Calls are designated ALPHA, then BRAVO, then CHARLIE, then DELTA; from lowest to highest. ALPHA calls are non-life threatening, DELTA calls are time-dependent, life-threatening calls, and Bravo and Charlie are somewhere in the middle. Sometime after the system was being used in many places, they added an ECHO response. When the caller spontaneously says "... and the person is not breathing," an ECHO response is triggered. The dispatcher immediately starts providing rescue breathing over the phone, so other details may or may not be known before the responding unit arrives.

Even though Medical Priority Dispatching is a nationwide program, administered by the National Academy of Emergency Medical Dispatch (NAEMD), it is left to individual departments or counties to determine their own response guidelines. For example: in places where they have both ALS (advanced life support) and BLS (basic life support) ambulances, a local County Health agency can determine that an ALS unit will go on heart attacks and a BLS unit will go on a leg injury.  It's often much more complicated than that, but not always.

["ECHO" was not in the original scheme of things but got added when it became clear that a "not breathing" response was needed. In some areas, non-water-carrying units respond, and in some areas, police, rescue squads or certain administrative units can respond.]

This is how it was in Jan 2003, for both County Fire & San Jose Fire, when I left.  I don't think it is significantly different now.

ECHO = Fire C3, ALS Ambulance C3
DELTA = Fire C3, ALS Ambulance C3
CHARLIE = Fire C3, ALS Ambulance C3
BRAVO = Fire C3, ALS Ambulance C2  
ALPHA = Fire C2, ALS Ambulance C2

[C3 = Code 3 (lights and sirens), C2 = Code 2 (no lights or sirens)]

If and when BLS ambulances get merged into this plan is a question that has not yet been determined. It may be some day, but I would not care to guess whether they would be invited to respond on Charlie, Delta or Echo calls, except in unusual circumstances.

Now we get to "OMEGA" responses.

Recently, San Jose Fire was heard to dispatch a "lift assist" as an "OMEGA" response. When I left the employ of Santa Clara County, we had not implemented “OMEGA” responses. It was my understanding at the time that "OMEGA" would be used for those circumstances when callers were referred to an advice nurse, or encouraged to contact their own physician, meaning that the problem was not an emergency requiring paramedic response.  So I asked one of my friends, a dispatcher supervisor at San Jose Fire, for an explanation of their current thinking on the topic. I'm going to paraphrase his response here, for your information.”

“Using the Medical Priority Dispatching System, a few "type" codes end up as "OMEGA" responses. They are found on the "fall" card, the "maternity" card and the "overdose" card.    When someone falls and is not injured, but needs help getting back on their feet (or into bed), San Jose Fire considers it an "OMEGA" response, and they dispatch a fire unit code two “on “OMEGA” response, lift assist." No ambulance is sent, as they do not regard this as a medical call.

On the maternity card, the Emergency Medical Dispatcher can work through the questions on the card and end up at an "OMEGA" response, meaning that there are no priority symptoms needing a response, and they recommend self-transport to the doctor's office or hospital emergency room.  All of the criteria on the card have been approved by SJFD's EMS medical director.

With an overdose call, if a child has (for example) eaten the silica gel that comes packed with certain products, after consulting with poison control, they may also end up at "OMEGA," and recommend to the caller that the parents consult with their pediatrician.”