Diabetics shown to have increased heart muscle mass

Diabetes strongly increases the risk of heart failure in all ethnic groups, but early effects of diabetes on the heart may differ depending on whether the subjects are white, African-American, Hispanic or Chinese.

These results emerged from the Multi-Ethnic Study of Atherosclerosis (MESA) when the investigators focused on heart mass - the weight of the heart muscle as measured by MRI, according to Alain Bertoni, M.D., M.P.H., at Wake Forest University School of Medicine.

"People with diabetes are recognized as having an increased risk of heart failure," Bertoni said. "We sought to better understand why.

We were especially interested in the role atherosclerosis may play."

In a report in the March issue of Diabetes Care, the researchers compared people with diabetes or impaired fasting glucose (which means their blood sugar levels were too high) with those who had normal blood sugar levels.

The investigators particularly looked at the muscle mass of the left ventricle, the part of the heart that pumps the blood through the aorta and out into the circulatory system. They measured the left ventricle itself, not the blood in it.

They also measured the volume of the ventricle when filled with blood just before it pumps the blood out. A lower volume indicates less blood is able to enter the ventricle, and suggests increased heart stiffness, said Bertoni.

"Increased left ventricular muscle mass suggests the future possibility of developing heart failure," he said. "We also think that if you have a stiffer heart, that could be an early indication that you have a propensity for developing heart failure."

MESA measured "subclinical" atherosclerosis - atherosclerosis that has yet to produce symptoms - through CT scans measuring the amount of calcium in the coronary arteries and ultrasound measuring the wall thickness of the carotid artery in the neck. Both are indications of atherosclerosis.

"Every ethnic group seems to have a set of abnormalities related to diabetes. While we think those with diabetes from all ethnic groups are at increased risk for heart failure, perhaps there is a different mechanism in play in each of the ethnic groups," Bertoni said.

"We found evidence that in whites, African-Americans, and Hispanics with diabetes there was increased heart muscle mass over those without diabetes," he said. In whites, the increased left ventricular mass was completely explained by subclinical atherosclerosis and high blood pressure, he said. With partial blockage of the coronary arteries, some areas of the heart muscle are getting less blood flow and are weakened, which means the rest of the heart muscle has to bulk up, he said.

In African-Americans and Hispanics, the increased mass was not fully explained by these factors. Among Chinese participants no differences in mass were observed.

In contrast, lower volumes, suggestive of increased stiffness, were seen in whites, blacks, and Chinese participants with diabetes, but not Hispanics.

Bertoni stressed that none of the MESA participants actually had heart failure. "We did not see any significant difference in the function of the heart, the squeeze of the heart."

"Other studies have in fact suggested that the incidence of heart failure is similar between whites and African-Americans with diabetes, but somewhat lower among Hispanics and Asians with diabetes."

The MESA investigators intend to follow the participants at least until 2008, with some participants in a sub-study called MESA Air being followed at least until 2012. If the measurements do predict heart failure, "they would help us target preventive therapies," Bertoni said.

"Further investigation will be required to determine whether there are differences in the incidence of heart failure by ethnicity in this [group], and if so, whether the observed differences at baseline will be predictive of the future risk of heart failure," the researchers said in Diabetes Care.

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