The relationship between heart disease and depression is like that between the chicken and the egg — hard to tell which comes first.

"There are areas we understand and areas where we still have questions," says Dr. Rhonda Robinson-Beale, chief medical officer for United Behavioral Health, a unit of UnitedHealth Group.

The National Institute of Mental Health has found that prevalence of depression in heart attack victims can be as high as 65 percent, while as many as 20 percent of patients with heart disease experience depression — a number that grows following heart attack or a heart bypass graft, among other heart events.

The reverse can be true as well: There's a "level of relative risk" for those who have major depression to have cardiovascular disease, she says.

One study found those over 55 with major depression were twice as likely to have a heart event, such as heart attack or stroke, as those without depression. Furthermore, a prior history of depression was found in 44 percent of those newly diagnosed with heart disease.

The ramifications are more serious than just being depressed after a heart attack, she says. For people who become depressed there's a significantly higher incidence of mortality within 6 to 18 months. And greater risk of another heart attack, as well.

Robinson-Beale admits the reasons are less clear than the results. Depression has been shown to affect heart rate and its variability. There's also a known effect on blood pressure. Where most people have a drop in blood pressure at night, those with depression don't. If you add in genetic factors like atherosclerosis, she says, it's worse.

Depression also makes it less likely a patient will follow doctor's orders after a heart event. He may not take medications on time or exercise or eat properly. Failure to do so puts him at risk for future complications, even death.

It's also harder to diagnose depression after someone survives a heart attack. "They don't present with a negative self-image, which is very common with chronic recurring depression, or with guilt. They have a high degree of positivity, but also feelings of exhaustion and fatigue. It's hard to tell if that's cardiac disease or a worsening of depression. It makes it difficult to monitor and adequately treat when someone has a myocardial infarction (heart attack), heart failure and depression."

It's also hard to prescribe medications, because some anti-depression medication, like the old tricyclates, are not safe with heart issues. And some newer drugs, while not as dangerous to the heart, can effect other cardiac medications and block their metabolism or cause drug interactions, she says.