Odds of surviving a coronary bypass operation increased greatly over the past 15 years because of improved surgical techniques, a study showed Thursday.

The study in the Journal of the American Medical Association studied 5,809 patients - 2,962 who underwent bypass operations to replace narrowed arteries - over a 15-year period beginning in 1969.It indicated surgical survival improved progressively and was significantly better for most heart patients than other forms of medical therapy - regardless of such factors as age and heart muscle pain.

"Patients who benefited most were those with more severe coronary disease, those who had the worst prognosis or those who were operated on more recently," said Dr. Robert Califf of Duke University Medical Center in Durham, N.C.