Soviet President Konstantin U. Chernenko suffers from irreversible emphysema but is expected to live at least six months, according to the latest confidential Reagan administration estimates.

The administration also believes that Chernenko's health will not affect forthcoming arms-control negotiations with the Soviets, certainly not in the short run.

The U.S. view, disclosed under ground rules that permitted neither identification of sources nor direct quotes, is that Chernenko, 73, is in control but may divest himself of some duties.

Reports have circulated that he might give up either the presidency of the Presidium or the post of general secretary of the Communist Party, although yielding the latter would strip him of much power. Some observers believe that the reports are being circulated by the KGB, the Soviet secret police.

U.S. officials anticipate continuity in Soviet policy. Their view is that the military, a major Soviet power center, is committed to arms control and that any prospective successor to Chernenko would take this into account.

Because Chernenko is viewed as a transitional leader by both the United States and the Soviet Union, his poor health is not considered decisive to superpower relationships.

In addition to minimizing the effect of Chernenko's health on U.S.-Soviet negotiations, American officials have noted that relations between the two countries appear to have stabilized. They anticipate that negotiations with the Soviets on arms control and other issues will continue throughout President Reagan's second term.

Much evidence that Chernenko has taken a turn for the worse is circumstantial. Speculation about his health, which has been almost constant since he took office a year ago Wednesday, rose anew when he failed to meet Greek Premier Andreas Papandreou last Tuesday.

Greek diplomats had expected that Papandreou would be granted an interview, and when he was not it was suggested that Chernenko's health was even worse than had been thought.

But U.S. officials are satisfied that the rumor that Chernenko had suffered a stroke is untrue and believe that his health is declining steadily but slowly, as is customary with emphysema. The best administration judgment is that Chernenko could have only six months to live, but there is no assurance that his debilitating disease will move at a predictable rate.

Emphysema, according to standard medical references, is a common, usually irreversible, often fatal disease in those whose lungs have been exposed to various irritants such as smoke or other chemicals.

Emphysema's main feature is a breakdown of the thin walls that form the lungs' countless tiny air sacs. The walls provide surfaces where blood vessels can come into contact with oxygen. As cavities form and grow larger, sometimes reaching diameters of several inches, less and less oxygen gets into the bloodstream.

Patients become permanently short of breath and can sustain only the mildest activity. Although progression of the disease sometimes slows or stops, it often resumes or continues until too little oxygen reaches the blood to sustain life.

Speculation about a sudden downturn in Chernenko's health also was fueled earlier this week when a prominent Soviet surgeon, Dr. Evgeny Chazov, former Soviet leader Leonid I. Brezhnev's physician, seemed to cut short a speaking tour in the United States to return to Moscow.

The State Department and Chazov's host group in the United States said Chazov returned to the Soviet Union as had been planned several weeks earlier.

Chazov, head of a branch of the ministry of health that looks after the health of Soviet VIPs, had been in the United States as the guest of American doctors campaigning against nuclear war.

A State Department spokesman said his agency had been assured by the Soviet Embassy that Chernenko's doctor hardly would be touring the United States while his patient lay ill. Chazov had said the same thing several times while here.

Chazov is a cofounder of the International Physicians for the Prevention of Nuclear War. His visit to the United States was hosted by an affiliated American group, Physicians for Social Responsibility.

Originally, the Soviet surgeon had been expected to tour with a Soviet-American delegation until Monday but, according to a PSR press aide, he notified the American group two weeks ago that he would have to go home Feb. 13. Chazov did leave about five hours ahead of schedule, when snowstorms threatened to close the airports in Ohio, where he was speaking.