His puffed face straining in concentration, the old man slowly moved an American-made pen over the thick, specially bonded paper, scrawling in single strokes of blue ink on the final page of the United States version of the SALT II treaty.

Only machine noises of cameras and the rustle of reporters and diplomats craning in the audience to watch disturbed the quiet as President Carter waited for his frail counterpart to complete the task of signing his name - Leonid L. Brezhnev.

Then the old man pushed himself away from the graceful, gold-leafed conference table once used by Austrian Emperor Franz Josef, got to his feet and in a moment now fixed in the world's retina, slowly embraced and exchanged kisses with the younger leader.

Unconcealable infirmity - in his limping, careful old man'walk; glassy-eyed, unfocused stare, garbled, sunken speech and the brief looks of puckered anxiety flickering over the faces of his entourage - haunted Brezhnev's summit with Carter from the first day to the last.

When the two men arrived last week, it was Carter who made an arrival speech and Brezhnev who walked silently and with seeming confusion through airport arrival ceremonies.

When they paid a joint state call on Austrian President Rudolf Kirchschlaeger Friday and attended the Vienna Opera the following night, it was the American who trooped the ceremonial marble steps to bystanders' applause and the Russian who rode the elevator.

Although the president kept a limited a schedule of public appearances that surely minimized the difference between the two men's vitality for the supersensitive Soviets, nothing could have underscored that disparity more than Carter's well-publicized morning jogging.

The Brezhnev seen here bore little resemblance to the Brezhnev of the 1973 Soviet-American summit in the United States. Then the Soviet leader enjoyed joking and mixing with his hosts, quipping with journalists and on not notable occassion, allowing himself to be bear-hugged off his feet by television cowboy star Chuck Connors.

During his brief public appearances here, the Communist Party general secretary seemed drawn into himself, oblivious to anyone except an everpresent military aide or Konstantin Chernenko, his administrative assistant who is now a full member of the ruling Politburo. He frequently turned to these men for protocol advice and then woodenly followed their instructions.

The Americans say Brezhnev in private was an alert participant in the talks but confined himself almost exclusively to reading from position papers. He was said to have been especially active on Saturday but his energy seemed to flag as the summit proceeded.

At the Saturday dinner at the U.S. Embassy, Brezhnev drank two or three glasses of wine but on Sunday at the Soviet Embassy's return dinner he seemed greatly subdued to one American official who was present.

Brehnev's frailty unexpectedly intruded during the climax of today's summit, when his eyes suddenly closed and he appeared to doze off after the SALT II documents were signed. CAPTION: Picture 1, Soviet President Brezhnev waves goodbye as he departs Vienna for Moscow. UPI; Picture 2, An aide assists Soviet President Brezhnev in boarding his plane at Vienna. AP