Rhonda Cooper-DeHoff of the University of Florida in Gainesville said the patients involved in the study had increased risk of heart attack, stroke or death when blood pressures were either too high or too low.
Blood pressure greater than 140 has been associated with a nearly 50 percent increase in cardiovascular risk in heart patients with diabetes, Cooper-DeHoff said.
Cooper-DeHoff said her study -- the International Verapamil SR-Trandolapril study -- revealed for the first time that these patients also had a similar increase in risk when their blood pressure was controlled to lower than 115 systolic, the top number.
However, efforts to reduce systolic blood pressure to below 130 did not appear to offer any additional benefit to diabetics with coronary artery disease compared with reduction of systolic blood pressure to between 130 and 140, the study said.
"Our data suggest that in patients with both diabetes and coronary artery disease, there is a blood pressure threshold below which cardiovascular risk increases," Cooper-DeHoff said in a statement.
The study findings were presented in Atlanta at the annual scientific session of the American College of Cardiology.
Normal blood pressure as defined by the American Heart Association is less than 120 systolic and less than 80 diastolic.