Explain hypovolemic shock

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Hypovolemic Shock

Hypovolemic shock is the most common form of shock encountered in surgical and emergency practice. It occurs when a reduction in intravascular volume falls to a level where the heart can no longer pump sufficient blood to meet the metabolic demands of vital organs. More than 20% of circulating blood volume must typically be lost before the full syndrome develops.

Definition & Core Mechanism

The fundamental derangement is reduced preload - less blood returning to the right heart - which, by the Frank-Starling mechanism, causes a fall in cardiac output. Since mean arterial pressure (MAP) = cardiac output × total peripheral resistance (TPR), falling cardiac output produces hypotension and tissue hypoperfusion.

MAP falls → inadequate oxygen delivery → anaerobic metabolism → lactic acidosis → organ dysfunction.

Guyton and Hall Textbook of Medical Physiology, p. 299

Causes

Hemorrhagic (most common):

Trauma, gastrointestinal bleeding, ruptured aortic aneurysm, obstetric hemorrhage

Non-hemorrhagic:

Plasma loss: severe burns (loss through denuded skin), intestinal obstruction (capillary leak into bowel wall), peritonitis ("third spacing")
Total body fluid depletion: severe vomiting/diarrhea, excessive sweating, osmotic diuresis (DKA), adrenal insufficiency (loss of aldosterone-driven Na/water reabsorption)
Traumatic: capillary disruption from tissue contusion even without overt hemorrhage

Note: plasma loss carries an additional complication - the increased red cell concentration raises blood viscosity, further impairing microvascular flow. - Guyton and Hall, p. 304

Compensatory (Neurohormonal) Responses

When arterial pressure falls, three major compensatory arms are activated simultaneously:

Cardiovascular responses to hemorrhage - showing baroreceptor reflex, renin-angiotensin-aldosterone system, and capillary fluid shift pathways all converging to restore arterial pressure

Fig. 4.37 - Cardiovascular responses to hemorrhage (Costanzo Physiology, 7th ed.)

1. Baroreceptor Reflex (Rapid - seconds)

Aortic arch and carotid sinus baroreceptors detect the fall in stretch
Increased sympathetic outflow → tachycardia, increased myocardial contractility, arteriolar vasoconstriction (↑TPR), and venous constriction (↓unstressed volume, ↑venous return)
Coronary and cerebral vessels are spared from sympathetic constriction - their autoregulation protects flow down to ~70 mmHg MAP

2. Renin-Angiotensin-Aldosterone System (Minutes to hours)

Renal hypoperfusion → renin release → angiotensin II → peripheral vasoconstriction + aldosterone release → renal Na⁺ and water retention → gradual blood volume restoration

3. Capillary Fluid Shift (Hours)

Fall in capillary hydrostatic pressure (Pc) favors reabsorption of interstitial fluid into the vascular compartment, helping restore blood volume

4. ADH / Vasopressin

Released from posterior pituitary in response to low blood pressure and increased plasma osmolality → renal free water retention + mild vasoconstriction

5. CNS Ischemic Response ("Last Stand")

When MAP falls below ~50 mmHg, the ischemic brainstem triggers extreme sympathetic discharge - visible as the "second plateau" on arterial pressure recordings

These reflexes extend survivable blood loss from ~15-20% (without reflexes) to ~30-40% (with intact reflexes). - Guyton and Hall, p. 300

ATLS Classification of Hemorrhagic Shock

Class	Blood Loss	Volume (70 kg adult)	Clinical Features	Treatment
I	up to 15%	up to ~750 mL	No measurable changes; normal cap refill	Self-correcting; minimal treatment
II	15-30%	800-1,500 mL	Tachycardia, tachypnea, narrowed pulse pressure, mild anxiety, reduced UO (20-30 mL/hr)	Crystalloid; some may need blood
III	30-40%	up to ~2,000 mL	Marked tachycardia, hypotension, confusion/combativeness, delayed cap refill	Crystalloid + blood transfusion
IV	>40%	>2,000 mL	Life-threatening; severe tachycardia, very low BP, cold/pale skin, negligible UO, depressed mental status	Immediate transfusion + surgical/interventional hemorrhage control

Class III is the minimum loss that consistently produces systolic hypotension. - Mulholland & Greenfield's Surgery, p. 530-531

Individual susceptibility varies with age, pregnancy, beta-blocker use, pre-existing disease, and other poorly characterized factors.

Progressive vs. Non-progressive Shock

This is one of the most important distinctions:

Nonprogressive (compensated) shock: Blood loss below a critical threshold. Compensatory mechanisms are sufficient; the patient gradually recovers with treatment.
Progressive shock: Blood loss exceeds a critical threshold (arterial pressure falls below ~45 mmHg in experimental models). Shock itself generates more shock in a vicious cycle:
- Cardiac ischemia → reduced contractility
- Tissue acidosis → vascular smooth muscle unresponsiveness
- Capillary stasis → microthrombi formation (DIC)
- Intestinal ischemia → bacterial translocation and endotoxin release
Irreversible shock: So much cellular and organ damage has accumulated that even restoration of normal blood volume cannot prevent death.

Clinical Features (Summary)

System	Signs
Cardiovascular	Tachycardia, hypotension, narrowed pulse pressure, weak pulses
Skin	Cool, pale, clammy; delayed capillary refill
Renal	Oliguria or anuria (UO < 0.5 mL/kg/hr)
Neurological	Anxiety → confusion → combativeness → obtundation → coma
Respiratory	Tachypnea (compensating for metabolic acidosis)
Labs	Metabolic acidosis, elevated lactate, rising BUN/creatinine, hemoconcentration (in plasma loss)

Hemodynamic profile: low CO, high SVR - this is a "cold shock." - Sabiston Textbook of Surgery, p. 2886

Treatment Principles

Hemorrhagic Shock

Control the source of bleeding (surgical, endovascular, or procedural) - this is the definitive step
Two large-bore IV lines (14-16G antecubital); intraosseous access if venous access fails
Permissive hypotension before hemorrhage control (target MAP ~50 mmHg in penetrating trauma to avoid clot dislodgement) - but maintain higher pressures with TBI
Blood products - in hemorrhagic shock, crystalloids alone worsen anemia and dilutional coagulopathy:
- 1:1:1 ratio - fresh frozen plasma : packed red blood cells : platelets (massive transfusion protocol, or MTP)
- Whole blood - emerging evidence suggests superiority over component therapy when available
- Viscoelastic testing (TEG or ROTEM) - can guide targeted component replacement; may offer mortality benefit in massive transfusion (>10 units pRBC)
Tranexamic acid (TXA) - stabilizes clot by inhibiting fibrinolysis; should be given within 3 hours of injury (CRASH-2 trial data); adult dose 1g IV over 10 min, then 1g over 8 hours
Vasopressors (norepinephrine, vasopressin) - a temporizing measure only, not a substitute for volume; vasopressin-deficient state may respond specifically to vasopressin

Non-hemorrhagic Hypovolemic Shock

Crystalloid resuscitation (lactated Ringer's or normal saline) is the primary treatment
Adult: 1-2L bolus and reassess; pediatric: 20 mL/kg bolus, repeat once, then 10 mL/kg pRBC if no improvement
Identify and treat the underlying cause (vomiting, diarrhea, burns, etc.)

Monitoring Resuscitation Endpoints

Urine output ≥0.5 mL/kg/hr (adult), ≥1 mL/kg/hr (child)
Normalization of lactate and base deficit
MAP ≥65 mmHg
Heart rate returning to normal

Sources

Mulholland & Greenfield's Surgery, 7th ed., pp. 530-533
Guyton and Hall Textbook of Medical Physiology, pp. 299-305
Costanzo Physiology, 7th ed., pp. 186-188
Sabiston Textbook of Surgery, pp. 2886-2894
Rosen's Emergency Medicine, pp. 2207-2223

Recent evidence note: A 2025 systematic review (PMID 40603759) confirmed point-of-care ultrasound (POCUS) has good diagnostic accuracy for differentiating shock types, supporting its use in early evaluation of suspected hypovolemic shock.

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