What is the primary consequence of pacemaker undersensing?

The primary consequence of pacemaker undersensing is inadequate cardiac support. This occurs when the pacemaker fails to detect the intrinsic electrical activity of the heart appropriately, which can lead to missed beats. In such cases, the pacemaker may not provide pacing stimuli when the heart requires it, resulting in a lack of support for maintaining an adequate heart rate and rhythm. This can compromise cardiac output and potentially lead to symptoms of bradycardia or other arrhythmias.

The other choices do not accurately reflect the consequences of undersensing. Unnecessary pacing generally occurs due to oversensing, which is the opposite of undersensing. A reduction in heart rate could happen if the pace-sensing function is compromised, but it's not the direct result of undersensing; rather, it may occur as a consequence of not pacing when needed. Normal heart function would not be the result of inadequate sensing, as undersensing implies that the pacemaker is not supporting the heart effectively, thereby disrupting normal physiological function.