Podcast: Play in new window | Download
Myocardial Infarction (MI)
- An ischemic event affecting cardiac tissue
- Cardiac muscle is dying due to insufficient supply of oxygen
Causes
- Most commonly a thrombotic event which occurs at the site of preexisting plaque causing a complete blockage of an artery.
- Vasospasm
- Severe hypotension
- Excessive metabolic demand
- Less often an embolic event
- Cocaine use (especially in young healthy individuals)
Risk Factors
- Male
- Increasing age
- Smoking
- Family History
- HTN
- DM
- Dyslipidemia
Clinical Presentation
- Crushing chest pain increasing in severity and lasting longer than thirty minutes
- 33% of patients do not have chest pain especially women and patients with diabetes
- “Indigestion”
- Often early in the morning
- Diaphoresis
- Weakness
- Anxiety
- Syncope
- Restlessness
- Dyspnea
- Nausea and vomiting
Physical Exam Findings
- Maybe hypertensive or hypotensive
- Maybe tachycardia or bradycardia
- Lungs may be clear or have rales or wheezing
- JVD
- Softened heart sounds
- S4 gallop
- After 12 hours – fever may develop
- After 24 hours pericardial friction rub may develop
- Dressler syndrome – post myocardial infarction syndrome
- 1–2 weeks after MI
- Pericarditis
- Fever
- Leukocytosis
- Pericardial effusion
- Pleural effusion
Labs and Studies
-
- Serial cardiac enzymes
- Myoglobin elevates in first 1–3 hrs peaks at 6–7 hours and normal by 24 hrs
- Cardiac troponin I and cardiac troponin K elevate within 2–12 hours and peak around 24 hours. They return to normal by two weeks
- CK-MB will elevate within 3–12 hours, peaks around 24 hours and normalizes by 72 hours.
Click here for the cardiac enzyme study worksheet
- ECG
- Progression from peaked T waves to ST segment elevation to Q waves to T wave inversion.
- ST elevation is defined as >0.1mv
- On a twelve lead you may be able to determine the location of the infarct by the lead that shows the changes
- Inferior – leads II,III and aVF
- Posterior – leads V1 and V2
- Anteroseptal – leads V1 and V2
- Anterior V1,V2 and V3
- Anterolateral V4, V5, V6
- Chest X-ray – may show signs of CHF
- Echo
- Catheterization
Treatment
- Approximately 50% of deaths from MI occur before the patient reaches the hospital.
- Aspirin 162 mg or 325 mg daily start immediately
- Clopidogrel (Plavix) 300 mg oral given once as a loading dose
- Titrate down to 75 mg/day
- Morphine
- Nitroglycerin
- Vasodilator helps to relieve ischemic pain
- Morphine for pain control if nitroglycerin not effective
- Enoxaparin (better than heparin) significant reduction in death and MI at day thirty
- Thrombolytic
- Most effective with the first three hours (50% reduction in mortality). Patients should be treated up until 12 hours after onset of symptoms (10% reduction in mortality)
- ST elevation suggests acute coronary occlusion warranting reperfusion
- Streptokinase no longer available in the U.S.
- Tissue plasminogen activator (t-PA)
- Contraindications
- Stroke within one year
- Intracranial neoplasm
- Recent head trauma
- Active internal bleeding
- Concern of aortic dissection
- Relative contraindications
- Blood pressure >180/110
- Intracerebral pathology
- Trauma within 2 weeks
- Major surgery in past 3 weeks
- CPR lasting more than 10 minutes
- Pregnancy
- Current use of anticoagulants.
- Percutaneous coronary intervention (PCI)
- Catheterization and stenting has been shown to be better than thrombolytic therapy when performed by experienced people and in a high volume center
- Door to balloon time < 90 minutes
- Coronary artery bypass grafting (CABG)
