The Fourth Universal Definition of Myocardial Infarction and the Emerging Importance of Myocardial Injury

The Fourth Universal Definition of Myocardial Infarction and the Emerging Importance of Myocardial InjuryArticle, see p 161 T he clinical relevance of any diagnosis stems predominantly from both its prognostic and therapeutic implications.The evolution of diagnostic criteria for acute myocardial infarction (MI) and the implementation of high sensitivity assays for cardiac troponin (hsTn) have engendered debate regarding these 2 aspects of a diagnosis of acute MI; particularly for type 2 MI related to supply-demand mismatch in the absence of acute atherothrombosis.Moreover, the 4 th Universal Definition of MI (UDMI) placed greater focus than prior iterations on the diagnosis of acute and chronic myocardial injury in the absence of MI. 1 Yet, few studies have rigorously examined outcomes associated with these diagnoses.Although estimates vary widely, some studies have suggested that as many as 70% of patients presenting for emergency evaluation who have abnormal values of hsTn will have a final diagnosis of myocardial injury other than MI. 2 In this issue of Circulation, Chapman and colleagues 3 report the largest prospective evaluation of the UDMI and outcomes among its diagnostic subsets in patients presenting for emergency evaluation who had cardiac troponin tested for suspected MI.An important, take-home finding is that patients with acute myocardial injury, a diagnosis often dismissed as inconsequential "troponinemia," had a qualitatively comparable 1-year rate of cardiovascular death or MI and a higher rate of death than patients with type 1 MI due to acute atherothrombosis.To be clear, acute myocardial injury caused by ischemia is an MI; 2 however, here and for the remainder of this editorial, the term myocardial injury will be used to indicate that from causes other than MI.Chapman and colleagues studied 48 282 consecutive patients who presented to any 1 of 10 hospitals in Scotland and had cardiac troponin tested for suspected MI. 3 Patients were enrolled as part of a stepped-wedge cluster randomized controlled trial, the primary results of which have already been published, 4 to evaluate implementation of a hsTnI assay and the recommendations of the UDMI.In the main trial, centers participated in a validation phase of ≥6 months in which they used a contemporary sensitive assay for cardiac troponin I before being randomized to early or late implementation of hsTnI using the sex-specific 99th percentile upper reference limit (URL) as a diagnostic threshold.Serial biomarker sampling was to occur at presentation and at 6 or 12 hours after symptom onset.The primary results of the trial showed no difference in the outcome of cardiovascular death or new MI with implementation of hsTnI versus the older assay using a cutpoint higher than the 99th percentile URL. 4 The focus of the present analysis was on the prognostic associations of the UDMI subtypes, as well as acute and chronic myocardial injury.