The MEV was designed to be able to evacuate casualties from a location while also providing medical aid. It could also perform routine surgical procedures if it had access to a hospital or medical bay.
It had four telescoping arms that allowed it to load passengers and reach almost anywhere on them to provide treatment. It had a built-in repulsorlift sled that folded up vertically behind. When deployed, it used a low level gravity field that prevented the patient from moving or falling off.
The MEV was released approximately a decade before the Clone Wars. It was very popular with mercenary groups and metropolitan emergency teams. Many of them were upgraded to improve their speed by boosting the repulsorlift engine or installing booster rockets. They could also be upgraded with an enclosed bed, to provide more protection to the patient. This modification would slow the droid down though.
Another popular modification was to install a secondary specialized subprocessor which would allow the droid to move more efficiently without any decrease in patient care.