The signals causing oversensing may not be visible on surface ECG. Oversensing occurs when the pacemaker detects electrical activity that it incorrectly interprets as atrial (P-wave) or ventricular activity (R-wave). Most pacemakers perform such calibrations several times daily, and they include a backup algorithm that delivers a stimulus when failure to capture is confirmed. The pacemaker is then set to the minimum energy needed to activate myocardium (a safety margin is often used). This is done by repeatedly stimulating with gradually decreasing amounts of energy until the stimulus no longer yields an activation. Modern pacemakers have built-in functions to calibrate the stimulus amplitude and width according to myocardial excitability. The most common cause of failure to capture is insufficient stimulus energy. A stimulus delivered during the refractory period should not result in activation because the myocardium is refractory. Functional failure to capture occurs when a stimulus is delivered during the myocardial refractory period.This may be due to insufficient stimulus energy, or lead dislodgement/fracture. ![]()
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