The man, aged about 29, went to the accident and emergency room claiming his elderly grandmother urgently needed help.
After nobody responded, he left the hospital, got into a private ambulance parked nearby and drove off.
He drove to a fire station about one kilometer from the hospital, where he asked again for someone to help his grandmother. But staff at the fire station opted to detain the man as he appeared incoherent and agitated.
Police were able to track the ambulance and arrest him at the fire station, where they found a 63-year-old patient , a resident of a local retirement home, in the back.
Police say the driver, who works at the local psychiatric unit, was suffering from psychiatric problems, and has since been put in hospital at the request of his family.