Wednesday, May 7, 2014

ECG Blog #87 — PVCs or Intermittent WPW?

Interpret the ECG shown in Figure 1. Unfortunately — No history is provided.
  • How certain are you of your interpretation?
  • In brief — What is going on?
  • Where is “the action” taking place?

Figure 1: 12-lead ECG with long lead II at the bottom. No history is provided. What is going on? 

Interpretation of Figure 1: There is a LOT going on in this tracing. Fortunately — a simultaneously-recorded long lead II rhythm strip appears at the bottom of the 12-lead ECG.
  • Although it might at first appear that the rhythm is either sinus with frequent end-diastolic PVCs (Premature Ventricular Contractions) vs sinus rhythm with a series of aberrantly-conducted supraventricular beats — neither of these theories account for what is happening on this ECG.
     In an attempt to clarify the situation — We label atrial activity with red arrows below in Figure 2. But BEFORE looking at Figure 2 — Be sure you have forced yourself to commit to an interpretation of the original ECG in Figure 1.

Figure 2: With P Waves and Other Findings from Figure 1 now Labeled
     Note in Figure 2 — that P waves in the original tracing (= Figure 1) remain regular throughout the entire tracing. Thus, there are no “early beats”.
  • The sinus-conducted beats in Figure 2 are beats #2,3; #6,7,8; and 11,12.
  • QRS morphology differs from that of the sinus-conducted beats for beats #1; 4,5; 9,10; and 13,14.
  • The reason for this difference in QRS morphology becomes more apparent when looking at other simultaneously-recorded leads on the 12-lead tracing (red ovals in Figure 2). This feature is perhaps best displayed by the red ovals highlighting the initial portion of the QRS complex in leads I, V4, and V5. Initial slurring of the QRS upslope in these leads is clearly the result of delta waves in this patient with WPW (Wolff-Parkinson-White) Syndrome.

Figure 2: Atrial activity in Figure 1 has been labeled with red arrows. Note that P waves remain regular throughout the tracing. There are no early beats. Instead — delta waves (red ovals) are apparent in multiple leads (See text).

We highlight the following additional points about Figure 2:
  • Delta waves may come in all shapes and forms. Delta waves are not always evident in all leads on a given tracing. It is worthwhile taking a good look at the red oval in each of the 12 leads in Figure 2 to see what the delta wave looks like in each lead. Note that in some leads (like I, aVR) — the presence of WPW doesn’t greatly alter the QRS complex. In contrast, in other leads (like lead V1) — the presence of WPW dramatically alters QRS morphology.
  • Conduction over an AP (Accessory Pathway) can be intermittent. A series of factors including drugs, exercise, stress, electrolyte abnormalities, underlying medical illness, etc. — may all contribute to favoring conduction over either the normal AV nodal pathway or the AP. This is well illustrated in the long lead II rhythm strip in Figure 2 — in which AP-conduction is most manifest for beats #1; 4,5; 9,10; and 13,14.
  • Conduction may occur over both the normal AV nodal pathway and the AP at the same time! That is — at any given instant in time, there may be relatively more or less conduction over one or the other pathway. That there is still some pre-excitation (ie, conduction over the AP) in Figure 2 is suggested by the blue ovals in leads I and V6, in which some slurring of initial QRS upslope is seen. Such partial pre-excitation may be subtle. As a result — it may not be obvious that WPW is present in some patients when the relative amount of pre-excitation is small. It may only be in retrospect (ie, after learning that a patient has WPW) — that we are able to recognize that some pre-excitation was present all along in prior tracings.

Localization of the Accessory Pathway:
     Final localization of the AP clearly lies in the domain of the EP (ElectroPhysiology) Cardiologist who is charged with evaluating a patient with WPW for a possible ablation procedure. That said — it is interesting indeed for us to use the surface ECG to predict the likely location of the AP. This can be easily done with surprising accuracy — as we describe in detail in ECG Blog #76. Applying the step-by-step approach we provide in Blog #76 to the delta waves (initial QRS deflection within the red ovals) in Figure 2 — We come up with the following.
  • NOTE: This method for localizing the AP will not work optimally if the relative amount of pre-excitation is minimal. That said, it appears that pre-excitation is probably maximal for the beats within the red ovals in Figure 2 (ie, for beats #1; 4,5; 9,10; and 13,14).
  • Look First for the point of transition in the precordial leads. Since the QRS complex in Figure 2 is upright in lead V1 — Begin with Step A-1. This Step A-1 already tells us that there is a LEFT-sided AP.
  • Step A-1 then asks us to measure the sum of delta wave polarities in the 3 inferior leads (II,III,aVF). Looking only at the delta waves (= the red ovalswhich correspond to the first 0.04 second of the QRS in each inferior lead) — We note that the delta wave is positive in all 3 inferior leads (even though in leads III and aVF, the QRS is predominantly negative). Therefore, the SUM of inferior lead polarities = +3. This tells us we have an AnteroLateral LV Free Wall AP!

Figure 3: Follow-Up ECG done a short while Later ...
     A short while later — a follow-up ECG was obtained on this patient (Figure 3). Note that beats #1-thru-#6 in this follow-up tracing are conducted normally (or perhaps conducted with some minimal amount of pre-excitation). Full pre-excitation is seen beginning with beat #7. This lasts for the remainder of the tracing.
  • Note QRS morphology in the chest leads for beats #7-thru-12. The QRS complex in each of these leads manifests full pre-excitation.
  • Given what we have seen in Figure 1 and Figure 3 — it is likely that this patient alternates between normal and accessory pathway conduction numerous times each day.

Figure 3: ECG obtained a short while later on this patient. Conduction is relatively normal for the first 6 beats. Full pre-excitation begins with beat #7 (See text).

Acknowledgment: I am grateful to Michael Frease, RN — who sent me these tracings and allowed me to use this case for teaching. Mike’s Float Nurse Blog is at: -

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