More than half of all heart pacemakers implanted by doctors are not clearly beneficial, according to the first detailed study of the increasingly common procedure.

A committee of doctors reviewing all pacemaker operations in Philadelphia County during a six-month period in 1983 found that 20 percent of the implants were unnecessary and that another 36 percent were of questionable value.

The results of the study, published in The New England Journal of Medicine today, support the widely held view that far too many such operations are performed.

"Many pacemakers are just put in for the wrong reasons," said Allan M. Greenspan, a cardiologist at the Albert Einstein Medical Center in Philadelphia and an author of the report. "Some doctors don't ask the right questions. And, in some cases, we don't know the right answers."

One in 500 Americans have permanent pacemakers, and 120,000 implants are performed each year, according to the study. At an average cost of $12,000, the annual expenditure for the operations approaches $1.5 billion.

Although the study notes that doctors can make large profits by performing the relatively simple operation, the authors suggest that ignorance and fear of malpractice suits may be the most common causes of the needless heart surgery.

The review examined in unusual detail the records of all 382 people who received pacemakers in Philadelphia during the first six months of 1983. The committee, organized by the city's Professional Standards Review Organization, found that 168 implants were clearly necessary, that 137 may have been necessary and that 77 should not have been made.

The article and the author of an accompanying editorial suggest several measures that could reduce the high rate of pacemaker implants.

Among these are better understanding of symptoms that would call for pacemakers, better education for physicians so they can understand arrhythmias, irregular heartbeats that can require the electric stimulation of a pacemaker, and more rigorous peer review.

In at least one New York hospital, the level of pacemaker implants dropped substantially after strict peer-review assessments were instituted, the Journal said.

Private and public health insurers now emphasisize cost reduction. Many insurance companies will not pay for pacemaker operations that they do not specifically approve.

"I don't think the situation has changed that much since 1983," said Dr. John A. Kastor, chairman of the department of medicine at the University of Maryland Medical School in Baltimore. "Doctors find themselves facing pressure to reduce costs on one side and the constant threat of malpractice suits on the other."

The study also found that many internists who ordered pacemaker implants did not consult with heart specialists. "That's a distressing finding," Kastor said. "The pacemaker business is 20 years old now, and there are many physicians who assume old knowledge will serve them well enough."

Pacemakers should often be installed when doctors examining the heart's electrical rhythms conclude that it beats too slowly, according to medical experts. But the study found that physicians often ordered them unnecessarily.

It also found that some people who suffered from dizziness and other symptoms were not given tests needed to pinpoint their problems.

The physicians who conducted the study said in the article that, in many cases, records of patients they examined were incomplete. But they added that it has been common in hospitals for doctors to read test results and forget to include them in the file.

"At times, the problem could be documenting the evidence," Greenspan said. "It doesn't always mean that the doctor didn't do the tests properly."