Updated: 2/21/2021

Myocardial Infarction

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Snapshot
  • A 55-year-old man presents to the emergency department due to substernal chest pain. His symptoms began a few hours ago. He describes the pain as "crushing" and it radiates down the left arm. Medical history is significant for type 2 diabetes and hypertension. On physical exam the patient is diaphoretic. An electrocardiogram demonstrates ST-segment elevations and cardiac troponins are significantly elevated.
Introduction
  • Clinical definition
    • death of myocardial tissue secondary to prolonged and severe ischemia 
      • also known as a "heart attack"
  • Types
    • ST-segment elevation myocardial infarction (STEMI)
      • an acute coronary syndrome (ACS) with ST-segment elevations found on electrocardiogram (ECG)
      • biomarkers of myocardial necrosis are present
    • Non-STEMI (NSTEMI)
      • an ACS without ST-segment elevations found on ECG
      • biomarkers of myocardial necrosis are present
    • unstable angina
      • an ACS    
        • without ST-segment elevations found on ECG
        • and no elevation biomarkers of myocardial necrosis
  • Epidemiology
    • incidence
      • increases with age
    • risk factors
      • hypertension
      • cigarette smoking
      • hyperlipidemia
      • hypercholesterolemia
      • male
      • postmenopause
      • genetic and behavioral predispositions to arteriosclerosis
        • e.g., high-fat diet
  • Etiology
    • occlusion of a coronary artery can be caused by
      • atheromatous plaque rupture with subsequent thrombi expansion 
      • vasospasm
      • emboli, which can be secondary to
        • atrial fibrillation, sending an embolus from the left atrium to the coronary arteries
        • vegetations from infective endocarditis
        • material from an intracardiac prosthetic
        • paradoxical emboli
  • Pathophysiology
    • occlusion of a coronary artery disrupts the blood supply to a region in the myocardium
      • ischemia ensues, the myocytes become rapidly dysfunctional
        • when ischemia persists, this can result in myocyte death
        • after 30 minutes of severe ischemia, the damage becomes irreversible 
    • infarction patterns
      • subendocardial
        • myocyte necrosis involving the inner cardiac wall
        • this is normally the least perfused portion of the myocardium
        • may be referred to as an NSTEMI
      • transmural
        • myocyte necrosis involving the full thickness of the cardiac wall
        • may be referred to as a STEMI
ECG Changes and STEMI
 
ECG Changes and STEMI
Infarction Location Involved ECG Leads
Involved Coronary Artery
Inferior wall
  • II, III, and aVF 
  • RCA
Antero-apical
  • V3 and V4
  • LAD (distal)
Antero-septal
  • V1 and V2
  • LAD
Antero-lateral
  • V5 and V6
  • LAD or LCX
Lateral
  • I and aVL
  • LCX
Posterior
  • ST depression and tall R waves in V1-3
  • V7-V9
  • Posterior descending artery
 
Evolution of MI
 
Morphological Myocardial Changes in an MI
Time
Gross Features Light Microscopy
Complications
0-24 hours
  • Initially no gross findings; however, over the course of the first 24 hours, dark mottling ensues
  • Early coagulation necrosis
  • Wavy fibers 
  • Elongated myocytes
  • Neutrophil infiltration
  • Arrhythmia
  • Heart failure
1-3 days
  • Mottling with a yellowish infarct center
  • Extensive coagulation necrosis  
  • Brisk neutrophil infiltration
  • Fibrinous pericarditis 
3-14 days
  • 3-7 days
    • hyperemic with central yellowing
  • 7-10 days
    • yellow-tan with reddish tan margins
  • 10-14 days
    • reddish gray infarct borders
  • Macrophage infiltration and tissue granulation  
  • Myocardial wall rupture
    • may lead to cardiac tamponade
  • Papillary muscle rupture 
    • mitral regurgitation 
  • Pseudoaneurysm of a ventricular wall
    • may rupture
2 weeks - several months
  • 2-8 weeks 
    • gray-white scar
  • > 2 months
    • complete scar
  • Collagenous scar
  • Dressler syndrome
  • Heart failure
  • True ventricular aneurysm 
    • a thrombus may form
 
Presentation
  • Symptoms 
    • chest pain
      • features
        • squeezing
        • crushing
        • substernal
      • radiation
        • jaw
        • neck
        • left shoulder or down the arm
    • nausea and vomiting
    • dyspnea
    • asymptomatic
      • typically seen in patients with diabetic neuropathy
        • nerve fibers are damaged and impair their ability to sense pain
  • Physical exam
    • diaphoresis
    • variable findings
      • e.g., S3 or S4, signs of heart failure, bradycardia (in cases of an inferior wall MI)
Imaging
  • Coronary angiography
    • indication
      • diagnostic study to assess coronary anatomy and to determine where the occlusion is
Studies
  • 12-lead ECG
    • perform as soon as possible
    • findings
      • STEMI 
        • hyperacute or peaked T-waves
          • earliest finding
        • ST elevation
        • Q waves
          • a late finding (~2 weeks post-MI)
        • new left bundle branch block (LBBB)
          • considered to be an equivalent to a STEMI
      • NSTEMI 
        • ST depression
        • T wave inversion
      • Late
        • pathologic Q waves 
          • Q wave in leads V2–V3 lasting longer than 0.02 seconds
          • Q wave anywhere lasting longer than 0.03 seconds with an amplitude greater than 0.1 mV
  • Biomarkers
    • Troponin
      • preferred marker as it has a high sensitivity and specificity for myocardial necrosis
      • troponin I increases after 4 hours and peaks around 24 hours
        • remains elevated for 7-10 days
    • CK-MB
      • a sensitive but not specific biomarker since skeletal muscle can also release it
      • useful for assessing reinfarction 
Differential
  • Unstable angina
    • differentiating factor
      • no elevation in cardiac biomarkers
  • Costochondritis
    • differentiating factor
      • chest pain that is reproducible with palpation 
Treatment
  • Conservative
    • lifestyle modification
      • e.g., smoking cessation
  • Medical
    • initial medical treatments include
      • aspirin
      • oxygen
      • nitroglycerin 
        • hypotension in an inferior wall infarction secondary to reduced preload  
      • morphine
        • only give if there is unacceptable pain 
          • appears to be associated with a mortality increase
    • P2Y12 (ADP) receptors blockers
      • indication
        • given in addition to aspirin
    • heparin
      • indication
        • given in addition to antiplatelet therapy
    • β-blockers
      • indication
        • given to all patients if there are no contraindications
    • statin
      • indication
        • given to all patients
    • angiotensin-converting enzyme (ACE) inhibitor
      • indication
        • given to patients with a myocardial infarction
        • recommended when there is
          • anterior infarction
          • heart failure
          • left ventricular ejection fraction < 40%
        • reduces mortality
      • contraindication
        • shock
        • bilateral renal artery stenosis
        • allergy
  • Reperfusion therapy
    • percutaneous coronary intervention (PCI)
      • indications
        • if STEMI symptoms developed in < 12 hours and the procedure can be performed within 90-120 minutes
        • if fibrinolytic therapy is contraindicated
    • coronary artery bypass graft (CABG)
      • indication
        • when coronary anatomy does not allow for PCI
        • 3 vessel occlusion or 2 vessel occlusion in a patient with diabetes
        • significant stenosis of the left main coronary artery
    • fibrinolytic therapy
      • indication
        • for patients who cannot receive PCI within 120 minutes
·· Three coronary vessels with > 70 percent stenosis
·· Left main coronary artery stenosis > 50–70 percent
·· 2 vessels in a diabetic
·· 2 or 3 vessels with low ejection fraction
Complications
  • Heart failure
  • Sudden cardiac death
  • Arrhythmia
  • Myocardial stunning 

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(M1.CV.15.75) A 68-year-old male with a history of type 2 diabetes, hypertension, and COPD presents to the emergency department with severe substernal chest pain. On exam, he is diaphoretic and in obvious distress. An EKG is obtained which is demonstrated in Figure F. He immediately receives treatment for his condition and is subsequently admitted to the cardiac intensive care unit (CICU). In the CICU, he does well until day 5 of hospitalization when he begins to complain of severe shortness of breath and chest pain. Vitals are noted to be T: 36 deg C, HR: 130 bpm, BP: 65/40 mmHg, RR: 12, SaO2: 98%. He is clearly in distress and has elevated JVP. Which of the following histologic images would you expect to see if a biopsy of the myocardium was performed on this patient?

QID: 106815
FIGURES:
1

Figure A

8%

(18/220)

2

Figure B

18%

(39/220)

3

Figure C

12%

(26/220)

4

Figure D

30%

(65/220)

5

Figure E

31%

(68/220)

M 1 D

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(M1.CV.13.138) A 62-year-old male collapses while mowing the lawn, and it is determined that he experienced sudden cardiac death. The patient's medical history is significant for a preceding myocardial infarction that was managed conservatively. Posthumous histologic evaluation of the patient's heart reveals extensive granulation tissue replacing dead myocardium as well as early evidence of neovascularization, which is shown in Figure A. How long prior to death did this patient most likely experience his myocardial infarction?

QID: 100654
FIGURES:
1

16 hours

2%

(2/80)

2

3 days

11%

(9/80)

3

8 days

26%

(21/80)

4

2 weeks

44%

(35/80)

5

2 months

11%

(9/80)

M 1 E

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(M1.CV.13.35) A 64-year-old male with a past medical history of obesity, diabetes, hypertension, and hyperlipidemia presents with an acute onset of nausea, vomiting, diaphoresis, and crushing substernal chest pain. Vital signs are temperature 37° C, HR 110, BP 149/87, and RR of 22 with an oxygen saturation of 99% on room air. Physical exam reveals a fourth heart sound (S4), and labs are remarkable for an elevated troponin. EKG is shown below. The pathogenesis of the condition resulting in this patient’s presentation involves:

QID: 100551
FIGURES:
1

Genetic inheritance of a mutation in ß-myosin or troponin expressed in cardiac myocytes

6%

(5/77)

2

A fully obstructive thrombus at the site of a ruptured, ulcerated atherosclerotic plaque

79%

(61/77)

3

A partially occlusive thrombus at the site of a ruptured, ulcerated atherosclerotic plaque

10%

(8/77)

4

Destruction of the vasa vasorum caused by vasculitic phenomena

3%

(2/77)

5

A stable atheromatous lesion without overlying thrombus

0%

(0/77)

M 1 C

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(M1.CV.13.25) A 53-year-old man with a past medical history significant for hyperlipidemia, hypertension, and hyperhomocysteinemia presents to the emergency department complaining of 10/10 crushing, left-sided chest pain radiating down his left arm and up his neck into the left side of his jaw. His ECG shows ST-segment elevation in leads V2-V4. He is taken to the cardiac catheterization laboratory for successful balloon angioplasty and stenting of a complete blockage in his left anterior descending coronary artery. Echocardiogram the following day shows decreased left ventricular function and regional wall motion abnormalities. A follow-up echocardiogram 14 days later shows a normal ejection fraction and no regional wall motion abnormalities. This post-infarct course illustrates which of the following concepts?

QID: 100541
1

Reperfusion injury

14%

(28/202)

2

Ventricular remodeling

17%

(35/202)

3

Myocardial hibernation

17%

(34/202)

4

Myocardial stunning

42%

(84/202)

5

Coronary collateral circulation

9%

(19/202)

M 1 E

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(M1.CV.13.102) A 60-year-old woman with history of type I diabetes currently on hormone replacement therapy is seen in your ED complaining of "bad indigestion", dizziness and nausea for the past several hours. Vitals are T98.9, HR 102, BP 130/100, RR 25 and she is mildly diaphoretic. An EKG is shown in Figure A. At what time point does the injury to her affected cells become irreversible?

QID: 100618
FIGURES:
1

5 seconds

4%

(11/258)

2

60 seconds

11%

(28/258)

3

10 minutes

22%

(57/258)

4

30 minutes

37%

(96/258)

5

1 hour

18%

(46/258)

M 2 C

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(M1.CV.13.34) An autopsy is being performed on an elderly man who died from a myocardial infarction. Biopsy of the heart is likely to reveal necrosis most similar to necrosis seen in which of the following scenarios?

QID: 100550
1

The central nervous system following a stroke

6%

(9/151)

2

The lung following a tuberculosis infection

6%

(9/151)

3

Acute pancreatitis resulting from release of enzymatically active enzymes into the pancreas

1%

(1/151)

4

A region of kidney where blood flow is obstructed

83%

(126/151)

5

An abscess

3%

(4/151)

M 1 B

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