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Shock for USMLE Step 1

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Shock for the USMLE Step 1 Exam
Shock is a critical condition characterized by inadequate tissue perfusion and oxygenation, leading to cellular and organ dysfunction. There are four main types of shock: hypovolemic, distributive, cardiogenic, and obstructive.
signs and symptoms of shock
Types of Shock
Hypovolemic Shock
  • Etiology: Caused by significant fluid loss, either from hemorrhage (e.g., trauma, gastrointestinal bleeding) or dehydration (e.g., vomiting, diarrhea, burns).
  • Pathophysiology: Reduced blood volume decreases preload, leading to decreased cardiac output and tissue hypoperfusion.
  • Clinical Features:
    • Signs: Tachycardia, hypotension, cool and clammy skin, delayed capillary refill.
  • Management:
    • Fluids: Rapid infusion of isotonic crystalloids (e.g., normal saline).
    • Blood Products: Indicated in hemorrhagic shock to restore circulating volume.
Distributive Shock
Distributive shock results from severe vasodilation, leading to a relative intravascular volume deficit despite normal blood volume.
Septic Shock
  • Etiology: Systemic response to infection (e.g., bacterial, fungal) causing widespread vasodilation and capillary leak.
  • Pathophysiology: Infection triggers inflammatory mediators, resulting in decreased systemic vascular resistance (SVR) and increased cardiac output.
  • Clinical Features: Fever, tachycardia, warm skin (early), progressing to cool extremities if shock is prolonged.
  • Management:
    • Antibiotics: Broad-spectrum within one hour of recognition.
    • Fluid Resuscitation: Initial bolus of 30 mL/kg of crystalloids.
    • Vasopressors: Norepinephrine is first-line if hypotension persists after fluids.
Anaphylactic Shock
  • Etiology: Severe allergic reaction, often from food, drugs, or insect stings.
  • Pathophysiology: IgE-mediated release of histamine causes systemic vasodilation, increased vascular permeability, and bronchoconstriction.
  • Clinical Features: Urticaria, angioedema, wheezing, and hypotension.
  • Management:
    • Epinephrine: First-line treatment, given intramuscularly.
    • Additional Support: IV fluids, antihistamines, and corticosteroids for symptom control and prevention of late-phase reactions.
Cardiogenic Shock
  • Etiology: Typically caused by myocardial infarction, severe heart failure, valvular disease, or arrhythmias.
  • Pathophysiology: Myocardial dysfunction leads to reduced cardiac output and elevated filling pressures, impairing tissue perfusion.
  • Clinical Features: Hypotension, tachycardia, cool and clammy skin, jugular venous distention, pulmonary crackles.
  • Management:
    • Inotropes: Dobutamine or milrinone to increase myocardial contractility.
    • Vasopressors: Norepinephrine for blood pressure support.
    • Revascularization: For MI, consider percutaneous coronary intervention (PCI) or thrombolytics.
Obstructive Shock
Obstructive shock occurs from mechanical impediments to blood flow, reducing cardiac output despite normal heart function.
  • Etiology:
    • Pulmonary Embolism (PE): Causes right ventricular outflow obstruction.
    • Tension Pneumothorax: Pressure within the pleural space compresses the heart and great vessels.
    • Cardiac Tamponade: Accumulation of fluid in the pericardial sac compresses the heart.
  • Clinical Features:
    • PE: Sudden dyspnea, pleuritic chest pain, signs of right heart strain.
    • Tension Pneumothorax: Tracheal deviation, unilateral breath sounds.
    • Cardiac Tamponade: Beck’s triad (hypotension, distended neck veins, muffled heart sounds).
  • Management:
    • PE: Anticoagulation or thrombolysis if massive.
    • Tension Pneumothorax: Immediate needle decompression followed by chest tube placement.
    • Cardiac Tamponade: Urgent pericardiocentesis.
Key Points
  • Types of Shock:
    • Hypovolemic: Caused by fluid loss; managed with IV fluids and blood products for hemorrhage.
    • Distributive: Characterized by vasodilation (e.g., septic, anaphylactic shock); treated with fluids and vasopressors as needed.
    • Cardiogenic: Results from myocardial pump failure; managed with inotropes, vasopressors, and revascularization if due to MI.
    • Obstructive: Due to mechanical blockage (e.g., PE, tension pneumothorax, cardiac tamponade); treated by relieving the obstruction.
  • Hemodynamics:
    • CVP: Typically low in hypovolemic shock; elevated in cardiogenic and obstructive shock.
    • SVR: Decreased in distributive shock; increased in hypovolemic and cardiogenic shock.
  • Management:
    • Fluid Resuscitation: First-line in hypovolemic and distributive shock.
    • Vasopressors: Norepinephrine is first-line for septic and neurogenic shock.
    • Inotropes: Used in cardiogenic shock to improve cardiac output.

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