The combination of depression and heart disease seems to be far more lethal than having either one of these conditions in isolation, suggests research published online in Heart.
Previous research has indicated that people who are depressed, but otherwise healthy, are more likely to develop coronary heart disease, irrespective of what other risk factors they might have.
And people who are depressed are more likely to die from all causes, but it still remains unclear as to whether depression is more fatal for those with heart disease than it is for those without.
The authors base their findings on just under 6,000 middle aged adults, whose mental and physical health were tracked for an average of five and a half years.
All the study participants were taking part in the British Whitehall II study, which is evaluating the impact of social and economic factors on the long term health of around 10,000 civil servants, aged between 35 and 55 in 1985.
Around one in seven of the 6,000 (14.9%) scored highly on a depressive symptom scale. And one in five (20%) of those with established heart disease were depressed compared with one in seven (14%) of those without heart problems.
During the five and a half year monitoring period, 170 people died. Heart attack or stroke accounted for 47 of these deaths.
Those with coronary heart disease alone were 67% more likely to die of all causes, while those who were depressed, but otherwise healthy, were twice as likely to do so as those who had neither condition.
But those who were both depressed and had heart disease were almost five times as likely to die as their mentally and physically healthy peers.
After taking account of age and sex, and other relevant influential factors, the combination of depression and heart disease tripled the risk of death from all causes and quadrupled the risk of dying from a heart attack or a stroke.
The biological explanations for the impact of depression on the risk of death are still not clear, say the authors, but may involve stimulating the inflammatory process and/or clot formation, or altering cellular responses and/or the metabolism of blood fats. Behavioural factors might also play a part, they add.
In the meantime, the results emphasise "the need for healthcare professionals to pay more attention to depression in their cardiac patients," they conclude.