Northern Virginia's health planning agency announced yesterday that it would sue to stop Fairfax Hospital's planned heart transplant unit, prompting the chairman of the county Board of Supervisors to threaten to cut off the agency's funds.

"From the evidence we have . . . Washington metropolitan area residents are well-served by existing programs," said Mark H. Epstein, associate director of the Northern Virginia Health Systems Agency. The agency contends the Fairfax program would duplicate heart transplant units in Baltimore and Richmond and that the hospital fails to meet nationally accepted guidelines for volume and previous transplant experience.

Epstein's agency had until yesterday to serve notice it would challenge Virginia Health Commissioner James B. Kenley's March 28 decision -- made over the agency's objections -- to allow the hospital to open the area's first heart transplant program. The suit will be filed within a few weeks in Fairfax County Circuit Court, Epstein said.

John F. Herrity, chairman of the Board of Supervisors, said that if the agency sues, he will ask his fellow supervisors to freeze its county payments, which account for about 7 percent of its $325,000 yearly operating budget. The supervisors supported the hospital's bid to perform transplants.

"For these bureaucrats to stick their nose into things and start using taxpayers' money to do it is inconsistent with our objectives," said Herrity. "Their budget must be too fat if they have money to waste on lawyers' fees."

Replied Epstein: "Holding up the purse strings is not a logical way to plan for the health care needs of the citizens of Fairfax County or the Washington area." He called Herrity's threat "inappropriate."

This is not the first the time the health planning agency has tangled with county supervisors. In 1984, the supervisors bucked an agency recommendation against any new hospitals in western Fairfax County and voted to approve the Fairfax Hospital Association's proposal to open a 160-bed institution in the Fair Oaks area. The supervisors later granted a zoning variance for a private hospital in Reston.

J. Knox Singleton, president of the Fairfax Hospital Association, said he did not think a lawsuit would delay plans to begin performing transplants as soon as patients and organs become available. He said he is confident the hospital will prevail.