Edith Joyce is an active older woman who hasn't let a stroke slow her down.

"To stay active, I walk, and I have my yoga class. I have my balance and dance class, where I'm headed after this," said Joyce.

Her stroke history, along with heart disease, puts Joyce in a tricky category. To protect her brain, blood pressure should be low but not too low.

"When you think about this, the lower the better. There must be something like a 'k' curve because, if your blood pressure is zero, mortality is 100 percent; everybody's dead," said cardiologist Dr. Franz Messerli.

Messerli looked at heart disease patients and their diastolic pressures, the lower number. He wanted to see the risk of pressure that's too low.

"If you have coronary artery disease and your diastolic blood pressure is low, then obviously the coronary circulation gets compromised. Not enough blood is flowing to the heart," said Messerli.

When he plotted the diastolic pressures of these patients, a pattern emerged.

"When we lowered blood pressure, diastolic, to below 70, the risk of heart attack doubled," he said. "When we lowered it below 60, it quadrupled."

Finding the right balance is difficult because medicines that reduce blood pressure to protect an organ, like the brain, can lower it dangerously for the heart.

"Heart attack may be the first and only sign that indeed the blood pressure is too low," said Messerli.

For now, the study puts "low pressure risk" on the map.

Heart disease patients who have other complications, like a previous stroke, need to closely track their blood pressures. Though lower is better most of the time, for these patients, it needs to stay in a safe range of about 70 to 80 for the lower number.