Last Thanksgiving, Cynthia Mosson had been on her feet all day in her kitchen in Frankfort, Ind., preparing dinner for nine. She was nearly finished — the ham in the oven, the dressing made — when she suddenly felt the need to sit down.

“I started hurting in my left shoulder,” said Ms. Mosson, 61. “It got really intense, and it started to go down my left arm.” She grew sweaty and pale and told her family, “I think I’m having a heart attack.”

An ambulance sped her to a hospital where doctors confirmed that she had suffered a mild heart attack. They said testing revealed serious blockages in all her coronary arteries and told her, “You’re going to need open-heart surgery,” Ms. Mosson recalled.

When such patients head into an operating room, what happens next has a lot to do with their sex, a recent study in JAMA Surgery reported. The study reinforced years of research showing that male and female patients can have very different outcomes following an operation called coronary artery bypass grafting.

C.A.B.G. (pronounced like the vegetable) restores blood flow by taking arteries from patients’ arms or chests, and veins from their legs, and using them to bypass the blocked blood vessels.

“It’s the most common cardiac operation in the United States,” taking place 200,000 to 300,000 times a year, said Dr. Mario Gaudino, a cardiothoracic surgeon at Weill Cornell Medicine and lead author of the study.

Twenty-five to 30 percent of C.A.B.G. patients are women. How do they fare? The mortality rate for C.A.B.G., though low, is much higher for women (2.8 percent) than men (1.7 percent), Dr. Gaudino and his colleagues found.

Analyzing results from about 1.3 million patients (average age: 66) from 2011 to 2020, the researchers also determined that after C.A.B.G., about 20 percent of men had complications that included strokes, kidney failure, repeat surgeries, infections of the sternum and prolonged respirator use and hospital stays. Among women, more than 28 percent did.

Of those complications, “many are relatively minor and self-resolving,” Dr. Gaudino said. But recovering from sternal wound infections can take months, he noted, and “if you have a stroke, that can affect you for a long time.” Though outcomes improved for both sexes over the decade, the gender gap remained.

The study “should be regarded as an exploding flare in the sky for all clinicians who care for women,” an accompanying editorial said. Yet to cardiac researchers, the results sounded familiar.

“This has been something we’ve known since the 1980s,” said Dr. C. Noel Bairey Merz, a cardiologist and researcher at Cedars-Sinai Medical Center. Heart disease, she pointed out, remains the leading cause of death for American women.

With C.A.B.G., “the general assumption was that it was getting better because the technology, the knowledge, the skills and training were all improving,” she said. To see the gender disparity persist “is very disappointing.”

Several factors help explain those differences. Women are three to five years older than men when they undergo bypass surgery, in part because “we recognize coronary artery disease more easily and earlier in men,” Dr. Gaudino said. “Men have the classic presentation we study in medical school. Women have different symptoms.” These may include fatigue, shortness of breath and pain in the back or stomach.

Fewer than 20 percent of patients enrolled in clinical trials have been female, so “what we’ve been taught is essentially based on research in men,” he added.

Partly because they’re older — about 40 percent are over 70 — women are more apt than men to have developed health problems like diabetes, high blood pressure and vascular conditions, “all factors that increase risk in cardiac surgery,” Dr. Gaudino said. They also have smaller, more fragile blood vessels, which can make surgery more complex.

The disparities affect other forms of cardiac treatment and surgery, too. Women have worse outcomes than men five years after receiving a stent, a 2020 review of randomized trials reported.

They’re “less likely to be prescribed and to take statins, and particularly less likely to take the high-intensity statins, which are the most lifesaving,” Dr. Bairey Merz said. “The list goes on and on.”

When C.A.B.G. works well, the results can feel miraculous. Rhonda Skaggs, 68, had a quadruple bypass in July 2022 and spent 12 days in intensive care before going home to Brooksville, Fla. Six months passed before she returned to work at a Home Shopping Network outlet store.

“Now, you’d never know I had open-heart surgery,” she said. “I walk 10,000 steps a day. I teach line dance classes twice a week. I have my life back.”

But Susan Leary, 71, a retired New York City teacher now living in Fuquay-Varina, N.C., is facing a second procedure after bypass surgery at Duke University last month.

“Women are less likely to get all the vessels that need to be bypassed bypassed,” said her cardiothoracic surgeon, Dr. Brittany Zwischenberger, co-author of the call-to-arms editorial in JAMA Surgery.

A few years before, Ms. Leary had sought a procedure to shrink away the “ugly-looking” varicose veins in her legs; now, she lacked viable blood vessels for grafting. “How did I know I was going to need some of those veins for my heart?” she said.

She had a double bypass, instead of the triple bypass she needed, which represents “incomplete revascularization.”

“It can contribute to worse outcomes and future interventions,” Dr. Zwischenberger said. “Fortunately, she’s a candidate for a stent” for the third blocked artery, which involves inserting a mesh tube into the vessel to widen it. The procedure is scheduled for next month.

Advocates of improved care for women argue that their surgical risks can be reduced.

Dr. Lamia Harik, a cardiothoracic surgery researcher at Weill Cornell Medicine, and her colleagues have found that nearly 40 percent of women’s mortality during C.A.B.G. stems from interoperative anemia. (Their study is in press.)

That occurs when operating teams administer fluids to dilute patients’ blood during the procedure, allowing them to use the large cardiopulmonary bypass machine (“the pump”) that keeps blood oxygenated and flowing while surgeons do the grafting.

“This is something modifiable,” Dr. Harik said. For women, surgeons might use smaller pumps or reduce the volume of added fluid, or both.

To learn more, Dr. Gaudino and other researchers have begun enrolling women, and only women, in two new clinical trials. The international ROMA study, the first all-female surgical trial, will investigate two C.A.B.G. techniques to see which produces better outcomes; the federally funded Recharge trial will compare stenting with C.A.B.G.

“In the past, a lot of surgeons thought this was inevitable,” Dr. Gaudino said of the differences between the sexes. “Maybe they will not disappear, but they can be minimized.”

Ms. Mosson said her surgeons were pleased with the results of her quadruple bypass, though she was readmitted to the hospital briefly for fluid in her lungs. She has begun a three-times-weekly cardiac rehab program, recommended for patients who’ve undergone bypass surgery, and finds that her stamina is improving.

She still contends with the psychological aftermath of her heart attack and surgery, as Ms. Skaggs did and Ms. Leary still does. They describe shock — none had a history of heart disease — depression and anxiety. “I’m still struggling with the fear it will happen again,” Ms. Mosson said.

One antidote, for Ms. Leary, was being recruited for ROMA; Duke is among the clinical trial sites. She jumped at the chance to enroll.

“Let me be a part of it,” she said. “Maybe my daughter will need this information someday.”