Acute Coronary Syndrome: Myocardial Infarction

Basics

Description

  • Imbalance in myocardial blood supply and oxygen requirement
  • Acute coronary syndrome (ACS) encompasses a spectrum of disease processes:
    • Unstable angina pectoris
    • Acute myocardial infarction (AMI)
    • ST elevation myocardial infarction (STEMI)
    • Non-STEMI

Etiology

  • Atherosclerotic narrowing of coronary vessels
  • Vasospasm (Prinzmetal or variant angina)
  • Microvascular angina or abnormal relaxation of vessels with diffuse vascular disease
  • Plaque disruption
  • Thrombosis
  • Arteritis:
    • Lupus
    • Takayasu disease
    • Kawasaki disease
    • Rheumatoid arthritis
  • Prolonged hypotension
  • Anemia/stress ischemia:
    • Hemoglobin <8 g/dL
  • Carbon monoxide/elevations in carboxyhemoglobin
  • Coronary artery gas embolus
  • Thyroid storm
  • Structural abnormalities of coronary arteries:
    • Radiation fibrosis
    • Aneurysms
    • Ectasia
  • Cocaine- or amphetamine-induced vasospasm
  • Cardiac risk factors include:
    • Hypercholesterolemia
    • DM
    • HTN
    • Smoking
    • Family history in a 1st-degree relative <55 yr old
    • Men, age >55 yr
    • Postmenopausal women

Diagnosis

Signs and Symptoms

  • Chest pain:
    • Most common presentation of MI
    • Substernal pressure
    • Heaviness
    • Squeezing
    • Burning sensation
    • Tightness
  • Anginal equivalents (MI without chest pain):
    • Abdominal pain
    • Syncope
    • Diaphoresis
    • Nausea or vomiting
    • Weakness
  • May localize or radiate to arms, shoulders, back, neck, or jaw
  • Associated symptoms:
    • Dyspnea
    • Syncope
    • Fatigue
    • Diaphoresis
    • Nausea
    • Vomiting
  • Symptoms are usually reproduced by exertion, eating, exposure to cold, or emotional stress.
  • Symptoms commonly last 30 min or more.
  • Symptoms may occur with rest or exertion.
  • Often preceded by crescendo angina
  • May be improved/relieved with rest or nitroglycerin
  • Symptoms generally unchanged with position or inspiration
  • Positive Levine sign or clenched fist over chest is suggestive of angina.
  • BP is usually elevated during symptoms.

Physical Exam
  • Physical exam is usually unrevealing.
  • Occasional physical findings include:
    • S3 or S4 due to left ventricular systolic or diastolic symptoms
    • Mitral regurgitation due to papillary muscle dysfunction
    • Diminished peripheral pulses
    • Physical findings of decompensated CHF

Essential Workup

History is critical in differentiating MI from noncardiac etiologies.

Diagnostic Tests and Interpretation

Lab
  • Electrolytes
  • Calcium, magnesium
  • Cardiac enzymes
  • Digoxin level

Imaging
  • CXR:
    • May identify cardiomyopathy or CHF
    • Often abnormal in aortic dissection

Diagnostic Procedures/Other
  • ECG:
    • Differentiate from nonischemic causes of ST elevation
      • Pericarditis
      • Benign early repolarization
      • Left ventricular hypertrophy with strain
      • Prior MI with left ventricular aneurysm
      • Hyperkalemia
  • ECG criteria for STEMI
    • New ST elevation at J point in at least 2 contiguous leads of 2 mm (0.2 mV) in men or 1.5 mm (0.15 mV) in women in leads V2–V3 and/or of 1 mm (0.1 mV) in other contiguous chest leads or the limb leads
    • ST depression in leads V1–V2 may indicate posterior injury
    • New or presumably new LBBB has been considered an STEMI equivalent. Most cases of LBBB at time of presentation, are not old but prior ECG is unavailable
    • Sgarbossa criteria for MI in LBBB are diagnostic
      • Concordant ST elevation >1 mm in leads with a positive QRS complex
      • Concordant ST depression >1 mm V1–V3
      • Excessively discordant ST elevation > 5 mm in leads with a negative QRS complex
  • Echo:
    • May identify regional wall motion abnormalities or valvular dysfunction

Differential Diagnosis

  • Aortic dissection
  • Anxiety
  • Biliary colic
  • Costochondritis
  • Esophageal spasm
  • Esophageal reflux
  • Herpes zoster
  • Hiatal hernia
  • Mitral valve prolapse
  • Peptic ulcer disease
  • Psychogenic symptoms
  • Panic disorder
  • Pericarditis
  • Pneumonia
  • Pulmonary embolus

Treatment

Pre Hospital

  • IV access
  • Aspirin
  • Oxygen
  • Cardiac monitoring
  • Sublingual nitroglycerin for symptom relief
  • 12-lead ECG, if possible, with transmission or results relayed to receiving hospital

Initial Stabilization/Therapy

  • IV access
  • Oxygen
  • Cardiac monitoring
  • Oxygen saturation
  • Continuous BP monitoring and pulse oximetry

Ed Treatment/Procedures

  • STEMI requires reperfusion therapy as soon as possible:
    • Percutaneous coronary intervention (PCI) is preferred diagnostic and therapeutic modality if available.
  • Goal is primary PCI within 90 min of 1st medical contact.
  • Aspirin should be administered 1st to all patients with suspected MI unless known allergy.
  • Glycoprotein IIb/IIIa inhibitors (e.g., Abciximab) may be started at time of PCI
  • Prasugrel or Clopidogrel should be started at the time of PCI
  • Prasugrel should not be given to patients with history of prior stroke or TIA
  • Clopidogrel is the recommended ADP receptor inhibitor for patients given fibrinolytics
    • Dose is reduced (age <75 yr: 300 mg, >75 yr: 75 mg)
  • If BP is >90–100 mm Hg systolic, administer sublingual nitroglycerin, nitropaste, or IV nitroglycerin assuming no ECG criteria or clinical evidence of right ventricular infarct:
    • Symptoms that persist after 3 sublingual nitroglycerin tablets are strongly suggestive of AMI or noncardiac etiology.
  • β-blockers should be initiated within 1st 24 hr if not contraindicated (e.g., heart block, heart rate <60, signs of heart failure, hypotension, or obstructive pulmonary disease) are present
    • No benefit of administration prior to PCI or in ED
  • Morphine may be given to relieve pain, anxiety, and increase oxygen carrying capacity.
  • Heparin (UFH) or Bivalirudin should be used in patients undergoing primary PCI. Bivalirudin is indicated in patients at high risk for bleeding.
  • In patients undergoing thrombolysis, Heparin (UFH), Enoxaparin, or Fondaparinux are appropriate.
  • If patient is in cardiogenic shock, patient should be transported to a cardiac catheterization laboratory for angioplasty and intra-aortic balloon pump as soon as possible (see “Congestive Heart Failure”).
  • Ventricular dysrhythmias:
  • Bradydysrhythmia associated with hypotension should be treated with atropine or external pacing.
  • Conduction disturbances:
    • 1st-degree atrioventricular (AV) block and Mobitz I (Wenckebach) are often self-limited and do not require treatment.
    • Mobitz II, complete heart block, new right bundle branch block (RBBB) in anterior MI, RBBB plus left anterior branch block or left posterior fascicular block, left bundle branch block plus 1st-degree AV block may require a temporary transvenous pacemaker.
  • Accelerated idioventricular rhythm (AIVR) may present after reperfusion, appearing as a ventricular rhythm with rate below 120 bpm
    • Only if sustained treat with electrical cardioversion or sodium bicarbonate
    • Lidocaine and other antidysrhythmics may cause asystole

Medication

  • Aspirin: 162–325 mg PO
  • ADP receptor inhibitors
    • Clopidogrel (Plavix): 600 mg PO
    • Prasugrel (Effient): 60 mg PO
    • Ticagrelor (Ticlid): 180 mg PO
  • Bivalirudin: 0.75 mg/kg IV bolus, then 1.75 mg/kg/h infusion
  • Enoxaparin (Lovenox): 1 mg/kg SC q12h
    • Fondaparinux: 2.5 mg IV
  • Glycoprotein IIb/IIIa inhibitors:
    • Abciximab (ReoPro) for use prior to PCI only: 0.25 mg/kg IV bolus
    • Eptifibatide (Integrilin): 180 μg/kg IV over 1–2 min, then 2 μg/kg/min up to 72 hr
    • Tirofiban (Aggrastat): 0.4 μg/kg/min for 30 min, then 0.1 μg/kg/min for 48–108 hr
  • Heparin: 60 units/kg IV bolus (max. 4,000 U), then 12 U/kg/h (max. 1,000 U/h)
  • Metoprolol: 5 mg IV q5–15min followed by 25–50 mg PO starting dose as tolerated (note: β-blockers contraindicated in cocaine chest pain)
  • Morphine: 2 mg IV, may titrate upward in 2 mg increments for relief of pain assuming no respiratory deterioration and SBP >90 mm Hg
  • Nitroglycerin: 0.4 mg sublingual q5min for max. 3 doses
  • Nitroglycerin: IV drip at 5–10 μg/min, USE NON-PVD tubing
  • Nitropaste: 1–2 in transdermal
  • Thrombolytics: See “Reperfusion Therapy, Cardiac,” for dosing

Ongoing Care

Disposition

Admission Criteria
  • Patients with an AMI require hospital admission.
  • If the diagnosis is unclear, admission to the hospital or an ED observation unit may be useful for serial cardiac enzymes, ECGs, and exercise stress testing and/or cardiac catheterization if needed.

Discharge Criteria
No patient with an AMI should be discharged from the ED.

Issues for Referral
  • If PCI is unavailable at the treating institution, particularly if the patient is in cardiogenic shock, he should be transported to another hospital if PCI can be initiated within 120 min of 1st medical contact.
  • Patients with failed reperfusion should be transported urgently to a PCI-capable facility
  • Patients undergoing reperfusion therapy may benefit from transfer to a PCI-capable facility within 3–24 hr as part of an invasive strategy

Pearls and Pitfalls

  • Goal of thrombolytic therapy is a 30 min door to needle time if PCI not possible.
  • New or presumably new LBBB at presentation occurs infrequently, and should not be considered diagnostic of AMI in isolation

Additional Reading

  • Filippo C, Giovanna L. Pathogenesis of acute coronary syndromes. J Am Coll Cardiol. 2013;61:1–11.  [PMID:23158526]
  • Hartman SM, Barros AJ, Brady WJ. The use of a 4-step algorithm in the electrocardiographic diagnosis of ST-segment elevation myocardial infarction by novice interpreters. Am J Emerg Med. 2012;30:1282–1295.  [PMID:22244224]
  • Mehta N, Huang HD, Bandeali S, et al. Prevalence of acute myocardial infarction in patients with presumably new left bundle-branch block. J Electrocardiol. 2012;45:361–367.  [PMID:22575807]
  • O’Gara PT,Kushner FG, AscheimDD, et al. 2013 ACCF/AHA Guideline for the Management of ST-Elevation Myocardial Infarction AReport of the American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines.Circulation.2013;127:e362–e425.  [PMID:23247304]
  • Smith SW, Dodd KW, Henry TD, et al. Diagnosis of ST-elevation myocardial infarction in the presence of left bundle branch block with the ST-elevation to S-wave ratio in a modified Sgarbossa rule. Ann Emerg Med. 2012;60:766–776.  [PMID:22939607]

See Also

  • Cardiac Testing
  • Reperfusion Therapy
  • Unstable Angina

Codes

ICD-9

  • 410.90 Acute myocardial infarction, unspecified site, episode of care unspecified
  • 410.91 Acute myocardial infarction of unspecified site, initial episode of care
  • 410.92 Acute myocardial infarction of unspecified site, subsequent episode of care
  • 411.1 Intermediate coronary syndrome
  • 410.00 Acute myocardial infarction of anterolateral wall, episode of care unspecified
  • 410.01 Acute myocardial infarction of anterolateral wall, initial episode of care
  • 410.02 Acute myocardial infarction of anterolateral wall, subsequent episode of care
  • 410.0 Acute myocardial infarction of anterolateral wall
  • 410.10 Acute myocardial infarction of other anterior wall, episode of care unspecified
  • 410.11 Acute myocardial infarction of other anterior wall, initial episode of care
  • 410.12 Acute myocardial infarction of other anterior wall, subsequent episode of care
  • 410.1 Acute myocardial infarction of other anterior wall
  • 410.20 Acute myocardial infarction of inferolateral wall, episode of care unspecified
  • 410.21 Acute myocardial infarction of inferolateral wall, initial episode of care
  • 410.22 Acute myocardial infarction of inferolateral wall, subsequent episode of care
  • 410.2 Acute myocardial infarction of inferolateral wall
  • 410.30 Acute myocardial infarction of inferoposterior wall, episode of care unspecified
  • 410.31 Acute myocardial infarction of inferoposterior wall, initial episode of care
  • 410.32 Acute myocardial infarction of inferoposterior wall, subsequent episode of care
  • 410.3 Acute myocardial infarction of inferoposterior wall
  • 410.40 Acute myocardial infarction of other inferior wall, episode of care unspecified
  • 410.41 Acute myocardial infarction of other inferior wall, initial episode of care
  • 410.42 Acute myocardial infarction of other inferior wall, subsequent episode of care
  • 410.4 Acute myocardial infarction of other inferior wall
  • 410.50 Acute myocardial infarction of other lateral wall, episode of care unspecified
  • 410.51 Acute myocardial infarction of other lateral wall, initial episode of care
  • 410.52 Acute myocardial infarction of other lateral wall, subsequent episode of care
  • 410.5 Acute myocardial infarction of other lateral wall
  • 410.60 True posterior wall infarction, episode of care unspecified
  • 410.61 True posterior wall infarction, initial episode of care
  • 410.62 True posterior wall infarction, subsequent episode of care
  • 410.6 True posterior wall infarction
  • 410.70 Subendocardial infarction, episode of care unspecified
  • 410.71 Subendocardial infarction, initial episode of care
  • 410.72 Subendocardial infarction, subsequent episode of care
  • 410.7 Subendocardial infarction
  • 410.80 Acute myocardial infarction of other specified sites, episode of care unspecified
  • 410.81 Acute myocardial infarction of other specified sites, initial episode of care
  • 410.82 Acute myocardial infarction of other specified sites, subsequent episode of care
  • 410.8 Acute myocardial infarction of other specified sites
  • 410.9 Acute myocardial infarction of unspecified site
  • 410 Acute myocardial infarction

ICD-10

  • ST elevation (STEMI) myocardial infarction of unspecified site
  • STEMI involving oth sites
  • Acute ischemic heart disease, unspecified
  • STEMI involving left main coronary artery
  • STEMI involving left anterior descending coronary artery
  • STEMI involving oth coronary artery of anterior wall
  • ST elevation (STEMI) myocardial infarction of anterior wall
  • STEMI involving right coronary artery
  • STEMI involving oth coronary artery of inferior wall
  • ST elevation (STEMI) myocardial infarction of inferior wall
  • STEMI involving left circumflex coronary artery
  • ST elevation (STEMI) myocardial infarction of other sites

SNOMED

  • 22298006 Myocardial infarction (disorder)
  • 57054005 Acute myocardial infarction (disorder)
  • 401303003 Acute ST segment elevation myocardial infarction (disorder)
  • 394659003 Acute coronary syndrome (disorder)
  • 15990001 Acute myocardial infarction of posterolateral wall (disorder)
  • 54329005 Acute myocardial infarction of anterior wall
  • 65547006 Acute myocardial infarction of inferolateral wall (disorder)
  • 70211005 Acute myocardial infarction of anterolateral wall (disorder)
  • 73795002 acute myocardial infarction of inferior wall (disorder)

Authors

Josh W. Joseph
Shamai A. Grossman


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