Undersensing occurs when the generator sensor is incorrectly programmed too high. This causes the device to become “insensitive” to the normal sensing of the spontaneous QRS complex. The pacemaker then runs in the “fixed-rate” mode. 

An ECG tracing from a pacemaker that is too insensitive displays randomly recorded spikes in the spontaneous cardiac cycle. This can be interpreted on the ECG as a loss of capture, especially when the spikes fall into the refractory period of the spontaneous ventricular cycle. 

Undersensing is dangerous. A spike that falls in a vulnerable period such as an R-on-T phenomenon can induce ventricular fibrillation. This has been documented, although very rarely, and is usually seen in patients with severe ischemia.

Undersensing can be avoided by setting the threshold at a lower voltage (typically from 1.5–2.5 mV).

Undersensing Pacemaker Rhythm