A leading cardiologist said yesterday that 25 per cent of coronary bypass operations - the most frequently performed heart surgery in the United States - are done "on the mistaken assumption" that patients will be less likely to have a heart attack or die suddenly.

Aside from the assumptions about the effects of the operation, Dr. Richard S. Ross told the Senate Human Resources health subcomittee that strong economic pressures help account for the "explosive growth" of bypass surgery. "We are dearling with a multimillion-dollar enterprise which is very hard to turn off." Ross said.

Ross dean of JOhns Hopkins University Medical School and former chief of cardiology at Johns Hopkins Hospital, estimated that 60,000 to 100,000 bypass operations - at an average cost of $12,500 each - were performed in 1976.

The coronary bypass operation involves removal of a portion of a vein from a patient's leg and use of the vein to construct a detour around an obstruction or blockage in one of arteries carrying blood away from the heart. The operation now accounts for about 60 per cent of all cardiac operations performed in this country, according to Ross.

Although the operation has been shown to be effective in treating patients suffering from chest pains resulting from heart disease, known as angina pectoris "there is no evidence that the operation prevents sudden death or makes patients with coronary artery disease live longer," Ross said.

He estimated that 25 percent of the operations are done on patients with no symptoms or pain after routine testing shows a blockage in a coronary artery or on patients with mild symptoms that might have been treated with medication rather than surgery.

The lack of proof that the operation prolongs life "has not been generally accepted by the medical profession or by the public and the number of operations continues to increase," Ross said.

"Hospitals have come to see cardiac surgery programs as status symbols," Ross aid. "They compete withe each other to have the latest equipment and the biggest case loads. Cardiac surgical programs are developing in suburban hospitals which 10 years ago would never have considered such a program to be an economic feasibility, and in most of these hopsitals the bypass procedure is the only heart operation being done.

"The administrators see the new program as a way to fill beds and increase revenue. The diagnostic and therapeutic equipment required for such a program is expensive and once purchased it must be used, to amortize the purchase cost. The need to generate a case load to justify the program undoubtedly results in overuse," Ross said.