Top health officials of the Department of Health and Human Services have recommended that the government begin paying for heart transplants for a limited number of Medicare patients -- those under about age 55 -- and Secretary Margaret M. Heckler is "favorably disposed" to the idea, sources said yesterday.

In addition, the Public Health Service is recommending that new multimillion-dollar Magnetic Resonance Imaging (MRI) machines, which provide far better pictures of the central nervous system and of the soft tissues inside the human body than X-rays or CAT-scan devices, be approved for use on Medicare patients generally. Sources said the department is likely to adopt that recommendation also.

Approval of the recommendations would be a watershed in introducing some of the most advanced medical technologies -- capable of producing enormous health improvements and generating huge costs -- into the nation's largest medical program, which now totals about $70 billion a year.

Although immediate costs for heart transplants would be small because of the age-55 limit, many politicians and health specialists fear political pressure eventually would lead to a change in the age cutoff to include far more patients.

Heckler has made no decision on the transplant recommendation but previously made clear that she would favor Medicare heart transplants under limited conditions that would keep costs from ballooning.

In May, she appeared ready to signal a go-ahead but postponed a decision -- at White House insistence according to some sources -- and ordered the Public Health Service and the Health Care Financing Administration to study the issue further.

Those two agencies now have developed a plan for Medicare coverage of heart transplants that would limit eligible recipients to those whom medical specialists consider the best candidates. Generally that means people who are under their "mid-50s with adequately young physiologic age to permit successful transplantation" and who are, in most cases, free of other serious health problems.

These limits, according to a memorandum prepared by the two agencies, would exclude the vast majority of Medicare's 30 million beneficiaries, most of whom are over 65, leaving only certain categories of beneficiaries, such as the disabled and certain young adults.

As a result, the memo estimated that 63 people in 1986, 91 the next year, 103 in 1988, 115 in 1989 and 128 in 1990 would be eligible for the Medicare-funded heart transplants. The costs, therefore, would be relatively small, ranging from no more than $10 million in 1986 to no more than $50 million -- and probably half that -- by 1990, depending on restrictions.

The plan would limit the number of medical centers eligible to perform Medicare transplants. Only about 10 with substantial transplant experience would qualify the first year, rising to 20 the next.

The use of the new MRI machines also would be a revolutionary step. The machines cost $800,000 to $2 million each and require a special room costing hundreds of thousands of dollars; in addition, each scan can cost from $360 to $1,400.

For certain conditions, the MRI is the most advanced diagnostic tool in the world. It is less intrusive on the human body than other imaging devices.