Atrial Undersensing Due to Amplifier Ringing

Alison Graves-Calhoun, PhD*, Judith Mackall, MD, Paul Kohanski, RN, MSN Harrington Heart and Vascular Institute, University Hospitals of Cleveland; *Medtronic, Inc. Cleveland, Ohio
Alison Graves-Calhoun, PhD*, Judith Mackall, MD, Paul Kohanski, RN, MSN Harrington Heart and Vascular Institute, University Hospitals of Cleveland; *Medtronic, Inc. Cleveland, Ohio

A failure to sense large amplitude atrial signals is described in the following patient presentation. 

Case Presentation

A 45-year-old man with a history of atrial fibrillation for one year was admitted to our center after an episode of syncope. The underlying rhythm showed atrial fibrillation with ventricular rates between 30 to 40 bpm. A dual chamber pacemaker (Adapta ADDRL1, Medtronic, Inc.) was selected. In the EP lab, post-implant P waves measured 1.4 mV–2 mV with the programmer analyzer. The pacemaker was programmed to DDDR at 60 bpm with mode switching active. Ventricular sensing and pacing values were acceptable. Three weeks later during a wound check appointment, the pacemaker was retested and atrial undersensing was noted as indicated by the absence of atrial sense markers (Figure 1). The bipolar sensitivity was increased to 0.18 mV, and testing was repeated without improved atrial sensing as noted by continued absence of atrial wave markers and continued atrial pacing. Unipolar testing had the same result. Pacing and sensing values from the RV lead were unchanged from implant and the ventricular sensing and pacing markers were present. PA and LAT chest films to check for lead dislodgement showed unchanged atrial lead position at the right atrial appendage. 

The patient returned to clinic, and again, no atrial sensing was seen with the sensitivity at 0.18 mV, the most sensitive value. Manual measurement on paper of the inscribed atrial electrogram was 7 mV (Figure 2). Thorough testing of the lead was repeated through a range of sensitivity values. It was found that decreasing the atrial sensitivity to 2 mV resulted in a return of atrial sensing and accurate marker channels. Consultation with the manufacturer identified the undersensing as a result of “amplifier ringing.” 


Amplifier “ringing” or overload occurs when the amplitude of a signal is too great to be processed at the front end of the sensing circuit. In the case described here, the atrial EGM measured 7 mV. This is not the first report of paradoxical atrial undersensing due to amplifier ringing, yet the phenomenon remains widely unrecognized. Using a sheep model, Willems et al found that increasing atrial sensitivity resulted in fewer atrial events being recognized by the pacemaker during atrial fibrillation.1 

This phenomenon is also not something unique to the Adapta pacemaker; Kolb and colleagues found that paradoxical atrial undersensing can occur in any dual chamber pacemaker. Kolb determined that the occurrence of atrial undersensing is 13% when using the most sensitive setting and 8% with the sensitivity set to 0.3 mV.2 

Beeman et al described the case of a 65-year-old man with progressive undersensing when the atrial sensitivity was set to 0.5 mV, worsened when set to 0.35 mV, and was completely lost at 0.25 mV. Total atrial sensing was recovered when the sensitivity was decreased to 1 mV.3 Barold described a patient with atrial fibrillation and sensed atrial signals of 4 mV. Undersensing was observed at all settings more sensitive than 1 mV. Correct sensing returned when the sensitivity was decreased to 1.4 mV.4 

Van Gelder noted that modern lead design favors large amplitude atrial signals and that amplifier ringing might be more common than suspected. He found that large amplitude atrial signals and high sensitivity leads can result in inappropriate atrial pacing and incorrect recording of atrial diagnostic data.1

In all of the published cases and in this case study, paradoxical atrial undersensing occurred during atrial fibrillation or atrial flutter. We speculate that this phenomenon occurs most often during atrial fibrillation or atrial flutter because, in this setting, both the amplitude and frequency of P waves are sufficiently large to overwhelm the sense amplifiers, thus resulting in amplifier ringing. As we observed, when atrial rhythm changes occurred (e.g., from normal sinus rhythm to atrial fibrillation/atrial flutter), there was a P wave amplitude change which resulted in paradoxical undersensing at the most sensitive settings. 

In the Adapta pacemaker, amplifier ringing initiates the “Quiet Timers” algorithm, which automatically extends the atrial blanking period to inhibit the input of large amplitude, high frequency signals to the sensing circuit. The absence of atrial sense markers is appropriate and reflects correct device operation, and can be seen as long as the large amplitude signals continue. 

The clinical challenge is to set a sensitivity value that allows for recognition of atrial signals during rhythm changes. Adequate sensing during sinus rhythm can result in undersensing if the rhythm changes to atrial fibrillation. Measures to help reduce the occurrence of paradoxical atrial undersensing due to amplifier ringing may include extending the atrial blanking period, changing sensitivity sensing polarities, and programming Sensing Assurance. The Sensing Assurance feature automatically adjusts atrial and ventricular sensitivities within defined limits to maintain an adequate sensing margin with respect to the patient’s sensed P and R waves.6

Acknowledgement: Thanks to Matt Falkinburg, BSN for calling this patient to our attention.

Disclosures: Alison Graves-Calhoun, PhD reports no conflicts of interest regarding the content herein. Outside the submitted work, she reports she is employed by and has received stock/stock options from Medtronic, Inc. Dr. Judith Mackall has no conflicts of interest regarding the content herein; she reports that outside the submitted work, she is a consultant with St. Jude Medical, Inc., has received honoraria as a course director for St. Jude Medical, and has had travel/accommodations expenses covered or reimbursed by Boston Scientific. Paul Kohanski, RN, MSN has no conflicts of interest to report. 

Editor’s note: This article underwent peer review by one or more members of EP Lab Digest®’s editorial board.  


  1. Willems R, Holemans P, Van De Werk F, Heidbuchel H. Paradoxical undersensing at a high sensitivity in dual chamber pacemakers. Pacing Clin Electrophysiol. 2001;24:308-315.
  2. Kolb C, Hafbfab P, Zrenner B, Schmitt C. Paradoxical atrial undersensing due to inappropriate atrial noise reversion of atrial fibrillation in dual-chamber pacemakers. J Cardiovasc Electrophysiol. 2005;16:696-700.
  3. Beeman A, Deutsch G, Rea R. Paradoxical undersensing due to quiet time blanking. Heart Rhythm. 2004;1:345-347.
  4. Barold S, Herweg B. Paradoxical atrial undersensing by a dual chamber pacemaker during atrial fibrillation. Cardiol J. 2012;19:207-209.
  5. Van Gelder B, Van Den Broek W, Bracke FA, Meijer A. Paradoxical atrial undersensing: noise rate reversion or amplifier ringing? J Cardiovasc Electrophysiol. 2006;6:1731-1734.
  6. Medtronic CRM Technical Services Tech Note, BTN-04-101 Quiet Timer Blanking, May 2004.