Women are more likely than men to die after coronary artery bypass surgery, according to a large new study.

Researchers still don't understand why women have these poorer outcomes.

"This should be a 'wake-up call' for cardiothoracic surgeons — women still have a higher risk of adverse outcomes following coronary artery bypass surgery, and there doesn't seem to have been any change in this trend over the past decade," said study leader Dr. Mario Gaudino. He is a cardiothoracic surgeon at NewYork-Presbyterian/Weill Cornell Medical Center in New York City.

The study found that, compared with men, women have a roughly 30% to 40% higher risk of death after bypass surgery. Women had a 2.8% death rate during or soon after surgery, compared to 1.7% for men, a nearly 50% difference.

When age and other health issues were taken into account, that risk only dropped 10% to 20%.

The findings stem from an analysis of nearly 1.3 million bypass surgeries performed between 2011 and 2020. More than 317,000 women were included.

The research team looked at deaths during or within 30 days of surgery among male and female patients. They also looked at a composite measure of deaths and major complications, such as stroke or kidney failure.

The findings showed that being female was tied to a significantly higher risk of death or major complications.

For death, being female was associated with a 28% to 41% higher risk depending on the year during the study period. For the combined outcome measure, being female was associated with a 2% to 9% higher risk, according to the report.

"We're clearly missing something here, and that means we need more data on women — data on the physiology of their coronary artery disease and how it tends to differ from men's, and data on their responses to different treatments and surgical techniques," said Gaudino, who is also director of the Joint Clinical Trials Office at Weill Cornell Medicine and NewYork-Presbyterian.

Since the 1990s, studies have shown that women tend to have worse outcomes during or after these surgeries than men. Women tend to be older when they have these surgeries and are more likely to have chronic diseases including diabetes and high blood pressure. After all this is factored in, they still have worse outcomes.

Doctors perform about 370,000 coronary artery bypass graft surgeries in the United States every year. Advances in surgical techniques and overall care have generally improved outcomes.

In the current study, Gaudino worked with surgeons in the United States, Canada and Austria. They now plan to begin a clinical trial in female patients to see if multiple bypasses improves outcomes over single-artery bypasses.

The study findings were published online March 1 in JAMA Surgery.