Scenario: Interpret the ECG and accompanying lead II rhythm strip in the Figure. Unfortunately, no history is provided. Can you explain the changing QRS morphology that is seen throughout the rhythm strip?

Interpretation: A lot is going on in this tracing. However, the constantly changing QRS morphology is due neither to frequent end-cycle premature ventricular contractions (PVCs) nor to aberrantly conducted supraventricular beats. Instead, the key clue lies with the fact that the P-P interval remains constant throughout. There are no "early beats."

  • The sinus-conducted beats in this ECG 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 the difference in QRS morphology becomes apparent when looking at simultaneously recorded leads on the 12-lead tracing. Note especially the last 2 beats in leads V4 and V5 (corresponding to beats #13 and 14 in the long lead II rhythm strip). Initial slurring of the QRS upslope in these two leads is clearly the result of delta waves in this patient with Wolff-Parkinson-White (WPW) Syndrome.

Impression: Conduction over an accessory pathway (AP) 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 lead II rhythm strip in which AP-conduction is most manifest for beats #1; 4, 5; 9, 10; and 13, 14.

  • Delta waves may come in all shapes and forms. They are not always evident in all leads on a given tracing. That said, this tracing is remarkable for how different QRS complexes look in some leads when there is sinus conduction compared to beats conducted over the AP.
  • Finally, 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 always at least some pre-excitation (i.e., conduction over the AP) in this tracing is suggested in leads I and V6, in which some slurring of initial QRS upslope is seen even in beats that were thought to be sinus conducted. Such partial pre-excitation may indeed 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 (i.e., after learning that a patient has WPW) that we are able to recognize that some pre-excitation was present all along in prior tracings.