Q4 LONG ANSWER 10M ★ High-yield

Define shock. Classify it and describe its pathogenesis, stages and morphology.

Pathology · 2nd Professional — Para-Clinical

Definition

Shock is a state of circulatory failure in which perfusion of the tissues is inadequate to meet their metabolic needs, leading to cellular hypoxia and, if unrelieved, to organ failure and death.

It is a failure of perfusion, not necessarily of blood pressure — a normal pressure does not exclude shock, particularly in the young, in whom compensation is efficient until it suddenly fails

Classification

Type	Mechanism	Causes
Hypovolaemic	Reduced circulating blood volume	Haemorrhage, burns, severe vomiting or diarrhoea, dehydration
Cardiogenic	Pump failure — reduced cardiac output	Myocardial infarction, arrhythmia, myocarditis, valve rupture
Obstructive	Mechanical obstruction to flow	Massive pulmonary embolism, cardiac tamponade, tension pneumothorax
Distributive — septic	Widespread vasodilatation and increased permeability	Gram-negative and gram-positive sepsis, fungi
Distributive — anaphylactic	IgE-mediated mast cell degranulation	Drugs, insect stings, foods
Distributive — neurogenic	Loss of vasomotor tone	Spinal cord injury, spinal anaesthesia

Feature	Hypovolaemic and cardiogenic	Septic (early, "warm")
Skin	Cold, clammy, pale	Warm, flushed, dry
Peripheral resistance	Increased	Decreased
Cardiac output	Reduced	Normal or increased
Pulse pressure	Narrow	Wide, bounding

CLINICAL PEARL

Septic shock is the one that feels wrong. The patient is warm, flushed and has a bounding pulse with a high cardiac output — the opposite of what "shock" suggests — because the problem is maldistribution, not pump failure or volume loss. Blood is flowing, but through shunts rather than through the capillary beds that need it.

Pathogenesis of Septic Shock

Endotoxin (lipopolysaccharide) from gram-negative bacteria → Binds TOLL-like receptor 4 on macrophages and endothelium → Massive release of TNF, IL-1, IL-6 and nitric oxide → Widespread vasodilatation → falling systemic vascular resistance → Increased permeability → loss of fluid into the interstitium → Endothelial activation with tissue factor expression → disseminated intravascular coagulation → Myocardial depression, mitochondrial dysfunction and metabolic derangement → multi-organ failure

Stages of Shock

Stage	Features	Reversible?
1. Non-progressive (compensated)	Baroreceptor reflexes, sympathetic activation, catecholamines, renin–angiotensin, ADH. Tachycardia, vasoconstriction, oliguria; blood pressure maintained	Yes — fully
2. Progressive	Tissue hypoperfusion, anaerobic glycolysis, lactic acidosis. Acidosis blunts the vasomotor response, arterioles dilate, blood pools. Confusion, falling urine output, hypotension	Yes, with prompt treatment
3. Irreversible	Widespread cell injury, lysosomal enzyme leakage, DIC, gut ischaemia allowing bacterial translocation, acute tubular necrosis, myocardial depression	NO — death follows even if the cause is corrected

Morphology

Kidney — acute tubular necrosis, the commonest and most important lesion
Lung — diffuse alveolar damage; "shock lung" or acute respiratory distress syndrome with hyaline membranes
Heart — subendocardial haemorrhage and contraction band necrosis
Gut — mucosal haemorrhagic necrosis; stress ulcers
Liver — centrilobular necrosis
Suprarenal — cortical lipid depletion; waterhouse–friderichsen syndrome, bilateral haemorrhagic necrosis in meningococcal sepsis
Brain — ischaemic encephalopathy

Compensatory Mechanisms in Shock

Mechanism	Trigger	Effect
Baroreceptor reflex	Fall in arterial pressure sensed in the carotid sinus and aortic arch	Tachycardia and increased contractility; arteriolar vasoconstriction
Sympathoadrenal discharge	Sympathetic activation	Catecholamine release; blood diverted from skin, gut and kidney to heart and brain
Renin–angiotensin–aldosterone	Reduced renal perfusion	Vasoconstriction and sodium and water retention
Antidiuretic hormone	Volume receptors and osmoreceptors	Water retention and vasoconstriction
Transcapillary refill	Fall in capillary hydrostatic pressure	Interstitial fluid moves into the circulation, restoring volume — and diluting the haemoglobin, which is why the haematocrit falls only hours after a bleed

The falling haematocrit after haemorrhage is a compensation, not the haemorrhage itself — immediately after a bleed, haemoglobin is normal because whole blood has been lost. This is a common and dangerous source of false reassurance

Applied Aspects

The single most useful marker of tissue perfusion is the serum lactate, and its fall with treatment predicts survival better than the blood pressure
Urine output is the cheapest continuous monitor of perfusion — below 0.5 mL/kg/hour indicates inadequate renal perfusion
Early goal-directed therapy in sepsis — fluids, cultures before antibiotics, antibiotics within the first hour, and vasopressors if fluid alone fails
Anaphylaxis is treated with intramuscular adrenaline first, not antihistamines or steroids; delay is the commonest cause of death
Cardiogenic shock is worsened by fluid, unlike every other type — which is why the diagnosis must be made before treatment begins
Compensation is more efficient in children and the young, so blood pressure is maintained until a very late stage and then falls precipitously
Capillary refill and skin temperature are useful bedside signs, and cost nothing — a cold, mottled periphery with a delayed refill indicates poor perfusion whatever the blood pressure
Prognosis — hypovolaemic shock has the best outcome if the bleeding is stopped; cardiogenic and septic shock carry mortality of 50% or more
Multi-organ dysfunction syndrome is the usual mode of death, and mortality rises steeply with each additional organ that fails
The kidney is the most reliable early indicator, which is why hourly urine output is measured in every shocked patient
Blood transfusion is given for haemorrhagic shock, but crystalloid first — and massive transfusion itself causes coagulopathy, hypothermia and hypocalcaemia
Neurogenic shock is distinguished by bradycardia with hypotension, because the sympathetic outflow is lost — the one shock without a tachycardia
Adrenal insufficiency must be considered in shock unresponsive to vasopressors, particularly in a patient recently on corticosteroids
Anaphylaxis can recur hours later (biphasic), so a patient must be observed even after apparently complete recovery
Obstructive shock is corrected mechanically, not pharmacologically — needle decompression for tension pneumothorax, pericardiocentesis for tamponade
The "golden hour" in trauma reflects the fact that shock is fully reversible early and progressively less so with time

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