In which scenario can "capture" be observed on an ECG?

Capture refers to the successful depolarization of the myocardium following a pacing impulse from a pacemaker. In this scenario, when the pacemaker fires, it generates an electrical impulse that travels through the conduction system of the heart, leading to the contraction of the heart muscle. On the ECG, this is visually represented as a distinct QRS complex that follows the pacing spike.

This phenomenon is critical in defining that the pacemaker is effectively stimulating the heart, which is essential for the management of patients with bradycardia or other dysrhythmias where pacing may be necessary. Successful capture indicates that the pacemaker is not only firing but that the electrical impulses are successfully causing myocardial depolarization and subsequent mechanical contraction.

The other scenarios do not demonstrate capture. Fibrillation refers to disorganized electrical activity within the heart that does not result in effective contraction, while induced bradycardia involves a slower heart rate without the specific context of pacing. In complete heart block, impulses from the atria do not reach the ventricles, making it impossible for a pacemaker to produce a capture effect if there is no connection between the two, hence no effective conduction occurs.