EBM Consult

Acute Coronary Syndrome

       Basic Definitions

        • Ischemia:  The presence of an imbalance between oxygen demand and oxygen delivery most commonly from reduced coronary artery blood flow secondary to:
          • Atherosclerotic plaque
          • Coronary thrombus
          • Arterial vasospasm
        • Angina: Chest discomfort associated with exertion and relieved with rest or analgesics or nitroglycerin
        • Unstable Angina:  Chest pain not associated with some other known etiology (e.g., injury or trauma) that usually:
          • Occurs at rest & lasts > 20 min
          • Is new in onset or increasing frequency
          • Increase in severity compared to prior episodes
        • Acute Myocardial Infarction:  Represents either ST segment elevation MI (STEMI) or non-ST segment elevation MI (NSTEMI).  There is a presence of cardiac enzymes (e.g., troponin) that rise and fall plus one or more of the following:
          • Presenting with ischemic type of chest pain
          • Changes in ECG such as dynamic T-wave changes or inversions, ST segment changes, new Q-waves or left bundle branch block.

         Clinical Presentation

        • Common descriptors and characteristics of patients presenting with cardiac chest pain:
          • Pain described as pressure or squeezing sensation
          • Pain that radiates to the shoulder, neck, and/or arm
          • Symptoms or pain similar to prior MI, especially if worse or changing
          • Patient factors:  older age, known CAD, diabetes mellitus, smoker

         EBM Core Knowledge: Percentage of Patients Without Chest Pain

        • 33% of patients with AMI will present without chest pain
        • 6 factors that increase the risk of presenting without chest pain:
          • Female, non-white, history of diabetes, hypertension, prior heart failure, and/or stroke
        • It is was also found that these patients took longer to get aspirin, ECGs, and heparin within the first 24 hours and had a 2-fold increased risk of "in-hospital" death compared to those presenting with chest pain (23% vs. 9%)
        • Evidence:  Prospective, observational study of 434,877 patients from the NRMI-2 national registry.  JAMA 2000;283(24):3223-9.
        • EBM Consult Article:  XXXX


         ECG Findings

        • It is important to consider doing serial ECGs (as early as within 15 mins of each other) in patients with a concerning presentation or with changes in symptoms and compare baseline ECGs to prior or older ECGs (if available).  Acute and dynamic changes should raise your level of concern.  Common ECG findings in ACS include:
          • Hyperacute T-waves especially early on in ACS
          • T-wave inversions
          • ST segment depressions
          • ST segment elevations in contiguous leads (e.g., V1 & V2 or V4 or V5 or leads II, III, and AVF)
            • Defined as an increase by at least 1 mm above isoelectric line
          • New Q-waves (note: these may not show up until 3-4 hours after an infarct)
            • 1 square wide or 1/3 the height of R-wave on ECG
          • New left bundle branch block (LBBB)
            • Presence of an R-R' in leads V5/V6
            • To determine if the LBBB involves the anterior fascicle there will be left axis deviation (lead I+ and aVF -) on ECG and if it involves the posterior fascicular hemi-block there will be right axis deviation (lead I- and aVF +) on ECG

         EBM Core Knowledge: Patients With MI Who Initially Have a Normal ECG

        • Summary:  5 - 10% of patients with MI will have a normal or non-diagnostic ECG.  Also, 20 - 30% of patients with MI will have ischemic changes on ECG
        • Clinical Application: Due to the above, the ECG alone is considered to have a relatively low diagnostic sensitivity for ACS and if the presentation suggests the chest pain is likely cardiac, then repeat the ECG in 15-30 min to make sure there is no evolution of ischemia.
        • Evidence:  
          • Prospective, single-center cohort study done over 32 month period in 3,814 patients presenting to the ED.  Ann Emerg Med 2004;44(3):206-12.
          • Observational cohort from single center over a 6 yr period in 1,641 patients.  Am J Emerg Med 2009;27(2):146-52.
        • EBM Consult Article:  ####


         Board Exam Review:

        • Remember STEMI and pericarditis can present with ST segment elevations on the ECG.
        • STEMI on a board exam will have ST segment elevations usually only in 2 - 3 contiguous leads along with reciprocal changes where as pericarditis will have diffuse ST segment elevations (i.e., elevations in all the leads) and PR depressions. 
        • This is important to differentiate as the treatment pathways are significantly different and failing to treat a STEMI would be a major error and medical liability.



        The diagnosis is unstable angina (UA) or acute myocardial infarction (AMI) is based on a number of factors such as:

        • Clinical Presentation
        • Risk factor assessment
        • ECG findings
        • Cardiac enzyme evaluation


        Types of Myocardial Infarction (MI)

        • Type 1 = MI due to coronary plaque rupture or dissection
        • Type 2 = Secondary to ischemia from increased oxygen demand
        • Type 3 = Sudden, unexpected cardiac death (i.e., cardiac arrest) with symptoms suggestive of MI, accompanied by STEMI, new LBBB, or evidence of a fresh thrombus in coronary artery by angiography or autopsy. 
        • Type 4 = MI Associated with coronary angioplasty or stents
          • 4a = MI associated with PCI
          • 4b = MI associated with stent thrombosis as documented by angiography or autopsy
        • Type 5 = MI Associated with CABG