The frail health of Soviet leader Leonid Brezhnev looms as a major factor shaping his summit conference next month in Vienna with President Carter, just as it is the dominant uncertainty within the Kremlin leadership today.

At 72, of halting step, increasing deafness and apparently flagging powers of concentration, the Soviet chief of state must search hard for the reserves of energy necessary for lengthy conversations with leaders of other countries.

This apparent limitation affecting his role as an active participant in prolonged talks emerged in some detail here during the visit three weeks ago of President Valery Giscard d'Estaing of France. Giscard, who has built up good personal relations with Brezhnev as the two pursued [LINE ILLEGIBLE] able to meet the Soviet leader privately for informal talks. Their meetings during the four-day visit were cordial and relaxed, but in formal setting surrounded by aides.

French sources later reported that Brezhnev's speech was slow and heavily slurred, his attention span was good for only about 90 minutes per session, and that he at times seemed disoriented. The menu at the state dinner made it possible for Brezhnev to use only a spoon for his food. The French trip to Moscow already had been abruptly delayed for three weeks while Brezhnev recovered from a reportedly serious bronchial illness.

While Brezhnev and Carter will discuss such major issues as the Camp David peace accords, which the Kremlin bitterly opposes, and human rights questions, the summit will center on signing the new strategic arms limitation agreement announced Wednesday in Washington by Secretary of State Cyrus Vance.

The substantive work of the accord has been completed, with exact wording and several minor issues to be resolved in Geneva before the summit set for June 15-18.

Well-informed Western diplomatic sources here suggest the question of Brezhnev's vitality may rule out the informal face-to-face talks favored by Carter as the best way to take measure of his Soviet counterpart and make his own points. American acquiescence to Vienna as the site of the talks instead of Washington, as protocol would have distated, is tacit acceptance of the Soviet leader's infirmity.

The precise state of Brezhnev's health cannot be authoritatively stated from here. The Soviets, long accustomed to routinely obscuring most details of their leaders' lives, have found ways to blur the truth about Brezhnev, refracting the image of a robust world leader for the Russian masses. Carefully edited television news clips seldom show him walking. His slurred speech is burned beneath announcers' prattle. He is never seen in tight closeup. The huge official portraits that adorn public gathering places in every city show a much younger man.

Brezhnev's health in recent years has swung unpredictably between sickness and zestful vitality. Sometime ago, he admitted in his memoirs that he suffered two serious heart attacks years ago. A long-time heavy smoker, he has quit after a long struggle. In public occasions he drinks sparingly.

Although he recently skipped or only briefly attended several important party functions, he was an animated spectator several nights running in April at the World Cup hockey matches. On May Day, he spent two hours atop the Lenin Mausqleum watching the annual Red Square demonstrations during a morning of intermittent showers.

But uncertainty over the leader's health may have prompted the official Tass news agency to insert the Russian term for "on or about" in its brief official statement yesterday announcing the June dates for the summit.

In the month between now and the summit, his first with an American president in 4 1/2 years, it is certain that Brezhnev will be, as always in recent years, under close medical supervision aiming to build his strength.

However, he will be forced to undertake several important state functions in between, including talks with Yugoslavia's Marshal Tito and Prime Minister Morarji Desai of India. Both visits are important: Yugoslavia's edgy relations with Moscow are strained because of Belgrade's warming ties with China and the Kremlin has long cultivated good relations with India. Western diplomats will be closely monitoring these sessions for clues to the Soviet leader's likely condition at the Vienna summit.

Brezhnev and his entourage will travel to Vienna by train, which he prefers to flying. The Soviet delegation will include Foreign Minister Andrei Gromyko and Politburo member Konstantin Chernenko, a close Brezhnev confidant who has passed under the leader's patronage into the Kremlin's inner circle in the last few years. Another likely member is Deputy Minister of Defense Nikolai Ogarkov, the principal military figure in fashioning the Soviet strategic arms negotiating position.

Brezhnev is an exceptional figure in Soviet history, a bureaucratic "vozhd" or supreme leader, who for almost 15 years has achieved or presided over Politburo consensus to wield power firmly yet cautiously in foreign and domestic affairs.

He has accumulated authority by isolating potential rivals, raising cronies such as Chernenko to position of influence, and adroitly maneuvering within the country's major power centers - the military, the Communist Party, the secret police, the economic planners.

Under Brezhnev, the Soviet Union has built up his conventional and strategic military power immensely. He has strengthened the grip of the Communist Party by taking the leadership posts of both a marshal of the Soviet Union. Brezhnev has tightened internal control over dissent and ideology, and backed party economists against would-be reformers who could diminish party role.

An experienced world leader, he finds no trouble espousing detente while exploiting America's post-Vietnam hesitancy over confrontation in such farflung places as Angola and the Horn of Africa.

Whatever his vitality, Brezhnev's Vienna summit conference undoubtly will provide him enormous personal satisfaction. He was presided over the Kremlin's end of detente - razryadki napreshonisti, or "relaxation of tensions" in Russian - for almost a decade. The cornerstone of that was the SALT I agreement, which lapsed in the fall of 1977. Carter's signature on the successor document. SALT II, promises to breathe new life into the bilateral relationship.

Western observers believe the Kremlin will pursue a policy of international restraint in the months between the signing and Senate ratification vote, lest some Soviet action touch off a furious Senate reaction. At the same time, as they have shown under Brezhnev's leadership the Soviets are ready to seize any advantage that comes their way, as in the revolutions in Ethopia and Afghanistan, and to move explosively to protect what they perceive to be their direct interests, as in the 1968 invasion of Czechoslovakia.