A new study concluded that the most commonly used method for extracting leg veins for grafts in coronary bypass surgery was more dangerous than the traditional method, and resulted in more deaths among patients.

Companies making vein-extraction devices, and some doctors, cast doubt on the study’s conclusions, saying that it wasn’t scientifically rigorous enough. But the finding is likely to raise concerns in the cardiology field and among patients undergoing cardiac bypass, an operation done about 300,000...