In the United States alone, this translates to an extra 6,000 deaths per year, they reported in the British Medical Journal (BMJ) yesterday.
The study was an overview of 48 published investigations into the outcome of nearly 1.9 million people hospitalised with an acute myocardial infarction, or heart attack.
Twenty of the investigations were carried out in the United States or Canada, 16 in European countries, and the others were either multinational or conducted in Australia, Brazil, Israel, Japan, South Korea, Thailand or Turkey.
Overall, though, the increased risk was far smaller but still significant at five percent, the researchers said.
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The study found that after-hours patients experienced a delay of nearly 15 minutes on average before getting a "balloon," a common procedure to inflate the coronary artery after infarction.
Patients admitted at night or on weekends "experience delays in urgent care and worse outcomes, and the gap seems to be increasing over time," doctors at the University of Toronto said in an accompanying BMJ editorial.