Background: Circadian processes are suggested to influence ischemic injury in ST-segment elevation myocardial infarction (STEMI). Observational data suspect a circadian dependence of the occurrence of microvascular obstruction (MVO) in patients with STEMI. However, these observations derive from a small sample size and circadian variations in left ventricular function and myocardial damage after STEMI are still a matter of controversy. We therefore aimed to investigate the association of time of day dependent symptom onset with left ventricular ejection fraction (LVEF), infarct size (IS) and MVO in a large cohort of STEMI patients treated with primary PCI.

Methods: This observational study (NCT04113356) investigated acute STEMI patients treated with primary percutaneous coronary intervention. Cardiac magnetic resonance (CMR) imaging was performed within 1 week after the index event for the determination of left ventricular function and infarct characteristics. The time of symptom onset was used to discriminate patients according to LVEF, IS as well as MVO occurrence and extent over a 24 hours cycle to evaluate circadian behavior. 

Results: In final analysis, 889 acute STEMI patients treated with primary PCI with a delay of 189 [120-315] minutes were included. Median age of the overall cohort was 57 [51-66] years and 17% were female. Median LVEF was 49 [42-55]%, IS assumed 15.5 [8.0-24.7]% of left ventricular myocardial mass (LVMM) and MVO occurred in 57% of STEMI patients. No hourly differences between symptom onset and MVO occurrence (p=0.108), MVO extent (p=0.735), LVEF (p=0.644) and IS (p=0.722) have been evaluated. When comparing 3 hourly segments of symptom onset, no variations in MVO occurrence (p=0.114), MVO extent (p=0.308), LVEF (p=0.547) and IS (p=0.405) were observed.

Conclusions: No circadian variations in symptom onset and the occurrence and extent of MVO, LVEF and IS were observed in this large cohort of STEMI patients treated with primary PCI. Further research is needed to investigate the complex interplay between circadian processes and ischemic injury in this patient population.