Failure to sense

Definition

A malfunction (undersensing) in which the pacemaker fails to detect the heart's own depolarizations and so paces asynchronously: spikes appear regardless of native P waves or QRS complexes, often too many of them and sometimes soon after a native beat. Causes include programming problems (an improper sensing threshold), a native signal too small to sense, lead or generator failure (for example fibrosis or fracture), and electrolyte abnormality. Sensitivity is set in millivolts, and an undersensing device has a high sensitivity value. A recognized hazard is a ventricular spike landing on the T wave (R-on-T), which can trigger ventricular fibrillation.

Clinical noteContrast with oversensing, in which signals other than the chamber's own depolarization (such as T waves, muscle potentials or electromagnetic interference) are sensed and inhibit pacing.