A deadly type of heart failure is more common than previously thought, new research suggests.

Heart failure refers to symptoms of shortness of breath, intolerance of exercise and fluid retention that occur when heart function is impaired.

Researchers in Canada and the U.S. focused on diastolic heart failure, where the heart is unable to fully relax and so does not properly fill with blood.

Heart failure may be detected by measuring the ejection fraction — the percentage of blood in the heart leaving with each beat. But in diastolic heart failure, this measurement may appear normal despite the problem.

Dr. Peter Liu of the University of Toronto and his colleagues looked at the incidence of diastolic heart failure among more than 2,800 patients admitted to hospitals in Ontario.

Liu's team found similar death rates for both types of heart failure after one year, 22 per cent compared to 26 per cent, they report in Thursday's issue of the New England Journal of Medicine.

A second, longer study by Dr. Margaret  Redfield of the Mayo Clinic in Rochester, Minn., and colleagues studied more than 4,500 patients discharged with a heart failure from 1987 through 2001.

The proportion of heart failure cases caused by diastolic heart failure increased from 38 per cent to 47 per cent to 54 per cent over the 15-year study, Redfield's team reports in the same issue. 

While survival rates for systolic heart failure improve, the same wasn't seen for diastolic heart failure, Redfield said.

"Research has helped us discover many therapies for systolic heart failure — drugs, devices and surgical procedures — to counteract the mechanisms that cause or worsen systolic heart failure," said Redfield, who reported receiving grant support from heart device and pharmaceutical manufacturers.

"This approach now needs to be expanded to the other half of the heart failure epidemic, patients with diastolic heart failure."

One possible explanation for the increased prevalence of diastolic heart failure is it is more common in older patients who are forming a greater percentage of the population, noted Dr. Gerard Aurigemma of the University of Massachusetts Medical School in a journal editorial.

Thanks to increased awareness, doctors are more likely to admit a patient to hospital for diastolic heart failure than in the past, and the growing availability of echocardiograms may increase the likelihood of diagnosis, he said.