How do vets deflate a cow in an emergency?

A vet will pass a tube down the cow's esophagus. If there is an obstruction this may force the obstruction down into the rumen. If there is a free gas bloat simply passing the tube will deflate the rumen. If there is a frothy bloat the vet will pour poloxane down the tube to help break up the surfactants and bubbles to release the gas.

If none of the above work or are not fast enough to release the gas, a trocar to the rumen wall will also work.