Pseudo pre-excitation with concertina effect in idioventricular tachycardia. The concertina effect is a phenomenon where the QRS complexes reflect alternating phases of gradual widening and narrowing. This is most commonly due to ventricular pre-excitation, and the changes in QRS morphology are due to variability of the ventricular zone that undergoes pre-excitation. This presentation reflects a case where the concertina effect is due to an idioventricular tachycardia at a rate nearly identical to the sinus rate. Variable degrees of ventricular fusion therefore occur, and the concertina effect ensues, in relation to slight variations of the sinus cycle. 