Fluid Replacement

A practical, pathophysiology-first guide to peri-operative fluid resuscitation, replacement, maintenance fluids, electrolyte correction, and monitoring in general surgery.

Fluid Replacement in General Surgery

Fluid replacement is not just "put up a drip." In surgery it is a core management skill because hypovolaemia, third-spacing, electrolyte loss, and iatrogenic over-resuscitation can all kill the patient before the definitive operation is even reached.

The central question is always:

What compartment has lost fluid, what has the patient already lost, what will they keep losing, and what physiological target proves that perfusion has been restored?

This page is written as one integrated management note because the topic is mainly Mx: assess volume status, resuscitate shock, calculate deficit, add maintenance, replace ongoing losses, choose the right fluid, correct electrolytes safely, and reassess continuously.

A. Body Fluid Compartments - The Anatomy of Fluid Prescribing

1. Normal Body Water Distribution

Compartment	Adult male	Adult female	Infant	Why it matters clinically
Total body fluid	60% body weight	50% body weight	80% at birth; about 65% by 1 year	Determines how large a fluid deficit or sodium deficit really is
Intracellular fluid	2/3 of total body fluid	2/3 of total body fluid	Higher proportion than adult	Dextrose water and hypotonic fluids distribute mainly here after glucose metabolism
Extracellular fluid	1/3 of total body fluid	1/3 of total body fluid	Larger than adult	This is the surgical resuscitation compartment
Interstitial fluid	3/4 of ECF	3/4 of ECF	Large and labile	Expands in oedema, sepsis, burns, pancreatitis, peritonitis
Intravascular plasma	1/4 of ECF	1/4 of ECF	Small absolute volume	The target compartment in shock: preload, stroke volume, cardiac output

2. Why Only a Small Fraction of Crystalloid Stays Intravascular

Normal saline and Hartmann's solution are crystalloids. They distribute through the extracellular compartment, not just the bloodstream.

If 1 L of isotonic crystalloid is infused:

Most distributes into the interstitial space
Roughly a quarter of the extracellular distribution remains intravascular
Therefore, a shocked patient may need repeated boluses before preload improves

This is why crystalloids are excellent first-line fluids but require reassessment after each bolus. If the capillary barrier is leaky, even less remains in the circulation.

3. The Starling Principle in One Surgical Sentence

Fluid stays inside capillaries when hydrostatic pressure pushing out is balanced by oncotic pressure pulling in and an intact endothelial barrier.

In surgical illness:

Sepsis / pancreatitis / peritonitis injure the endothelium -> capillary leak
Albumin and water leave the vascular space -> interstitial oedema
Plasma volume falls despite total body water excess
The patient can be simultaneously oedematous and intravascularly depleted

Core Concept

Hypovolaemia is a problem of effective circulating volume, not just total body water. A patient with septic capillary leak may look swollen but still have poor venous return, poor stroke volume, low urine output, and rising lactate.

B. Pathophysiology of Volume Depletion

1. The Haemodynamic Cascade

Loss of intravascular volume causes:

Reduced venous return
Reduced right ventricular preload
Reduced left ventricular filling
Reduced stroke volume by the Frank-Starling relationship
Reduced cardiac output
Reduced tissue oxygen delivery
Anaerobic metabolism -> lactate production -> metabolic acidosis

The body initially compensates:

Response	Mechanism	Bedside manifestation
Sympathetic activation	Baroreceptor unloading -> catecholamine release	Tachycardia, cool peripheries, sweating, anxiety
Peripheral vasoconstriction	Alpha-1 mediated arteriolar constriction	Narrow pulse pressure, delayed capillary refill, cold clammy skin
RAAS activation	Renin -> angiotensin II -> aldosterone	Sodium and water retention
ADH release	Osmotic and non-osmotic stimulation	Concentrated urine, oliguria
Thirst	Hypothalamic osmoreceptors and angiotensin II	Strong thirst unless obtunded

This is why tachycardia and falling urine output often precede hypotension. Blood pressure is maintained until compensation fails. Hypotension is a late and dangerous sign.

2. Third-Space Loss - Why Surgical Patients Need More Fluid Than Expected

Third-spacing is pathological expansion of the interstitial space due to capillary leak and inflammation.

Classic causes:

Acute pancreatitis
Peritonitis
Septicaemia
Major trauma
Burns
Bowel obstruction
Major abdominal surgery

Pathophysiological sequence:

Inflammation -> endothelial glycocalyx injury -> increased permeability -> protein-rich fluid leaves plasma -> interstitial oncotic pressure rises -> more water follows -> oedema -> intravascular depletion -> reduced organ perfusion.

This is why a patient with pancreatitis may require litres of fluid even without visible bleeding, vomiting, or diarrhoea.

C. Clinical Assessment of Hypovolaemia

Fluid prescribing begins at the bedside. The lab values support the assessment; they do not replace it.

1. Adult Volume Depletion

Sign	About 5% body weight fluid loss	About 10% body weight fluid loss	About 15% body weight fluid loss
Mucous membrane	Dry	Very dry	Parched
Pulse rate	Normal or mildly increased	Clearly increased	Markedly increased
Blood pressure	Normal	Low	Very low
Orthostatic change	Absent	Present	Marked: HR rise more than 15 bpm or SBP fall more than 10 mmHg
Urinary flow rate	Reduced	Very reduced	Minimal / anuria
Sensorium	Normal	Lethargic	Obtunded

2. Infants and Children - Why They Deteriorate Suddenly

Children have high total body water, higher metabolic rate, limited renal concentrating ability, and small absolute circulating volume. They can maintain blood pressure with vasoconstriction until late, then crash quickly.

Finding	Mild dehydration 3-5%	Moderate dehydration 6-9%	Severe dehydration 10% or more
Pulse	Normal	Rapid	Rapid and weak or absent
Heart rate	Normal or 10-15% above baseline	Increased	Markedly increased
Systolic BP	Normal	Normal to low	Low
Respiration	Normal	Deep and faster	Deep tachypnoea, or reduced / absent if peri-arrest
Anterior fontanelle	Normal	Sunken	Markedly sunken
Eyes	Normal	Sunken	Markedly sunken
Buccal mucosa	Slightly dry / tacky	Dry	Parched
Skin turgor	Normal	Reduced	Tenting
Skin perfusion	Normal	Cool	Cool, mottled, acrocyanotic, delayed capillary refill
Urine output	Normal or mildly reduced, concentrated	Oliguria	Anuria
Mental state	Normal	Listless and irritable	Grunting, lethargic, coma
Thirst	Thirsty	Moderately increased	Very thirsty or too lethargic to indicate thirst

Exam and Ward Pearl

In children, hypotension is late. A tachycardic child with poor perfusion, dry mucosa, reduced urine output, and lethargy is already significantly volume depleted even if the blood pressure still looks acceptable.

D. Management Algorithm

E. Resuscitation vs Replacement vs Maintenance

These are three different prescriptions.

Term	What it means	Typical indication	Fluid logic
Resuscitation	Rapid restoration of effective circulating volume	Shock, severe hypovolaemia, sepsis, bleeding	Isotonic crystalloid bolus, reassess after each bolus
Replacement	Correction of a known or estimated deficit	Dehydration, vomiting, diarrhoea, NG aspirate, stoma loss, third-space loss	Replace deficit plus ongoing losses
Maintenance	Daily water and electrolytes for a patient who cannot drink	NPO peri-operative patient	Covers basal water, sodium, potassium, glucose needs

Do Not Confuse the Three

A shocked patient does not need maintenance fluids first. They need resuscitation. A stable NPO patient does not need repeated boluses. They need maintenance. A patient with an ileostomy pouring 2 L/day does not need a standard bag only. They need replacement of ongoing losses.

F. Calculating Fluid Replacement

1. The Master Formula

Fluid replacement = prior fluid deficit + maintenance fluid requirement + ongoing or anticipated losses

This formula is the whole topic.

2. Prior Fluid Deficit

Prior fluid deficit in litres = body weight in kg x percentage dehydration

Examples:

Patient	Estimated dehydration	Calculation	Deficit
50 kg adult	5%	50 x 0.05	2.5 L
50 kg adult	10%	50 x 0.10	5.0 L
20 kg child	6%	20 x 0.06	1.2 L

This is only the pre-existing deficit. You still need to add maintenance and ongoing losses.

3. Maintenance Requirement - Hourly Rule

Body weight band	Hourly rule	Daily rule
First 10 kg	4 mL/kg/hr	100 mL/kg/day
Second 10 kg	2 mL/kg/hr	50 mL/kg/day
Each kg above 20 kg	1 mL/kg/hr	20 mL/kg/day

Fever increases insensible losses:

Add about 10% extra fluid for every 1 degree Celsius above 37 degrees Celsius

4. Maintenance Examples

Weight	Hourly calculation	Hourly maintenance	Daily maintenance
10 kg	10 x 4	40 mL/hr	1000 mL/day
20 kg	10 x 4 + 10 x 2	60 mL/hr	1500 mL/day
50 kg	10 x 4 + 10 x 2 + 30 x 1	90 mL/hr	2160 mL/day
70 kg	10 x 4 + 10 x 2 + 50 x 1	110 mL/hr	2640 mL/day

5. Ongoing or Anticipated Losses

Loss type	Examples	Replacement thinking
Blood loss	Trauma, operative bleeding, GI bleeding	Restore circulating volume first; consider blood products if significant haemorrhage
Upper GI loss	Vomiting, NG aspirate	Often chloride-rich and hydrogen-rich -> hypochloraemic metabolic alkalosis; normal saline with potassium is often logical
Lower GI loss	Diarrhoea, high-output ileostomy	Bicarbonate and potassium loss -> metabolic acidosis and hypokalaemia; replace volume and electrolytes
Surgical drains / stoma	Biliary drain, pancreatic drain, ileostomy, fistula	Measure output and replace like-for-like when large
Third-space loss	Pancreatitis, peritonitis, sepsis	May be invisible; guided by perfusion, urine output, lactate, haematocrit, urea/creatinine

6. Rate of Replacement

If not in shock:

Give half of the calculated replacement requirement in the first 8 hours
Give the remaining half over the next 16 hours

Why front-load it? Because tissue hypoperfusion is time-sensitive. Waiting 24 hours to correct a large deficit prolongs renal, gut, and microcirculatory ischaemia.

If in shock:

Do fluid resuscitation first
Then calculate replacement after perfusion is restored

G. Fluid Resuscitation - The Emergency Prescription

1. Adult Resuscitation

Step	Action
Access	Establish 2 large-bore peripheral IV cannulae, ideally 14G or 16G
Initial bolus	Adult: 10 mL/kg crystalloid; practically 500 mL in a 50 kg patient
If severe shock	Run 1 L 0.9% normal saline or balanced crystalloid full-rate while preparing definitive haemorrhage/sepsis control
Reassess	Pulse, BP, mentation, capillary refill, JVP, lung crepitations, urine output
Target	Restored perfusion, improving urine output, improving lactate/base deficit

2. Paediatric Resuscitation

Step	Action
Initial bolus	20 mL/kg isotonic crystalloid
Reassess after each bolus	HR, capillary refill, respiratory effort, hepatomegaly, lung crepitations, mental state, urine output
Escalate early	Persistent shock after boluses needs senior help, inotropes, blood products if bleeding, and treatment of the cause

3. Endpoints of Resuscitation

Endpoint	Why it matters
Urine output at least 0.5 mL/kg/hr in adults	Kidney perfusion is a useful real-time marker of effective circulating volume
Improving mental state	Brain perfusion improving
Warm peripheries and capillary refill improving	Peripheral vasoconstriction is resolving
Falling lactate / improving base deficit	Tissue oxygen delivery improving
No new pulmonary oedema	Avoids crossing from resuscitation into fluid overload

The Bolus Is a Test

A fluid bolus is both treatment and diagnostic test. If stroke volume, BP, mentation, and urine output improve, the patient was fluid responsive. If they do not improve and the lungs become wet, the problem may be pump failure, ongoing bleeding, vasodilatation, or obstructive shock rather than simple volume depletion.

H. Choice of Fluid

1. Crystalloids vs Colloids

Feature	Colloids	Crystalloids
Examples	Albumin, hetastarch, dextran, gelatin / Gelofusine	Normal saline, dextrose, Ringer's lactate / Hartmann's
Main compartment effect	Expands intravascular compartment more effectively by increasing oncotic pressure	Expands extracellular compartment; only part remains intravascular
Volume needed	Lower infusion volume	Higher infusion volume
Cost	More expensive	Less expensive
Key adverse effects	Allergic/anaphylactoid reactions; AKI/coagulopathy with starches; crossmatch interference with dextran; infection risk with natural products is theoretical/modern screened products are safer	Pulmonary oedema, peripheral oedema, dilutional hypoalbuminaemia if excessive
Practical role	Select situations, not routine first-line	First-line in most surgical resuscitation and replacement

2. Normal Saline - 0.9% Sodium Chloride

Normal saline is isotonic but not physiologically balanced. It contains a high chloride load.

Feature	Detail
Best use	Initial resuscitation; hyponatraemia; hypochloraemia; metabolic alkalosis from vomiting/NG loss
Why useful in vomiting	Gastric loss removes H+ and Cl- -> metabolic alkalosis. NS replaces chloride, allowing renal bicarbonate excretion
Main risk	Large volumes cause hyperchloraemic metabolic acidosis
Mechanism of acidosis	Excess chloride reduces strong ion difference and promotes renal bicarbonate handling changes -> non-anion gap metabolic acidosis
Sterile water warning	Never use sterile water IV for resuscitation: it is profoundly hypotonic and causes haemolysis

3. Saline Concentrations

Concentration	Tonicity	Use and warning
0.2%, 0.33%, 0.45% saline	Hypotonic	May be used in selected replacement settings to reduce chloride load, often with dextrose to improve tonicity. Rapid infusion can cause water shifts and haemolysis risk if too hypotonic
0.9% saline	Isotonic	Good for initial resuscitation; risk of sodium/chloride overload and hyperchloraemic metabolic acidosis with large volumes
3% saline	Hypertonic	For symptomatic severe hyponatraemia or urgent sodium correction; requires careful monitoring

4. Dextrose Solutions

5% dextrose is initially isotonic in the bag, but after glucose is metabolised it behaves like free water.

Point	Explanation
Role	Provides water and small amount of calories; helps prevent starvation ketosis in maintenance fluids
Why not for shock	Glucose is rapidly metabolised by the liver -> remaining water distributes across all compartments -> minimal intravascular expansion
Common concentrations	D5, D10, D20, D50 depending on purpose
Risk	Hyponatraemia if excessive free water is given, especially post-operatively when ADH is high

5. Hartmann's Solution / Lactated Ringer's

Hartmann's is a balanced crystalloid. It more closely resembles plasma electrolyte composition than normal saline.

Feature	Detail
Osmolality	About 278 mmol/kg; plasma is usually about 285-295 mmol/kg
Best use	Replacement of fluid similar to plasma: blood loss before blood products, oedema fluid, small bowel losses, general balanced resuscitation
Buffer	Lactate is metabolised to bicarbonate, helping buffer acidosis
Chloride advantage	Lower chloride than NS -> less hyperchloraemic metabolic acidosis
Cautions	Prolonged use may contribute to hyponatraemia or hyperkalaemia, especially with renal impairment; consider liver failure/lactic acidosis context carefully

Why Balanced Crystalloids Often Feel Better Physiologically

Normal saline is "normal" only by tonicity. It has much more chloride than plasma. Balanced crystalloids are closer to plasma, are chloride-restrictive, and contain a buffer. That is why large-volume resuscitation is often better tolerated with Hartmann's/LR than with repeated litres of NS.

6. Albumin

Albumin is a natural colloid that increases plasma oncotic pressure.

Benefit	Risk / caution
Promotes retention of fluid in the intravascular space	If capillary permeability is high, albumin can leak into interstitium and worsen oedema
May reduce interstitial oedema when the barrier is intact	Caution in ARDS, burns, sepsis, and other microvascular leak states
Useful in selected hypoalbuminaemic or cirrhotic settings	Not routine first-line resuscitation for ordinary surgical dehydration

I. Standard Ward Fluid: "2D1S Q8H"

The Felix Lai notes describe standard therapy as 2D1S Q8H:

D = 5% dextrose
S = 0.9% normal saline
Practical meaning: a repeating maintenance pattern of two dextrose bags and one saline bag, usually each over 8 hours, adjusted to patient size, comorbidities, electrolytes, and losses

Why this pattern exists:

Dextrose supplies free water and prevents starvation ketosis
Saline supplies extracellular sodium and chloride
It is a maintenance starting point, not an automatic prescription for shock, sepsis, renal failure, heart failure, high-output stoma, or major electrolyte abnormality

Important

No adult surgical patient should be left on "standard fluids" without daily review. Fluid charts, weight, renal function, Na+, K+, acid-base status, and ongoing losses should change the prescription.

J. Electrolyte Replacement

1. Daily Requirements and Normal Values

Electrolyte	Daily requirement	Normal serum value	Key surgical relevance
Na+	1-2 mmol/kg/day	136-148 mmol/L	Predominantly extracellular; often already included in replacement fluids
K+	0.5-1.0 mmol/kg/day	3.6-5.0 mmol/L	Predominantly intracellular; prone to deficiency from NG loss, diarrhoea, ileostomy, insulin, alkalosis
Ca2+	About 5 mmol/day	2.11-2.55 mmol/L	Usually stable, but may fall in pancreatitis, massive transfusion, hypoparathyroidism
Mg2+	About 1 mmol/day	0.7-1.1 mmol/L	Usually stable; low Mg makes hypokalaemia hard to correct

2. Potassium - The Commonly Missed Deficit

Hypokalaemia is common in surgical patients because of:

Vomiting / NG aspirate
Diarrhoea / high-output stoma
Intracellular shift from insulin
Metabolic alkalosis driving K+ into cells
Secondary hyperaldosteronism from volume depletion

Why it matters:

Low K+ impairs gut smooth muscle -> ileus
Causes weakness and respiratory muscle dysfunction
Predisposes to arrhythmias, especially with digoxin or cardiac disease

Principles:

Do not add potassium until urine output is adequate unless specialist-directed
Correct magnesium if potassium remains low despite replacement
Monitor ECG and renal function when giving IV potassium

3. Correction of Hyponatraemia

Na+ deficit = total body weight x correction factor x (desired serum Na+ - actual serum Na+)

Correction factor:

Male: 0.6
Female: 0.5
Child: 0.6

Desired serum Na+:

Usually 136-148 mmol/L, but acute symptomatic hyponatraemia is corrected initially to remove symptoms, not instantly to normal

Pathophysiology:

Hyponatraemia means extracellular fluid is hypotonic relative to cells
Water shifts into brain cells -> cerebral oedema -> headache, confusion, seizures, coma
If chronic hyponatraemia is corrected too fast, brain cells cannot re-accumulate osmoles quickly -> osmotic demyelination syndrome

Clinical rule:

Symptomatic severe hyponatraemia needs urgent hypertonic saline under close monitoring
Chronic/asymptomatic hyponatraemia needs slow correction and treatment of cause

4. Correction of Hypernatraemia

Body water deficit in litres = total body weight x correction factor x [(actual serum Na+ / 140) - 1]

Correction factor:

Male: 0.6
Female: 0.5
Child: 0.6

Replacement schedule from the notes:

Replace half of the body water deficit over the first 24 hours
Replace the remaining deficit over the next 1-2 days

Pathophysiology:

Hypernatraemia means extracellular fluid is hypertonic
Water leaves brain cells -> cellular dehydration -> confusion, irritability, seizures, coma
If chronic hypernatraemia is corrected too rapidly, water rushes back into brain cells -> cerebral oedema

5. Chloride and Acid-Base Patterns

Clinical setting	Likely electrolyte / acid-base issue	Fluid logic
Vomiting / NG aspirate	H+ and Cl- loss -> hypochloraemic metabolic alkalosis; K+ loss	Normal saline plus potassium often corrects the physiology
Diarrhoea / ileostomy	Bicarbonate loss -> metabolic acidosis; K+ loss	Replace volume, potassium, and bicarbonate-equivalent deficit if severe
Large-volume NS	Hyperchloraemic metabolic acidosis	Switch to balanced crystalloid if appropriate
Pancreatitis / sepsis	Lactic acidosis from hypoperfusion, capillary leak	Balanced resuscitation and source control

K. Monitoring - The Prescription Is Never Finished

1. Minimum Monitoring for IV Fluid Therapy

Monitor	Why
Vitals	HR and BP show compensation/failure; tachycardia often precedes hypotension
Strict input/output chart	Detects under-replacement and hidden high-output losses
Urine output	Target at least 0.5 mL/kg/hr in adults
Daily weight	Best simple marker of net fluid balance
Urea/creatinine	Pre-renal AKI improves with resuscitation; worsening may mean ongoing hypoperfusion or overload/congestion
Na+, K+, Cl-, HCO3-	Guides fluid choice and electrolyte replacement
Glucose	Dextrose fluids, sepsis, pancreatitis, diabetes
Lactate / ABG or VBG if unwell	Tissue perfusion and acid-base physiology
Chest exam / oxygen requirement	Detects pulmonary oedema from over-resuscitation

2. Signs of Under-Resuscitation

Persistent tachycardia
Orthostatic hypotension
Cool peripheries
Dry mucosa
Poor skin turgor
Oliguria
Rising urea/creatinine ratio
Rising lactate or worsening base deficit
Lethargy or confusion

3. Signs of Over-Resuscitation

New oxygen requirement
Basal crepitations
Raised JVP
Peripheral oedema
Rapid weight gain
Hyponatraemia from excess hypotonic fluid
Abdominal compartment physiology in severe pancreatitis/trauma
Worsening renal function from venous congestion

Fluid Overload Has Pathophysiology Too

Over-resuscitation raises venous pressure. High venous pressure reduces organ perfusion gradient, worsens tissue oedema, impairs oxygen diffusion, and can worsen renal function through renal venous congestion. More fluid is not always more perfusion.

L. Special Surgical Scenarios

1. Post-Operative NPO Patient

Goal:

Maintenance water
Maintenance sodium and potassium
Some glucose to prevent starvation ketosis
Avoid hyponatraemia from excessive dextrose/free water

Daily review:

Can the patient drink?
Is there ileus?
What is the urine output?
Are electrolytes stable?
Are there drains, NG losses, fever, sepsis, or bleeding?

2. Vomiting or NG Aspirate

What is lost:

H+
Cl-
Na+
Water
K+ indirectly through renal compensation

Result:

Hypovolaemia
Hypochloraemic metabolic alkalosis
Hypokalaemia

Best physiological replacement:

0.9% normal saline to restore chloride
Add potassium when renal output is adequate
Replace measured NG loss if high

3. High-Output Ileostomy or Diarrhoea

What is lost:

Water
Na+
K+
Bicarbonate

Result:

Hypovolaemia
Hyponatraemia
Hypokalaemia
Metabolic acidosis
Pre-renal AKI

Management principle:

Measure output
Replace ongoing loss
Correct potassium and magnesium
Use oral rehydration with sodium/glucose coupling if gut is usable

4. Acute Pancreatitis / Peritonitis / Sepsis

Problem:

Third-space loss and capillary leak
Intravascular depletion despite oedema
SIRS-driven vasodilatation
Risk of renal, gut, and pancreatic hypoperfusion

Management:

Early isotonic crystalloid resuscitation
Prefer balanced crystalloid when large volumes are expected
Use urine output, lactate, haematocrit, urea/creatinine, and clinical perfusion to guide therapy
Avoid both under-resuscitation and blind over-resuscitation

5. Blood Loss

Crystalloid is a bridge, not definitive replacement for major haemorrhage.

Principles:

Control bleeding
Activate major haemorrhage protocol when appropriate
Give blood products when oxygen-carrying capacity and coagulation factors are being lost
Avoid dilutional coagulopathy from excessive crystalloid

M. Common Pitfalls

Pitfall	Why it is dangerous	Better approach
Giving maintenance fluids to a shocked patient	Too slow and too little for hypoperfusion	Bolus resuscitation first
Continuing "standard fluids" for days	Causes Na+/water imbalance and misses changing losses	Daily reassessment and tailored prescription
Using D5 for resuscitation	Minimal intravascular expansion after glucose metabolism	Use isotonic crystalloid
Ignoring NG/stoma/drain output	Ongoing losses recreate the deficit	Measure and replace
Correcting chronic Na+ disorders too fast	Osmotic demyelination or cerebral oedema	Controlled correction with serial sodium checks
Adding K+ in anuric AKI	Hyperkalaemic arrest	Confirm urine output and renal function
Repeated NS litres without checking chloride/HCO3-	Hyperchloraemic metabolic acidosis	Consider balanced crystalloid
Treating oedema as proof of overload in sepsis	Capillary leak can coexist with intravascular depletion	Assess effective circulating volume

High Yield Summary

Core formula: Fluid replacement = prior deficit + maintenance + ongoing/anticipated losses.

Body water: Adult male 60% body weight, adult female 50%, infant 80% at birth. ICF = 2/3 TBW; ECF = 1/3 TBW; interstitial = 3/4 ECF; intravascular = 1/4 ECF.

Hypovolaemia pathophysiology: Reduced venous return -> reduced preload -> reduced stroke volume -> reduced cardiac output -> tissue hypoperfusion -> lactate/metabolic acidosis. Tachycardia and oliguria precede hypotension.

Adult dehydration signs: 5% = dry mucosa, mild tachycardia, reduced urine; 10% = very dry, tachycardic, orthostatic/low BP, lethargic; 15% = parched, severe tachycardia, marked hypotension, obtunded/anuric.

Children: BP is preserved until late. Look for tachycardia, delayed capillary refill, sunken eyes/fontanelle, reduced urine, lethargy.

Resuscitation: Adults 10 mL/kg crystalloid bolus (about 500 mL in 50 kg). Children 20 mL/kg. Use 2 large-bore IV cannulae in shock and reassess after each bolus.

Replacement rate: If not shocked, give half the requirement in the first 8 hours and the remainder over the next 16 hours.

Maintenance: 4-2-1 rule: first 10 kg = 4 mL/kg/hr, second 10 kg = 2 mL/kg/hr, each extra kg = 1 mL/kg/hr. Add 10% for each degree Celsius above 37 degrees.

Ongoing losses: Blood, vomiting/diarrhoea, NG aspirate, stoma, drains, and third-space loss from pancreatitis, peritonitis, sepsis.

Fluid choice: D5 is not for shock. NS is useful for resuscitation and chloride-depletion alkalosis but can cause hyperchloraemic metabolic acidosis. Hartmann's/LR is balanced, lower chloride, lactate-buffered, and useful for large-volume replacement.

Electrolytes: Daily Na+ 1-2 mmol/kg/day, K+ 0.5-1.0 mmol/kg/day, Ca2+ about 5 mmol/day, Mg2+ about 1 mmol/day.

Sodium formulas: Na+ deficit = weight x correction factor x (desired Na+ - actual Na+). Water deficit in hypernatraemia = weight x correction factor x [(actual Na+ / 140) - 1]. Correction factor: male 0.6, female 0.5, child 0.6.

Do not overfill: Fluid overload causes pulmonary oedema, tissue oedema, impaired oxygen diffusion, renal venous congestion, and worse outcomes.

Active Recall - Fluid Replacement

References

^[1] Felix Lai notes: General Surgery - Fluid replacement.

^[2] NICE Clinical Guideline CG174: Intravenous fluid therapy in adults in hospital.

^[3] Surviving Sepsis Campaign Guidelines 2021: international guidelines for management of sepsis and septic shock.

^[4] Nelson Textbook of Pediatrics / standard paediatric dehydration assessment principles.

^[5] Bailey & Love's Short Practice of Surgery: peri-operative fluid and electrolyte management principles.

Fluid Replacement

Active Recall - Fluid Replacement

On this page