Conducting Small-Group Discussions in a Large Room

Applicantus asks:

Fellow smokers, I need a bit of teaching advice or some help brainstorming. My class will be taking place in a very large classroom where the chairs are pinned to the floor. I was planning on quite a bit of small group discussion during class, but that would now be a challenge. Or will it? How realistic will it be to ask students to turn around in their chairs and discuss with those seated behind them? How many at a time? This is a medical ethics class, and so I was planning on many case studies. (The reason I only found it out now is that the classroom was being renovated and last year the chairs were not bolted to the floor in that particular room; it is now much fancier, though not very conducive for my purposes - and it's too late to change rooms, since I have 80 students).

Any thoughts?