Depression after myocardial infarction (MI) is associated with poor cardiovascular prognosis. There is some evidence that specifically depressive episodes that develop after the acute event are associated with poor cardiovascular prognosis. The aim of the present study was to evaluate whether an increase in the number of depressive symptoms after MI is associated with new cardiac events.

In 442 depressed and 325 non-depressed MI patients the Composite International Diagnostic Interview interview to assess post-MI depression was extended to evaluate the presence of the ICD-10 depressive symptoms just before and after the MI. The effect of an increase in number of depressive symptoms during the year following MI on new cardiac events up to 2.5 years post-MI was assessed with Cox regression analyses.

Each additional increase of one symptom was significantly associated with a 15% increased risk of new cardiac events, and this was stronger for non-depressed than for depressed patients. This association was independent of baseline cardiac disease severity. There was no interaction with the number of depressive symptoms pre-MI.

Our findings suggest that an increase in depressive symptoms after MI irrespective of the state of depression pre-MI explains why post-MI depression is associated with poor cardiovascular prognosis. Also increases in depressive symptoms after MI resulting in subthreshold depression should be evaluated as a prognostic marker. Whether potential mechanisms such as cardiac disease severity or inflammation underlie the association remains to be clarified.