Pacemaker "Failure to capture" Failure to capture is when the pacemaker signal fires but there is no response. This is identified by having pacing spikes present with no resulting QRS, (capture). The example below shows the first two ventricular paced beats capture, then the next two spikes fire but there is no capture. The last beat captures.

Pacemaker "Failure to Sense" In most cases, sensing problems are less threatening and can be corrected by performing a pacemaker interrogation. Sensing problems include “undersensing” where the pacemaker fails to sense the native cardiac rhythm. “Oversensing”, which occurs when a foreign signal (non-cardiac) such as a muscle twitch, are mistaken for real cardiac activity. Sensing issues are identified when there are pacemaker spikes showing up where they do not belong. The example below (undersensing) shows the first two spikes fire out of place. The underlying rhythm is running. The next three spikes are sensing correctly and pace the ventricles.

Pacemaker "Failure to Output" Failure to output (failure to pace) simply means there is no output signal being generated from the pacemaker. Somehow, the electrodes have no current flowing through them. There is no particular rhythm to define this. It could be an intermittent problem or failure altogether. In general, you will have no pacing spikes where they need to be. It may be the pacemaker battery, the leads or it could be an over sensing problem. If the pacemaker fails to pace, its absence will result in the underlying rhythm running (if any). This should be addressed immediately. The ekg strip below shows the first two spikes pacing, then the pacemaker fails to pace. This results with the underlying rhythm left on its own, with a long pause followed by a ventricular escape beat. .

Pacemaker problems and malfunction. Pacemaker malfunction can occur for a wide variety of reasons, ranging from equipment failure to changes in underlying native rhythm. Diagnosis of pacemaker malfunction is challenging and often associated with non-specific clinical symptoms while ECG changes can be subtle or absent. Problems with Undersensing Undersensing occurs when the pacemaker fails to sense native cardiac activity. Results in asynchronous pacing. Causes include increased stimulation threshold at electrode site (exit block), poor lead contact, new bundle branch block or programming problems. ECG findings may be minimal, although presence of pacing spikes within QRS complexes is suggestive of undersensing. Oversensing Oversensing occurs when electrical signal are inappropriately recognised as native cardiac activity and pacing is inhibited. These inappropriate signals may be large P or T waves, skeletal muscle activity or lead contact problems. Abnormal signals may not be evident on ECG. Reduced pacemaker output / output failure may be seen on ECG monitoring if the patient stimulates their rectus or pectoral muscles (due to oversensing of muscle activity). Problems with Pacing Output failure Output failure occurs when a paced stimulus is not generated in a situation where expected. Results in decreased or absent pacemaker function. Multiple causes including oversensing, wire fracture, lead displacement, or interference. Failure to capture Failure to capture occurs when paced stimulus does not result in myocardial depolarisation. Multiple causes

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