Fairfax County's Board of Supervisors agreed yesterday to consider buying a new computer software system for its 911 operators that proponents say would enable them to better evaluate medical emergencies and provide improved help over the telephone.

However, police and fire officials said they probably couldn't implement the new system without additional staff.

The move came in response to arguments by the Fairfax firefighters union that a computerized system called Emergency Medical Dispatch--which prompts call-takers to ask more questions and to provide specific medical instructions while help is on the way--could enhance public safety. Fairfax is the only county in the Washington area without the system.

In a memo to the board, County Executive Robert J. O'Neill Jr. noted that Fairfax operators already receive training in medical dispatching during their initial communications schooling. They are taught how to give instructions for CPR, choking, poisons, overdoses and assaults, he said, and 29 of the current operators have voluntarily taken training as emergency medical technicians.

But the firefighters union said that sometimes call-takers still don't ask enough questions of callers or don't send the proper equipment. Union President Michael Mohler said yesterday that he was dispatched to the Fairfax County Government Center this month for a woman trapped in an elevator with an injured ankle. He said three firetrucks were dispatched to the scene, but no ambulance.

Mohler said that as he approached the Government Center, "The dispatcher asked me, 'Do you need a medical unit [ambulance]?' I said, 'Yeah, unless you want me to throw her in the cab of the fire engine.' . . . The equipment that gets dispatched is still not correct."

Mohler was irate that county officials played down the need for a more formalized dispatch system, which he said would enable operators to better judge the type of emergency equipment needed at an incident. "As technology changes," he said, "this county has a responsibility to provide the best training and equipment they can afford to protect the citizens of Fairfax County."

During yesterday's board meeting, O'Neill told the supervisors that the county's emergency operators already have a "substantial" training requirement. The operators currently meet federal standards for medical dispatching, and Police Chief J. Thomas Manger said, "I don't believe there are any cases we've had where a person who called 911 was not given proper information."

Board Chairman Katherine K. Hanley (D) said of the operators, "We need to be very clear about what they are getting and what they are not getting." Supervisor Sharon S. Bulova (D-Braddock) said the operators are "adequate, they do a good job. On the other hand, we can do better, we can enhance the training."

O'Neill said he would propose purchasing the software system, at a cost of up to $200,000. The proposal would be voted upon in September. "The only unanswered question," O'Neill said, "is if it has additional staffing requirements. Even if you authorized it, we have unfilled positions."

Keeping 911 operators has been difficult for the county in recent years. Communications director Michael B. Fischel said yesterday that even with 14 new trainees, there are still 12 vacancies.

The number of operators also hasn't been increased by the county since 1987, Fischel said. Manger said purchasing the software may allow communications officials to determine how many more employees they will need, since the Emergency Medical Dispatch system requires call-takers to stay on the line longer, possibly delaying them from answering other calls.

Supervisor Robert B. Dix Jr. (R-Hunter Mill) asked Fire Chief Edward L. Stinnette what rank-and-file firefighters thought of the issue. "Our men and women believe we need to move forward" on getting the new system, Stinnette said.