Although moderate exercise is of benefit to people with chronic heart failure (CHF), researchers have advised that such people should avoid strenuous workouts. That's because there is new evidence that vigorous exercise affects properties of the blood that may increase the chance of clotting.

People with heart failure are already at risk of developing clots that may cause a stroke. Researchers in England studied 22 patients with stable CHF, comparing them to 20 patients with vascular disease and 20 healthy controls. They took a treadmill exercise test until they were exhausted or until a threatening condition such as arrhythmias or a drop in blood pressure occurred.

The patients with CHF showed an increase in those factors associated with increased clotting and blood flow abnormalities. Further, that increase was greater than the changes in the other patients and controls.

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