People with coronary artery disease who develop depression face twice the risk of death as compared to heart patients without depression, a major study has found.

The increased risk of death from any cause holds true whether the depression immediately follows the heart disease diagnosis or occurs even years later, according to Heidi May, from Intermountain Medical Center Heart Institute in the US.

The findings point out the importance of screening for and treating depression even years after someone is diagnosed with heart disease, she said.

Researchers found that post-coronary artery disease depression was the single biggest predictor of death, and remained so even after they controlled for the other factors.

"No matter how long or how short it was, patients were found to have twice the risk of dying compared to those who did not have a follow-up diagnosis of depression," said May, lead author of the study published in the European Heart Journal – Quality of Care & Clinical Outcomes.

"Depression was the strongest risk factor for dying, compared to any other risk factors we evaluated. That included age, heart failure, diabetes, high blood pressure, kidney failure, or having a heart attack or stroke," she said.

That association did not change for patients who were previously diagnosed with depression before their heart disease diagnosis or for patients whose angiograms were performed for various reasons, which included stable angina, unstable angina, or heart attack, researchers said.

May and colleagues studied 24,138 patients who underwent angiographies, which determined they had coronary artery disease.

Patients with depression were also placed into subcategories based on how long after their heart disease diagnosis the depression was identified.

In all, 15 per cent, or 2,646 patients, were diagnosed with depression at some point during follow-up.

This study reinforces previous research investigating the link between depression, heart disease, and increased risks of death, researchers said.

The study did not explain the reason for the elevated risk of death, although May said one possibility is that depression impacts how closely patients follow their treatment plans.