U.S. Rep. Gladys Noon Spellman of Prince George's County, who was struck down by a massive heart arrest Friday evening, remained in critical condition yesterday but showed gradual signs of improvement throughout the day, according to doctors at Greater Laurel-Beltsville Hospital.

Cardiologist Michael Schwartz said last night Spellman's blood pressure had returned to normal and that the 62-year-old Democrat, while still in a "sleep-like state" and unable to talk, seemed to be aware of her surroundings.

"The heart is pumping well," said Schwartz. "It seems to be doing what it should." He said that while he was encouraged by her progress, it was still impossible to determine whether Spellman had suffered any brain damage or would ever make a full recovery from the heart arrest, which hit her early Friday evening as she was making a Halloween appearance at a Laurel shopping mall.

Heart arrest occurs when the heart muscle stops pumping, depriving the body, perhaps fatally, of a fresh blood supply. It can have a number of causes, including a heart attack, which specifically implies damage to the heart muscle itself.

Before the news of Spellman's gradual improvements were made public last evening, family and friends gathered outside her room of the modern hospital's third floor, and county officials began quietly preparing for the possibility that Spellman might not survive the incident.

"The question is whether she'll wake up," said Francis B. Francois, a longtime friend of Spellman who served in local government with her. "We really won't know until she wakes up" how seriously the heart arrest affected Spellman.

Until late yesterday afternoon, hospital authorities had declined to release any information on Spellman's condition, with sources reporting grave concerns for her survival. Schwartz's statement, the first optimistic note to be heard since the incident Friday, also revealed that Spellman had been suffering from angina, heart pains associated with heart disease, for the last few years.

County and state election officials spent the day researching election laws to determine what procedures they would have to put into effect should Spellman not survive until Tuesday's election. According to Willard Morris, the state administrator of elections, the Maryland Democratic Central Committee, possibly in conjunction with the county central committee, could immediately appoint a candidate to replace Spellman. That candidate's name would be printed on stickers and placed over Spellman's name in all voting booths in the Fifth Congressional District.

There is a consensus in Prince George's County that if Spellman's name is on the ballot -- regardless of whether she survives -- she will win a landslide victory over her Republican opponent, Ken Igoe. Should she live through the election but then succumb, Gov. Harry Hughes would have 60 days to call a special primary and general election. The special primary would have to be held 35 to 80 days after the governor's announcement, with a similar period allotted for the general election.

If such a scenario were to evolve, the county would be thrown into political turmoil. Spellman is the county's top elected Democrat and most popular politician. A vacancy in her congressional seat would prompt major changes in the county's political structure and alliances. It also would cause the Republican Party, which has essentially abandoned all efforts to win the seat in the face of Spellman's overwhelming popularity, to make its first serious bid for the post in years.

Spellman suffered the arrest Friday evening at about 6:30 as she made her way toward a Halloween costume contest for local children that she was supposed to help judge in a Laurel shopping mall. According to several accounts, Spellman began to descend the stairs leading to the pavilion area where the show was being held when suddenly she collapsed.

Kay Bienen, a state legislator for Laurel who was one of the three other judges at the contest, said that the judging had just begun when she heard a commotion behind her. "We looked over and heard someone ask if anyone there knew CPR [cardio-pulmonary resuscitation]. There was a big crowd and I couldn't see anything."

Laurel Mayor Robert DiPietro, who had been emceeing the costume contest, immediately walked toward the scene and told one of Bienen's colleagues to take over the announcing, Bienen said. About 10 minutes later, when things had calmed down, Bienen said she turned to one of the show's organizers and asked, "Was it a parent or a kid," and someone responded, "It was Gladys." Bienen, stunned, looked over and saw that Spellman was being given mouth-to-mouth resuscitation by two nurses who happened to be in the audience.

A short time later, a rescue squad arrived and parametics administered electric shocks and injections in an effort to stimulate Spellman's heart as she was spread out, unconscious, on the floor of the mall. Then they took her to the emergency room at Greater Laurel-Beltsville Hospital.

It apparently took emergency room doctors more than an hour to improve Spellman's pulse, and then, at about 8:30, she was moved to the hospital's coronary care unit, where she was being treated yesterday.

All day yesterday, Spellman's friends and political associates reacted with astonishment to the news of her illness. Most said that she seemed generally fit and healthy, despite a bout last summer with pancreatitis, an inflammation of the pancreas that is not related to heart disease. But despite her apparent good health, rumors had surfaced frequently in recent years that Spellman had been pushing herself too hard and was considering resigning from Congress. Such rumors were usually floated by persons interested in obtaining her seat.

If a special election were held to fill Spellman's seat, the race, particularly the primary, would be "a blood bath," as one county politician put it. Dozens of Democratic hopefuls who have kept their congressional aspirations in check over the past six years, might jump into the wide-open fray.

Among the possible candidates mentioned by county politicians yesterday were Francois, who recently retired from the County Council; former state Senate president Steny H. Hoyer, who does not live in Spellman's district but could run for the seat anyway; former county executive Winfield M. Kelly Jr.; and the party's U.S. Senate nominee, state Sen. Edward T. Conroy.

"It's a very ghoulish business," said one county politician who did not want to be identified. "The buzz is already out." Added another politician: "Everyone's funny because they don't want to say anything, but you know it's popped up in everyone's mind." CAPTION: Picture 1, GLADYS NOON SPELLMAN . . . shows gradual improvement; Picture 2, GLADYS SPELLMAN . . . condition improves slightly