PRESIDENT REAGAN is spending the Easter weekend on the Caribbean island of Barbados. According to official Soviet sources, his counterpart, Soviet President Leonid Brezhnev, is also otdich, taking a rest.

Unlike Reagan's rest, however, which is reported by the world press, there are no pictures of Brezhnev warming himself in the sun of the Black Sea coast. Instead there are reports published in the West that the Soviet leader is still in Moscow recuperating from severe fatigue and a possible stroke suffered while on a recent trip to Soviet Central Asia. According to these unofficial sources, the 75- year-old Brezhnev was so weak on his return he had to be carried off the plane on a stretcher.

Evidence supporting a deterioration in the Soviet president's health is strong. During and since his return from Uzbekistan, there have been no photographs of the 75-year-old leader. A meeting of the Communist Party Central Committee was postponed. Brezhnev's personal physician, cardiologist Yevgeniy Ivanovich Chazov, abruptly canceled a trip to England where he was to have participated in an international meeting of Physicians for the Prevention of Nuclear War, a meeting both Brezhnev and Reagan endorsed with letters of support.

Exact details of the Soviet leader's condition are unknown. Unlike the current situation in the United States, where the medical nuances of president's health, from Jimmy Carter's hemorrhoids to Ronald Reagan's recent urinary tract infection, are public knowledge, the health status of Soviet leaders is never discussed. When Brezhnev takes ill, it is claimed, as it was last week, that he is resting or on vacation. Even if he becomes critically ill, the Soviet people will not know about it until he dies.

Death could come soon for the aging leader. From interviews with Soviet physicians who have been consulted on Brezhnev's care and from conversations with American physicians who have frequent contacts with high- ranking Soviet physicians like Dr. Chazov, it is clear that Brezhnev is a sick man whose performance over the past 10 years is the result of a strong will combined with careful medical treatment.

Brezhnev's major medical problem is generalized atherosclerosis, "hardening of the arteries." The disease affects his coronary arteries and has led to two heart attacks. It also involves the blood vessels to his brain, causing one major and an unknown number of minor strokes.

Although many thousands of persons have atherosclerosis (atherosclerotic heart disease is now the leading cause of death in both the United States and the Soviet Union), it is not an invariable accompaniment to aging. At 71, President Reagan is only a few years younger than Brezhnev but shows no evidence of the problem. Reegan's cholesterol level, a risk factor for atherosclerosis, has always been normal and he has never smoked or had high blood pressure, two other important contributing factors. Exercise or "stress test" electrocardiograms done on the president since 1969 have been negative. Finally, Reagan's prompt recovery from last year's assassination attempt is dramatic evidence of his superior physical condition and an excellent example of the large difference in health status between the two world leaders.

When Reagan was shot, the bullet punctured his left lung, which began bleeding. Deapite the rapid loss of 20 percent of his blood volume, the president was able to maintain consciousness throughout the ride to the hospital, becoming weak only upon entering the emergency room. There, after the president was placed on a stretcher and given a liter of fluid, his blood pressure returned to normal and his mental confusion cleared.

A similar wound would have killed Brezhnev immediately. His heart, weakened by the two myocardial infarctions, would not have been able to make up for the sudden blood loss by pumping harder. Besides, the Soviet president has severe lung disease, caused by years of heavy smoking.

Although Brezhnev quit cigarettes 10 years ago, their constant use during most of his life left him with emphysema (destruction of lung tissue) and chronic bronchitis. These problems make Brezhnev a respiratory cripple, frequently short of breath and unable to walk more than a few steps without stopping. The lung problems are the reason the Soviet president is so slow to walk down the ramps of airplanes. They are why he frequently stops in mid-sentence when making speeches. His respiratory disease is so advanced that a severe cold, much less a bullet wound, could be fatal.

The aftereffects of Brezhnev's strokes are also evident in his infrequent public appearances. A severe right-sided stroke during the mid-1970s left Brezhnev with weakness on the left side of his body. For a time he also had slurring of his speech -- a problem he has largely overcome but that reappears during long speeches or when he is tired.

To conserve his limited strength, visits of foreign dignitaries are tightly controlled, seldom exceeding an hour. When Brezhnev begins to rise, signaling an end to the meeting, there are two persons at his side lest he fall. Other precautions are taken to avoid any public embarrassment. Television cameras film him mainly face-on or from the right side and move to shots of the audience if he falters. At state dinners, Brezhnev frequently eats with a spoon to avoid dropping his food.

Because of the fragility of Brezhnev's condition, physicians limit the number and length of trips. Long plane rides are dangerous because of the risk of a pulmonary embolus or blood clot to his lungs. During his 1980 visit ot India, he took blood thinners to reduce the risk of a fatal clot. He did not take them during his recent trip to Central Asia and that, combined with an unusually heavy speaking and meeting schedule, is thought to have contributed to his current problems.

Brezhnev's many medical problems require constant supervision. Along with the elimination of cigarettes, he follows a low-salt diet. Once a regular drinker, he now carefully limits his intake of alcohol.

After one of his heart attacks, Brezhnev developed heart block, a serious disturbance that required a pacemaker to restore a regular heartbeat. Pacemakers are not generally available to Soviet citizens and Brezhnev's model was imported from the United States. During the late 1960s, Soviet physicians considered performing coronary artery bypass graft surgery on Brezhnev, an operation that is now very popular in the United States. At that time, however, the surgery was considered too high risk. Today Brezhnev is too ill to consider any type of operation.

Today the best that can be done for the Soviet president is to prescribe frequent courses of antibiotics for his recurrent lung infections and make sure his high blood pressure is carefully controlled.

The tension of domestic and international crises make blood pressure control difficult. During the recent Polish crisis, there was much discussion among Brezhnev's physicians over the best way to handle this problem. Some feel that it was, in part, the failing health and uncontrolled hypertension of President Franklin D. Roosevelt that may have give Stalin important advantages during their February 1945 meetings in Yalta. (The lack of drugs to treat Roosevelt's hypertension led to his death from a large stroke in April that year.) To prevent a similar occurrence with Brezhnev, his blood pressure medication was increased and his regular rest periods prolonged.

During periods of high stress, Brezhnev is reported to have angina (heart pains). These attacks must give persons close to the Soviet president very anxious moments. As one of the physicians who had a small role in treating President Reagan after his emergency surgery, I can imagine the apprehension Soviet physicians face waiting to see if the nitroglycerin tablet relieves Brezhnev's angina.

All of this is also reported to ge taking its toll on Brezhnev's mood. He is said to be depressed, despondent over his own failing health and discouraged by the death of many of his long-time colleagues. To help, he has turned to regular counseling and hypnosis by an Assyrian woman, a sort of modern-day Rasputin.

Until this most recent episode, however, there was no indication that Brezhnev's many health problems affected his ability to reason. World leaders who had spoken to him prior to March reported a determined, almost fierce, concentration in his eyes as he listened to discussion. During the translations, which, because of a progressive hearing loss, were spoken directly into his ear, Brezhnev looked straight ahead. His responses, although short, were to the point and occasionally mixed with rough humor and a coarse laugh that frequently collapsed into a short coughing spell. Although clearly limited in stamina, he was still the man in charge.

Given the curtain of secrecy that surrounds the Kremlin, it is impossible to know how much of this, if any, is now changed.

What we are more sure of is that, regardless of his current medical aituation, Brezhnev is receiving high-quality medical care. The Soviet president's personal physician, Dr. Chazov, maintains close professional contacts with a number of prominent American doctors and physicians in other countries. If there is any treatment the Soviet president needs, Chazov, an excellent cardiologist, will make sure he gets it. But even the most advanced medical care cannot stop the relentless decline in Brezhnev's health.

The Soviet leader may or may not recover from this latest setback (his tenacity should not be underestimated). But a face-to-face meeting between the two leaders, as suggested by President Reagan, would be a severe challenge -- even for a man who has had to constantly battle personal as well as political threats to remain alive and in office.