Cause of exercise intolerance in heart failure patients

A new study shows that blood flow to the legs is relatively normal in people with diastolic heart failure, suggesting other potential causes of their inability to do everyday activities, according to researchers at Wake Forest University Baptist Medical Center.

"Reduced tolerance for physical activity is the primary symptom of diastolic heart failure, and it greatly affects quality of life," said Dalane Kitzman, M.D., professor of cardiology and senior researcher on the study. "This condition will increase as our population ages, so it's important to pinpoint the reasons for their symptoms and to develop effective treatments."

The study results are reported on-line in American Journal of Physiology - Heart Circulation Physiology and will be published in an upcoming print issue.

There are an estimated 5 million heart failure patients in the United States, and about half have diastolic heart failure, in which the heart muscle is stiff and doesn't take in enough blood with each beat. The other type, systolic heart failure, is when the heart muscle is too weak to effectively pump blood out into the body. Diastolic heart failure is the more common form in the elderly and is expected to increase as the population ages.

With both types of heart failure, patients' bodies aren't getting enough oxygenated blood, which reduces their ability to perform everyday activities. But even when heart function is treated and improves - many patients still have symptoms of exercise intolerance.

"This is a complex puzzle and we need to understand whether part of the picture is the blood vessels that deliver flow or perhaps how the muscles are using the oxygenated blood," said Greg Hundley, M.D., lead author and an associate professor of cardiology.

Previous research has shown that the degree of blood flow to the legs contributes significantly to exercise performance in healthy young subjects as well as in patients with systolic heart failure. Researchers suspected that the same would be true of patients with diastolic heart failure, but the hypothesis had not been tested.

For the current study, researchers examined blood flow in the femoral artery, which delivers a majority of the blood received by the leg. The study involved 30 participants over age 60. Eleven of the participants were healthy, nine had diastolic heart failure and 10 had systolic heart failure.

All participants had a test to measure the volume of oxygen they consumed during exhaustive exercise. They also had a magnetic resonance imaging (MRI) test to assess blood flow through their femoral artery before and after flow was temporarily stopped with a blood pressure cuff. This test measures small changes in the size of the vessel interior and in blood flow velocity.

Patients with both types of heart failure had severely reduced peak oxygen consumption - confirming their reduced exercise tolerance. But, surprisingly, blood flow to the legs following release of the occlusion cuff was relatively similar in the healthy participants and in those with diastolic heart failure. It was approximately 75 percent lower in those with systolic failure.

"This is important because exercise intolerance is the primary symptom of patients with diastolic failure," said Hundley. "It suggests that factors other than leg blood flow may be responsible for the severely reduced exercise tolerance in this increasingly important disorder."

The researchers tested to see if other variables - such as tobacco use, diabetes, or certain medications - could have affected the results. After making these adjustments, the gap in leg blood flow between the two types of heart failure actually widened.

"If these results are confirmed, they could help us turn our attention to evaluating other factors that may contribute to reduced exercise tolerance in diastolic heart failure," said Kitzman. "This might include, in addition to other parts of the cardiovascular system, the leg muscles themselves. Such information should help guide treatment of the severe physical disability experienced by the increasing number of patients with this newly recognized form of heart failure."