The pacemaker is either “oversensing” by interpreting an external signal as an impulse and not providing a necessary shock, or “undersensing” by pacing the heart unnecessarily when not sensing the heart’s natural impulse. Oversensing is due to decreased sensitivity of the pacemaker and will show as the lack of a pacer spike in conjunction with the lack of natural conduction on the ECG. Undersensing is due to increased sensitivity in the pacemaker and will show a pacer spike when there is already a naturally conducted waveform on the ECG.