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Shock for USMLE Step 3

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Shock for the USMLE Step 3 Exam
Shock is a life-threatening condition of inadequate tissue perfusion that can lead to organ dysfunction. It is divided into four main types: hypovolemic, distributive, cardiogenic, and obstructive, each with unique causes, hemodynamic profiles, and treatment strategies.
signs and symptoms of shock
Types of Shock
Hypovolemic Shock
  • Etiology: Caused by significant fluid loss, typically due to hemorrhage (e.g., trauma, gastrointestinal bleeding) or non-hemorrhagic causes like dehydration (e.g., severe vomiting, diarrhea, burns).
  • Pathophysiology: Reduced blood volume lowers preload, reducing stroke volume and cardiac output, leading to decreased tissue perfusion.
  • Clinical Features:
    • Signs: Tachycardia, hypotension, cool and clammy skin, delayed capillary refill.
  • Management:
    • Fluids: Rapid administration of isotonic crystalloids (e.g., normal saline or lactated Ringer’s).
    • Blood Products: For hemorrhagic shock, packed red blood cells or whole blood are given to restore volume and oxygen-carrying capacity.
Distributive Shock
Distributive shock is characterized by systemic vasodilation, leading to a relative intravascular volume deficit.
Septic Shock
  • Etiology: Systemic infection (often bacterial or fungal) leading to excessive cytokine release and capillary leak.
  • Pathophysiology: Decreased systemic vascular resistance (SVR) and increased capillary permeability lead to hypotension, while cardiac output may initially be increased.
  • Clinical Features: Fever, tachycardia, hypotension, warm skin in early stages progressing to cool extremities in prolonged shock.
  • Management:
    • Antibiotics: Broad-spectrum antibiotics within one hour of diagnosis.
    • Fluid Resuscitation: 30 mL/kg bolus of crystalloids.
    • Vasopressors: Norepinephrine is first-line if hypotension persists after fluids.
Anaphylactic Shock
  • Etiology: Severe IgE-mediated allergic reaction, often triggered by food, medications, or insect stings.
  • Pathophysiology: Release of histamine and other mediators causes vasodilation, increased vascular permeability, and bronchoconstriction.
  • Clinical Features: Angioedema, urticaria, wheezing, hypotension, and stridor.
  • Management:
    • Epinephrine: First-line treatment, given intramuscularly.
    • Additional Support: IV fluids, antihistamines, and corticosteroids to manage symptoms and prevent delayed reactions.
Cardiogenic Shock
  • Etiology: Most commonly due to acute myocardial infarction (MI), but can also result from severe heart failure, valvular disease, or arrhythmias.
  • Pathophysiology: Myocardial dysfunction reduces cardiac output, increasing filling pressures and leading to systemic hypoperfusion.
  • Clinical Features: Hypotension, tachycardia, cool and clammy skin, jugular venous distention, pulmonary crackles.
  • Management:
    • Inotropes: Dobutamine or milrinone to enhance myocardial contractility.
    • Vasopressors: Norepinephrine for hypotension.
    • Revascularization: Percutaneous coronary intervention (PCI) or thrombolytics in the case of MI.
Obstructive Shock
Obstructive shock results from a physical obstruction to blood flow, decreasing cardiac output despite normal cardiac function.
  • Etiology:
    • Pulmonary Embolism (PE): Causes right ventricular (RV) outflow obstruction.
    • Tension Pneumothorax: Increased intrathoracic pressure compresses the heart and great vessels.
    • Cardiac Tamponade: Fluid accumulation in the pericardial sac limits cardiac filling.
  • Clinical Features:
    • PE: Sudden dyspnea, pleuritic chest pain, right heart strain signs.
    • Tension Pneumothorax: Unilateral chest pain, absent breath sounds, tracheal deviation.
    • Cardiac Tamponade: Beck’s triad (hypotension, jugular venous distention, muffled heart sounds).
  • Management:
    • PE: Anticoagulation or thrombolysis in massive PE.
    • Tension Pneumothorax: Immediate needle decompression followed by chest tube insertion.
    • Cardiac Tamponade: Emergency pericardiocentesis.
Key Points
  • Types of Shock:
    • Hypovolemic Shock: Caused by fluid loss; managed with rapid IV fluids and blood transfusion for hemorrhagic cases.
    • Distributive Shock: Includes septic and anaphylactic shock; treated with fluids, antibiotics (in sepsis), and vasopressors if needed.
    • Cardiogenic Shock: Due to heart pump failure; managed with inotropes, vasopressors, and revascularization if related to MI.
    • Obstructive Shock: Caused by mechanical obstructions (e.g., PE, tension pneumothorax, cardiac tamponade); managed by relieving the obstruction.
  • Hemodynamic Profiles:
    • CVP: Low in hypovolemic shock; elevated in cardiogenic and obstructive shock.
    • SVR: Low in distributive shock; high in hypovolemic and cardiogenic shock.
  • Management Principles:
    • Fluid Resuscitation: Primary intervention in hypovolemic and distributive shock.
    • Vasopressors: Norepinephrine is first-line in septic and neurogenic shock.
    • Inotropes: Used in cardiogenic shock to increase cardiac output.

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