People with heart failure are also more likely to be diagnosed with cancer, according to a new study that followed older adults with and without heart problems.
The findings don't prove that heart failure, when the heart can't pump enough blood to the rest of the body, causes cancer. Researchers said more studies are needed to determine what might explain the link.
"People have not really considered any association of heart failure and cancer together, at least not developing cancer after diagnosis," said Dr. Adrian Hernandez, a cardiologist at the Duke University School of Medicine in Durham, North Carolina.
But Dr. Sudhir Kushwaha, who worked on the study at the Mayo Clinic in Rochester, Minnesota, said the association makes sense, because a lack of blood and oxygen could create problems in many organs.
"The (heart failure) patient should be aware or alert to any new symptoms that might develop," he told Reuters Health.
Close to six million Americans have heart failure, according to the U.S. Centers for Disease Control and Prevention. Symptoms include trouble breathing and fatigue.
For the new study, the researchers matched 961 newly-diagnosed heart failure patients with people of their same age and gender that didn't have the disease.
A similar proportion of those participants - 22 to 23 percent - had already had cancer.
There were 596 cancer-free study pairs, who the researchers then followed, starting when participants were an average of 73 years old.
Over the next eight years, 244 people still in the study were diagnosed with cancer, including colon cancer, prostate cancer, breast cancer and blood cancers.
After accounting for certain disease risks such as people's weight and whether they smoked, Kushwaha and his colleagues calculated that heart failure patients were 68 percent more likely to be diagnosed with cancer than their heart failure-free pairs.
Although people with heart failure were sicker in general - with more diabetes and high blood pressure, for example - that didn't explain their greater cancer risk, the study team wrote in the Journal of the American College of Cardiology.
Kushwaha's group said there are a few possible explanations for the link, all of which need more study. Certain heart drugs could increase cancer risks, or stress and inflammation from heart failure itself might play a role, as could lack of oxygen.
It's also possible the link can be explained by people with heart failure seeing their primary care doctors more often and thus getting more screening tests, researchers said.
"Sicker people tend to be seen in medical encounters all the time … they get more lab tests, more people ask them whether they want to be screened," said Dr. Jersey Chen, a cardiology researcher from Kaiser Permanente's Mid-Atlantic Permanente Research Institute in Rockville, Maryland.
"That may explain why the difference seems to be this high," Chen, who wasn't involved in the new research, told Reuters Health.
Kushwaha, however, said that explanation was unlikely - both because the difference in cancer rates took a couple of years to show up, and because people without heart failure still saw their doctors regularly.
According to Hernandez, who didn't participate in the study, the next step will be to follow people with and without heart failure, taking into account exactly how many tests they receive.
Chen agreed that type of data is needed to figure out the underlying association.
"I wouldn't make patients worry about this, that either they have a higher risk of cancer right now or that they should change their medications or treatments," he said.
"I think it's way too preliminary to invite those kinds of clinical changes."
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