Which of the following could indicate a malfunctioning pacemaker due to undersensing?

A malfunctioning pacemaker due to undersensing can typically manifest as skipped heartbeats or increased pauses. When a pacemaker undersenses inherent cardiac activity, it fails to detect the natural heartbeats that occur and subsequently does not respond by pacing accordingly. This can lead to periods where the heart is not adequately stimulated, resulting in noticeable pauses or missed beats.

In the context of pacemaker function, it's essential to understand that the device relies on sensing its programmed thresholds to determine whether it needs to initiate a pacing impulse. When a pacemaker undersenses, the failure to recognize the heart's rhythm can cause increased intervals without electrical stimulation, leading to unsynchronized contractions that may feel like skipped heartbeats to the patient. This clinical symptom is a direct consequence of the device not responding appropriately to the physiological demands of the heart.

The other options do not directly correlate with undersensing. Frequent ventricular contractions might reflect the opposite issue of oversensing or inappropriate pacing. Prolonged pacing intervals can suggest programming settings that extend the pacing intervals, and elevated heart rates could be due to other factors unrelated to sensing issues. Focusing on the association between undersensing and the specific manifestation of skipped heartbeats clarifies why it is the correct indicator of pacemaker malfunction.