beat #12 Illustrates the Ashman Phenomenon
     We magnify events from Figure-2 below in Figure-4. Note that the “funniest beat” (beat #12) follows the longest pause (the R‑R interval between beats #10-11). Therefore in addition to the very short coupling interval of 0.22 second for the PAC preceding beat #12 (red arrow) the relatively longer preceding R-R interval favors conditions that predispose to aberrant conduction (via the Ashman phenomenon). Cycle-sequence comparison for other PACs in Figure-4 (white arrows) is not nearly as favorable for aberrant conduction.
Figure-4: Magnification of part of Figure-2. (Figures reproduced from ACLS-2013-Arrhythmias: Expanded Version- pg 215-217.)
FINAL Point: Ashman Phenomenon Not Applicable in AFib
     Utilization of the Ashman phenomenon may be extremely helpful diagnostically when applied to assessing wide beats in arrhythmias obtained from patients who are in sinus rhythm.
  • Be aware that the Ashman phenomenon is of uncertain value with AFib (Atrial Fibrillation). This is because the length of the R-R interval in AFib is continually influenced by another phenomenon known as 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 (Marriott and Conover).
  • Please see our ECG Blog #71 — in which we further explore why the Ashman phenomenon is not diagnostic in the setting of AFib.
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