Definition
Acute Myocardial Infarction (AMI)
- Myocardial infarction (MI) is death of cardiac myocytes (irreversible necrosis) caused by prolonged ischemia
- The term “acute” usually refers to the time 6 hours to 7 days following pathologic appearance of the infarct
- A prior or unrecognized/silent MI is a condition that has the following criteria:
- Abnormal Q waves with or without symptoms in the absence of non-ischemic causes
- Loss of viable myocardium on imaging consistent with an ischemic cause
- Patho-anatomical findings of a previous MI
- A recurrent MI is an MI occurring 28 days after an incident MI while a re-infarction is an acute MI occurring within 28 days of an incident or recurrent MI
Signs and Symptoms
Characteristics of Ischemic-type Chest Discomfort
- Retrosternal/substernal chest pain lasting 10 to 20 minutes or longer
- Pain is usually described as heaviness, pressure, dull, sharp, stabbing, squeezing, tearing, tightness or burning in nature
- Pain may occur at rest or during activity and does not respond fully to Glyceryl trinitrate (GTN)
- The pain which is usually central or in the left chest may radiate to the jaw, neck, left arm, back or shoulder
- Discomfort is diffuse, not localized, positional nor affected by movement of the region
- Occasionally, symptoms are mistaken for indigestion or heartburn if pain occurs in the epigastric region
- Accompanying symptoms may include nausea, vomiting, dyspnea, diaphoresis, palpitations, lightheadedness, dizziness, confusion, syncope, fatigue and weakness
- Atypical patterns may occur especially in females, diabetics and elderly patients; the pain develops in the arm, shoulder, wrist, jaw or back without occurring in the chest
- MI should be suspected especially if the symptoms are severe and occur suddenly
- MI may present with autonomic nervous system activation (eg pallor, sweating), hypotension or narrow pulse pressure, bradycardia or tachycardia, third heart sound (S3), basal rales or occasionally syncope in the elderly
Risk Stratification
- Identifying patients who are at increased risk of further reinfarction or death is essential in order that it can be prevented or intervention can be done accordingly
- Referral of high-risk patients to specialty centers should be made for early coronary angiography and revascularization
- Risk stratification of post-ST-segment elevation MI (STEMI) patients can be done clinically or by using the GRACE or the Thrombolysis in Myocardial Infarction (TIMI) risk scores
- Relevant for patients who did not receive PCI, including those with >48 hours presentation
High-Risk Patients
- Left ventricular ejection fraction (LVEF) <35%, ischemia that affects >50% of viable myocardium, post-revascularization (percutaneous coronary intervention [PCI] or coronary artery bypass graft [CABG] surgery)
- Clinical indicators include:
- Advanced age
- Hypotension and cardiogenic shock
- Anterior infarction
- Elevated initial serum creatinine
- Malignant arrhythmias
- Early angina on minimal exertion/post-infarct angina
- Tachycardia
- Killip class >1
- Previous infarction
- Heart failure history
- Persistent chest pain
- Peripheral arterial disease
Medium-Risk Patients
- Patients not considered low risk or high risk based on imaging criteria should be treated based on symptomatic status
Low-Risk Patients
- LVEF >50% or mild inducible ischemia that affects <20% of viable myocardium