SHOCK

(15-Mark University Question - Pharmacology)

Definition

• Goodman & Gilman's, p. 286: "Shock is an immediately life-threatening impairment of delivery of O₂ and nutrients to the organs of the body."
• Robbins & Kumar Basic Pathology: "Shock is a state of circulatory failure that impairs tissue perfusion and leads to cellular hypoxia."

Classification / Types of Shock

Pathophysiology

1. Cellular Level

2. Microvascular Level

3. Systemic Responses

• Cardiovascular: Decreased preload/afterload triggers baroreceptor reflexes, with increased sympathetic activity and catecholamine release - causing tachycardia and vasoconstriction (except in distributive shock, where vasodilation predominates).
• Respiratory: Metabolic acidosis drives increased respiratory rate, producing a compensatory respiratory alkalosis.
• Renal: Decreased glomerular filtration pressure reduces urine output; the renin-angiotensin-aldosterone axis is activated, increasing vasoconstriction and sodium/water reabsorption.
• Endocrine: Vasopressin (ADH) is released, causing vasoconstriction and water reabsorption. Cortisol is released from the adrenal cortex, sensitizing cells to catecholamines.

Pathogenesis of Septic Shock (in detail)

1. Microbial triggers: Endotoxin (LPS from gram-negative bacteria), staphylococcal enterotoxin B, streptococcal M protein, and fungal antigens all activate innate immune cells via Toll-like receptors (TLRs) and pattern recognition receptors that detect PAMPs (pathogen-associated molecular patterns) and DAMPs (damage-associated molecular patterns).
2. Cytokine storm: Activation of NF-kB causes release of TNF, IL-1, IL-6, IL-12, IL-18, IFN-γ, and HMGB1. These cause widespread endothelial activation, increased adhesion molecule expression, and further cytokine amplification.
3. Complement activation: C3a/C5a act as anaphylatoxins and chemotactic fragments; C3b acts as an opsonin - all amplifying the proinflammatory state.
4. Coagulation activation: Widespread thrombin activation via factor XII and altered endothelial function leads to DIC.
5. Immunosuppression paradox: The hyperinflammatory state triggers counter-regulatory mechanisms shifting from Th1 to Th2 cytokines, producing IL-10, IL-1 receptor antagonist, soluble TNF receptor, and lymphocyte apoptosis. Patients may oscillate between hyperinflammatory and immunosuppressed states.
6. Metabolic derangements: Proinflammatory cytokines suppress insulin release and promote insulin resistance; hyperglycemia impairs neutrophil function. Adrenal insufficiency may supervene (Waterhouse-Friderichsen syndrome via DIC). Lactic acidosis results from impaired oxidative phosphorylation.

Stages of Shock

Stage 1 - Non-progressive (Compensated) Stage

• Baroreceptor reflexes, catecholamine release (NE, Epi)
• ADH and RAAS activation
• Generalized sympathetic stimulation
• Result: tachycardia, peripheral vasoconstriction, maintaining BP and cerebral blood flow

Stage 2 - Progressive Stage

• Widespread tissue hypoxia and anaerobic glycolysis
• Lactic acidosis blunts vasomotor responses
• Arteriolar dilation with blood pooling in the microcirculation
• Endothelial hypoxic injury → DIC risk
• Vital organ dysfunction begins

Stage 3 - Irreversible Stage

• Lysosomal enzyme leakage worsens the shock state
• Myocardial contractile function deteriorates (increased NO synthesis)
• Ischemic bowel allows intestinal flora into circulation → superimposed bacteremic shock
• Progressive renal failure (acute tubular necrosis)
• Death despite maximal intervention

Morphological (Organ) Changes in Shock

Clinical Features

• Hypovolemic/Cardiogenic shock: Hypotension, weak and rapid pulse (tachycardia), tachypnea, cool, clammy, cyanotic skin, oliguria, altered sensorium
• Septic/Distributive shock: Warm, flushed skin (early, due to peripheral vasodilation), fever, later progressing to hypotension and cool peripheries
• Progressive shock: Worsening oliguria, lactic acidosis, electrolyte imbalances
• Prognosis: >90% survival in young patients with hypovolemic shock with appropriate management; substantially poorer in septic or cardiogenic shock

Treatment / Pharmacological Management

General Principles

1. Maintain airway and ventilation (supplemental O₂ or mechanical ventilation)
2. Restore blood volume (IV crystalloids, colloids, blood transfusion)
3. Monitor hemodynamic parameters (CVP, MAP, urine output, lactate)
4. Treat underlying cause (antibiotics for sepsis, revascularization for MI, etc.)

Vasoactive/Pharmacological Agents

1. Dopamine (DA)

• Dose-dependent effects:
  • Low dose (1-5 µg/kg/min): D₁ receptor activation → renal and splanchnic vasodilation (preserves renal function)
  • Moderate dose (5-10 µg/kg/min): β₁ agonism → increased cardiac output and inotropy
  • High dose (>10-20 µg/kg/min): α₁ agonism → peripheral vasoconstriction (similar to NE)
• Preferred in cardiogenic shock for its combined inotropic and renal-protective effects

2. Dobutamine

• Selective β₁ agonist (with some β₂ and α effects from its stereoisomers)
• Increases myocardial contractility with minimal increase in heart rate or peripheral resistance
• Drug of choice in cardiogenic shock to improve cardiac output
• Less arrhythmogenic than isoproterenol

3. Norepinephrine (NE)

• Potent α₁ + β₁ agonist
• Increases peripheral vascular resistance and blood pressure
• Risk: intensifies peripheral vasoconstriction, potentially worsening organ ischemia (kidneys, gut)
• Useful in septic shock to restore MAP when vasodilation is refractory

4. Epinephrine (Epi)

• α + β agonist
• Increases heart rate and myocardial contractility
• Risk: predisposes to dangerous arrhythmias; increases myocardial O₂ demand
• Drug of choice in anaphylactic shock (reverses bronchospasm, vasodilation, and urticaria)

5. Phenylephrine, Metaraminol, Vasopressin

• Pure α₁ agonists (except vasopressin - V1 receptor agonist)
• Used when hypotension is so severe that vasoconstriction is needed to maintain CNS perfusion
• Particularly useful in neurogenic shock (after spinal anesthesia/injury) where sympathetic tone is lost
• Caution: in cardiogenic shock, further vasoconstriction increases cardiac work and may be detrimental

6. Vasodilators (Nitroprusside, Nitrates)

• Used in cardiogenic shock when reflex vasoconstriction is already intense
• Decrease afterload, reduce cardiac work, improve blood flow to kidneys and gut
• Can improve cardiac output if a minimal acceptable blood pressure can be maintained

7. Corticosteroids

• Relative adrenal insufficiency is common in septic shock
• Low-dose hydrocortisone (200-300 mg/day) is used in vasopressor-dependent septic shock refractory to fluids and vasopressors (Surviving Sepsis Campaign guidelines)
• Restores vascular sensitivity to catecholamines

8. Antibiotics

• In septic shock: prompt, broad-spectrum antibiotics must be started within 1 hour of recognition
• Choice guided by suspected source and local antibiogram

Summary Table - Drug Choices by Shock Type

Conclusion

• Goodman & Gilman's The Pharmacological Basis of Therapeutics, pp. 285-286
• Robbins, Cotran & Kumar - Pathologic Basis of Disease, pp. 134-137
• Bailey and Love's Short Practice of Surgery, 28th ed., pp. 32-34
• Robbins & Kumar Basic Pathology, pp. 134-137