Merlin has been at the vets all day, he has been sedated, airways checked, xray performed and blood tests done.

Nothing in airway. Blood tests a couple of abnormalities but the vet said nothing to cause concern. Xray, the vet could not see the bottom part of Merlins heart as there is fluid on his one lung blocking the xray. He is being treated with Frusemide and i have to go back Monday for the next step.

The vet could give me no explanation as to why only one lung is affected and also whether there are any heart problems to be frank he was stumped. He said that Merlins heart was beating very fast and it was very quiet but the quietness could be down to the fluid.

Feeling very worried and scared of the next steps and hoping/praying that the frusemide works. It is so upsetting that we do not know why and how he has this and whether there are more problems to come.

Merlin is currently motionless sitting on a blanket looking very very sad.

Fingers crossed and thank you to everyone who took the time to answer my original post.

Is it fluid or could it be blood in the pleural cavity?
http://en.wikipedia.org/wiki/Hemothorax - caused by trauma.
Have they sampled the lung fluid?