Cardiac Rhythm Management
Articles Articles 2013 October

Role of Unipolar Electrograms in Modern Electrophysiology


Heart Rhythm Service, Kingston General Hospital, Queen's University Kingston, Ontario, Canada

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Unipolar electrograms (EGMs) have been part of the armamentarium of electrophysiologists for decades. 1 However, their use in modern electrophysiology has decreased due to the development of new techniques that facilitate endocardial and epicardial mapping.

In this case report presented in the Journal involving a 25-year-old female patient, the authors describe an uncommon presentation of a premature ventricular focus with a left bundle morphology arising from the non-coronary cusp (NCC) of the aortic root. 2 They also highlight the usefulness of utilizing the unipolar EGM in order to differentiate far-field from local signals, thereby optimizing timing, which led to a successful outcome following ablation.

Knowledge on the anatomy and physiology of the cusps as well as their role in arrhythmogenesis was gained in the last decade. In their series, Yamada and coworkers, 3 evaluating the origin of idiopathic ventricular arrhythmias (VA) arising in structurally normal hearts, found that only one (2.3%) out of 265 patients had a focal VA arising from the NCC. The left coronary cusp was far more common (54.5%), and the right coronary cusp was involved in 11.4% of the PVCs found to arise within the aortic root. This uncommon site for PVC foci (NCC) was also identified in the case series of only two patients by Alasady and colleagues. 4

Anatomically, the NCC lies anteriorly and superior to the interatrial septum as well as in close proximity to the atrioventricular junction. Therefore, some reports have described ablation of atrial tachycardias from within the NCC. 4 Ablation of PVCs from the NCC are however rare.

Stavrakis et al 2 postulate that there is a persistence of muscular connections between the left ventricle and the NCC, and this failure of regression results in the substrate for ectopic ventricular foci.

The procedure described in this case report involved the use of careful delineation of the aortic root. An aortogram was performed in two views (left anterior oblique and right anterior oblique) in order to define the location and orientation of the mapping catheter within the cusps as well as its relationship to the coronary ostia. A multipole spiral catheter was also placed within the right ventricular outflow tract (RVOT) and used as a continuous timing reference in addition to an electroanatomic mapping system (CARTO). Although not displayed in the tracings in the index case, the use of a distally placed coronary sinus catheter would also have helped as a timing reference if it was wrapped around to the anterior interventricular region over the left ventricle.

The authors demonstrate that the bipolar signal in the RVOT was misleading, and the unipolar EGM was more reliable in differentiating the far-field component, thus giving a more accurate assessment of activation timing. However, once the NCC was mapped and the culprit focus was arrived at, a timing reference 70 ms ahead of the surface ECG was demonstrated.

This was evident on both unipolar and bipolar configurations. Arguably, the bipolar EGMs, in their case report appear more discrete than the unipolar signals.

Having both unipolar and bipolar signals available for activation as well as pace mapping for complex VT cases can thus be appreciated. It provides a means of cross-referencing and validating low amplitude and confusing intracardiac EGMs which increase the difficulty in addition to the obvious anatomical constraints.

Regardless, the outcome of this case was highly satisfying after a detailed and meticulous mapping strategy. The authors should be congratulated for employing an old but effective tool in the form of unipolar sensing to help differentiate difficult signals.


Heart Rhythm Service, Kingston General Hospital
Queen's University
Kingston, Ontario, Canada

Heart Rhythm Service, Kingston General Hospital
Queen's University
Kingston, Ontario, Canada


  1. Varriale P, Niznik J. Unipolar ventricular electrogram in the diagnosis of right ventricular ischemic injury. Pacing Clin Electrophysiol 1978; 1:335–41.
  2. Stavrakis S, Dusa AC, Garabelli P, Po SS. Idiopathic ventricular tachycardia originating from a myocardial extension into the non-coronary aortic cusp: the significance of unipolar electrograms. J Innov Card Rhythm Manage 2013.
  3. Yamada T, Lau YR, Litovsky SH, et al. Prevalence and clinical, electrocardiographic, and electrophysiologic characteristics of ventricular arrhythmias originating from the noncoronary sinus of Valsalva. Heart Rhythm 2013; doi:pii: S1547-5271(13)00892-8. 10.1016/j.hrthm.2013.08.017.
  4. Alasady M, Singleton CB, McGavigan AD. Left ventricular outflow tract ventricular tachycardia originating from the noncoronary cusp: electrocardiographic and electrophysiological characterization and radiofrequency ablation. J Cardiovasc Electrophysiol 2009; 20:1287–90.