Diagnosis: Septic Shock

Pathogenesis of Septic Shock

1. Inflammatory and Counter-inflammatory Responses

• TLR ligation activates NF-kB, driving transcription of pro-inflammatory cytokines: TNF, IL-1, IL-12, IL-18, IFN-γ, and HMGB1
• Reactive oxygen species (ROS), prostaglandins (PGs), and platelet-activating factor (PAF) are elaborated
• Complement is activated - producing anaphylatoxins (C3a, C5a), chemotactic fragments (C5a), and opsonins (C3b)
• Acute phase reactants - CRP and procalcitonin - rise in the blood

2. Endothelial Activation and Injury

• Pro-inflammatory cytokines loosen endothelial tight junctions, causing widespread vascular leakage and protein-rich edema throughout the body
• Activated endothelium overproduces nitric oxide (NO) and other vasodilators (C3a, C5a, PAF), causing vascular smooth muscle relaxation and systemic hypotension
• Microvascular dysfunction develops: heterogeneous capillary flow, loss of autoregulation, mismatch between oxygen delivery and demand

3. Induction of a Procoagulant State (DIC)

• Pro-inflammatory cytokines increase tissue factor (TF) expression on monocytes and endothelium
• Anticoagulant factors are suppressed: TFPI (tissue factor pathway inhibitor), thrombomodulin, and Protein C are downregulated
• PAI-1 (plasminogen activator inhibitor-1) is increased, dampening fibrinolysis
• Neutrophil extracellular traps (NETs) promote both intrinsic and extrinsic coagulation pathways
• Result: Systemic thrombin activation → fibrin-rich microthrombi in small vessels → further ischemia
• Full-blown DIC causes consumption of clotting factors and platelets → simultaneous bleeding and clotting

4. Metabolic Abnormalities

• TNF and IL-1 + stress hormones (glucagon, growth hormone, glucocorticoids, catecholamines) drive gluconeogenesis → hyperglycemia
• Insulin release is suppressed; GLUT-4 expression is impaired → insulin resistance
• Hyperglycemia suppresses neutrophil bactericidal activity and upregulates endothelial adhesion molecules
• Lactic acidosis develops due to cellular hypoxia and impaired oxidative phosphorylation
• Adrenal insufficiency may supervene; in severe DIC, adrenal necrosis causes Waterhouse-Friderichsen syndrome

5. Organ Dysfunction - Stages of Shock

Pathogenesis Diagram (Robbins)

Morphology of Septic Shock

Except for neuronal and cardiomyocyte loss (irreversible), affected tissues can recover completely if the patient survives.

Other Causes of Shock (Classification)

1. Cardiogenic Shock

• Myocardial infarction (most common cause)
• Myocarditis
• Cardiac tamponade
• Severe arrhythmias
• Dilated cardiomyopathy
• Massive pulmonary embolism

2. Hypovolemic Shock

• Hemorrhagic: trauma, ruptured aortic aneurysm, GI bleeding, obstetric hemorrhage
• Non-hemorrhagic: severe burns (plasma loss), vomiting/diarrhea, Addison's disease (salt-wasting)

3. Septic Shock (Distributive)

• Gram-negative bacteria (current case): E. coli, Klebsiella, Pseudomonas, Neisseria meningitidis, Proteus - via endotoxin (LPS)
• Gram-positive bacteria (most common overall): Staphylococcus aureus, Streptococcus pyogenes - via superantigens (toxic shock syndrome), teichoic acid
• Fungi: Candida (especially in immunocompromised)
• Viruses: SARS-CoV-2, influenza (emerging causes)
• Anaerobes: Clostridium, bacteroides

4. Anaphylactic Shock (Distributive)

• Drugs (penicillin, NSAIDs), bee stings, food allergens (peanuts)
• Histamine release → venodilation → reduced venous return → severe hypotension

5. Neurogenic Shock (Distributive)

• Spinal cord injury (above T6)
• Deep general anesthesia
• Brain injury

6. Obstructive Shock

• Massive pulmonary embolism
• Tension pneumothorax
• Cardiac tamponade

Key Summary

Diagnosis: Hypovolemic (Hemorrhagic) Shock

Classification of Shock

ATLS Classification of Hemorrhagic Shock (American College of Surgeons)

This patient - unconscious, cold/clammy/cyanotic, rapid pulse, hypotension, tachypnea = Class III-IV hemorrhagic shock (>30-40% blood volume loss)

Pathogenesis of Hypovolemic Shock

Initial Trigger

Stage 1: Compensatory (Nonprogressive) Phase

• Arterial baroreceptors in carotid sinus and aortic arch detect the fall in BP
• Increased sympathetic outflow → tachycardia, increased myocardial contractility
• Arteriolar vasoconstriction (except coronary and cerebral vessels) → increased total peripheral resistance (TPR)
• Venous constriction → increased venous return to heart
• Result: Blood shunted from skin, gut, and skeletal muscle to vital organs (heart and brain)
• Cutaneous vasoconstriction explains the cold, pale, clammy skin

• Reduced renal perfusion → renin release → Angiotensin II → arteriolar vasoconstriction + aldosterone release → Na⁺ and water retention

• Released in response to decreased blood volume and osmolality → vasoconstriction + renal water conservation

• Release of epinephrine and norepinephrine → peripheral vasoconstriction, tachycardia, increased cardiac contractility

• Reduced capillary hydrostatic pressure → net interstitial fluid absorption into capillaries → auto-transfusion effect, partially restoring blood volume

• When BP falls below 50 mmHg, cerebral ischemia itself triggers extreme sympathetic discharge ("last ditch stand"), producing a temporary secondary plateau in arterial pressure at ~50 mmHg

Cardiovascular Compensation Diagram (Costanzo Physiology)

Stage 2: Progressive Phase - The Vicious Cycle

1. Cardiac depression: Prolonged low coronary blood flow → myocardial ischemia → further fall in cardiac output → more ischemia. By 4 hours at arterial pressure of 30 mmHg, cardiac output falls ~40%; then rapidly to zero.
2. Vasomotor centre failure: Prolonged cerebral hypoperfusion → vasomotor centre in medulla depresses → arteriolar dilation → more venous pooling → further fall in cardiac output
3. Microvascular sludging and thrombosis:

• Tissue hypoxia + lactic/carbonic acid accumulation → local blood agglutination → small vessel plugging
• Intravascular clotting → further impairs microvascular flow

1. Increased capillary permeability:

• Prolonged capillary ischemia damages the capillary walls → protein-rich fluid leaks into interstitium → blood volume further falls

1. Release of inflammatory toxins:

• Ischemic tissues release histamine, serotonin, cytokines, and lysosomal enzymes → further vascular damage
• In prolonged shock: intestinal ischemia → bacterial translocation → endotoxin absorption → cardiac depression (superimposed endotoxemia)

1. Generalized cellular deterioration:

• Failure of Na⁺/K⁺-ATPase → cellular sodium and water accumulation, cellular swelling
• Mitochondrial dysfunction → impaired oxidative phosphorylation → lactic acidosis
• Lysosomal rupture → intracellular release of hydrolases → cell death
• Insulin resistance and glucose utilization failure

Stage 3: Irreversible Phase

• Widespread cell death reaches a point where even if hemodynamics are restored, the patient cannot survive
• Characterized by: anuria, profound metabolic acidosis, refractory hypotension, multiorgan failure
• The conscious patient from Stage 2 (confusion, combativeness) progresses to unconsciousness - as seen in this case - indicating Stage 3 involvement

Morphology of Hypovolemic Shock

Brain

• Ischemic encephalopathy - neuronal cell death in areas most vulnerable to hypoxia (hippocampus CA1, Purkinje cells of cerebellum, layers 3, 5, 6 of cortex)
• This explains the unconscious state in this patient
• Neuronal loss is irreversible

Heart

• Subendocardial hemorrhage and coagulative necrosis (myocardial infarction-like changes in subendocardial zones)
• These areas are most remote from blood supply and most vulnerable
• Cardiomyocyte loss is irreversible

Kidney

• Acute Tubular Necrosis (ATN) - the most common renal manifestation
• Proximal tubular and ascending loop of Henle cells are most susceptible
• Histology: dilated tubular lumens with flattened epithelium, loss of brush border, sloughed epithelial cells as granular casts in tubules
• Fibrin thrombi may be seen in glomerular capillaries
• Clinically → oliguria → anuria

Lung

• In pure hypovolemic shock: relatively resistant (well protected by collateral circulation)
• In trauma-associated shock: Diffuse Alveolar Damage (DAD) / ARDS ("Shock Lung")
• Precipitated by the combination of trauma and hypoperfusion
• Histology: hyaline membranes, interstitial edema, hemorrhage, type II pneumocyte hyperplasia

Liver

• Centrilobular (zone 3) necrosis - as this zone is furthest from blood supply (perivenular)
• Illustrated by Guyton's Fig. 24.5: patchy necrosis of central portions of hepatic lobules near central veins
• Hepatic ischemia → reduced metabolic and detoxification function → worsens overall metabolic derangements

GI Tract

• Mucosal ischemia → hemorrhagic erosions (stress ulcers), primarily in stomach and duodenum
• Intestinal barrier failure → bacterial/endotoxin translocation → superimposed sepsis/endotoxemia

Adrenal Glands

• Cortical cell lipid depletion - stored cholesterol esters used for emergency cortisol synthesis
• In severe DIC: bilateral adrenal hemorrhage (Waterhouse-Friderichsen syndrome)

Evolution Summary

RTA → Hemorrhage
    ↓
Reduced Blood Volume (>30-40%)
    ↓
Stage 1: Compensatory (Nonprogressive)
  • Baroreceptor reflex → tachycardia, vasoconstriction
  • RAAS → fluid retention
  • ADH → fluid conservation
  • Adrenal catecholamines → tachycardia, vasoconstriction
  [Clinical: Rapid pulse, clammy skin, tachypnea - CONSCIOUS]
    ↓ (if untreated / massive hemorrhage)
Stage 2: Progressive (Vicious Cycle Begins)
  • Cardiac depression, vasomotor failure
  • Microvascular sludging/DIC
  • Capillary leak, cellular deterioration
  • Lactic acidosis, cytokine storm
  [Clinical: Confusion, worsening hypotension]
    ↓
Stage 3: Irreversible
  • Widespread cell death (neurons, cardiomyocytes)
  • Multiorgan failure (ATN, DAD, hepatic necrosis)
  • Refractory metabolic acidosis
  [Clinical: UNCONSCIOUS, no urine - as in this patient]

Key Hemodynamic Profile of Hypovolemic Shock