The initial three quarters of the tracing in Figure 1 could be either ventricular bigeminy with a pseudo-delta wave or ventricular preexcitation of every other beat. Luckily, the last three beats (V4, 5, and 6) prove that it is ventricular preexcitation of every other beat. If it were ventricular bigeminy with a pseudo-delta wave, the rhythm in V4, 5, and 6 would reflect a ventricular bigeminy leading into an accelerated idioventricular rhythm. It would be too much of a coincidence for a sinus rhythm and an accelerated idioventricular rhythm to have an identical rate. Therefore, the answer is ventricular preexcitation of every other beat.
As noted in October's ECG of the month, some QRS complexes have a slurred upstroke simulating a delta wave without being preexcited, which can be called a pseudo delta wave (Figure 2).
If this happens with ventricularly paced complexes or premature ventricular contractions (PVCs) without a P wave in front of them, it does not pose any confusion. If it occurs with QRS complexes during sinus rhythm, it also does not lead to misinterpretation because the PR interval is not short and the QRS complex is not wide. However, if it happens with a PVC with a sinus P wave just in front of it, it simulates a preexcited complex.
Of course, one has to know that ventricular preexcitation can occur intermittently. How intermittently? Sometimes only every other beat is preexcited; sometimes several beats at a time or for several days at a time.
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Cite this: A Partial Read May Mislead - Medscape - Dec 18, 2014.