Shock & Fluid Resuscitation

Shock is acute circulatory failure causing inadequate tissue oxygen delivery and cellular hypoxia — a state of global tissue hypoperfusion, NOT merely hypotension. This is one of the highest-yield topics across Surgery, Anaesthesia and Emergency Medicine, blending haemodynamic physiology, the Parkland formula, and resuscitation endpoints.

Definition & core concept

Shock = an imbalance between oxygen delivery (DO₂) and oxygen consumption (VO₂), leading to anaerobic metabolism, lactate accumulation and cellular injury. Crucially, a patient can be in shock with a normal or even high blood pressure (compensated/cryptic shock), and conversely be hypotensive without shock. The defining marker is tissue hypoperfusion — clinically read off as raised lactate, low mixed venous saturation, oliguria, altered mentation and cold/mottled skin.

• DO₂ = Cardiac Output × CaO₂, where CaO₂ ≈ (1.34 × Hb × SaO₂) + (0.003 × PaO₂).
• Cardiac Output = Heart Rate × Stroke Volume; stroke volume depends on preload, afterload and contractility.

High-yield: Hypotension is a LATE sign in young patients. Up to 30% of blood volume (Class III) can be lost before systolic BP falls, because intense sympathetic vasoconstriction maintains BP. Tachycardia and a narrowed pulse pressure appear earlier.

Classification of shock

Shock is classified into four physiological categories. The distinction is most easily memorised by the behaviour of Cardiac Output (CO), Systemic Vascular Resistance (SVR) and filling pressures (CVP/PCWP).

Type	Primary defect	CO	SVR	CVP/PCWP	SvO₂	Skin
Hypovolaemic	↓ Preload (volume loss)	↓	↑	↓	↓	Cold, clammy
Cardiogenic	Pump failure	↓	↑	↑	↓	Cold, clammy
Obstructive	Mechanical obstruction to flow	↓	↑	↑*	↓	Cold, clammy
Distributive (septic/anaphylactic/neurogenic)	↓ SVR (vasodilation)	↑ or ↔	↓	↓	↑ (septic)	Warm (early)

*In obstructive shock CVP is typically raised (tamponade, tension pneumothorax, massive PE), but PCWP may be low.

High-yield: Warm shock with high CO, low SVR and HIGH SvO₂ = early/hyperdynamic septic (distributive) shock. This high SvO₂ reflects impaired peripheral oxygen extraction, a classic distractor in exams.

Sub-types of distributive shock

• Septic — commonest cause of distributive shock; inflammatory vasodilation.
• Anaphylactic — IgE-mediated; treat with intramuscular adrenaline 0.5 mg (0.5 mL of 1:1000) into the anterolateral thigh.
• Neurogenic — loss of sympathetic tone after spinal cord injury (above T6). The hallmark is hypotension WITH bradycardia (unopposed vagal tone) and warm, dry skin — distinguishing it from hypovolaemic shock where tachycardia is expected.

High-yield: Spinal shock ≠ neurogenic shock. Spinal shock is a neurological phenomenon — transient loss of all reflexes/flaccidity below the injury, recovery heralded by return of the bulbocavernosus reflex. Neurogenic shock is the haemodynamic state (hypotension + bradycardia).

Aetiology & pathophysiology

Stages of shock

1. Compensated (non-progressive): baroreceptor reflex → sympathetic surge → tachycardia, vasoconstriction, RAAS + ADH activation → preserved BP and brain/heart perfusion.
2. Decompensated (progressive): compensation fails → hypotension, worsening acidosis, capillary sludging.
3. Irreplaceable / refractory: cellular damage, mitochondrial failure, multi-organ dysfunction; even restored perfusion cannot reverse it.

Cellular events

Hypoperfusion → switch to anaerobic glycolysis → lactic acidosis → failure of the Na⁺/K⁺-ATPase pump → cellular swelling, Ca²⁺ influx, lysosomal enzyme release and ultimately apoptosis/necrosis. Reperfusion brings its own injury via oxygen free radicals.

High-yield: Serum lactate is the single best biochemical marker of tissue hypoperfusion and a powerful prognosticator. Lactate clearance ≥10% over the first 2 hours is a validated resuscitation endpoint comparable to ScvO₂-targeted therapy.

Clinical features & the ATLS haemorrhage classes

The ATLS classification of haemorrhagic shock is among the most asked tables in NEET PG.

Parameter	Class I	Class II (mild)	Class III (moderate)	Class IV (severe)
Blood loss (%)	<15%	15–30%	30–40%	>40%
Blood loss (mL, 70 kg)	<750	750–1500	1500–2000	>2000
Heart rate	<100	100–120	120–140	>140
Blood pressure	Normal	Normal	↓	↓↓
Pulse pressure	Normal/↑	↓	↓	↓
Respiratory rate	14–20	20–30	30–40	>35
Urine output (mL/h)	>30	20–30	5–15	Negligible
Mental status	Slightly anxious	Mildly anxious	Anxious, confused	Confused, lethargic

High-yield: The earliest reliable sign of haemorrhage is a narrowed pulse pressure (rising diastolic from vasoconstriction) and tachycardia — appearing in Class II. Frank hypotension marks Class III onward. Urine output <0.5 mL/kg/h signals inadequate renal perfusion.

Mnemonic for the four ATLS classes — think of a game of tennis scoring (0–15–30–40) for the percentage blood loss boundaries: 15, 30, 40.

Diagnosis & investigations

Shock is a clinical diagnosis; investigations confirm aetiology, severity and guide resuscitation.

• Bedside: vitals, capillary refill (>2 s), mottling score, mental status, urine output via catheter.
• Lactate & base deficit: quantify hypoperfusion; base deficit worse than −6 correlates with severe shock.
• ABG: metabolic acidosis with raised anion gap.
• Haemodynamic monitoring: CVP (crude preload marker), arterial line, ScvO₂/SvO₂.
• POCUS/FAST: rapid identification of free fluid (trauma), tamponade, pneumothorax, IVC collapsibility and cardiac contractility — the modern bedside tool of choice.
• Echocardiography: for cardiogenic/obstructive differentiation.

Approach flow: Identify shock (perfusion signs) → classify by examination + POCUS → secure airway/breathing → large-bore IV access (two 14–16 G cannulae) → fluid challenge → reassess (dynamic response) → vasopressor/source control if refractory.

High-yield: Dynamic measures of fluid responsiveness (pulse pressure variation >13%, stroke volume variation, passive leg raise, IVC distensibility) predict volume response far better than static measures (CVP, PCWP), which are poor predictors.

Fluid resuscitation

Crystalloids vs colloids

Feature	Crystalloids	Colloids
Examples	Normal saline, Ringer lactate, Plasma-Lyte	Albumin, gelatins, HES, dextrans
Cost	Cheap	Expensive
Volume to expand plasma	~3× deficit	~1× (stay intravascular longer)
Risk	Dilutional, oedema	Anaphylaxis, coagulopathy, renal injury (HES)
Evidence	First-line	No survival benefit over crystalloids

High-yield: The SAFE trial showed albumin and saline are equivalent overall, but albumin increased mortality in traumatic brain injury. The CHEST/VISEP trials showed hydroxyethyl starch (HES) increases acute kidney injury and need for renal replacement therapy — HES is now largely abandoned. Crystalloids remain first-line.

High-yield: Normal saline in large volumes causes hyperchloraemic metabolic acidosis (SID effect). Balanced solutions (Ringer lactate / Plasma-Lyte) are preferred in most resuscitation (SMART/SPLIT trials favoured balanced fluids for kidney outcomes). Avoid Ringer lactate with massive blood transfusion (calcium may clot citrated blood) and in severe liver failure (lactate metabolism).

Massive transfusion & damage control

For severe haemorrhagic shock, modern practice favours damage control resuscitation:

• Balanced transfusion ratio PRBC : FFP : platelets = 1 : 1 : 1.
• Permissive hypotension (target SBP ~80–90 mmHg / palpable radial pulse) until surgical control — avoids "popping the clot" and dilutional coagulopathy. Contraindicated in traumatic brain injury, where cerebral perfusion must be maintained.
• Early tranexamic acid (CRASH-2: give within 3 hours of trauma; benefit lost/harmful after 3 h).
• Avoid the lethal triad: hypothermia + acidosis + coagulopathy.

High-yield: CRASH-2 — tranexamic acid 1 g over 10 min then 1 g over 8 h, within 3 hours of injury, reduces mortality from bleeding.

Parkland formula (burns)

For thermal burns, fluid resuscitation uses the Parkland (Baxter) formula:

Total fluid (first 24 h) = 4 mL × body weight (kg) × % TBSA burned, using Ringer lactate.

• Give HALF in the first 8 hours (from the time of burn, not from admission), and the remaining half over the next 16 hours.
• %TBSA estimated by the Rule of Nines (adults) or Lund–Browder chart (children, more accurate).

High-yield: Parkland uses only the first 8 hours from the time of injury for the first half — if presentation is delayed, the rate is recalculated to deliver the remaining volume in the time left.

Worked example: 70 kg adult, 40% TBSA → 4 × 70 × 40 = 11,200 mL Ringer lactate in 24 h. First 8 h: 5600 mL (≈700 mL/h); next 16 h: 5600 mL (≈350 mL/h).

• Best endpoint of burns resuscitation = urine output (0.5 mL/kg/h adults; 1 mL/kg/h children). Titrate fluids to urine output, NOT rigidly to the formula.
• The modified Brooke and Galveston (children) formulas are alternatives; the trend now is "fluid creep" awareness and conservative titration.

Management & drug of choice

General principles: VIP rule → Ventilate, Infuse, Pump. Treat the cause.

Shock type	First-line management	Vasopressor / DOC
Hypovolaemic/haemorrhagic	Control bleeding + balanced blood products	Fluids (vasopressors are a temporising measure)
Septic	30 mL/kg crystalloid, source control, broad-spectrum antibiotics within 1 h	Noradrenaline (1st line); add vasopressin / adrenaline
Cardiogenic	Treat cause (PCI for MI), cautious fluids	Noradrenaline if hypotensive; dobutamine for low output; consider IABP/ECMO
Anaphylactic	Remove trigger, airway	IM adrenaline 0.5 mg (1:1000)
Neurogenic	Fluids + maintain spinal precautions	Noradrenaline / phenylephrine; atropine for bradycardia
Obstructive	Relieve obstruction	Tamponade → pericardiocentesis; tension pneumothorax → needle/finger thoracostomy; massive PE → thrombolysis

High-yield: Noradrenaline (norepinephrine) is the first-line vasopressor in septic shock (Surviving Sepsis Campaign). Adrenaline is the drug of choice in anaphylaxis and in cardiac arrest. Dopamine is no longer preferred (more arrhythmias, higher mortality).

High-yield: Surviving Sepsis "Hour-1 bundle": measure lactate, blood cultures before antibiotics, broad-spectrum antibiotics, 30 mL/kg crystalloid for hypotension/lactate ≥4, start vasopressors to keep MAP ≥65 mmHg.

Endpoints of resuscitation

• Macro: MAP ≥65 mmHg, urine output ≥0.5 mL/kg/h, normalising HR.
• Micro (better): lactate clearance ≥10%/normalisation, ScvO₂ ≥70%, base deficit correction, capillary refill normalisation.
• The ProCESS, ARISE and ProMISe trials showed protocolised EGDT (early goal-directed therapy with mandatory CVP/ScvO₂ catheters) offered no mortality benefit over good usual care — early antibiotics and early fluids matter most.

Complications

• Acute kidney injury (acute tubular necrosis) — commonest organ failure.
• Acute respiratory distress syndrome (ARDS).
• Multi-organ dysfunction syndrome (MODS) — leading cause of late death.
• Disseminated intravascular coagulation (DIC).
• Ischaemic hepatitis ("shock liver"), gut ischaemia, bacterial translocation.
• Reperfusion injury and abdominal compartment syndrome (over-resuscitation).

Key differentials

• Hypovolaemic vs cardiogenic: both cold with low CO and high SVR — distinguished by filling pressures (low CVP/PCWP in hypovolaemia, high in cardiogenic) and POCUS contractility.
• Septic vs neurogenic: both can be warm/vasodilated; septic = tachycardia, neurogenic = bradycardia.
• Obstructive mimics: tamponade (Beck's triad — hypotension, muffled heart sounds, raised JVP), tension pneumothorax (tracheal deviation, absent breath sounds), massive PE.
• Vasovagal syncope and adrenal crisis (Addisonian) can mimic shock — consider in refractory hypotension.

Recently asked / exam angle

• Type of shock from haemodynamic profile (CO/SVR/CVP/SvO₂ table) — a perennial single-best-answer.
• Earliest sign of haemorrhagic shock → narrowed pulse pressure / tachycardia.
• Parkland formula calculation — given weight and %TBSA, compute first 8-hour volume.
• Neurogenic shock = hypotension + bradycardia; vs spinal shock with bulbocavernosus reflex return.
• First-line vasopressor in septic shock = noradrenaline; DOC anaphylaxis = IM adrenaline.
• HES → AKI (CHEST trial); albumin harmful in TBI (SAFE).
• Best resuscitation endpoint = lactate clearance / urine output (not CVP).
• CRASH-2 / tranexamic acid within 3 hours; 1:1:1 transfusion ratio; permissive hypotension (avoid in head injury).
• Warm shock with high SvO₂ = septic shock (impaired O₂ extraction).
• Surviving Sepsis MAP target ≥65 mmHg.

Rapid revision

1. Shock = global tissue hypoperfusion; hypotension is a late sign (Class III, >30% loss).
2. Four types: hypovolaemic, cardiogenic, obstructive, distributive; classify by CO/SVR/CVP.
3. Distributive (septic) shock: ↑CO, ↓SVR, ↑SvO₂ ("warm shock").
4. Neurogenic shock: hypotension + bradycardia + warm dry skin (cord injury > T6).
5. Earliest haemorrhage sign = narrowed pulse pressure + tachycardia.
6. Lactate clearance ≥10%/2 h and urine output are the best resuscitation endpoints.
7. Crystalloids first-line; balanced fluids preferred; HES causes AKI; albumin harmful in TBI.
8. Parkland: 4 × wt × %TBSA Ringer lactate, half in first 8 h from burn.
9. Noradrenaline = 1st-line pressor in septic shock; adrenaline IM for anaphylaxis.
10. Massive transfusion 1:1:1, permissive hypotension (NOT in head injury), avoid lethal triad.
11. Tranexamic acid within 3 hours of trauma (CRASH-2).
12. Surviving Sepsis Hour-1 bundle: lactate, cultures, antibiotics, 30 mL/kg fluid, MAP ≥65.