If we're going to be pedantic, a passenger "medical emergency" is normally not a MAYDAY call. PAN PAN would suffice.
A distressed aircraft in grave and imminent danger (MAYDAY) receives immediate assistance at the highest ATC priority (priority 1). The aircraft will be marked DETRESFA ("distress phase") and emergency procedures are activated. The definition of distress phase can be found in PANS-ATM (ICAO Doc. 4444). An aircraft in distress has right of way over all other traffic.
A distressed passenger with medical illness may receive priority 2 service, as necessary, if the pilot declares PAN PAN or formally declares an emergency. This is a higher level of priority than normal traffic, but does not meet the distressed aircraft criteria. Heavy traffic may preclude controllers from providing immediate assistance. This is similar to priority given to MEDEVAC flights, for example.
Last edited by peekay4; 21st Dec 2014 at 18:08.