Reassessing the clinical significance of electrocardiographically unrecognized myocardial infarctions: Radionuclide infarct size and its impact on long-term prognosis.

BACKGROUND: Silent or unrecognized myocardial infarction (UMI) diagnosed by surveillance electrocardiography (ECG) carries similarly poor prognosis as recognized MI (RMI) for poorly understood reasons.

METHODS: This study included 5430 consecutive patients who presented to the nuclear laboratory and underwent 2-day stress and rest Tc-99m sestamibi and ECG studies between March 1991 and June 1999. UMI was diagnosed if ECG showed Q-wave MI in the absence of a history of RMI. We measured infarct size (% defect size as compared with the entire left ventricular sestamibi uptake), ejection fraction (EF, %), and summed difference score (SDS, sestamibi uptake by myocardium in stress minus sestamibi uptake in rest images as a marker of ischemia). Survival was determined by follow-up survey (median 6 years).

RESULTS: We identified 346 UMIs, 628 RMIs, and 4456 subjects without MI (No MI). As compared with RMI, UMI patients had lesser abnormalities on nuclear scans (p < .0001 for all), including smaller infarct size (5.7% vs. 12.2%), higher EF (58% vs. 53%), and lesser ischemia (SDS; 3.9% vs. 2.7%). UMI prognosis was as poor as that of RMI (annual mortality rate 4.7% vs. 4.8% with No MI rate of 2.9%; p < .001 for all comparisons), and this persisted after multivariate analysis. Infarct size quantification successfully risk-stratified ECG-UMI patients, but UMI patients continued to predict mortality even if the infarct size was 0%.

CONCLUSIONS: Although UMI patients have lesser abnormalities on nuclear scans, ECG-based UMI continues to independently predict mortality, indicating the continuing relevance of ECG in clinical practice.