Which of the following is a potential outcome if a pacemaker fails to sense an intrinsic beat?

When a pacemaker fails to sense an intrinsic beat, it may lead to the pacemaker providing electrical impulses inappropriately, which can cause it to compete with the heart's natural rhythm. This competition occurs because the device continues to deliver pacing stimuli even when the heart is already beating effectively on its own. As a result, the patient may experience a combination of natural heartbeats and pacemaker-induced beats, which can disrupt the normal cardiac cycle and lead to irregular heart rhythms or even increased demand on the heart.

The other outcomes listed do not directly relate to sensing failure in the same way. Increased heart rate might suggest a response to stimulation, while reduced pacemaker output would imply it is functioning correctly in sensing beats. Cardiac arrest is a more extreme and less common outcome, typically resulting from other critical failures in the pacemaker system or severe underlying cardiac issues, rather than a simple failure to sense. Thus, the most relevant outcome of a pacemaker failing to sense an intrinsic beat is its competition with the patient's heart.