Failure to capture

Definition

A malfunction in which a pacemaker spike is delivered but is not followed by depolarization of the intended chamber, so the ECG shows a spike with no following P wave (atrial noncapture) or QRS complex (ventricular noncapture). It occurs when the stimulus does not reach the myocardial capture threshold, the minimum energy that consistently depolarizes the chamber. Causes include lead dislodgement or malposition (the most common cause soon after implant), lead fracture or insulation failure, an output programmed too close to the threshold, a threshold that rises over time (fibrosis at the lead tip, cardiomyopathy, rarely infarction at the lead site), electrolyte disturbance (hyperkalemia most often), acidosis, drugs such as flecainide, and battery depletion.

Clinical noteNoncapture is judged on spikes that fall when the paced chamber is not expected to be refractory.