shock types pathogenesis of each type & management

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Shock: Types, Pathogenesis & Management

Shock is a state of circulatory failure in which diminished cardiac output or reduced effective circulating blood volume impairs tissue perfusion, leading to cellular hypoxia. At the outset the cellular injury is reversible; prolonged shock leads to irreversible tissue injury and death. — Robbins, Cotran & Kumar Pathologic Basis of Disease

Classification

The four major physiologic categories are:

Type	CO/CI	SVR	Preload (PAOP)	SvO₂
Hypovolemic	↓	↑	↓	↓
Cardiogenic	↓	↑	↑	↓
Distributive (septic/anaphylactic/neurogenic)	N–↑	↓	N–↓	N–↑
Obstructive	↓	↑–N	N–↓ (PAOP) / ↑ RAP	N–↓

— Washington Manual of Medical Therapeutics; Harriet Lane Handbook

1. Hypovolemic Shock

Pathogenesis

Results from a critical reduction in intravascular volume — either blood (hemorrhagic) or fluid (burns, vomiting, diarrhea, polyuria in DKA/DI). Loss of circulating volume → ↓ venous return → ↓ stroke volume → ↓ cardiac output. Compensatory mechanisms activate:

Baroreceptor reflexes → sympathetic activation → tachycardia, peripheral vasoconstriction
Renin-angiotensin-aldosterone axis → sodium and water retention
ADH release → renal fluid conservation
Catecholamine surge → redistribution of blood to heart and brain (away from skin, gut, kidneys)

Net effect: cool, clammy, pale skin; oliguria; weak rapid pulse. If uncorrected, anaerobic metabolism produces lactic acidosis → arteriolar dilation → blood pools in microcirculation → endothelial injury → DIC → irreversible stage.

Management

Rapid volume resuscitation with isotonic crystalloid (balanced/buffered preferred, e.g., Lactated Ringer's)
Hemorrhagic shock: Replace blood loss with 10 mL/kg pRBC boluses; in massive hemorrhage, use a 1:1:1 ratio of pRBCs : FFP : platelets (damage control resuscitation)
Identify and control the source of bleeding (surgical/interventional if needed)
Colloids considered if poor response to crystalloids and protein-containing fluid loss is suspected
Target: MAP ≥65 mmHg, UO ≥0.5 mL/kg/hr, lactate clearance

Prognosis: >90% of young, otherwise healthy patients with hypovolemic shock survive with appropriate management. — Robbins, Cotran & Kumar

2. Cardiogenic Shock

Pathogenesis

Results from myocardial pump failure → ↓ cardiac output despite adequate intravascular volume. Causes:

Intrinsic myocardial damage: MI (most common — occurs in ~7% of STEMI), myocarditis, cardiomyopathy
Arrhythmia: sustained ventricular tachycardia/fibrillation, complete heart block
Extrinsic compression: cardiac tamponade
Mechanical: ventricular rupture, severe valvular disease (acute aortic/mitral regurgitation)

Pump failure → ↓ CO → compensatory ↑ SVR (afterload) and fluid retention (↑ preload/PAOP) → further worsens pump function → vicious cycle: ischemia → dysfunction → ↑ filling pressures → pulmonary edema → further ischemia. 90% of patients develop cardiogenic shock during hospitalization rather than on presentation. — Harrison's Principles of Internal Medicine 22E

Management

Goals: restore perfusion, reduce ischemia, offload the failing heart.

Revascularization (first priority in AMI): Emergency PCI of infarct-related artery (STEMI or NSTEMI-CS)
Vasopressors: Norepinephrine is first-line (fewer arrhythmias vs. dopamine; start 2–4 µg/min, titrate to MAP ≥65)
Inotropes: Dobutamine (2–20 µg/kg/min, β1 agonist) for normotensive patients with ↓ perfusion; Milrinone (PDE3 inhibitor) — no benefit over dobutamine in recent trials
Avoid: dopamine as first-line (proarrhythmogenic); IABP (no mortality benefit in IABP-SHOCK II trial — no longer recommended for LV failure)
Mechanical Circulatory Support (MCS): Impella or ECMO in refractory cases
Diuretics + nitrates: For pulmonary congestion (furosemide IV; nitroglycerin for preload/afterload reduction — use cautiously to avoid hypotension)
Arrhythmia management, treat tamponade with pericardiocentesis, correct mechanical defects surgically

— Harrison's Principles of Internal Medicine 22E; Fuster & Hurst's The Heart 15th Ed.

3. Distributive Shock

Distributive shock is unique: there is a compensatory increase in CO, but SVR drops profoundly → maldistribution of blood flow → functional tissue hypoperfusion despite high flow. Subcategories:

3a. Septic Shock

Pathogenesis

Defined as sepsis with circulatory, cellular, and metabolic abnormalities that increase mortality risk. Triggered most often by gram-positive bacteria > gram-negative bacteria > fungi (SARS-CoV-2 also implicated).

Key mechanisms (Robbins, Cotran & Kumar):

Inflammatory cascade: Microbial PAMPs (LPS/endotoxin from gram-negatives, teichoic acid from gram-positives, fungal β-glucans) engage Toll-like receptors (TLRs), G-protein-coupled receptors, and C-type lectin receptors (Dectins) on macrophages, neutrophils, dendritic cells, and endothelial cells → activation of NF-κB → massive cytokine release:
- TNF, IL-1, IL-6, IL-12, IL-18, IFN-γ, HMGB1
- Reactive oxygen species, prostaglandins, platelet-activating factor (PAF)
- Complement activation → C3a, C5a (anaphylatoxins), C3b (opsonin)
Counter-inflammatory response: To modulate the hyperinflammatory state, immunosuppressive mechanisms activate (IL-10, soluble TNF receptor, IL-1 receptor antagonist, lymphocyte apoptosis, Th1→Th2 shift) → oscillation between hyperinflammatory and immunosuppressed states.
Endothelial activation and injury: Cytokines upregulate adhesion molecules (ICAM-1, VCAM-1), promote leukocyte infiltration, and cause endothelial apoptosis → vascular leakage → edema, decreased circulating volume, further hypotension.
Coagulation activation: Microbial components activate factor XII and alter endothelial function → widespread thrombin generation → DIC (disseminated intravascular coagulation) → microvascular thrombosis → organ ischemia.
Metabolic derangements: Insulin resistance, mitochondrial dysfunction, impaired O₂ utilization ("cytopathic hypoxia") → lactic acidosis despite adequate delivery.
Cardiovascular effects: Initial high-output, low-SVR state ("warm shock" — warm flushed skin, bounding pulse). As shock progresses: myocardial depression (NO-mediated), relative hypovolemia (capillary leak) → "cold shock."

Management (Surviving Sepsis Campaign framework)

Hour-1 Bundle:

30 mL/kg IV crystalloid (balanced preferred) within first 3 hours
Blood cultures before antibiotics (do not delay antibiotics)
Broad-spectrum antibiotics within 1 hour (choice guided by suspected source — see table below)
Measure lactate (target lactate <2 mmol/L; repeat if >2)
Apply vasopressors if MAP <65 despite fluids

Vasopressors (initiate for MAP <65 after initial fluid):

Norepinephrine (0.01–3 µg/kg/min): First-line vasopressor
Vasopressin (0.03–0.04 U/min): Second-line; may reduce norepinephrine requirements
Epinephrine: Third-line; or switch if cardiac dysfunction (warm shock → epinephrine; cold shock → add dobutamine to norepinephrine)
Dopamine: Avoid (higher adverse events)

Adjuncts:

Hydrocortisone 200 mg/day (divided or continuous infusion) if vasopressor-refractory shock persists
Dobutamine (2–20 µg/kg/min) if signs of low CO/cardiac dysfunction despite adequate MAP
Glycemic control (target 140–180 mg/dL)
Lung-protective ventilation if ARDS develops
Source control: drain abscesses, remove infected devices, surgical debridement for necrotizing infections

— Goldman-Cecil Medicine; Harrison's Principles of Internal Medicine 22E; Washington Manual

3b. Anaphylactic Shock

Pathogenesis

IgE-mediated (Type I hypersensitivity) → massive mast cell and basophil degranulation → release of histamine, leukotrienes, prostaglandins, tryptase, PAF → profound systemic vasodilation + increased vascular permeability + bronchospasm → ↓ SVR, ↓ preload → ↓ CO → tissue hypoperfusion. Can also be IgE-independent (direct mast cell degranulation by certain drugs/contrast media). — Robbins, Cotran & Kumar

Management

Epinephrine IM (0.3–0.5 mg; 1:1000) — cornerstone; given immediately into lateral thigh; reverses vasodilation and bronchospasm (α1 + β2 effects)
IV crystalloid boluses for relative hypovolemia
Supplemental O₂ / airway management (intubation if laryngeal edema)
Antihistamines (H1 blocker: diphenhydramine; H2 blocker: ranitidine) — adjunctive only, do not replace epinephrine
Corticosteroids (methylprednisolone) — to prevent biphasic reaction (secondary wave)
Remove/stop offending agent
Vasopressors (norepinephrine) if refractory to epinephrine and fluids

3c. Neurogenic Shock

Pathogenesis

Caused by acute spinal cord injury (typically above T6) or high spinal/epidural anesthesia → disruption of descending sympathetic pathways → loss of sympathetic tone → vasodilation (↓ SVR) + bradycardia (unopposed vagal tone, loss of cardiac sympathetic innervation). Unlike other shock types: HR is normal or low (not compensatory tachycardia) and skin may be warm/dry. — Robbins, Cotran & Kumar; Harriet Lane Handbook

Management

Position patient flat or head-down (Trendelenburg) to augment venous return
IV crystalloid trial (judicious — avoid fluid overload given intact cardiac function)
If fluid-refractory: Vasopressors (norepinephrine or epinephrine) to restore vascular tone; vasopressin also used
Atropine or cardiac pacing if bradycardia is hemodynamically significant
Maintain normothermia (poikilothermia due to sympathetic loss)
Spinal cord stabilization and neuroprotection

4. Obstructive Shock

Pathogenesis

Caused by mechanical obstruction to cardiac filling or outflow → ↓ CO despite normal/hypervolemic intravascular volume and normal myocardium.

Causes and mechanisms:

Cause	Mechanism
Tension pneumothorax	Air in pleural space shifts mediastinum → compresses SVC/IVC → ↓ venous return
Cardiac tamponade	Pericardial fluid compression → ↓ diastolic filling → ↓ SV
Massive pulmonary embolism	Right ventricular outflow obstruction → RV failure → ↓ LV filling (septal shift)
Constrictive pericarditis / aortic dissection	Impaired filling / outflow obstruction

Hemodynamics: ↓ CO, ↑ SVR, ↑ RAP, ↑ RVP, ↑ PAP, normal or low PAOP (except tamponade where equalization of diastolic pressures occurs). — Harrison's Principles of Internal Medicine 22E; Washington Manual

Management

Treat the cause immediately:

Tension pneumothorax: Immediate needle decompression (2nd intercostal space, midclavicular line) → chest tube
Cardiac tamponade: Pericardiocentesis (or surgical drainage for traumatic hemopericardium)
Massive PE: Systemic thrombolysis (tPA) if hemodynamically unstable and no contraindications; catheter-directed thrombolysis or surgical embolectomy for failed/contraindicated thrombolysis
Bridging: IV crystalloid (cautious — may worsen RV overload in PE/tamponade); vasopressors (norepinephrine) for refractory hypotension
Prostaglandin E₁ if ductal-dependent congenital cardiac lesion suspected (pediatrics)

Stages of Shock (All Types)

Unless massive and rapidly lethal, shock evolves through three stages:

Stage	Features
1. Nonprogressive (Compensated)	Baroreceptor reflexes, catecholamines, RAAS, ADH activation → tachycardia, vasoconstriction, renal fluid conservation; vital organ perfusion maintained; skin cool/pale
2. Progressive	Tissue hypoxia exceeds compensation → anaerobic glycolysis → lactic acidosis → arteriolar dilation → microvascular pooling → endothelial injury → DIC; multi-organ dysfunction begins
3. Irreversible	Lysosomal enzyme leakage; myocardial depression (↑ NO); bowel ischemia → bacterial translocation; renal failure; death despite intervention

— Robbins, Cotran & Kumar Pathologic Basis of Disease

Morphologic Consequences of Shock

Brain: Ischemic encephalopathy, neuronal loss
Heart: Subendocardial necrosis, contraction bands
Kidneys: Acute tubular necrosis → acute kidney injury; fibrin thrombi in glomeruli
Lungs: Diffuse alveolar damage ("shock lung" / ARDS) — especially in septic/traumatic shock
GI tract: Hemorrhagic mucosal necrosis; bacterial translocation
Adrenals: Cortical lipid depletion (increased steroid synthesis from stress)

Except for neurons and cardiomyocytes, affected tissues can recover completely if the patient survives. — Robbins, Cotran & Kumar

Sources: Robbins & Kumar Basic Pathology; Robbins, Cotran & Kumar Pathologic Basis of Disease; Harrison's Principles of Internal Medicine 22E; Goldman-Cecil Medicine; Washington Manual of Medical Therapeutics; Harriet Lane Handbook 23E; Fuster & Hurst's The Heart 15E; Sabiston Textbook of Surgery.

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