One of my hens, Edith, became more and more swollen over about a week. She looked healthy, but she was barely breathing, her body was swelling up, and could barely move around. I thought it was another egg bound situation, so I tried using the method for helping her pass an egg (check out my "egg bound" post)--at least three different times. But there was no egg and she kept getting worse. Unfortunately, I was leaving (out of the country) within a few days, and didn't have the time to try to fix her. So I had to put her down. Then I proceeded to do an autopsy on her, and as soon as I inserted the knife, a whole bunch of fluid flooded out and her body totally deflated. The liquid was yellow-ish and watery. Her organs looked perfectly healthy. So her lungs and every other organ were still working, but were slowly being suffocated by fluid build up. So I looked around online and in my books to find out what was wrong with her. I don't know specifically what it was, but I have two similar diagnostics. It was either Ascites or a type of egg yolk (EYP) Peritonitis.
Ascites-Fluid pooling in the abdomen
Sterile Peritonitis-Fluid in her abdomen that is not filled with bacteria or caused by internal infection
Septic Peritonitis-Fluid with bacteria
I found a very detailed forum entry (scroll down to the entry by "crazychick"): http://www.thepoultrysite.com/forums/showthread.php?t=4340
describing the diseases and how to fix it. I recommend reading the whole thing very thoroughly.
The idea for fixing this is to drain the fluid out of the hen using a catheter. You will need to buy supplies, and set up a sterile area since you will be performing surgery.
Although I don't believe it is inhumane or unfair if you decide to put your hen down and not do this procedure (since it is very time consuming, and risky) I will definitely try it next time this happens. I think that if you're new to chickens, then it might be a good idea to get another chicken owner/expert's advice or help before trying anything.