ST-elevation myocardial infarction (STEMI) patients with a prior MI history have worse outcomes. The prognostic significance of silent MI (pathologic Q waves outside the ST-elevation territory) in STEMI is unclear.

A total of 5,733 STEMI patients from 296 clinical centers in 17 countries were classified as (1) silent MI—baseline Q waves outside the infarct-related artery territory and no history of prior MI, (2) history of prior MI (HxMI), or (3) no prior MI.

Of 5,733 STEMI patients, 419 (7.3%) had silent MI, 693 (12.1%) had HxMI, and 4,621 (80.6%) had no prior MI. Ninety-day death and death/congestive heart failure/shock were higher in patients with HxMI (8.4% and 15.3%, respectively) and silent MI (6.7% and 13.9%, respectively) compared with patients with no prior MI (4.0% and 9.1%, respectively) (P ≤ .001 for all). After baseline adjustment, patients with HxMI were at increased risk for 90-day death (adjusted hazard ratio [HR] 1.62, 95% CI 1.18-2.21), whereas both those with HxMI and those with silent MI had increased risk of 90-day death/congestive heart failure/shock compared with those with no prior MI (adjusted HR 1.54, 95% CI 1.23-1.93 and adjusted HR 1.46, 95% CI 1.10-1.93, respectively).

Seven percent of STEMI patients had a silent MI. They represent a novel subgroup at increased risk comparable to those with known prior MI. Hence, in future studies, acquiring baseline Q wave data outside the distribution of acute injury should broaden the prognostic insights from STEMI patients with a prior MI.