Wilson ’ s Central Terminal , the keystone to electrogram recording – What , where and why ?

Even amongst Cardiologists and Cardiac Physiologists there is considerable inconsistency in the understanding of Wilson’s Central Terminal (WCT), an unusual situation considering the ubiquity with which it used both in electrocardiography and invasive electrophysiology. (1) (Figure 1) The WCT was developed, by Frank Wilson in 1934, in order to allow the possibility of straightforward and consistent unipolar recording. Eighty years later, it is still the keystone to electrogram referencing. Conceptually, the unipolar recording is that of a point source. Intra-cardiac point source or unipolar recordings may be particularly beneficial when information regarding directionality is needed, as in the ablation of accessory pathways or focal atrial and ventricular tachycardias (looking for the QS pattern of retreating activation), and when examining farfield events, such as using endocardial recordings to examine epicardial depolarization. (2) On the other hand, the more commonly used bipolar recording, which is the difference between to two closely spaced unipolar signals, is useful for assessing local events as the two ‘near identical’ farfield signals are effectively ‘subtracted out’. Furthermore, a unipolar signal can be made to approximate a bipolar signal by increasing the high pass filter from the normal value of 0.5-1Hz to around 30Hz, when directionality information will be lost. This occurs because high pass filters effectively differentiate the signal such that the new signal becomes proportional to the rate of amplitude change rather than the amplitude itself.