Hypovolumic shock

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Hypovolemic Shock

Hypovolemic shock is the most common type of shock encountered in surgical and emergency practice. It occurs when intravascular volume falls to a level at which the heart can no longer maintain adequate cardiac output (CO) to meet the oxygen demands of vital organs.

Definition & Core Pathophysiology

Hypovolemic shock is defined by:

Reduced CO due to decreased preload (low circulating volume)
Elevated SVR (sympathetic compensatory vasoconstriction)
Low CVP and PCWP (low filling pressures)
Low mixed venous O2 saturation (SvO2)

Harrison's Principles of Internal Medicine 22E summarizes it: "Hypovolemic shock encompasses disease processes that reduce CO (and oxygen delivery) via a reduction in preload... characterized by an elevated SVR and low CVP and PCWP related to decreased intravascular volume."

Causes

Hemorrhagic (most common):

External trauma
Internal bleeding (upper/lower GI hemorrhage)

Non-hemorrhagic:

GI losses: profuse vomiting, diarrhea
Renal losses: osmotic diuresis (DKA, diabetes insipidus)
Skin losses: severe burns, Stevens-Johnson syndrome
Plasma sequestration: soft tissue injury, peritonitis, bowel obstruction

The essential threshold: loss of >20% of circulating volume is required to produce the full shock syndrome. Blood volume is estimated at ~7% of ideal body weight (~4,900 mL in a 70-kg adult).

Compensatory Mechanisms

When blood volume and arterial pressure fall, three major systems are activated:

Cardiovascular responses to hemorrhage - showing baroreceptor reflex, renin-angiotensin-aldosterone activation, and capillary fluid absorption

Fig. Cardiovascular responses to hemorrhage (Costanzo Physiology)

1. Baroreceptor Reflex (immediate):

Increased sympathetic outflow
Tachycardia + increased contractility
Arteriolar constriction → increased TPR
Venous constriction → increased venous return

2. Renin-Angiotensin-Aldosterone System:

Angiotensin II raises TPR
Aldosterone promotes Na+ retention → restores blood volume

3. Capillary Fluid Shift:

Decreased capillary hydrostatic pressure draws interstitial fluid into vasculature, restoring volume

Key point: Sympathetic reflexes preferentially maintain arterial pressure over cardiac output - CO falls first, BP falls later. Cardiac and cerebral blood flows are protected until MAP falls below ~70 mmHg.

Effect of Blood Loss on Hemodynamics

Effect of hemorrhage on cardiac output and arterial pressure as percentage of blood volume removed

Fig. Effect of hemorrhage on cardiac output and arterial pressure (Guyton & Hall)

Up to ~10% blood loss: minimal change in BP or CO
20-30%: CO begins to fall significantly; BP partially maintained by reflexes
40-45%: Both CO and BP fall to zero - lethal without intervention

ATLS Classification (Hemorrhagic Shock)

Class	Blood Loss	Volume (70-kg adult)	HR	BP	Pulse Pressure	Urine Output	Mental Status
I	<15%	<750 mL	Normal	Normal	Normal	>30 mL/hr	Normal
II	15-30%	750-1,500 mL	>100	Slightly ↓	Narrowed	20-30 mL/hr	Anxious
III	30-40%	1,500-2,000 mL	>120	Hypotensive	Narrowed	5-15 mL/hr	Confused
IV	>40%	>2,000 mL	>140	Severely ↓	Unobtainable	Negligible	Lethargic/Comatose

From Mulholland and Greenfield's Surgery / ATLS classification

Class I-II: crystalloid resuscitation usually sufficient
Class III: blood transfusion + crystalloid
Class IV: immediate blood transfusion + surgical/angiographic intervention

Progressive vs. Nonprogressive Shock

Nonprogressive (Compensated) Shock:

Blood loss within tolerable limits
Compensatory mechanisms (baroreceptors, RAAS, stress-relaxation) maintain circulation
Patient can recover with adequate resuscitation

Progressive Shock:

Blood loss exceeds a critical threshold (~45% in experimental models)
"Shock causes more shock" - a vicious cycle of deterioration
Mechanisms include: cardiac ischemia → decreased contractility, gut ischemia → toxin release, microvascular failure, coagulopathy
Without intervention, leads to irreversible shock and death

Irreversible Shock:

So much cellular and tissue damage that survival is impossible even with aggressive treatment
Characterized by refractory hypotension, multiorgan failure

Hemodynamic Profile (Comparison Table)

Shock Type	CO	SVR	PAOP	CVP	SvO2
Hypovolemic	↓	↑	↓	↓	↓
Cardiogenic (LV MI)	↓	↑	↑	N/↑	↓
Cardiogenic (RV MI)	↓	↑	N/↓	↑	↓
Tamponade	↓	↑	↑	↑	↓
Distributive (early)	↑/N	↓	N	N	↑

Source: Mulholland and Greenfield's Surgery

The unique signature of hypovolemic shock: low CO + high SVR + low filling pressures (CVP, PCWP).

Clinical Features

Tachycardia (earliest and most sensitive sign)
Tachypnea
Hypotension (late sign - appears after Class III)
Narrowed pulse pressure
Cool, pale, clammy skin (cutaneous vasoconstriction)
Delayed capillary refill
Oliguria/anuria (reduced renal perfusion)
Altered mental status (anxiety → confusion → coma)

For non-hemorrhagic hypovolemia (plasma/fluid losses), additional features include:

Hemoconcentration
Elevated BUN and creatinine
Hypernatremia (in free water losses)

Treatment

General Principles:

Stop the source of loss (hemorrhage control, treat underlying cause)
Restore intravascular volume (fluid resuscitation)
Restore oxygen-carrying capacity (blood transfusion when indicated)

Specific Management:

Class	Intervention
I	Observation; fluid replacement usually not needed
II	Crystalloid (isotonic saline or lactated Ringer's)
III	Crystalloid + packed red blood cells (pRBCs)
IV	Immediate blood products + emergent surgical/interventional radiology

Hemorrhagic shock additional considerations:

Damage control resuscitation: balanced ratio of pRBCs:FFP:platelets (1:1:1)
Permissive hypotension: tolerate MAP 50-65 mmHg until hemorrhage controlled (avoid fluid overload and clot disruption before surgical control)
Tranexamic acid (TXA): given within 3 hours of injury to reduce hemorrhage-related mortality
Vasopressors: generally NOT first-line; used only if volume replacement is insufficient and patient remains unstable

For non-hemorrhagic losses: crystalloid replacement tailored to the specific electrolyte abnormality (e.g., hypotonic fluids for hypernatremia, repleting specific electrolytes for GI losses).

Monitoring Endpoints

Urine output (target >0.5 mL/kg/hr in adults)
Lactate clearance (target normalization within 6 hours)
Heart rate normalization
Mean arterial pressure (target >65 mmHg)
CVP/PCWP if central monitoring available
Point-of-care ultrasound (POCUS): a recent 2025 systematic review (PMID: 40603759) confirms POCUS has high diagnostic accuracy in differentiating shock types

Key Sources:

Guyton and Hall Textbook of Medical Physiology, Ch. 24
Mulholland and Greenfield's Surgery, Ch. 9
Harrison's Principles of Internal Medicine 22E
Costanzo Physiology, 7th Ed., Box 4.3
Sabiston Textbook of Surgery

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