![]() ![]() With undersensing, the pacemaker is unable to correctly interpret native cardiac activity due to changes to intracardiac signals (myocardial ischemia, new bundle-branch blocks, PVCs ). With oversensing, the pacemaker misinterprets other signals (peaked T-waves, electromagnetic interference, skeletal muscle activity) to be QRS complexes, inhibiting response and failing to initiate pacing. Failure to sense includes both oversensing and undersensing. ![]() Fibrosis from the local inflammatory response weeks after implantation (decreased incidence due to steroid-eluting leads ) in addition to lead fracture/dislodgment, twiddler’s syndrome, and cardiac perforation may be the culprits. Once again, the patient’s native rhythm is seen on the ECG. Pacer spikes are seen on an ECG, but there is no cardiac response. Failure to capture occurs when a pacemaker pulse is given, but the impulse is unable to depolarize non-refractory myocardial tissue.The ECG shows neither pacer spikes or pacer-induced QRS complexes, but rather the native rhythm of the patient. Failure to pace suggests that the pulse generator is not providing sufficient voltage output to depolarize myocardium.Pacemaker failure has traditionally been divided into 3 categories: This is a patient with a normally-functioning DDD pacemaker: If no native beat is sensed after a set time interval, the pacemaker delivers a triggered beat. If a native beat is sensed, the pacemaker is inhibited. The most common setting for dual-chamber devices is DDD, wherein the pacemaker paces and senses both the atrium and the ventricle. These can be remembered by the mnemonic PaSeR 1 – pacing function, sensing position, and response to sensing. Pacemakers are fully described by a 5-position code, with the first 3 positions defining the function of the device. ![]() These features are programmable and affect battery longevity. The stimulation threshold is the minimum amount of energy required to depolarize myocardium, described by amplitude (volts) and duration.
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