Shock

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Shock

Definition

Shock is a state of inadequate tissue perfusion leading to cellular hypoxia and, if uncorrected, irreversible organ injury and death. At the subcellular level, it first impairs mitochondrial function, replacing aerobic respiration with anaerobic glycolysis and producing lactic acidosis. Mortality rates exceed 20% across all types. - Rosen's Emergency Medicine, p. 58; Robbins, Cotran & Kumar Pathologic Basis of Disease, p. 134

Key point from Rosen's: Shock can occur with a normal blood pressure, and not all patients with hypotension are in shock. A base deficit more negative than -4 mEq/L or serum lactate >4.0 mmol/L warrants a presumptive diagnosis.

Classification

Four classic categories (with a fifth used clinically):

Type	Mechanism	Clinical Examples
Hypovolemic	Low blood/plasma volume	Hemorrhage, burns, vomiting, diarrhea
Cardiogenic	Pump failure - low cardiac output	MI (>40% myocardium lost), arrhythmia, tamponade, pulmonary embolism
Distributive (Septic)	Peripheral vasodilation + blood pooling	Gram-positive/negative sepsis, fungal sepsis, SIRS
Obstructive	Mechanical outflow obstruction	PE, tension pneumothorax, cardiac tamponade
Neurogenic	Loss of sympathetic tone	Acute spinal cord injury
Anaphylactic	IgE-mediated acute vasodilation	Allergen exposure

Robbins, Cotran & Kumar, Table 4.3, p. 134; Rosen's Emergency Medicine, Box 3.1

Pathophysiology

Cellular Level

Mitochondria are the first subcellular target
Oxygen deficit drives anaerobic glycolysis -> lactic acid accumulation
Lactic acidosis blunts vasomotor response -> arteriolar dilation -> microcirculatory pooling
Endothelial injury -> DIC risk
Lysosomal enzyme leakage worsens the spiral

Three Stages of Shock (Hypovolemic as prototype)

1. Nonprogressive (Compensated) Stage

Baroreceptor reflexes, catecholamine release, ADH release, RAAS activation, sympathetic stimulation
Result: tachycardia, peripheral vasoconstriction, renal fluid conservation
Blood is shunted away from skin to heart and brain
Skin: cool, pale, clammy (Note: early septic shock may show warm, flushed skin due to vasodilation)

2. Progressive Stage

Tissue hypoxia becomes widespread
Anaerobic glycolysis -> lactic acidosis -> arteriolar dilation -> microvascular pooling
Cardiac output worsens further; endothelial anoxic injury -> DIC
Vital organs begin to fail

3. Irreversible Stage

Lysosomal enzymes leak into cells
Myocardial contractility worsens (nitric oxide overproduction)
Ischemic bowel -> bacterial translocation -> superimposed bacteremic shock
Renal failure (ischemic ATN)
Death despite intervention
Robbins, Cotran & Kumar Pathologic Basis of Disease, p. 134-135

Septic Shock - Pathogenesis in Detail

Septic shock is the most common distributive shock. Mortality ~40%. Triggered most often by gram-positive bacteria, then gram-negative, then fungi.

Key mechanisms:

Pattern recognition: Microbial PAMPs and host DAMPs activate TLRs, G-protein-coupled receptors, and C-type lectin receptors on macrophages, neutrophils, dendritic cells
Cytokine storm: NF-kB activation -> TNF, IL-1, IL-12, IL-18, IFN-gamma, HMGB1, reactive oxygen species, prostaglandins, PAF
Endothelial activation: Upregulated adhesion molecules, increased permeability, capillary leak
Complement activation: C3a/C5a (anaphylatoxins), C3b (opsonins) -> further inflammation
Coagulation activation: Factor XII + altered endothelial function -> DIC
Immunosuppression oscillation: Counter-regulatory shift from Th1 to anti-inflammatory cytokines (IL-10, soluble TNF receptor) -> patients oscillate between hyperinflammatory and immunosuppressed states

Three primary effects of septic shock to address:

Hypovolemia (absolute from GI losses/sweating + relative from venous capacitance + capillary leak)
Direct myocardial depression (TNF-alpha, IL-1beta, iNOS, mitochondrial dysfunction)
Systemic inflammation -> ARDS, AKI, liver failure
Robbins, Cotran & Kumar Pathologic Basis of Disease, p. 135; Rosen's Emergency Medicine, p. 59

Morphologic Effects (Organ Pathology)

Organ	Finding
Brain	Ischemic encephalopathy
Heart	Subendocardial hemorrhage/necrosis
Kidney	Acute tubular necrosis ("shock kidney") - fibrin thrombi in glomeruli
Adrenals	Cortical cell lipid depletion (increased steroid synthesis)
Lung	Diffuse alveolar damage ("shock lung"/ARDS) - especially in septic/traumatic shock
GI tract	Mucosal hemorrhage, ulceration, bacterial translocation

Neuronal and cardiomyocyte loss is irreversible; most other tissues can recover if the patient survives. - Robbins, Cotran & Kumar, p. 135

Clinical Features

Finding	Hypovolemic/Cardiogenic	Septic (early)
Skin	Cool, clammy, cyanotic	Warm, flushed
Pulse	Weak, rapid	Bounding initially
BP	Hypotension	May be normal early
Mental status	Confusion, obtundation	Variable
Urine output	<0.5 mL/kg/h (severe)	Decreased
Lactate	Elevated (>4 mmol/L)	Elevated

Diagnostic criteria (Rosen's Box 3.3):

Septic shock: Sepsis + hypotension requiring vasopressors after fluid loading + lactate >2 mmol/L
Hemorrhagic shock: Suspected bleeding with base deficit <4 mEq/L or HR >100 consistently
Cardiogenic shock: Cardiac failure + shock criteria

Monitoring

Urine output - most reliable index of vital organ perfusion; normal = 1.0 mL/kg/h; <0.5 mL/kg/h = severe renal hypoperfusion
Lactate - serial measurement; persistent elevation = ongoing shock
Base deficit - worsening base deficit + rising lactate + low UO = persistent or worsening shock
Bedside ultrasound (FAST/echo) - screens for tamponade, LV/RV dysfunction, fluid volume

Management Principles

General Approach

Secure airway and IV access (2 large-bore IVs or central line)
Identify and treat the underlying cause (source control in sepsis, PCI in cardiogenic, hemorrhage control in hypovolemic)
Restore tissue perfusion with targeted resuscitation

Type-Specific Treatment

Hemorrhagic Shock:

Preferentially treated with blood products using a balanced transfusion approach - pRBC : FFP : platelets in a 1:1:1 ratio
Avoid excessive crystalloid (worsens coagulopathy)
Damage control surgery for ongoing bleeding

Septic Shock (Surviving Sepsis Bundle):

Early balanced fluid resuscitation (crystalloid; 30 mL/kg bolus as initial guide)
Vasopressors (norepinephrine first-line) to target MAP ≥65 mmHg
Prompt antimicrobial therapy (within 1 hour of recognition)
Source control (drain abscess, remove infected device)
Steroids (hydrocortisone) if refractory to vasopressors

Cardiogenic Shock:

Treat underlying cause (primary PCI for STEMI)
Inotropes (dobutamine, milrinone)
Mechanical circulatory support (IABP, Impella, ECMO) in refractory cases
Avoid aggressive fluid loading (worsens pulmonary edema)

Neurogenic Shock:

IV fluids + vasopressors (norepinephrine or phenylephrine)
Atropine for symptomatic bradycardia
Spinal immobilization, definitive spine management

Anaphylactic Shock:

Epinephrine IM (0.3-0.5 mg) is first-line - immediately
Antihistamines (H1 + H2 blockers) as adjuncts
Steroids to prevent biphasic reaction
IV fluids for volume support
Rosen's Emergency Medicine, p. 58-61; Sabiston Textbook of Surgery, p. 563

Summary Table

Type	CO	SVR	PCWP	CVP	Treatment
Hypovolemic	Low	High	Low	Low	Fluids / blood products
Cardiogenic	Low	High	High	High	Inotropes / PCI / MCS
Distributive/Septic	High (early)	Low	Low	Low	Fluids + vasopressors + antibiotics
Obstructive	Low	High	Variable	High	Relieve obstruction
Neurogenic	Low/normal	Low	Low	Low	Fluids + vasopressors

CO = cardiac output; SVR = systemic vascular resistance; PCWP = pulmonary capillary wedge pressure; CVP = central venous pressure

Recent Evidence: A 2024 review in Intensive Care Medicine (PMID: 39254735) covers the state-of-the-art management of cardiogenic shock, including mechanical circulatory support strategies. A 2025 review (PMID: 39720581) addresses the emerging entity of mixed cardiogenic-vasodilatory shock, which is increasingly recognized in ICU patients.

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