http://spermup.blogspot.com/ The Wolff-Parkinson-White syndrome PSVT due to accessory pathways (the Wolff-Parkinson-White syndrome, WPW) This syndrome is the second most common cause of PSVT. The mechanism of the tachyarrhythmia relates to the presence of two pathways between the atria and the ventricles that have different conducting properties (see figure1). Figure 1 click for big picture Usually the period of these pathways during which they can not respond to stimulus to conduct (refractory period) exceeds that of the normal AV nodal-His pathway. Thus, a premature atrial impulse may block at the accessory pathway and conduct antegrade down the normal pathway and enter the accessory one in a retrograde direction and reentering the atrium to cause a circus movement tachycardia (orthodromic, see figure 3). Figure 3 click for big picture Figure 3a click for big picture Since the accessory pathway provides retroconduction to the atria, P waves (if seen) are usually inverted in EKG