Dr. Howard H. Hiatt, dean of the School of Public Health, met with President Reagan yesterday as part of a delegation sponsored by the Vatican urging him to strengthen efforts to abolish nuclear weapons.

Hiatt said last night that Reagan "acknowledged" during the 20-minute White House meeting that a nuclear exchange would "end civilization as we know it" and that it is impossible to "reconcile this with thoughts that one can win or survive a nuclear war."

The meeting was the first of several emissaries of Pope John Paul II plan with leaders of four of the five recognized nuclear powers. As in the discussion with Reagan, representatives of the Pontifical Academy of Scientists will join the Vatican delegation.

At yesterday's meeting, Hiatt and three other experts told the president that the casualties of even a single megaton explosion in Washington, D.C., would overwhelm the majority of the area's medical facilities. "In addition to which all the hospitals would be gone," he said.

Reagan "did not attempt to dispute this," Hiatt added, saying that while the president expressed his goal to limit the roliferation of nuclear arms, he remained faithful to that of negotiating "from a position of strength."

The Pope's message, formed after an October meeting of the Pontifical Academy of Scientists, did not search for mechanisms to control nuclear arms, but rather "expressed the underlying need to do so," Hiatt said. The academy chose Hiatt, who is not a member, as a technical adviser.

Reagan accepted the message as a "statement of moral concern," David Baltimore, professor of biology at MIT and a member of the Vatican group, said last night, adding that the president seemed to clearly understand "that a nuclear exchange would mean the end of the world as we know it."

He added that Reagan did, not answer the pontiff's call for limiting nuclear weapons with any specific policy suggestions. Reagan's response, Baltimore said, was a comment on "the Biblical notion of Armaggedon."

Other members of the delegation to the White House were Marshall W. Nirenberg, professor of genetics and biochemistry at the National Institutes of Health; Victor F. Weisskopf, professor of physics at MIT; and the Papal Nuncio, Archbishop Pio Laghi.