Assessment of Coronary Microvascular Dysfunction After STEMI Using Continuous Saline Thermodilution

Inclusion Criteria: * Adults (persons \>18 years old) presenting to Cleveland Clinic cardiac catheterization lab within 24 hours of symptom onset, with clinical and EKG findings concerning for STEMI, with intent to undergo emergent primary PCI, who have culprit artery identified on diagnostic angiography * Adults (persons \>18 years old) presenting to Cleveland Clinic cardiac catheterization lab for pharmaco-invasive management of failed fibrinolysis, with intent to undergo emergent PCI, who have culprit artery identified on diagnostic angiography * On-call treating interventional cardiologist is trained in the method of continuous saline thermodilution for coronary microvascular assessment Exclusion Criteria: * No evidence of coronary obstruction on diagnostic angiography (e.g., Takotsubo, myocarditis leading to STEMI activation). * Patients in hemodynamic shock * Culprit artery \<3.0 mm in diameter. * Culprit artery being a bypass graft * Patients physically unable to tolerate additional time required to conduct coronary microvascular testing after primary PCI. * Patients with eGFR \<30 mL/min/1.73m2 are excluded from contributing cardiac MRI data * Patients with standard contraindications to CMR (e.g., pacemakers, cochlear implants, certain prosthetic heart valves, certain surgical implants) are excluded from contributing cardiac MRI data * Unable to provide verbal and written consent * Pregnancy