Shock pathophysiology

Shock Essentials - The Vicious Cycle

Shock is a self-perpetuating cycle where decreased tissue perfusion causes cellular injury, which worsens the perfusion deficit, spiraling into multi-organ failure.

• Trigger: ↓ Cardiac Output (CO) or ↓ Systemic Vascular Resistance (SVR).
• Cellular Hypoxia: Forces switch to anaerobic glycolysis → ↑ lactic acid.
• Metabolic Acidosis: $H^+$ ions depress myocardial contractility and blunt catecholamine response.
• Mediator Release: Cellular injury releases inflammatory cytokines (TNF-α, IL-1), fueling vasodilation and capillary leak, culminating in Multi-Organ Dysfunction Syndrome (MODS).

⭐ The Lethal Triad: In trauma, the shock cycle is amplified by the "lethal triad" of hypothermia, acidosis, and coagulopathy. Each component worsens the others, accelerating decline.

Shock Types - Pump, Tank, or Pipes?

A framework for classifying shock based on the primary circulatory defect.

• CVP: Central Venous Pressure (estimates Right Atrial Pressure).

⭐ Septic Shock: A type of distributive shock, is unique for its initial hyperdynamic state ("warm shock") with high cardiac output and low SVR, which can later decompensate into a hypodynamic state ("cold shock").

Septic Shock - The Cytokine Storm

• A distributive shock from a dysregulated host response to infection, primarily driven by Pathogen-Associated Molecular Patterns (PAMPs) like LPS.
• Initiation: PAMPs bind to Pattern Recognition Receptors (e.g., TLRs) on immune cells (macrophages, neutrophils), triggering a massive release of pro-inflammatory cytokines (TNF-α, IL-1, IL-6).
• Vasodilation: Cytokines upregulate inducible Nitric Oxide Synthase (iNOS), causing a surge in Nitric Oxide ($NO$). This leads to profound systemic vasodilation and ↓ Systemic Vascular Resistance (SVR).
• Consequences:
  • ↑ Capillary permeability → fluid extravasation, edema, and relative hypovolemia.
  • Myocardial depression from circulating cytokines.
  • Potential for Disseminated Intravascular Coagulation (DIC).

⭐ Hemodynamic Profile (Early "Warm" Shock): Unlike other shock states, early septic shock classically presents with an ↑ Cardiac Output, ↓ Systemic Vascular Resistance (SVR), and a high mixed venous oxygen saturation (SvO₂).

End-Organ Damage - The Final Fallout

• Universal Pathway: ↓ Perfusion → Cellular Hypoxia → ↑ Lactic Acid → Organ Dysfunction.
• Brain: Ischemic encephalopathy (confusion → coma).
• Heart: Subendocardial ischemia/infarction (↓ CO).
• Kidneys: Acute Tubular Necrosis (ATN) → oliguria, ↑ BUN/Cr.
• Lungs: "Shock Lung" → Acute Respiratory Distress Syndrome (ARDS).
• Liver: Centrilobular necrosis (↑ LFTs).
• GI Tract: Ischemic bowel, mucosal necrosis.

⭐ The kidneys are highly vulnerable, manifesting as Acute Tubular Necrosis (ATN). Look for muddy brown granular casts in urine sediment.

High‑Yield Points - ⚡ Biggest Takeaways

• Shock is inadequate organ perfusion and tissue oxygenation, leading to widespread cellular injury.
• The core metabolic problem is a shift to anaerobic glycolysis, causing lactic acidosis.
• Shock progresses through three stages: compensated, progressive, and irreversible.
• Hypovolemic shock is defined by low preload (↓ CVP, ↓ PCWP) from volume loss.
• Cardiogenic shock results from primary pump failure (↓ cardiac output).
• Distributive shock features massive vasodilation and low systemic vascular resistance (↓ SVR).
• The final common pathway is often multi-organ dysfunction syndrome (MODS).