Fluids, Nutrition & Metabolism

Balanced Crystalloids vs Normal Saline

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Definitions

Normal Saline (NS / 0.9% NaCl): 154 mEq/L each of Na&spplus; and Cl&supminus;; slightly hyperchloremic and hyperosmolar (~308 mOsm/L); pH ~5.5.

Balanced Crystalloids (BC): Ringer’s Lactate (RL) and Plasma-Lyte — electrolyte composition closer to plasma; chloride partially replaced by metabolisable buffers (lactate in RL; acetate + gluconate in Plasma-Lyte).

Composition Comparison

Parameter	Balanced Crystalloids	Normal Saline	Plasma
Na&spplus; (mmol/L)	130–140	154	140
Cl&supminus; (mmol/L)	98–110	154	103
Buffer	Lactate (RL) / Acetate+Gluconate (PL)	None	Bicarbonate
pH	6.5–7.4	~5.5	7.4
Osmolarity (mOsm/L)	273–294	308	295
Volume distribution	25% intravascular, 75% interstitial	Same	100% intravascular

Pathophysiology — Why NS Causes Problems

High chloride load → hyperchloraemic metabolic acidosis via dilutional bicarbonate loss and direct renal tubular effects
Renal vasoconstriction → reduced GFR → increased AKI risk in some settings
Balanced crystalloids mitigate these effects — electrolyte profile closer to physiology

Clinical Evidence by Setting

Setting	BC Advantage	Caution
Adult sepsis	Reduced 28-day mortality, lower AKI (SSC 2021 weak recommendation)	Low-quality evidence, more RCTs needed
Paediatric sepsis	Lower mortality, AKI, hyperchloraemia, shorter stay	—
DKA (adults)	Faster metabolic resolution, faster insulin weaning	Strong support for BC
TBI	Potentially harmful — BC may be hypotonic → cerebral oedema	Prefer NS in TBI
Critically ill (general)	Possibly lower composite mortality/AKI	Mixed, inconclusive overall

Practical Recommendations

Balanced crystalloids generally preferred in adult sepsis, paediatric sepsis, and DKA
Normal saline remains appropriate in: TBI (cerebral oedema risk), alkalosis/hypochloraemia needing chloride load, hyperkalaemia (avoid lactate in RL)
Choice should be individualized and context-specific

Exam Tip BC = more physiologic composition, reduces hyperchloraemic acidosis risk. SSC 2021 gives a weak recommendation for BC in adult sepsis. NS still indicated in TBI — balanced fluids may be relatively hypotonic and worsen cerebral oedema. Key differentiator: RL contains lactate (avoid in severe liver failure); Plasma-Lyte contains acetate+gluconate (safer in liver failure).

Colloids vs Crystalloids

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Definitions

Crystalloids: solutions of small molecules (electrolytes ± buffers) that pass freely through semipermeable membranes. Volume expands both intravascular and interstitial compartments.

Colloids: solutions of large-molecular-weight substances (proteins or polysaccharides) that stay in the intravascular space by exerting oncotic pressure.

Classification

Crystalloids: Isotonic (0.9% NS, RL), Hypotonic (0.45% NS, D5W), Hypertonic (3% NS, 7.5% NS)
Colloids — Natural: Albumin (4%, 5%, 20%), Fresh Frozen Plasma
Colloids — Synthetic: Gelatins (Haemaccel, Gelofusine), Dextrans (Dextran-40, -70), HES (hydroxyethyl starch — withdrawn/restricted)

Volume Effect

Crystalloids: 1L expands plasma volume by ~250–300 mL (rest redistributes to interstitium)
Colloids: 1L expands plasma volume by ~800–1000 mL if endothelium is intact
In capillary leak (sepsis, burns), colloids escape into interstitium — advantage is negated, oedema worsened

Key Trial Evidence

SAFE trial (NEJM 2004): 4% albumin vs NS in 7,000 ICU patients — no overall mortality difference. Subgroup: TBI — albumin associated with increased mortality
VISEP, 6S, CHEST trials: HES linked to increased AKI, increased RRT, possibly increased mortality — HES now restricted/avoided in sepsis and renal dysfunction

Comparison Table

Feature	Crystalloids	Colloids
Oncotic pressure	Low	High
Volume expansion	Short-lived; ~25% remains intravascular	More sustained; ~80% intravascular (if intact endothelium)
Cost	Low	High (esp. albumin)
Allergy risk	Minimal	Yes (gelatins, dextrans)
Coagulopathy	Dilutional with large volumes	Dextrans, HES worsen coagulation
Renal toxicity	Rare (hyperchloraemia with NS)	HES — significant AKI risk

Current Recommendations

First line: balanced crystalloids for initial resuscitation in most surgical/ICU patients
Normal saline: TBI, hyponatraemia, metabolic alkalosis needing chloride
Albumin: adjunct in sepsis with hypoalbuminaemia after crystalloids; cirrhosis with large-volume paracentesis
Synthetic colloids (HES, dextran, gelatin): avoid in sepsis, burns, renal dysfunction

Exam Tip No strong mortality benefit of colloids over crystalloids in most settings. Synthetic colloids (esp. HES) largely abandoned in critical care — AKI and coagulopathy risk (CHEST, 6S trials). Albumin has a niche role: sepsis with hypoalbuminaemia; cirrhosis + large-volume paracentesis (give 8g albumin per litre drained). In capillary leak states, colloids offer no advantage over crystalloids.

Goal-Directed Fluid Therapy (GDFT)

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GDFT perioperative fluid bolus challenge decision algorithm — GDFT perioperative fluid bolus challenge — stroke volume-guided decision algorithm — *AI-generated diagram, verify with textbook*

Definition

An individualised, dynamic approach to perioperative fluid administration guided by specific physiological targets (haemodynamic and perfusion parameters), aiming to optimise tissue oxygen delivery while avoiding both hypovolaemia and fluid overload.

Rationale

Traditional fixed-formula fluid therapy (body weight × estimated losses) leads to under-resuscitation (hypoperfusion, AKI) or overload (pulmonary oedema, impaired wound healing). GDFT uses real-time monitoring to titrate fluids to the patient’s physiological response.

GDFT Algorithm

Step 1: Give fluid bolus (200–250 mL crystalloid) over 5–10 min
Step 2: Assess response — measure SV, CO, SVV or PPV
Step 3a: Stroke volume rises ≥10% → fluid responsive → repeat bolus
Step 3b: No significant SV rise → fluid unresponsive → stop fluids, consider inotropes/vasopressors
Step 4: Reassess after each intervention

GDFT Monitoring Parameters

Parameter	Target	Interpretation
Urine output	≥0.5 mL/kg/hr	Adequate renal perfusion
MAP	≥65 mmHg	Ensures organ perfusion
SVV (Stroke Volume Variation)	<10–12% after optimisation	>13% → fluid responsive
PPV (Pulse Pressure Variation)	<12%	>12% suggests preload responsiveness
Stroke volume	Optimise to plateau	No further rise after bolus = target met
Cardiac index	>2.2 L/min/m²	Adequate cardiac output
ScvO₂	>70%	DO₂/VO₂ balance
Serum lactate	<2 mmol/L	Elevated → tissue hypoperfusion
BUN:Creatinine ratio	10–20 normal	>20 hypovolaemia; <10 overload
PaO₂:FiO₂	>300	<300 suggests pulmonary overload/ARDS

Devices Used

Oesophageal Doppler (CardioQ) — non-invasive; standard for GDFT in UK
LiDCO, PiCCO, FloTrac — minimally invasive cardiac output monitors
Pulmonary artery catheter — gold standard but invasive; largely replaced

Evidence

OPTIMISE trial and multiple meta-analyses show GDFT reduces postoperative renal/pulmonary complications and hospital stay. Recommended by ERAS protocols. Effect on mortality less clear in lower-risk surgery.

Exam Tip GDFT mnemonic “U-MaSS-COLaB-Pa”: Urine ≥0.5 · MAP ≥65 · SVV <12% · SV optimised · CO >2.2 · ScvO₂ >70% · Lactate <2 · BUN:Cr 10–20 · PaO₂/FiO₂ >300. SVV and PPV require mechanical ventilation with tidal volumes ≥8 mL/kg to be reliable — not valid in spontaneously breathing patients.

IV Fluid Therapy in Adults — NICE CG174

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The 5 R’s of IV Fluid Therapy

Resuscitation — restore intravascular volume in compromised patients
Routine maintenance — meet daily basic fluid and electrolyte needs
Replacement — correct existing deficits (dehydration, bleeding, losses)
Redistribution — manage abnormal fluid shifts (oedema, third spacing)
Reassessment — continuous monitoring and adjustment

Indications for Resuscitation

Any of: SBP <100 mmHg; HR >90/min; capillary refill >2 sec; RR >20/min; NEWS ≥5; positive passive leg raise response.

Resuscitation fluid: crystalloid (Na&spplus; 130–154 mmol/L); 500 mL bolus over <15 min; reassess after each bolus.

Routine Maintenance Prescription

Standard daily requirements:

Water: 25–30 mL/kg/day
Na&spplus;, K&spplus;, Cl&supminus;: ~1 mmol/kg/day each
Glucose: 50–100 g/day (prevents ketosis)

Typical day-1 fluid: 0.18% NaCl + 4% dextrose + 27 mmol/L K&spplus;, ≤2.5 L/day.

Reduce to 20–25 mL/kg/day in: elderly, cardiac failure, renal disease, refeeding risk.

Maintenance Rule “30 : 1 : 100” — 30 mL/kg water · 1 mmol/kg Na/K/Cl · 100 g glucose per day. Never add K&spplus; directly to IV bag — always use pre-mixed bags.

Common IV Fluids — Composition

Fluid	Na&spplus; (mmol/L)	Cl&supminus; (mmol/L)	Use
0.9% NaCl	154	154	Resuscitation (risk: hyperchloraemic acidosis)
Hartmann’s / RL	131	111	Balanced resuscitation (preferred)
5% Dextrose	0	0	Free water replacement (not resuscitation)
4% Dextrose + 0.18% NaCl	30	30	Routine maintenance

Complications of IV Fluid Therapy

Complication	Definition	Time Frame
Hypovolaemia	Clinical dehydration, ↑urea/Cr, low urine output	During therapy
Pulmonary oedema	Breathlessness, X-ray infiltrates	During / ≤6h post-IVF
Hyponatraemia	Na&spplus; <130 mmol/L	≤24h post-IVF
Hypernatraemia	Na&spplus; ≥155 mmol/L	≤24h post-IVF
Hypo/hyperkalaemia	K&spplus; <3.0 or >5.5 mmol/L	≤24h post-IVF
Peripheral oedema	Pitting oedema without cardiac/renal cause	≤24h post-IVF

Exam Tip NICE CG174 applies to adults ≥16 yr in hospital (excludes pregnancy, severe renal/liver disease, DKA, burns, ICU). The 5 R’s are the framework — Resuscitation, Routine maintenance, Replacement, Redistribution, Reassessment. Resuscitation: 500 mL balanced crystalloid bolus over <15 min, reassess. Check serum chloride daily if 0.9% NaCl is used for maintenance.

Artificial Nutritional Support

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Introduction & Indications

Adequate nutrition is vital for postoperative recovery and wound healing. Artificial nutritional support is indicated when oral intake is inadequate for >5 days, or earlier in malnourished patients.

Commonly required in: major head/neck/GI surgery (oesophagectomy, gastrectomy, Whipple’s), severe trauma/burns, sepsis, neurological impairment, prolonged ileus, high-output fistulae.

Routes — Enteral vs Parenteral

Feature	Enteral Nutrition	Parenteral Nutrition
Route	GI tract	Intravenous (peripheral or central)
Gut integrity	Maintains mucosal integrity and GALT	Mucosal atrophy, bacterial translocation risk
Infection risk	Lower	Higher (catheter-related sepsis)
Cost	Cheaper	Expensive
Indication	When gut is functional	When gut unusable or contraindicated
Main complications	Aspiration, diarrhoea, tube issues	Metabolic, hepatic, catheter-related

Core Principle Enteral nutrition is always preferred over parenteral when the gut is functional. “If the gut works, use it.” Enteral feeding preserves gut mucosal integrity, prevents bacterial translocation, and is cheaper with fewer complications.

Exam Tip Key points to remember: (1) Start nutritional support within 5 days of inadequate oral intake; earlier in malnourished patients. (2) Prevent refeeding syndrome with slow initiation and electrolyte monitoring. (3) Central venous access required for TPN >14 days (high osmolarity). (4) Regular MDT input — surgeon, dietician, physician.

Enteral Nutrition

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Definition

Delivery of nutrients directly into the GI tract through oral, gastric, or post-pyloric routes, when normal oral intake is inadequate but the gut is functional.

Types by Nutrient Composition

Polymeric: intact protein, polysaccharides, long-chain TG — requires normal digestion; standard for most patients
Elemental/monomeric: amino acids, monosaccharides, MCT — for malabsorption, short bowel, pancreatitis
Semi-elemental (oligomeric): peptides, oligosaccharides, MCT — intermediate absorption
Disease-specific: renal (low protein/K), hepatic (BCAA-enriched), pulmonary (high fat/low CHO), diabetic (low GI, fibre-added)

Access Routes by Duration

Route	Duration	Indication
Nasogastric (NG)	≤4–6 weeks	Short-term; intact gag reflex; most common
Nasojejunal (NJ)	≤4–6 weeks	High aspiration risk; acute pancreatitis
PEG (Percutaneous Endoscopic Gastrostomy)	>4–6 weeks	Long-term; intact gastric emptying; neurological conditions
RIG (Radiologically Inserted Gastrostomy)	>4–6 weeks	Endoscopy not feasible
Jejunostomy (PEJ / surgical)	Long-term	Gastroparesis; gastric outlet obstruction; post-oesophagectomy

Advantages of Enteral over Parenteral

Physiological: preserves gut mucosal integrity; maintains GALT; decreases bacterial translocation; stimulates bile flow and pancreatic secretions
Clinical: lower infection incidence; better glycaemic control; early EN reduces post-op ileus and improves wound healing
Practical: cheaper; easier administration; fewer metabolic complications

Complications

Category	Complications
Mechanical	Malposition, kinking, blockage, dislodgement; nasal/oesophageal ulceration, sinusitis; aspiration pneumonia
Gastrointestinal	Nausea, vomiting, distension; diarrhoea (osmotic/rapid infusion); constipation; GI bleeding
Metabolic	Electrolyte imbalance; hyperglycaemia; refeeding syndrome; dehydration or overload
Infective	Sinusitis, otitis (nasal tubes); peristomal infection (PEG/jejunostomy); peritonitis (leak)

Contraindications

Absolute: intestinal obstruction (mechanical or paralytic ileus); severe mesenteric ischaemia; peritonitis or intra-abdominal sepsis; severe GI haemorrhage; inability to access GI tract safely.

Relative: high aspiration risk with unprotected airway; haemodynamic instability on high-dose vasopressors (bowel ischaemia risk); severe uncontrolled diarrhoea; severe pancreatitis (NJ feeding often still feasible).

Exam Tip NG tube is the commonest enteral access — use for ≤4–6 weeks. PEG if >4–6 weeks needed (endoscopic placement, internal bumper). Jejunostomy placed intraoperatively during oesophagectomy or gastrectomy for post-op feeding. NJ tube for pancreatitis or high aspiration risk. Refeeding syndrome risk: start at 10 kcal/kg/day and increase slowly over 4–7 days; supplement thiamine.

Parenteral Nutrition (PN)

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Definition

Administration of nutrients intravenously, bypassing the GI tract, to provide substrates for energy, growth, and tissue repair when enteral feeding is not possible.

Types

Partial PN (PPN): supplies part of daily requirements; peripheral access acceptable; lower osmolarity (<900 mOsm/L)
Total PN (TPN): full caloric/nutritional requirements; requires central venous access (high osmolarity); contains dextrose + amino acids + lipids + electrolytes + vitamins + trace elements

Access Routes

Route	Duration	Comment
Central Venous Catheter (CVC)	Long-term	Preferred; reduces thrombophlebitis
PICC line	Weeks–months	Inserted via basilic/cephalic vein; good for home TPN
Peripheral line	<14 days	Low osmolarity feeds only (<900 mOsm/L)
Hickman / Implanted port	>3 months	Long-term home TPN; tunnelled

Indications

Intestinal obstruction, severe short bowel syndrome, high-output enterocutaneous fistulae
Severe pancreatitis where EN not tolerated; intractable vomiting/diarrhoea
Perioperative: preoperative severe malnutrition; prolonged postoperative NPO after major GI surgery
Critical illness: severe trauma, burns, sepsis where EN not feasible
Oncology: malignant bowel obstruction, radiation enteritis

Complications

Type	Examples
Insertion-related	Pneumothorax (0.5–1%; subclavian route), arterial puncture, misplacement — confirm position on CXR
Line-related	Catheter-related bloodstream infection (CRBSI) up to 15%; thrombosis (SVC occlusion/PE); line blockage
Metabolic	Hyperglycaemia (most common); refeeding syndrome; electrolyte derangements (Na, K, Mg, PO₄)
Hepatic (IFALD)	Fatty liver → fibrosis → cirrhosis (~25% incidence in long-term TPN; especially children)
Metabolic bone disease	Osteoporosis/osteomalacia; supplement Ca²&spplus;, PO₄, Vit D

Refeeding Syndrome Pathophysiology: rapid feeding in severely malnourished patients → ↓serum phosphate (driven into cells with glucose) → fluid and electrolyte shifts → arrhythmias, cardiac failure, seizures, weakness.

Risk factors: BMI <16; weight loss >15% in 3–6 months; no intake >10 days; low baseline K&spplus;/Mg/PO₄; alcoholism.

Prevention: start at 10 kcal/kg/day; increase slowly over 4–7 days; supplement thiamine, vitamins, trace elements before and during refeeding; monitor electrolytes daily.

Exam Tip Confirm CVC tip position on CXR before starting TPN — tip should be in lower SVC or atriocaval junction. CRBSI: take paired blood cultures (central + peripheral); remove line if Staphylococcus aureus or candida grows. IFALD (intestinal failure-associated liver disease): manage by cycling TPN (resting liver), omega-3 lipid emulsions, and promoting any enteral feeding. Hyperglycaemia: use variable rate insulin infusion; adjust when TPN is interrupted to prevent hypoglycaemia.

Caloric & Protein Requirements

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Metabolic Phases After Surgery

Phase	Duration	State	Features
Ebb phase	0–24 hrs	↓ Metabolism	↓ O₂ consumption, ↓ energy expenditure, ↓ body temperature
Flow phase	2–10 days	↑ Catabolism	↑ cortisol, ↑ catecholamines, ↑ gluconeogenesis, ↑ protein breakdown
Anabolic phase	After 7–10 days	↑ Protein synthesis	Tissue repair, positive nitrogen balance

Energy Requirements

Total Energy Requirement (TER) = BEE × (stress factor + activity factor)

Condition	Stress Factor	Approx. kcal/kg/day
Post-op elective surgery	1.1–1.2	25–30
Moderate infection/trauma	1.3–1.5	30–35
Sepsis / major burns / multiple trauma	1.5–2.0	35–45
Severe burns (>40% BSA)	2.0–2.5	Up to 50

Protein Requirements

Condition	Protein (g/kg/day)
Normal adult	0.8–1.0
Post-operative mild stress	1.0–1.2
Moderate stress (infection/trauma)	1.3–1.5
Severe stress / burns / sepsis	1.5–2.0
Protein-losing conditions	Up to 2.5

Macronutrient Summary

Parameter	Normal	Post-op mild	Severe stress/sepsis
Calories (kcal/kg)	25	30–35	40–50
Protein (g/kg)	1.0	1.2–1.5	2.0
Carbohydrates (g/kg)	3–4	4–5	5–6
Fat (g/kg)	1.0	1–2	2.0
Water (mL/kg)	30–35	30–40	40–50

Nitrogen balance: goal is positive nitrogen balance (nitrogen in > nitrogen out). Nitrogen intake (g) = Protein intake (g) ÷ 6.25.

Exam Tip Energy needs rise with surgical stress severity. Average post-op requirement: 30–35 kcal/kg/day. Average protein: 1.5 g/kg/day. Early enteral feeding within 24–48 hrs preferred to maintain gut integrity. Indirect calorimetry is gold standard if available. Overfeeding → hyperglycaemia, CO₂ retention, fatty liver. Underfeeding → poor wound healing, infection, muscle wasting. Use ideal body weight (not actual) in obese patients.

Nutritional Assessment & MUST Tool

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ABCD Approach to Nutritional Assessment

A — Anthropometry

BMI = weight (kg) ÷ height² (m²)
MUAC (Mid-Upper Arm Circumference)
TSF (Triceps Skinfold Thickness)
MAMC (Mid-Arm Muscle Circumference) = MUAC − (3.14 × TSF)
Caveat: anthropometry is altered by fluid shifts in critically ill patients

B — Biochemistry

Albumin: falls in malnutrition/inflammation; not reliable acutely (half-life 20 days)
Prealbumin: shorter half-life (2 days) — better acute marker
CRP, WBC: inflammation markers
Electrolytes (Na&spplus;, K&spplus;, Mg, PO₄): assess refeeding risk

C — Clinical evaluation

Symptoms: nausea, vomiting, dysphagia, early satiety, diarrhoea
Past history: malignancy, IBD, liver disease, stroke, Parkinson’s
Malabsorptive states: short bowel, high-output stoma, enterocutaneous fistula, pancreatic insufficiency

D — Dietary assessment

Diet diary or 24-hour dietary recall
Compare to requirement (25–35 kcal/kg lean body weight)
Recent or anticipated decreased intake >5 days → initiate nutritional support

MUST Tool (Malnutrition Universal Screening Tool)

Parameter	Score 0	Score 1	Score 2
BMI (kg/m²)	≥20	18.5–20	<18.5
Weight loss (3–6 mo)	≤5%	5–10%	>10%
Acute disease effect	–	–	Add 2 if likely no intake ≥5 days

Total Score	Risk	Action
0	Low	Routine care; weekly hospital reassessment
1	Medium	Observe; document intake for 3 days
≥2	High	Dietician referral; initiate nutritional support

Follow-up Frequency

Hospital: weekly
Care homes: monthly
Community: yearly (age >75 years)

Exam Tip MUST is the standard malnutrition screening tool in UK practice — three components: BMI + weight loss + acute disease effect. Score ≥2 = high risk → dietician + nutritional support. ABCD nutritional assessment: Anthropometry, Biochemistry, Clinical, Dietary. In critically ill patients, albumin is unreliable as a nutritional marker (falls with inflammation regardless of intake) — use prealbumin or trend of CRP instead.