Apparent undersensing due to oversensing of low amplitude pulses in a thoracic impedance-sensing, rate-responsive pacemaker.

A case in which apparent undersensing was caused by detection of low amplitude impedance-measuring pulses emitted by a thoracic impedance-sensing rate-modulated pacemaker is described. The detection of these high frequency pulses was interpreted as "noise" by the pacemaker and led to asynchronous pacing in the interference reversion mode. Increasing the sensitivity failed to correct the problem. Decreasing the sensitivity eliminated oversensing of the low amplitude pulses and the apparent undersensing. Recognition of this phenomenon is important to prevent a misdiagnosis of undersensing.