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SHOCK - A Comprehensive Clinical Note

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1. DEFINITION

"A manifestation of the rude unhinging of the machinery of life." - Gross, 1872 (Sabiston Textbook of Surgery, p. 563)

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2. HISTORICAL BACKGROUND

• The term "shock" first appeared in a 1743 translation of Henri Francois Le Dran's French treatise on battlefield wounds, used to designate the act of collision.
• In 1815, Guthrie used it to describe physiologic instability in Gunshot Wounds of the Extremities.
• In 1830, Herman attempted IV rehydration with water for cholera - one of the first documented attempts at intravascular volume replacement.
• In 1872, Gross defined shock as "a manifestation of the rude unhinging of the machinery of life."
• In 1899, George Crile proposed that profound decline in blood pressure accounted for all symptoms of shock, leading to the routine clinical measurement of BP.

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3. CLASSIFICATION / TYPES OF SHOCK

1. At the heart - cardiogenic abnormalities (intrinsic or extrinsic)
2. At large/medium vessels - hemorrhage and loss of blood volume
3. At small vessels - neurologic dysfunction or sepsis causing vasodilatation and maldistribution

A. Cardiogenic Shock

• Results from low cardiac output due to myocardial pump failure
• Intrinsic causes: Myocardial infarction, ventricular rupture, arrhythmia, myocardial contusion/laceration
• Extrinsic causes: Cardiac tamponade, tension pneumothorax, hemothorax
• Obstructive causes: Pulmonary embolism (outflow obstruction)
• Hemodynamics: Low CO, high SVR, high PCWP

B. Hypovolemic Shock

• Results from low cardiac output due to loss of blood or plasma volume
• Causes: Hemorrhage (most common), fluid loss from severe burns, vomiting, diarrhea, third-spacing
• Subdivided into:
  • Hemorrhagic shock (most studied; caused by blood loss)
  • Non-hemorrhagic hypovolemic shock (plasma loss, GI losses)
• Hemodynamics: Low CO, high SVR, low PCWP

C. Distributive (Vasodilatory) Shock

1. Septic Shock

• Most common form of distributive shock
• Caused by overwhelming microbial infections (gram-positive bacteria most common, followed by gram-negative bacteria and fungi)
• Sepsis: life-threatening organ dysfunction caused by a dysregulated host response to infection
• Septic shock: a subset of sepsis with profound circulatory, cellular, and metabolic abnormalities
• Hemodynamics: High CO (hyperdynamic), low SVR, warm periphery

2. Neurogenic Shock

• Loss of sympathetic vascular tone due to spinal cord injury or anesthesia
• Acute vasodilation leads to hypotension and tissue hypoperfusion
• Hemodynamics: Low CO (due to bradycardia), very low SVR
• Classic triad: Hypotension + Bradycardia + Warm extremities

3. Anaphylactic Shock

• IgE-mediated hypersensitivity reaction
• Triggered by drugs (penicillin), foods (peanuts), insect stings
• Systemic vasodilation and increased vascular permeability
• Features: Urticaria, bronchospasm, angioedema, hypotension

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4. ATLS CLASSIFICATION OF HEMORRHAGIC SHOCK

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5. PATHOPHYSIOLOGY

5.1 General Mechanism

Oxygen delivery (DO₂) fails to meet oxygen consumption (VO₂) at the cellular level

1. Shift from aerobic to anaerobic glycolysis
2. Excessive production of lactic acid → metabolic acidosis
3. Failure of Na⁺/K⁺ ATPase pumps → cellular swelling
4. Lysosomal enzyme release → cellular necrosis
5. Eventual irreversible cell death if not corrected

5.2 Pathogenesis of Septic Shock

a) Inflammatory & Counter-inflammatory Responses

• Microbial constituents (PAMPs) activate Toll-like receptors (TLRs), G-protein-coupled receptors, and C-type lectin receptors on innate immune cells
• Activated macrophages, neutrophils, and dendritic cells release:
  • Cytokines: TNF, IL-1, IL-12, IL-18, type I interferons
  • Reactive oxygen species (ROS)
  • Lipid mediators: prostaglandins, platelet-activating factor (PAF)
• Complement cascade is activated → produces C3a (mast cell activation), C5a (chemotaxis), C3b (opsonin)
• Elevated acute phase reactants: C-reactive protein, procalcitonin
• With time, the hyperinflammatory state triggers counter-regulatory immunosuppression involving both innate and adaptive immune cells

b) Endothelial Activation and Injury

• Inflammatory cytokines loosen tight junctions → vascular leakage and protein-rich edema
• Activated endothelium upregulates NO and other vasoactive mediators (C3a, C5a, PAF) → smooth muscle relaxation → systemic hypotension
• Microvascular dysfunction: increased capillaries with intermittent flow, heterogeneous flow, loss of autoregulation → oxygen supply-demand mismatch

c) Induction of a Procoagulant State

• Proinflammatory cytokines increase tissue factor production by monocytes/endothelial cells
• Decrease anticoagulant factors: tissue factor pathway inhibitor (TFPI), thrombomodulin, protein C
• Dampen fibrinolysis by increasing PAI-1 expression
• Result: systemic thrombin activation and fibrin-rich thrombi in small vessels → further compromise tissue perfusion
• In up to 50% of septic patients: Disseminated Intravascular Coagulation (DIC) - consumption of clotting factors and platelets leads to paradoxical bleeding

d) Metabolic Abnormalities

• Insulin resistance and hyperglycemia: driven by TNF, IL-1, catecholamines, glucocorticoids, growth hormone
• Proinflammatory cytokines suppress insulin release and promote insulin resistance
• Sepsis is associated with rises in blood glucose, triglycerides, and lactate
• Hyperglycemia decreases neutrophil bactericidal function and increases endothelial adhesion molecule expression
• Adrenal insufficiency may follow initial glucocorticoid surge, sometimes from Waterhouse-Friderichsen syndrome (adrenal necrosis from DIC)
• Cellular hypoxia + diminished oxidative phosphorylation → lactic acidosis

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6. COMPENSATORY MECHANISMS (Stages of Shock)

Stage 1: Initial Nonprogressive (Compensated) Stage

• Reflex compensatory mechanisms are activated
• Vital organ perfusion is maintained
• Neurohumoral responses activated:
  • Baroreceptor reflexes
  • Release of catecholamines and ADH (antidiuretic hormone)
  • Activation of renin-angiotensin-aldosterone axis
  • Generalized sympathetic stimulation
• Net effects:
  • Tachycardia
  • Peripheral vasoconstriction (skin becomes cool, pale, clammy)
  • Renal fluid conservation (oliguria)
  • Blood shunting away from skin to heart and brain (coronary and cerebral vessels less sensitive to sympathetic signals)
• Note: In septic shock, initial cutaneous vasodilation may produce warm, flushed skin

Stage 2: Progressive (Decompensated) Stage

• Underlying cause not corrected → widespread tissue hypoxia
• Intracellular aerobic respiration replaced by anaerobic glycolysis → excessive lactic acid production
• Metabolic lactic acidosis blunts the vasomotor response
• Arterioles dilate → blood pools in the microcirculation
• Peripheral pooling worsens cardiac output and risks DIC from endothelial anoxic injury
• Vital organs affected and begin to fail

Stage 3: Irreversible Stage

• Widespread cell injury → lysosomal enzyme leakage, further aggravating shock
• Myocardial contractile function worsens (partly from increased NO synthesis)
• Ischemic bowel → intestinal flora enters circulation → superimposed bacteremic shock
• Renal failure from ischemic kidney injury
• Despite best interventions, the downward spiral culminates in death

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7. CLINICAL FEATURES

General Features

• Hypotension (SBP <90 mmHg or MAP <65 mmHg)
• Tachycardia (>100 beats/min; not always reliable in isolation)
• Tachypnea
• Altered mental status (anxiety, confusion, obtundation)
• Oliguria (<0.5 mL/kg/h, indicating reduced renal perfusion)
• Metabolic acidosis with elevated lactate (>2 mmol/L)

Type-Specific Features

Markers of Shock Severity

• Serum lactate: best marker; >4 mmol/L = severe; measures anaerobic metabolism
• Base deficit: correlates well with lactate; >8 mmol/L associated with ~25% mortality
• pH: lactic acidosis reflects severity
• ScvO₂ (central venous oxygen saturation): <70% indicates inadequate oxygen delivery
• Procalcitonin & CRP: markers of sepsis

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8. MORPHOLOGICAL CHANGES (Pathology)

Organs Affected:

• Neuronal and cardiomyocyte loss is irreversible; other affected tissues can recover completely if patient survives
• Lungs are relatively resistant to hypoxic injury in pure hypovolemic shock (hemorrhage alone) - but are vulnerable in sepsis or trauma

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9. INVESTIGATIONS / DIAGNOSIS

Bedside Assessment

• Blood pressure (systolic, diastolic, pulse pressure, MAP)
• Heart rate, respiratory rate, pulse oximetry
• Mental status, urine output (catheter)
• Temperature

Laboratory

• Full blood count: Hb, Hct (hemorrhagic shock); WBC (sepsis)
• Serum lactate: elevated (>2 mmol/L in shock; >4 mmol/L = severe)
• Arterial blood gas (ABG): metabolic acidosis (↓ pH, ↓ HCO₃, ↑ base deficit)
• Electrolytes, urea, creatinine: acute kidney injury
• Coagulation profile (PT, aPTT, D-dimers, fibrinogen): DIC screen
• Blood cultures x 2: in suspected septic shock (ideally before antibiotics)
• Procalcitonin, CRP: infection markers
• Troponin, BNP: cardiogenic shock
• Cross-match/Group & Screen: for blood transfusion

Imaging

• ECG: MI, arrhythmia (cardiogenic shock)
• Chest X-ray: pneumothorax, hemothorax, pulmonary edema, infiltrates (ARDS)
• FAST ultrasound (Focused Assessment with Sonography in Trauma): fluid in peritoneum/pericardium
• Echo (bedside): cardiac function, tamponade, volume status (IVC collapsibility)
• CT scan: source of hemorrhage, aortic dissection, PE (CT-PA)

Monitoring (ICU)

• Central venous catheter: CVP, ScvO₂
• Arterial line: continuous arterial BP, ABG access
• Pulmonary artery catheter (Swan-Ganz): CO, PCWP, SVR (in complex cases)
• Point-of-care lactate clearance (target >10% every 2 hours)

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10. MANAGEMENT

General Principles

1. Airway and Breathing: Supplemental oxygen (high-flow), intubation if GCS ≤8 or respiratory failure
2. Circulation: Two large-bore IV lines (16G or larger), fluid resuscitation
3. Control of bleeding (in hemorrhagic shock): The most critical step is identifying and stopping the source
4. Monitoring: Continuous BP, pulse oximetry, ECG, urine output
5. End-point resuscitation goals: Normalize lactate, base deficit, urine output, MAP ≥65 mmHg

Resuscitation Endpoints

• MAP ≥65 mmHg (or ≥80 mmHg in neurogenic shock due to spinal cord perfusion needs)
• Urine output >0.5 mL/kg/h
• Serum lactate normalization (<2 mmol/L)
• ScvO₂ >70%
• Correction of base deficit within 48 hours (persistent elevation = ongoing shock)

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A. Hemorrhagic Shock Management

• Maintain SBP 80-90 mmHg (or MAP ~50 mmHg) in penetrating trauma until surgical hemorrhage control
• Prevents "popping the clot" before definitive surgical control
• Not applied in traumatic brain injury (requires higher MAP for cerebral perfusion)

• Initial: Crystalloid (Normal saline or Lactated Ringer's) - Class I and II
• Class III/IV: Crystalloid + blood products
• Massive transfusion protocol (MTP): Packed red blood cells : Fresh frozen plasma : Platelets in 1:1:1 ratio
• Avoid excessive crystalloid (hyperchloremic acidosis, abdominal compartment syndrome)

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B. Septic Shock Management (Surviving Sepsis Campaign Bundles)

1. Measure serum lactate (re-measure if >2 mmol/L)
2. Obtain blood cultures before antibiotics
3. Administer broad-spectrum antibiotics
4. Administer 30 mL/kg crystalloid for hypotension or lactate ≥4 mmol/L
5. Apply vasopressors if patient remains hypotensive to maintain MAP ≥65 mmHg

• Within 1 hour of septic shock recognition
• Broad-spectrum empirical coverage (e.g., piperacillin-tazobactam + aminoglycoside)
• De-escalate based on culture results

• 30 mL/kg crystalloid (Lactated Ringer's preferred over normal saline)
• Reassess with dynamic indicators of fluid responsiveness (pulse pressure variation, IVC collapsibility)

• Norepinephrine: first-line vasopressor for septic shock (titrate to MAP ≥65 mmHg)
• Vasopressin (0.03 units/min): second-line, added to reduce norepinephrine requirements
• Dopamine: alternative in select patients (bradycardia + low cardiac output)
• Dobutamine: if myocardial dysfunction (low CO with high filling pressures)

• Hydrocortisone 200 mg/day IV if refractory to vasopressors (vasopressor-dependent septic shock)
• Fludrocortisone 50 mcg/day (oral) may be added

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C. Cardiogenic Shock Management

• Treat the underlying cause: Reperfusion therapy for MI (primary PCI), pericardiocentesis for tamponade, thrombolysis/embolectomy for PE
• Fluids: cautious (small boluses) - avoid fluid overload (worsens pulmonary edema)
• Vasopressors: Norepinephrine (first-line), Dopamine
• Inotropes: Dobutamine (positive inotrope to improve CO)
• Intra-aortic balloon pump (IABP): increases diastolic coronary perfusion, reduces afterload
• Mechanical circulatory support: Impella device, ECMO (extracorporeal membrane oxygenation) in refractory cases

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D. Neurogenic Shock Management

• Immobilize spine (spinal precautions)
• Aggressive fluid resuscitation: restoration of intravascular volume
• Vasopressors: Norepinephrine preferred (phenylephrine for pure vasoconstriction)
• Atropine/cardiac pacing: for associated bradycardia
• Target MAP ≥85 mmHg (for first 7 days post-spinal injury) to maintain spinal cord perfusion
• Avoid secondary spinal cord injury

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E. Anaphylactic Shock Management

• Epinephrine (adrenaline) 0.5 mg IM (anterolateral thigh): first-line, immediately
• Remove/neutralize trigger
• Airway: early intubation if laryngeal edema
• IV fluids: large-volume crystalloid resuscitation
• Antihistamines: Chlorphenamine (H1), Ranitidine/Famotidine (H2)
• Corticosteroids: Hydrocortisone 200 mg IV (slow; reduces biphasic reactions)
• Bronchodilators: Salbutamol nebulization for bronchospasm

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11. COMPLICATIONS

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12. SPECIAL CONSIDERATIONS

Hypothermia in Shock ("Lethal Triad")

• Hemorrhagic shock patients are prone to hypothermia
• Lethal Triad of Trauma: Hypothermia + Acidosis + Coagulopathy - mutually reinforcing
• Even a few degrees drop in core temperature causes 40% inefficiency in coagulation enzymes
• Cold affects coagulation by decreasing enzyme activity, enhancing fibrinolysis, inhibiting thromboxane B2 (platelet dysfunction)
• Prevention: Warm all IV fluids; warming blankets; heated humidified ventilation

Lactate as a Biomarker

• Not merely a waste product - may be a "pseudohormone" affecting cellular redox state
• Lactate levels depend on equilibrium between production AND elimination (liver metabolizes lactate)
• Elevated in:
  • Anaerobic conditions (tissue hypoxia)
  • High adrenergic states (stress response)
  • Liver failure (decreased clearance)
• Used to guide resuscitation: target lactate clearance of >10-20% every 2 hours
• Lactate normalization within 6 hours is a key resuscitation target

Fluid Choice Considerations

• Lactated Ringer's: preferred over normal saline (avoids hyperchloremic acidosis)
• Balanced crystalloids: reduce AKI risk vs. normal saline (SMART trial)
• Albumin: may be used in septic shock after adequate crystalloid loading
• Colloids (HES - hydroxyethyl starch): Avoid in sepsis (increased AKI risk and mortality)

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13. PROGNOSIS

• Persistent elevated lactate (failure to clear)
• Persistent base deficit >6-8 mmol/L after 24-48 hours
• Development of ≥3 organ dysfunction (MODS)
• Age extremes
• Multidrug-resistant organisms (sepsis)
• Delayed recognition and treatment

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SUMMARY TABLE

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• Robbins, Cotran & Kumar Pathologic Basis of Disease (10th ed.), Chapter 4 - Hemodynamic Disorders, Thromboembolic Disease, and Shock
• Robbins & Kumar Basic Pathology (11th ed.), Chapter 3
• Sabiston Textbook of Surgery - Chapter 33: Shock
• Goodman & Gilman's Pharmacological Basis of Therapeutics
• Goldman-Cecil Medicine
• Ganong's Review of Medical Physiology