Rosalynn Carter, a face rarely seen on Washington's ladies luncheon circuit, made what may have been her debut appearance yesterday at a fashion benefit here. It was the annual Affair of the Heart luncheon at the Sheraton Park Hotel, which netted an estemated $20,000 for the local afficiate of the American Heart Association's Heart Saver program.

Mrs. Carter's appearance was something of a coup for the association's women's board chairman, Ruthanna Weber, who had first extended the invitation to her "She's interested in serious things," explained Weber when asked how she'd gotten the First Lady to come.

At a pre-luncheon reception, Mrs. Carter watched a demonstration of "Resusci-Anne," a Norwegian invention used in cardiopulmonary resuscitation training. Dr. Ross Fletcher, president of the heart association's Nation's Capital Affiliate, demonstrated the technique, then invited Mrs. Carter to try her hand. After several unsuccessful attempts, the First Lady finally got a yellow light, indicating that she had applied the proper pressure on "Annie's" chest.

Fletcher said the association hopes to train 20 percent of the District population in such lifesaving techniques by the end of next year with the aid of these mannequins. The ambassador from Norway, Soren Christian Sommerfelt, presented the display mannequin, valued at $1,000, as a gift from the Norwegian inventor, and announced that the inventor would also contribute $5,000 to the association.

Woodward & Lothrop presented the spring collections of three designers --Jerry Silverman, Sal Cesarani and Anthony Muto -- in their fashion show for the 1,400 guests. Of the three, only Silverman did not attend the show.

Cesarani, a menswear designer who presented his first women's collection last fall, said he thought Mrs. Carter could relate to his collection of "easily constructed" sportswear pieces. "She could wear them in the country in Maryland or with her children on the weekends."

Muto also felt the First Lady could wear his collection of evening clothes designed for the A/M P/M and Marita firms. "I can see her in a lot of them," he said. "It will be interesting to see how she reacts to them."

Mrs. Carter said that she "enjoyed the show" and "saw a lot of things she liked," but gave no hint as to what they were. She has bought from designer Jerry Silverman in the past, including purchases made a year ago during a well-publicized spring buying trip to Seventh Avenue.

Since then, Mrs. Carter has enlisted the professional assistance of Georgia Young, of Lewis & Thomas Saltz, as her personal fashion scout. "Now georgia can find it faster for me," she said.

At the head table yesterday along with Mrs. Carter and Susan Goldwater were the wives of the secretaries of Agriculture, Transportaion, Energy, Labor and Defense.