I came across this thought about Bikur Cholim at Revach.
The Rosh in Parshas Vayeira says that if you visit a sick person who is sleeping, you still have the mitzva of Bikur Cholim because when he wakes up they will tell the sick person that you visited and he will feel good. What about someone who is in a coma? Is there a mitzva to visit them?
The gemara implies that the mitzva of bikur cholim is to take care of the sick person's needs. If there is something you can do for the comatose patient, says Rav Chaim Kanievsky, you certainly have performed the mitzva. The Shita of the Rambam is that the mitzva is to daven for a sick person's recovery. Rav Chaim says that if your visit will cause you to daven, this may be part of the mitzva and you should go visit.
Important Note: We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. Please also understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.
http://www.revach.net/halacha/tshuvos/Rav-Chaim-Kanievsky-Is-There-a-Mitzva-of-Bikur-Cholim-To-Visit-Someone-Lying-in-a-Coma/1301
This reminded me of a visit I paid to an old woman in the hospital. There is an organization in my neighborhood which coordinates hospital and home visits to the sick and elderly.
One evening I received a call and the woman at the other end of the line asked me if I would visit a woman in a hospital nearby. I acquiesced and found myself at the bedside of a woman who showed no signs of recognition. I spoke to her for a few minutes but got no response. As I left, I couldn't help wondering whether I had accomplished anything. But then I thought to myself that the nurse had seen me entering and leaving the woman's room. As long as they were aware that the woman was receiving visitors, they would not neglect her.
