Case 11.7 – Electrolyte Management [SDL]

Systematic Assessment of IV Fluid Prescription

1. Review Patient History and Current Status

• Type of surgery: Sigmoid colon resection – major abdominal surgery with potential for significant fluid shifts
• Time since surgery: Post-operative day 3
• Co-morbidities: Otherwise fit and well (no cardiac, renal, or liver disease)
• Current losses: Nasogastric tube on free drainage
• Oral intake: Likely minimal or nil by mouth
• Medications: Any drugs affecting fluid/electrolyte balance

2. Clinical Assessment

Assess Volume Status

Clinical Feature	Volume Depletion	Volume Overload
Vital signs	• Tachycardia • Hypotension • Postural drop ≥20mmHg systolic • Low urine output	• Hypertension • Tachypnoea
Examination	• Dry mucous membranes • Reduced skin turgor • Sunken eyes • Reduced JVP • Weak/thready pulse • Prolonged capillary refill	• Raised JVP • Peripheral oedema • Pulmonary oedema (crackles) • Third heart sound (S3) • Ascites
Weight	Decreased from baseline	Increased from baseline

Assess for Complications

• Surgical complications: Anastomotic leak, bleeding, ileus
• Respiratory status: Signs of pulmonary oedema or chest infection
• Mental status: Confusion (hyponatremia, dehydration, sepsis)
• Abdominal examination: Distension, tenderness, bowel sounds

3. Review Fluid Balance Chart

• Input: All IV fluids, oral intake, enteral feeds
• Output:
  • Urine output (should be >0.5mL/kg/hr or >30mL/hr)
  • NG tube drainage
  • Drain output
  • Stoma output
• Insensible losses: ~500-800mL/day normally; increased with fever (10mL/kg/day per °C above 37°C)
• Daily balance: Calculate cumulative fluid balance

4. Review Laboratory Results

Test	Why Important	Action if Abnormal
Sodium	Hyponatremia common post-op due to ADH secretion and hypotonic fluids	Adjust fluid type; consider fluid restriction if dilutional
Potassium	May be depleted (NG losses, inadequate replacement); may be elevated (renal impairment, cell lysis)	Supplement or restrict as needed
Urea & Creatinine	Assess renal function and hydration status	↑ Urea:Cr ratio suggests dehydration; ↑ both suggests renal impairment
Chloride	May be depleted with NG losses	Replace with 0.9% saline if deficient
Bicarbonate/Blood gas	Assess acid-base status	May indicate metabolic alkalosis (NG losses) or acidosis (sepsis, renal failure)
Haemoglobin/Haematocrit	May be elevated in dehydration; low if bleeding	Transfuse if bleeding; consider dehydration if elevated
Glucose	May be elevated in stress response; risk of hypo if poor intake	Consider dextrose-containing fluids if hypoglycemic

5. Review Current IV Fluid Prescription

Check the following components:

• Type of fluid: Crystalloid vs colloid; isotonic vs hypotonic
• Rate: Appropriate for patient's weight and clinical status
• Additives: Potassium, dextrose, other electrolytes
• Total daily volume: Meeting maintenance requirements plus replacing losses
• Duration: Is IV fluid still needed or can patient take orally?

6. Calculate Fluid Requirements

Maintenance Fluids (Adult)

Standard requirement: ~25-30mL/kg/day

For 70kg adult: ~2000-2500mL/day (including oral intake)

Alternative Calculation (4-2-1 Rule)

• 4mL/kg/hr for first 10kg
• 2mL/kg/hr for next 10kg
• 1mL/kg/hr for remaining kg

Example for 70kg patient:

• First 10kg: 4 × 10 = 40mL/hr
• Next 10kg: 2 × 10 = 20mL/hr
• Remaining 50kg: 1 × 50 = 50mL/hr
• Total: 110mL/hr or ~2640mL/day

Plus Replacement of Ongoing Losses

• NG tube losses: Replace mL for mL with 0.9% saline + KCl
• Drain output: Replace based on volume and composition
• Fever: Add 10mL/kg/day per °C above 37°C

7. Consider Electrolyte Requirements

Electrolyte	Daily Requirement (Adult)	Notes
Sodium	1-2 mmol/kg/day (70-140 mmol/day)	Provided by 0.9% saline (154mmol/L) or Hartmann's (131mmol/L)
Potassium	1 mmol/kg/day (70 mmol/day)	Usually added to IV fluids (typically 20-40mmol/L) Check K+ before supplementing
Chloride	1-2 mmol/kg/day	Provided by 0.9% saline (154mmol/L)
Glucose	50-100g/day	Prevents ketosis; provided by 5% dextrose (50g/L)

8. Formulate a Plan

• Continue current fluids if appropriate
• Adjust rate, type, or additives based on assessment
• Set clear endpoints for reassessment
• Plan for transition to oral intake when appropriate
• Arrange follow-up bloods to monitor electrolytes

Key Differences in Paediatric vs Adult IV Fluid Prescribing

1. Fluid Requirements

Maintenance Fluid Calculation

Method	Paediatric (Holliday-Segar Formula)	Adult
4-2-1 Rule	• 4 mL/kg/hr for first 10kg • 2 mL/kg/hr for next 10kg • 1 mL/kg/hr for each kg >20kg	Same rule applies, or use 25-30mL/kg/day
Example: 15kg child	First 10kg: 4×10 = 40mL/hr Next 5kg: 2×5 = 10mL/hr Total: 50mL/hr	N/A
Example: 70kg adult	N/A	25-30mL/kg/day = 1750-2100mL/day OR use 4-2-1 rule ≈ 110mL/hr

2. Fluid Composition

Choice of Fluid

Aspect	Paediatric	Adult
Maintenance fluid	0.9% saline + 5% dextrose + KCl (Previously used hypotonic fluids, but risk of hyponatremia) NICE guideline: Isotonic fluids (0.9% saline with glucose and K+) to reduce hyponatremia risk	0.9% saline or Hartmann's for resuscitation and replacement 0.9% saline + 5% dextrose + KCl or 0.45% saline + 5% dextrose + KCl for maintenance
Dextrose	ALWAYS include dextrose in maintenance fluids Children have limited glycogen stores and are at higher risk of hypoglycemia	Dextrose optional in maintenance fluids Adults can maintain blood glucose longer without exogenous glucose
Resuscitation fluid	0.9% saline or Hartmann's Bolus: 10-20mL/kg over 10-20 minutes Reassess after each bolus	0.9% saline or Hartmann's Bolus: 500-1000mL over 15-30 minutes Reassess after bolus

3. Special Considerations in Children

Higher Risk of Complications

• Hyponatremia: Children more susceptible to cerebral oedema from rapid correction; avoid hypotonic fluids
• Hypoglycemia: Limited glycogen stores; always include dextrose in maintenance fluids
• Volume overload: Smaller circulating volume means less margin for error
• Hypothermia: Higher surface area to volume ratio; warm fluids if giving large volumes

4. Electrolyte Requirements

Electrolyte	Paediatric	Adult
Sodium	2-4 mmol/kg/day	1-2 mmol/kg/day
Potassium	1-2 mmol/kg/day (Usually add 20mmol/L to fluids)	1 mmol/kg/day (Usually add 20-40mmol/L to fluids)
Glucose	ESSENTIAL 5-10g/kg/day Use 5% or 10% dextrose	50-100g/day (optional) Usually 5% dextrose sufficient if needed

5. Monitoring

Parameter	Paediatric	Adult
Frequency	More frequent monitoring required • Electrolytes: Every 12-24 hours initially • Blood glucose: Every 4-6 hours if on IV fluids • Fluid balance: Hourly in acute setting	• Electrolytes: Daily or twice daily • Blood glucose: If diabetic or on TPN • Fluid balance: Daily, hourly if unstable
Weight	Daily weights essential Acute weight change primarily reflects fluid status	Daily weights helpful Especially in cardiac/renal patients
Urine output	Target: >1mL/kg/hr (Infants >2mL/kg/hr)	Target: >0.5mL/kg/hr

6. Prescribing Approach

Paediatric

• Weight-based: All calculations must be based on accurate weight
• Age-specific: Neonates, infants, children, and adolescents have different requirements
• Senior review: Junior doctors should discuss all paediatric IV fluid prescriptions with seniors
• Clear documentation: Document weight used for calculations

Adult

• More standardized: Often use fixed volumes (e.g., 1L bags)
• Weight-based in specific situations: Critical illness, obesity, renal impairment
• Greater physiological reserve: More tolerance for small errors

7. Recent Guideline Changes

NICE Guidance on Paediatric IV Fluids (Updated)

• Avoid hypotonic fluids: Use isotonic (0.9%) saline with 5% glucose for maintenance
• Reason: Reduce risk of hyponatremia and associated cerebral oedema
• Previous practice: 0.45% saline or 0.18% saline (now avoided)

8. Common Mistakes to Avoid

In Children

• ❌ Using hypotonic maintenance fluids (risk of hyponatremia)
• ❌ Forgetting glucose (risk of hypoglycemia)
• ❌ Calculation errors (wrong weight, decimal point errors)
• ❌ Not monitoring glucose regularly
• ❌ Giving excessive fluid boluses without reassessment

In Adults

• ❌ Excessive 0.9% saline (risk of hyperchloremic acidosis)
• ❌ Adding potassium without checking levels first
• ❌ Not reducing fluids when patient starts eating/drinking
• ❌ Fluid overload in cardiac/renal patients

Summary - Key Differences

Feature	Paediatric	Adult
Fluid calculation	Always weight-based (4-2-1 rule)	Fixed volume or 25-30mL/kg/day
Maintenance fluid	0.9% saline + 5% dextrose + K+	0.9% saline or Hartmann's ± dextrose
Glucose	Essential (always include)	Optional
Tonicity	Isotonic ONLY	Isotonic or hypotonic
Monitoring	More frequent	Less frequent
Complications	Higher risk (hyponatremia, hypoglycemia)	Lower risk (more physiological reserve)
Senior input	Always discuss with senior	Independent for routine cases

Volume Depletion

Recognition of Volume Depletion

Clinical Features

System	Mild (3-5% loss)	Moderate (6-9% loss)	Severe (\>10% loss)
Cardiovascular	• Mild tachycardia • Normal BP • Postural hypotension	• Tachycardia (\>100bpm) • Hypotension (systolic 90-100) • Prolonged capillary refill (\>2 sec)	• Marked tachycardia • Hypotension (systolic <90) • Weak, thready pulse • Shock
Renal	• Mild oliguria	• Oliguria (urine <0.5mL/kg/hr)	• Anuria or severe oliguria
Skin/mucosa	• Dry mucous membranes • Reduced skin turgor	• Very dry mouth • Tenting of skin • Sunken eyes	• Extreme dryness • Cold, clammy extremities • Mottled skin
Neurological	• Thirst • Fatigue	• Confusion • Lethargy	• Severe confusion • Coma • Seizures
Other	• Decreased JVP	• Absent JVP • Weight loss	• Collapsed veins • Significant weight loss

Laboratory Findings

• Elevated urea:creatinine ratio (\>20:1): Pre-renal azotemia
• Elevated haematocrit: Haemoconcentration
• Hypernatremia: If losses are hypotonic (e.g., sweat, insensible losses)
• Hyponatremia: If fluid losses replaced with free water
• Metabolic alkalosis: If gastric losses (vomiting, NG aspiration)
• Low urine sodium (\<20mmol/L): Kidneys conserving sodium
• High urine osmolality (\>500mOsm/kg): Kidneys concentrating urine
• Elevated lactate: Tissue hypoperfusion in severe depletion

Management of Volume Depletion

1. Assess Severity

Use clinical features and vital signs to determine if mild, moderate, or severe.

2. Choose Appropriate Fluid

Situation	Fluid Choice	Rationale
Hypovolemic shock	0.9% saline or Hartmann's	Isotonic crystalloid for rapid volume expansion
Hemorrhagic shock	Blood products ± crystalloid	Replace lost blood; target 1:1:1 ratio (RBC:FFP:platelets)
Gastric losses	0.9% saline + KCl	Replaces Na+, Cl-, and K+ lost in gastric secretions
Diarrhea	Hartmann's or 0.9% saline + KCl	Replaces electrolytes lost in stool
Burns	Hartmann's (Parkland formula)	Preferred for large burns
Diabetic ketoacidosis	0.9% saline initially	Correct dehydration; switch to 0.45% saline if hypernatremic

3. Rate and Volume of Replacement

Mild-Moderate Depletion

• Bolus: 500-1000mL over 15-30 minutes
• Reassess after each bolus (vital signs, urine output, clinical status)
• Repeat boluses as needed until euvolemic
• Then switch to maintenance fluids

Severe Depletion/Shock

• Rapid bolus: 1000mL (or 20mL/kg in children) as fast as possible
• Consider two large-bore IV cannulas
• Reassess frequently
• May need several litres in first hour
• Early senior/ICU involvement

4. Monitoring During Resuscitation

• Vital signs: HR, BP, RR every 15-30 minutes initially
• Urine output: Target >0.5mL/kg/hr (insert catheter)
• Lactate: Should decrease with adequate resuscitation
• Mental status: Should improve
• Capillary refill: Should normalize (\<2 seconds)
• CVP monitoring: Consider in severe cases or ICU

5. Identify and Treat Underlying Cause

• Bleeding → control source, transfuse
• Vomiting/diarrhea → antiemetics, treat cause
• Sepsis → antibiotics, source control
• Burns → specialized burn management
• DKA → insulin, ongoing fluid replacement

6. Complications to Avoid

• Fluid overload: Especially in elderly or cardiac/renal disease
• Electrolyte disturbances: Monitor K+, Na+ closely
• Hyperchloremic acidosis: From excessive 0.9% saline; consider balanced solutions

Volume Overload

Recognition of Volume Overload

Clinical Features

System	Clinical Signs
Cardiovascular	• Elevated JVP (\>3cm above sternal angle) • Third heart sound (S3 gallop) • Hypertension • Tachycardia
Respiratory	• Dyspnoea • Orthopnoea • Paroxysmal nocturnal dyspnoea • Tachypnoea • Bibasal crackles (pulmonary oedema) • Reduced oxygen saturation • Pleural effusions
Peripheral	• Peripheral oedema (ankles, legs, sacrum if bedbound) • Pitting oedema • Weight gain
Abdominal	• Ascites • Hepatomegaly (tender liver edge) • Hepatojugular reflux

Investigations

• Chest X-ray:
  • Cardiomegaly
  • Pulmonary oedema (bat wing appearance, Kerley B lines)
  • Pleural effusions
  • Upper lobe blood diversion
• BNP/NT-proBNP: Elevated in heart failure
• Echocardiogram: Assess cardiac function
• Electrolytes: Hyponatremia may occur (dilutional)
• Renal function: May be impaired

Management of Volume Overload

1. Stop Contributing Factors

• Cease IV fluids or reduce rate significantly
• Review medications (NSAIDs, steroids can worsen)
• Identify underlying cause (heart failure, renal failure, liver disease, excessive IV fluids)

2. Fluid Restriction

• Restrict total fluid intake to 1000-1500mL/day (including IV and oral)
• More severe restriction (800-1000mL) in severe hyponatremia
• Daily weights to monitor

3. Diuretic Therapy

Loop Diuretics (First-line)

• Furosemide:
  • Oral: 40mg once or twice daily (titrate up to 80-120mg BD if needed)
  • IV: 20-40mg initially; can increase to 80-120mg or give as infusion
  • Monitor response: Expect diuresis within 1 hour of IV, 1-2 hours of oral
• Bumetanide: Alternative if furosemide resistance (40:1 potency ratio)

Thiazide Diuretics

• Add to loop diuretic if inadequate response
• Synergistic effect
• E.g., Metolazone 2.5-5mg daily

Potassium-Sparing Diuretics

• Spironolactone: 25-50mg daily (useful in heart failure, liver disease)
• Monitor potassium closely (risk of hyperkalemia)

4. Positioning and Oxygen

• Sit patient upright (reduces venous return, improves breathing)
• Supplemental oxygen to maintain SpO2 >94%
• Consider CPAP/BiPAP for severe pulmonary oedema

5. Treatment of Underlying Cause

Cause	Specific Management
Acute heart failure	• Diuretics • GTN (reduces preload) • ACE inhibitor (once stable) • Beta-blocker (once compensated) • Consider inotropes if cardiogenic shock
Chronic kidney disease	• Higher doses of diuretics often needed • May require dialysis (ultrafiltration) • Restrict sodium and fluid
Liver disease	• Spironolactone (first-line for ascites) • Furosemide (second-line or in combination) • Therapeutic paracentesis if tense ascites • Sodium restriction
Iatrogenic (excessive IV fluids)	• Stop IV fluids • Diuretics • Usually resolves quickly

6. Monitoring During Treatment

• Daily weights: Target 0.5-1kg loss per day
• Fluid balance: Aim for negative balance
• Electrolytes: Daily monitoring
  • Risk of hypokalemia (with loop diuretics)
  • Risk of hyponatremia (dilutional or from diuretics)
  • Risk of hyperkalemia (with K-sparing diuretics)
• Renal function: Monitor for worsening (over-diuresis)
• Clinical status: Improvement in dyspnoea, oedema, JVP

7. Additional Measures

• Sodium restriction: <2g/day (90mmol/day)
• Dialysis/ultrafiltration: If refractory to diuretics or severe renal impairment
• Optimize cardiac output: Treat arrhythmias, optimize medications

8. Complications to Monitor

• Electrolyte disturbances (hypo/hyperkalemia, hyponatremia)
• Renal impairment from over-diuresis
• Hypotension from excessive diuresis
• Hepatic encephalopathy (in liver disease)

Summary Comparison

Feature	Volume Depletion	Volume Overload
Key signs	↓JVP, ↓BP, tachycardia, dry mucosa, ↓urine output	↑JVP, crackles, oedema, dyspnoea, weight gain
Management principle	Replace fluids	Remove fluids
Key treatment	IV crystalloid bolus	Diuretics + fluid restriction
Monitoring	Response to fluid bolus (BP, HR, UO)	Daily weights, fluid balance

Post-Operative Hyponatremia

Definition and Severity

Severity	Serum Sodium (mmol/L)	Clinical Features
Mild	130-135	Often asymptomatic; mild nausea, headache
Moderate	125-129	Nausea, confusion, lethargy
Severe	<125	Confusion, seizures, coma, cerebral oedema

This patient (Na+ 118mmol/L) has SEVERE hyponatremia requiring urgent treatment.

Causes of Post-Operative Hyponatremia

1. Syndrome of Inappropriate ADH Secretion (SIADH)

Most common cause post-operatively

Mechanism

• Excessive ADH (vasopressin) release
• Increased water reabsorption in kidneys
• Dilutional hyponatremia

Post-operative Triggers for SIADH

• Surgical stress: Major surgery is a potent stimulus for ADH release
• Pain: Uncontrolled pain stimulates ADH
• Nausea: Powerful ADH stimulus
• Medications:
  • Opioid analgesics (morphine, fentanyl)
  • NSAIDs
  • SSRIs
  • Carbamazepine
• Respiratory causes: Pneumonia, atelectasis, positive pressure ventilation
• CNS causes: If neurosurgery, head trauma, meningitis

Diagnostic Criteria for SIADH

• Hyponatremia with low serum osmolality (\<275mOsm/kg)
• Inappropriately concentrated urine: Urine osmolality >100mOsm/kg
• High urine sodium (\>40mmol/L)
• Euvolemic clinically (no signs of dehydration or overload)
• Normal thyroid and adrenal function
• No recent diuretic use

2. Excessive Hypotonic Fluid Administration

• Giving too much free water (5% dextrose, 0.45% saline)
• Particularly dangerous when combined with SIADH
• Water retained, sodium diluted

3. Volume Depletion

• Blood loss, third-space losses
• Stimulates ADH release (appropriate response to maintain circulating volume)
• If replaced with hypotonic fluids → hyponatremia

4. Other Causes (Less Common Post-op)

• Adrenal insufficiency: Hypocortisolism → ↓ Na+, ↑ K+
• Hypothyroidism: Low T4 → ↓ metabolic rate → water retention
• Diuretics: Especially thiazides
• Psychogenic polydipsia: Excessive water intake (rare post-op)

Clinical Assessment

Symptoms of Hyponatremia

Severity	Symptoms
Mild-Moderate	• Headache • Nausea and vomiting • Confusion • Lethargy • Muscle cramps
Severe	• Severe confusion or delirium • Seizures • Reduced consciousness/coma • Respiratory arrest • Cerebral oedema (papilloedema, focal neurology)

Assess Volume Status

Volume Status	Likely Cause	Clinical Features
Hypovolemic	• True sodium loss • Vomiting, diarrhea • Third-space losses • Diuretics	• ↓JVP • ↓BP, postural drop • Tachycardia • Dry mucous membranes • ↓urine output • Urine Na+ <20mmol/L
Euvolemic	• SIADH (most common post-op) • Hypothyroidism • Adrenal insufficiency	• Normal JVP • No oedema • Normal BP • Urine Na+ >40mmol/L
Hypervolemic	• Heart failure • Liver failure • Renal failure • Excessive IV fluids	• ↑JVP • Peripheral oedema • Ascites • Pulmonary oedema • Urine Na+ variable

Investigations

• Serum: Na+, K+, osmolality, glucose, urea, creatinine
• Urine: Sodium, osmolality
• Thyroid function: TSH, free T4
• Cortisol: 9am cortisol or short Synacthen test if adrenal insufficiency suspected
• Paired serum and urine osmolality: Essential for diagnosis

Management of Severe Hyponatremia (Na+ 118mmol/L)

Immediate Assessment

1. Is the patient symptomatic? Confusion = symptomatic → URGENT treatment required
2. Volume status: Likely euvolemic (SIADH) or hypervolemic (excessive fluids)
3. Speed of onset: Post-op day 2 → likely acute (\<48 hours)

Management Strategy

Step 1: Immediate Measures for Symptomatic Severe Hyponatremia

If seizures, reduced consciousness, or signs of cerebral oedema:

• Hypertonic saline (3% NaCl):
  • Give 150mL (or 2mL/kg) IV over 20 minutes
  • Can repeat twice if no improvement (total 3 doses)
  • Goal: Raise Na+ by 4-6mmol/L in first 1-4 hours to stop symptoms
  • Do NOT raise by more than 10mmol/L in first 24 hours (risk of osmotic demyelination syndrome)
• Monitor closely: Check Na+ every 2-4 hours initially
• Senior/ICU involvement: Severe hyponatremia requires senior input

Step 2: Identify and Treat Underlying Cause

If SIADH (Most Likely)

• Fluid restriction: 800-1000mL/day (including IV and oral)
• Stop hypotonic fluids: No 5% dextrose or 0.45% saline
• Switch to isotonic saline if IV fluids needed
• Address triggers:
  • Optimize pain control
  • Treat nausea (antiemetics)
  • Treat respiratory complications (pneumonia, atelectasis)
  • Review medications (stop NSAIDs, SSRIs if possible)

If Volume Depleted

• Isotonic saline: 0.9% saline to restore volume
• Once euvolemic, fluid restriction for SIADH

If Volume Overloaded

• Fluid restriction
• Diuretics: Furosemide (causes loss of free water)

Step 3: Monitor Correction Rate

CRITICAL: Avoid overly rapid correction (risk of osmotic demyelination syndrome)

Safe Correction Limits

• First 24 hours: Increase Na+ by maximum 10mmol/L
• First 48 hours: Increase Na+ by maximum 18mmol/L
• Target rate: 0.5mmol/L/hour or 6-8mmol/L per day

Higher Risk for Osmotic Demyelination

• Chronic hyponatremia (\>48 hours)
• Severe hyponatremia (\<120mmol/L)
• Alcoholism
• Malnutrition
• Liver disease

Step 4: Frequent Monitoring

• Serum Na+: Every 2-4 hours initially, then every 6-8 hours
• If correction too rapid: Give 5% dextrose or desmopressin (DDAVP) to slow correction
• Neurological status: Monitor for improvement or deterioration
• Fluid balance: Strict input/output monitoring

Step 5: Consider Additional Therapies (if SIADH persists)

• Salt tablets: Oral sodium chloride 1-2g TDS
• Demeclocycline: Induces nephrogenic diabetes insipidus (specialist use)
• Vaptans (e.g., tolvaptan): V2 receptor antagonists; promote free water excretion (specialist use, expensive)

Osmotic Demyelination Syndrome (ODS)

Previously called central pontine myelinolysis

Cause

• Overly rapid correction of chronic hyponatremia
• Damage to myelin sheath in pons and other brain regions

Presentation

• Symptoms appear 2-6 days after correction
• Dysarthria, dysphagia
• Quadriparesis or paraparesis
• Confusion, coma
• "Locked-in" syndrome in severe cases

Prevention

• Slow correction: <10mmol/L in 24 hours, <18mmol/L in 48 hours
• If accidental overcorrection: Relower sodium with 5% dextrose or desmopressin

Summary Management Algorithm for This Patient

1. Immediate:
   • Patient has confusion (symptomatic) + severe hyponatremia (118mmol/L)
   • Consider hypertonic saline if severely symptomatic (seizures, coma)
   • Senior/ICU review
2. Investigations:
   • Paired serum and urine osmolality + urine sodium
   • Assess volume status
3. Likely diagnosis: SIADH (post-op day 2, euvolemic)
4. Management:
   • Fluid restriction 800-1000mL/day
   • Stop hypotonic fluids
   • Treat pain, nausea
   • Review medications
5. Monitoring:
   • Na+ every 2-4 hours initially
   • Target correction: 6-8mmol/L per day, <10mmol/L in 24 hours
6. Continue until: Na+ >130mmol/L and patient asymptomatic