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World’s largest transfusion study in cardiac surgery changes transfusion practices around the world

Lower thresholds for blood transfusion result in positive patient outcomes six months after heart surgery

Toronto, August 27, 2018

By Ana Gajic

Dr. David Mazer
Dr. David Mazer

Lower thresholds for blood transfusions for cardiac surgery patients compared to traditional thresholds provide positive patient outcomes and safety at six months after surgery, according to the world’s largest research study on this topic.

The research found that in addition to providing good patient outcomes six months after hospital discharge, the lower threshold - known as ‘restrictive transfusion therapy’ - reduces the amount of blood transfused and money spent on blood per procedure. The higher, traditional threshold is called ‘liberal transfusion therapy.’

Physicians who practice the liberal transfusion approach give blood transfusions early in the surgery to prevent patients’ hemoglobin level from falling. Hemoglobin is the protein that allows red blood cells to deliver oxygen to body tissues. Physicians who practice a restrictive approach wait longer to see if the hemoglobin level remains stable or if the patient has further bleeding.

These findings were presented today at the European Society of Cardiology Annual Congress in Munich, Germany by Dr. David Mazer, principal investigator on the study, anesthesiologist at St. Michael’s Hospital, associate scientist in its Keenan Research Centre for Biomedical Science, and Professor of Anesthesia and Physiology at the University of Toronto, with simultaneous publication in the New England Journal of Medicine.

“Our research question was, at what point does the risk of anemia, or the risk of a lower hemoglobin, outweigh the risk of transfusion?” Dr. Mazer said. “We wanted to know whether it is safe to let your hemoglobin go to a lower level before you transfuse. The answer is yes. It'll save blood, make blood more available, reduce costs of transfusion and result in similar or better outcomes."

This work builds on Dr. Mazer’s research published less than a year ago in the New England Journal of Medicine, which analyzed immediate postoperative patient outcomes. The randomized trial involved more than 5,200 patients at 74 sites in 19 countries and every continent in the world except Antarctica. At the six-month mark, data was available for 96 per cent of the patients.

Dr. Mazer, who co-led this study with Dr. Nadine Shehata, a hematologist at the Sinai Health System, and the research team found no clinical or statistical difference in four patient outcomes (death, heart attack, stroke and new kidney failure), whether the patients had contemporary restrictive therapy or traditional liberal practices. In fact, use of the restrictive transfusion protocol during and after heart surgery may actually reduce the incidence of complications in older patients, including heart attack, stroke, kidney failure and death.

“This research has already started to change transfusion practice around the world,” said Dr. Mazer. “With this data at six months, we’ve proven the longer term safety of restrictive therapy. This approach has already been adopted into guidelines and will likely become the standard of care worldwide.”

The large size of this study provides Dr. Mazer and his team additional opportunity to answer several other important questions related to transfusion and cardiac surgery.

This study received funding from the Canadian Institutes of Heart Research, Canadian Blood Services, the National Health and Medical Research Council in Australia and the Health Research Council of New Zealand.


This paper is an example of how St. Michael's Hospital is making Ontario Healthier, Wealthier, Smarter.

About St. Michael's Hospital

St. Michael’s Hospital provides compassionate care to all who enter its doors. The hospital also provides outstanding medical education to future health care professionals in more than 29 academic disciplines. Critical care and trauma, heart disease, neurosurgery, diabetes, cancer care, care of the homeless and global health are among the Hospital’s recognized areas of expertise. Through the Keenan Research Centre and the Li Ka Shing International Healthcare Education Centre, which make up the Li Ka Shing Knowledge Institute, research and education at St. Michael’s Hospital are recognized and make an impact around the world. Founded in 1892, the hospital is fully affiliated with the University of Toronto.

St. Michael’s Hospital with Providence Healthcare and St. Joseph’s Health Centre now operate under one corporate entity as of August 1, 2017. United, the three organizations serve patients, residents and clients across the full spectrum of care, spanning primary care, secondary community care, tertiary and quaternary care services to post-acute through rehabilitation, palliative care and long-term care, while investing in world-class research and education.


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