Dr. Leland Clark, inventor of the platinum "Clark electrodes" used to detect oxygen in liquid (and allows measurement of blood oxygen levels) also did this famous experiment in "liquid breathing":
Dr. Leland Clark of Cincinnati invented a blood substitute allowing this living mouse to breathe in the liquid, while goldfish inhabit the water floating on top.
After bubbling oxygen through the fluorocarbon, the oxygenated fluid was pumped into the animals' lungs, and recirculated (about 6 cycles of inhalation and exhalation per minute). Most of the animals who were kept in the fluid for up to an hour survived for several weeks after their removal, before eventually succumbing to pulmonary damage.
Oh, and in medicine - vast improvement on the water bed (trivia: Robert Heinlein designed what may have been the first while in hospital before being discharged).
And they were not causing the rat any pain.
Don't refute brilliant experiments done in the name of science simply because you have the means to, please.