[THIS WILL BE A NEW PAGE AND WILL NEED A NEW GRAPHIC] The previous page illustrated that pre-excitation may be alternating. It may also be: 1) Intermittent – Will be present on some occasions and in some tracings but totally absent at other times and on other tracings. 2) Variable in extent - Some beats may demonstrate only a minimal degree of pre-excitation with a small delta wave and a minimally altered QRS complex while other beats will demonstrate more pronounced degrees of pre-excitation with a more obvious delta wave and more significantly altered QRS complex. This variable ECG change, which may occur on a beat to beat basis is sometimes referred to as the “accordion effect” 3) Associated with multiple accessory pathways – In some studies, approximately 10% of patients with ventricular pre-excitation will be found to have more than one accessory pathway at the time of electrophysiologic study 4) Concealed – This indicates that the accessory pathway may not conduct from atrium to ventricle and may not be associated with any ECG abnormality, but will be capable of conducting in a retrograde direction, i.e. from the ventricle to the atrium and of forming the retrograde limb of a re-entry circuit. This is a not uncommon phenomenon in patients with supraventricular tachyarrhythmias.