An acute ST-elevation myocardial infarction (STEMI) is an event in which transmural myocardial ischemia results in myocardial injury or necrosis. The current 2018 clinical definition of myocardial infarction (MI) requires the confirmation of the myocardial ischemic injury with abnormal cardiac biomarkers.
An ST-elevation myocardial infarction occurs from occlusion of one or more of the coronary arteries that supply the heart with blood. The cause of this abrupt disruption of blood flow is usually plaque rupture, erosion, fissuring or dissection that results in an obstructing thrombus. The major risk factors for ST-elevation myocardial infarction are dyslipidemia, diabetes mellitus, hypertension, smoking, and family history of coronary artery disease.
The estimated annual incidence of myocardial infarction (MI) in the United States is 550,000 new and 200,000 recurrent. In 2013, 116,793 persons in the United States suffered a fatal MI with 57% occurring in men and 43% in women. The average age of incidence of a first MI is 65.1 for men and 72 for women. Approximately 38% of patients who present to the hospital with acute coronary syndrome have an ST-elevation myocardial infarction.
For a thrombotic coronary event to cause ST-segment elevation on a surface ECG, there needs to be a complete and persistent occlusion of blood flow. Coronary artery occlusion in animal models shows a "wave-front" of myocardial injury that spreads from the sub-endocardial myocardium to the sub-epicardial myocardium resulting in a transmural infarction that appears as an ST elevation on surface ECG. Myocardial damage begins to occur as soon as the blood flow is interrupted which makes timely management a necessity.
Prior to performing an ECG and collecting troponins the history and physical provide the only clues that lead to a diagnosis of myocardial infarction. Initial evaluation should include a focused physical examination and a brief history. Patients should be asked about the characteristics of the pain and associated symptoms, risk factors or history of cardiovascular disease, and recent drug use. Risk factors for an ST-elevation myocardial infarction include age, gender, family history of premature coronary artery disease, tobacco use, dyslipidemia, diabetes mellitus, hypertension, abdominal obesity, sedentary lifestyle, a diet low in fruits and vegetables, psychosocial stressors. Cocaine use can cause an ST-elevation myocardial infarction regardless of risk factors.
Evaluation of patients with acute onset of chest pain should begin with an electrocardiogram (ECG) and troponin level. The American College of Cardiology, American Heart Association, European Society of Cardiology, and the World Heart Federation committee established the following ECG criteria for ST-elevation myocardial infarction (STEMI):
After making the diagnosis of acute ST-elevation myocardial infarction, intravenous access should be obtained, and cardiac monitoring started. Patients that are hypoxemic or at risk for hypoxemia benefit from oxygen therapy; however, recent studies show possible deleterious effects in normoxic patients. Patients should undergo percutaneous coronary intervention (PCI) within 90 minutes of presentation at a PCI capable hospital or within 120 minutes if transfer to a PCI capable hospital is required. If PCI is not possible within the first 120 minutes of first medical contact, fibrinolytic therapy should be initiated within 30 minutes of patient arrival at the hospital.
All patients with an acute myocardial infarction should be started on a beta blocker, statin, aspirin, and a P2Y inhibitor as soon as possible. Nitroglycerin administration can reduce anginal pain however it should be avoided in patients who have used phosphodiesterase inhibiting medication within the last 24 hours and in cases of right ventricular infarction. Further pain relief with morphine can be given for patients that continue report discomfort after nitroglycerin administration however judicious use is recommended as it may adversely affect outcomes. P2Y inhibiting antiplatelet medication choice depends on whether the patient underwent PCI or fibrinolytic therapy. Ticagrelor and prasugrel are preferred to clopidogrel in patients who undergo PCI due to recent trials showing superiority. Patients undergoing fibrinolytic therapy should be started on clopidogrel.
Other pathologies that can cause ST-segment elevations include: myocarditis, pericarditis, stress cardiomyopathy (Takotsubo), early repolarization, left bundle branch block, various channelopathies, and electrolyte abnormalities.
Mortality rates at 30 days for patients presenting with ST-elevation myocardial infarction are between 2.5% to 10%. The most commonly used scoring system for 30-day mortality is the TIMI risk score. The TIMI scoring system considers:
There are 3 life-threatening mechanical complications of myocardial infarction: ventricular free wall rupture, interventricular septum rupture, and acute mitral regurgitation. Ventricular free wall rupture occurs within 5 days in half the cases and within 2 weeks in 90% of cases with an overall mortality rate of greater than 80%. Rupture of the interventricular septum is reported about half as often as free wall rupture and typically occurs 3 to 5 days with an overall mortality rate greater than 70%. Prompt surgery reduces the mortality rate in both conditions. Acute mitral regurgitation following a myocardial infarction is most commonly due to ischemic papillary muscle displacement, left ventricular dilatation, or rupture of the papillary muscle of chordae. In ST-elevation myocardial infarction, the degree of mitral regurgitation is usually severe and associated with a 30-day survival of 24%.
A door-to-balloon time of fewer than 90 minutes is the goal of a PCI capable facility and is one of the core measures of the Joint Commission on Accreditation of Healthcare Organizations. This is the time between the patient's arrival in the emergency department and the crossing of the culprit lesion by a guidewire in the cardiac cath lab. Teamwork between EMS, emergency department physician, and interventional cardiologist sets the groundwork for optimal door-to-balloon times (Level 1). As of 2010, the median door-to-balloon time was 64 minutes with 91% of patients receiving PCI in under 90 minutes and 70% in under 75 minutes.
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