Shock
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Shock — A Comprehensive Overview
1. Definition
Shock is a state of systemic circulatory failure resulting in inadequate tissue perfusion and cellular hypoxia. As Cannon first described in 1929, it is "inadequate blood flow resulting in cellular hypoxia." If uncorrected, the resulting oxygen deficit leads to progressive, irreversible organ dysfunction and death.
"Shock is a state of circulatory failure that impairs tissue perfusion and leads to cellular hypoxia." — Robbins & Kumar Basic Pathology
2. Classification / Types of Shock
The most widely accepted classification (Weil & Shubin, 1971) divides shock into four categories:
| Type | Mechanism | Examples |
|---|---|---|
| Hypovolemic | Inadequate blood/plasma volume → ↓ preload → ↓ CO | Hemorrhage, burns, vomiting, diarrhea, trauma |
| Cardiogenic | Pump failure → ↓ CO | MI (>40% LV involvement), arrhythmias, cardiac tamponade, pulmonary embolism, acute valvular disease |
| Distributive | ↓ SVR with maldistribution of flow | Septic shock, anaphylaxis, neurogenic (spinal cord injury), adrenal insufficiency |
| Obstructive | Mechanical impediment to flow | Tension pneumothorax, cardiac tamponade, massive PE |
Some modern classifications (Schwartz's Principles of Surgery) describe six types, adding septic (vasodilatory), neurogenic, and traumatic shock as distinct entities.
3. Hemodynamic Profiles (Harrison's, 22nd ed.)
| Type | CVP | PCWP | Cardiac Output | SVR |
|---|---|---|---|---|
| Distributive | ↓ | ↓ | ↑ | ↓ |
| Cardiogenic | ↑ | ↑ | ↓ | ↑ |
| Obstructive | ↑ | ↓/↑ | ↓ | ↑ |
| Hypovolemic | ↓ | ↓ | ↓ | ↑ |
This hemodynamic fingerprint is key to differentiating shock types at the bedside and with invasive monitoring.
4. Pathophysiology
Common Final Pathway
Regardless of type, all shock leads to oxygen delivery (DO₂) failing to meet oxygen consumption (VO₂), causing cells to shift to anaerobic metabolism → lactic acidosis → cellular dysfunction → organ failure.
Neuroendocrine Response
The body's compensatory response is triggered by baroreceptors and chemoreceptors detecting hypoxia, hypotension, or hypovolemia:
- Sympathetic activation: norepinephrine-mediated vasoconstriction of low-priority beds (skin, gut, kidneys); reflex tachycardia to increase CO
- Catecholamines (epinephrine/NE): glycogenolysis, gluconeogenesis → stress hyperglycemia
- ACTH → Cortisol: further gluconeogenesis, lipolysis
- Renin → Angiotensin II → Aldosterone: sodium/water retention, vasoconstriction
- ADH (vasopressin): water reabsorption, splanchnic vasoconstriction
Immunoinflammatory Response (Distributive/Septic)
In septic and traumatic shock, simultaneous massive systemic inflammation occurs:
- TNF-α, IL-1, chemokines, nitric oxide are released by macrophages, neutrophils, and endothelial cells
- Coagulation cascade activation → DIC
- PMN adhesion to endothelium → transmigration into remote tissues
- This hyperdynamic pro-inflammatory state drives Multiple Organ Dysfunction Syndrome (MODS)
5. Stages of Shock
(Harrison's Principles of Internal Medicine, 22nd ed.)
Stage 1: Compensated Shock (Preshock)
- Body's compensatory mechanisms are intact
- No overt organ dysfunction clinically
- Subtle lab changes: mildly elevated creatinine, troponin, or lactate
- Examples: early sepsis with ↑ HR and ↑ CO compensating for ↓ SVR; early hemorrhage with ↑ SVR and ↑ HR compensating for volume loss
Stage 2: Decompensated Shock (True Shock)
- Compensatory responses are overwhelmed
- Overt organ dysfunction present
- Hypotension, oliguria, confusion, rising lactate
- Reversible with prompt treatment
Stage 3: Irreversible Shock
- Permanent organ dysfunction
- No longer responsive to resuscitation — as Wiggers stated: "adequate circulation cannot be restored by merely filling the system, as one does an automobile radiator"
- Progresses to multisystem organ failure and death
6. Classes of Hemorrhagic Shock
(Mulholland & Greenfield's Surgery / ATLS classification)
| Class I | Class II | Class III | Class IV | |
|---|---|---|---|---|
| Blood Loss (mL) | ≤750 | 750–1,500 | 1,500–2,000 | >2,000 |
| Blood Loss (%) | ≤15% | 15–30% | 30–40% | >40% |
| Heart Rate | <100 | >100 | >120 | >140 |
| Blood Pressure | Normal | Normal | ↓ | ↓ |
| Pulse Pressure | Normal | ↓ | ↓ | ↓ |
| Respiratory Rate | 14–20 | 20–30 | 30–40 | >35 |
| Urine Output (mL/hr) | >30 | 20–30 | 5–15 | Minimal |
| Mental Status | Normal | Mildly anxious | Anxious/confused | Confused/lethargic |
| Fluid Replacement | Crystalloid | Crystalloid | Crystalloid + blood | Crystalloid + blood |
Key point: Hypotension does not appear until Class III (>30% blood loss). Class I and II represent compensated shock where the clinician must rely on tachycardia and other early signs.
7. Clinical Features
| Feature | Sign |
|---|---|
| Skin | Cool, clammy, mottled (vasoconstriction); warm/flushed in distributive |
| HR | Tachycardia (earliest sign) |
| BP | Initially maintained; hypotension = late sign |
| Pulse pressure | Narrowed early (increased SVR) |
| RR | Tachypnea (compensatory) |
| Urine output | Oliguria (<0.5 mL/kg/hr) |
| CNS | Anxiety → confusion → obtundation |
| Metabolic | Elevated lactate, metabolic (lactic) acidosis, stress hyperglycemia |
8. Management Principles
(Harrison's; Mulholland; Sabiston)
General Principles
- Recognize shock early — before decompensation
- Identify the type — determines specific treatment
- Initiate therapy simultaneously with workup — do not delay
- Goal: restore oxygen delivery (DO₂) to tissues
Monitoring
- Two large-bore IV lines (14–16 gauge) minimum
- Arterial line for continuous BP and ABG
- Central venous catheter for CVP, ScvO₂
- Urinary catheter for urine output
- Serial lactate levels (clearance = resuscitation adequacy)
Fluid Resuscitation
- Hypovolemic/hemorrhagic: Crystalloid bolus (LR or NS) + packed RBCs; in massive hemorrhage — 1:1 PRBC:FFP ratio (damage control resuscitation)
- Tranexamic acid (TXA): indicated in hemorrhagic shock within first 3 hours; prevents fibrinolysis
- Avoid overresuscitation — associated with ARDS, ACS, MODS
- Coagulopathy correction: FFP (all soluble factors), cryoprecipitate (Factor VIII, fibrinogen), platelets (target >50,000 for surgery)
Vasopressors/Inotropes
| Agent | Indication |
|---|---|
| Norepinephrine | First-line for septic/distributive shock |
| Vasopressin | Adjunct in septic shock (reduces NE requirements) |
| Dopamine | Cardiogenic or mixed; now less preferred |
| Dobutamine | Cardiogenic shock with ↓ CO |
| Epinephrine | Anaphylactic shock (first-line IM); refractory shock |
Type-Specific Treatment
| Type | Priority Treatment |
|---|---|
| Hypovolemic | Volume replacement; control hemorrhage source |
| Cardiogenic | Inotropes; revascularization (PCI for MI); mechanical support (IABP, Impella) |
| Septic | Early antibiotics (within 1 hr), source control, 30 mL/kg crystalloid bolus, norepinephrine |
| Anaphylactic | IM epinephrine, corticosteroids, antihistamines, airway management |
| Obstructive | Relieve obstruction (pericardiocentesis, needle decompression, thrombolytics) |
| Neurogenic | IV fluids + vasopressors; spinal cord management |
Endpoints of Resuscitation
- MAP ≥65 mmHg
- Lactate clearance (>10% per 2 hours; target <2 mmol/L)
- Urine output >0.5 mL/kg/hr
- ScvO₂ >70%
- Resolution of mental status changes
9. Complications of Shock
- Ischemia-reperfusion injury from oxidative stress on reperfusion
- Hypothermia and coagulopathy ("lethal triad" with acidosis)
- Abdominal Compartment Syndrome (ACS)
- ARDS (especially with over-resuscitation)
- Multiple Organ Dysfunction Syndrome (MODS)
- Secondary immunosuppression → increased nosocomial infection risk
- Acute Kidney Injury (AKI)
- DIC (particularly in septic/traumatic shock)
Sources
- Robbins, Cotran & Kumar — Pathologic Basis of Disease, p. 134
- Robbins & Kumar Basic Pathology, p. 134
- Harrison's Principles of Internal Medicine, 22nd ed., pp. 2354–2355
- Mulholland & Greenfield's Surgery, 7th ed., pp. 525–556
- Sabiston Textbook of Surgery, p. 563
- Schwartz's Principles of Surgery, 11th ed., p. 152
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