Shock

Definition

Shock is a state of circulatory failure in which diminished cardiac output and/or reduced effective circulating blood volume impairs tissue perfusion, leading to cellular hypoxia. At the outset, cellular injury is reversible; prolonged shock leads to irreversible tissue injury and is often fatal.

Robbins, Cotran & Kumar Pathologic Basis of Disease, p. 134
Guyton and Hall Textbook of Medical Physiology

Classification

Shock falls into four major categories:

Type	Mechanism	Key Clinical Examples
Cardiogenic	Pump failure - low CO due to intrinsic or extrinsic myocardial dysfunction	MI, ventricular arrhythmia, cardiac tamponade, pulmonary embolism
Hypovolemic	Inadequate blood/plasma volume - low CO	Hemorrhage, burns, vomiting, diarrhea
Distributive (Septic)	Peripheral vasodilation + cytokine cascade; maldistribution of blood flow	Gram-positive/negative bacteremia, fungal sepsis
Distributive (Others)	Loss of vascular tone	Neurogenic shock (spinal cord injury, anesthesia), anaphylactic shock (IgE-mediated, systemic vasodilation)

Robbins & Kumar Basic Pathology, Table 3.3
Sabiston Textbook of Surgery, Fig. 33.5

Stages of Shock

Three stages are classically described (Guyton & Hall):

Nonprogressive (Compensated) Stage - Normal compensatory mechanisms (baroreceptors, sympathetic reflexes, RAAS) are sufficient to restore perfusion without external therapy. Arteriolar constriction, venoconstriction, and tachycardia maintain cardiac output.
Progressive Stage - Persistent oxygen deficit shifts aerobic to anaerobic glycolysis, producing lactic acidosis. Tissue pH falls, vasomotor response is blunted, arterioles dilate, blood pools in the microcirculation. Endothelial injury triggers DIC. Vital organs begin to fail.
Irreversible Stage - Widespread cell injury with lysosomal enzyme leakage. Myocardial contractility worsens (increased NO synthesis). Ischemic bowel allows translocation of intestinal flora (bacteremic superinfection). Renal failure ensues. Death follows despite maximal therapy.

Pathophysiology of Septic Shock (Most Complex)

Sepsis affects >750,000 patients/year in the US; mortality remains ~20-40%. Most frequently triggered by gram-positive bacteria, then gram-negative bacteria and fungi.

Key Mechanisms:

1. Inflammatory cascade

Microbial PAMPs (LPS, peptidoglycans, fungal wall components) bind Toll-like receptors (TLRs), G-protein-coupled receptors, and C-type lectin receptors (dectins) on macrophages, neutrophils, and dendritic cells.
This triggers release of TNF, IL-1, IL-6, IL-12, ROS, prostaglandins, and PAF.
A counterinflammatory response (IL-10, soluble TNF-R, IL-1 receptor antagonist) is simultaneously activated; patients may oscillate between hyper- and hypo-inflammatory states.

2. Endothelial activation and injury

Cytokines loosen endothelial tight junctions → vascular leakage and protein-rich edema throughout the body
Activated endothelium upregulates nitric oxide (NO) and other vasodilators (C3a, C5a, PAF) → smooth muscle relaxation → systemic hypotension
Microvascular heterogeneity of flow and loss of autoregulation → oxygen delivery-demand mismatch

3. Procoagulant state

Increased tissue factor on monocytes and endothelium
Decreased endothelial anticoagulants (TFPI, thrombomodulin, protein C)
Increased PAI-1 suppresses fibrinolysis
Neutrophil extracellular traps (NETs) activate both coagulation pathways
Result: DIC in up to 50% of septic patients

4. Metabolic abnormalities

TNF, IL-1, and stress hormones → insulin resistance and hyperglycemia

Major pathogenic pathways in septic shock - PAMPs activate TLRs leading to proinflammatory state, endothelial activation, procoagulant state (DIC), and metabolic abnormalities, culminating in multiorgan failure

Fig: Major pathogenic pathways in septic shock. Microbial products activate innate immune cells; the resulting proinflammatory state leads to endothelial activation, procoagulant state (DIC), and metabolic derangements converging on multiorgan failure. - Robbins, Cotran & Kumar, Fig. 4.20

ATLS Classification of Hemorrhagic Shock

Parameter	Class I	Class II	Class III	Class IV
Blood loss (%)	0-15	15-30	30-40	>40
Pulse (bpm)	<100	>100	>120	>140
Blood pressure	Normal	Normal	Decreased	Decreased
Pulse pressure	Normal	Decreased	Decreased	Decreased
Respiratory rate	14-20	20-30	30-40	>35
Urine output (mL/hr)	>30	20-30	5-15	Negligible
CNS	Slightly anxious	Mildly anxious	Anxious/confused	Confused/lethargic
Fluid	Crystalloid	Crystalloid	Crystalloid + blood	Crystalloid + blood

Note: The ATLS classification was empirically derived and not rigorously validated. Children compensate longer (large fluid reserve) and decompensate abruptly; elderly patients show signs of higher-class shock at lower blood loss due to reduced cardiac compensation. - Sabiston Textbook of Surgery, Table 33.1

Compensatory Mechanisms

Sympathetic reflexes are the first line of compensation. When blood pressure drops, baroreceptors and vascular stretch receptors trigger:

Arteriolar constriction - increases total peripheral resistance
Venoconstriction - maintains venous return
Tachycardia - heart rate can rise to 160-180 bpm

These reflexes extend the survivable blood loss from ~15-20% (without reflexes) to 30-40% of total blood volume. - Guyton & Hall

Additional mechanisms include RAAS activation (angiotensin II, aldosterone), ADH release, and stress hormones (cortisol, epinephrine) promoting fluid retention.

Morphological (Organ) Effects

Organ injury is essentially that of hypoxic injury combined with microvascular thrombosis. Key organs affected:

Organ	Effect
Brain	Encephalopathy, ischemic injury
Heart	Subendocardial necrosis, decreased contractility
Kidneys	Acute tubular necrosis (oliguria, anuria) - most clinically dominant
Adrenals	Cortical lipid depletion (increased steroid synthesis from stores)
Lungs	Resistant in pure hypovolemia; in sepsis/trauma - diffuse alveolar damage ("shock lung" / ARDS)
GI tract	Ischemia, mucosal loss, bacterial translocation in irreversible stage

Fibrin thrombi form most visibly in renal glomeruli. Except for neurons and cardiomyocytes, affected tissues can fully recover if the patient survives.

Clinical Features

Feature	Hypovolemic / Cardiogenic	Septic
Skin	Cool, clammy, cyanotic	Warm, flushed (early; peripheral vasodilation)
Pulse	Weak, rapid	Bounding (early), weak (late)
BP	Hypotension	Hypotension
Respiration	Tachypnea	Tachypnea

Late manifestations: oliguria, metabolic acidosis (lactic), electrolyte imbalances, multiorgan failure.

Prognosis

Hypovolemic shock in young, healthy patients: >90% survival with appropriate management
Septic shock or cardiogenic shock: substantially worse outcomes even with optimal care

Summary - Key Concepts

Shock = systemic tissue hypoperfusion from reduced CO and/or circulating volume
Three major types: cardiogenic, hypovolemic, septic (distributive); also neurogenic and anaphylactic
Septic shock - dysregulated host response involving TLR activation, cytokine storm, endothelial injury, DIC, metabolic derangements
Three stages: compensated → progressive (lactic acidosis, DIC) → irreversible (organ failure, death)
Management targets the underlying cause: volume resuscitation (hypovolemic), vasopressors (distributive), inotropes (cardiogenic), antibiotics + source control (septic), epinephrine (anaphylactic)

Sources: Robbins, Cotran & Kumar Pathologic Basis of Disease; Robbins & Kumar Basic Pathology; Sabiston Textbook of Surgery; Guyton & Hall Textbook of Medical Physiology

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