Soviet Communist Party chief Leonid I. Brezhnev is a chronically ill, rapidly aging man with a failing heart, speech and hearing difficulties and other medical problems of uncertain nature that hindered him severely in Moscow this week.

Washington doctors who watch the health of Soviet leaders drew that picture this week of the 70-year-old man whose appearance shocked American diplomats and journalists at the U.S. Soviet arms control talks.

Brezhnev's health is considered vital to the success of the arms negotiations, since it is widely believed that he has been strongly committed to signing an agreement in Washington to ca phis career.

"Mr. Brezhnev has had hear failure periodically for some time, and his constant companion is a cardiologist who is deputy health minister and also in charge of the Kremlin polyclinic," said one government source.

"Heart failure" in medical terms means a heart that chronically or periodically fails to supply the body with enough blood.

Brezhnev's heart failure was called "mild" but still disabling to the point where Western observers think it highly possible, though they are not certain, that he has had an electronic "cardiac pacemaker" implanted inside his chest.

Reporters noted Wednesday that Brezhnev's movements and speech were slurred when he appeared before them. Such observations made medical observers think of the possibility of a stroke or a brain tumor, although they are far from ready to state a conclusion.

Brezhnev also failed to respond to reporters' questions. Apparently he did not hear them, it was later learned.

He sometimes wears a hearing aid, and during negotiations, it was reported, American officials had to raise their voices at him to make themselves heard.

"He has had slurred speech caused by recurrent jaw and dental problems for a long time," one medical source said. "That's probably what is bothering him now rather than a stroke, though we don't know for certain. We must just sit back and watch."

The "we," it was explained, refers to a small group of doctors in various federal agencies who scrutinize films and television clips of the Russian leader, interview persons who have seen him and gather other snippets of medical information.

Brezhnev's chronic speech problem was triggered by a jaw infection about five years ago, it was stated. That in turn was part of a series of dental problems that made the Russian leader's jaw hurt so much that on one public occasion he took his teeth out and stepped on them.

Even before the Strategic arms limitation Talks, it was said, Brezhnev had not been well and had missed meetings.

"This is far from a diagnosis," another source cautioned. "He did look bad this week, but he has looked bad sometimes in the past. There is much we don't know. There are probably not very many people even in Russia who do know."