The goal of this observational study to measure the heart's microvascular function in the setting of a myocardial infarction (MI), or heart attack, using a method called continuous saline thermodilution (CST). The participants will include people who are experiencing MI from sudden and complete blockage of a coronary artery requiring immediate balloon and/or stent therapy. After getting the balloon and/or stent therapy, participants will have their heart's microvascular system tested using CST. The main questions it aims to answer are:

• What measurements using CST can we expect from the heart's microvascular system during a treated MI?
• Can CST measurements during a treated MI predict the amount of heart muscle that is injured and that recovers?

For this study, participants will undergo measurement of their heart's microvascular function after balloon and/or stent therapy for the MI. They will then receive an MRI scan of the heart several days after the MI.

This will be a non-interventional, prospective, descriptive analysis on a cohort of patients experiencing ST-elevation myocardial infarction (STEMI) who present to the Cleveland Clinic cardiac catheterization laboratory (CCL) for emergent primary percutaneous coronary intervention (PCI). The aim is to study acute coronary microvascular dysfunction using the method of continuous saline thermodilution to provide absolute measurements of coronary blood flow, coronary microvascular resistance, and microvascular resistance reserve (MRR) following primary PCI for STEMI patients.

Adult patients experiencing acute STEMI who present to the catheterization laboratory for emergent cardiac catheterization for STEMI, <24 hours after symptom onset, who undergo primary PCI to an IRA. Only those able to provide initial oral consent upon CCL presentation and confirmatory written consent following cardiac catheterization will be included in the study.

Study endpoints are part of usual standard practice, and include clinic visits with updated medical histories and physical examination, and echocardiography. Clinical endpoints of interest will include mortality, incident heart failure, anginal symptoms, and standard major adverse cardiac events including cardiac death, nonfatal MI, urgent coronary revascularization, or hospitalization for unstable angina.

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

Adults presenting to catheterization lab within 24 hours of ischemic symptom onset with intent for emergent primary PCI due to STEMI on ECG or who have failed thrombolytic therapy and present for pharmaco-invasive management of STEMI.