In which pacing mode would "R" indicate a rate-responsive function?

In the context of cardiac pacing modes, the designation "R" specifically indicates that the pacemaker has a rate-responsive feature. This means that the pacemaker can adjust its pacing rate based on physiological demands, such as physical activity or changes in the patient's heart rate.

When considering the options given, DDDR is the correct answer because it includes the "R" notation. The DDDR mode signifies that the pacemaker can sense both atrial and ventricular activity, and it can also respond to changes in bodily demand by adjusting the pacing rate accordingly. This is particularly beneficial in maintaining appropriate heart rates during various levels of activity, providing support to patients with different hemodynamic needs.

The other options, while they denote various pacing functions, do not incorporate the "R" that signifies rate responsiveness. For instance, VVIR can respond to ventricular events but does not feature the same comprehensive atrial sensing and response capability. Similarly, DDD allows for dual sensing and pacing but lacks the specific rate-responsive function signaled by the presence of "R." VVI, on the other hand, indicates ventricular pacing only and does not include the rate-responsive adjustment, making DDDR the only choice that embodies the full parameters expected for rate responsiveness.