Why Fluid Balance Is Examined
IV fluid prescribing is the most common prescription written in hospital medicine and one of the most common sources of iatrogenic harm. NICE CG174 (2013) highlighted that inappropriate IV fluid prescribing causes significant morbidity. OSCEs examine fluid balance in prescribing stations, ward-round clinical reasoning, and acute deterioration scenarios. Examiners test systematic fluid status assessment, NICE-aligned prescribing, and electrolyte correction.
💡 Tip
The NICE 5R framework for IV fluids: Resuscitation, Routine maintenance, Replacement, Redistribution, Reassessment. Always state which indication you are prescribing for and reassess every 4-6 hours.
Clinical Assessment of Fluid Status
Signs of Dehydration (Hypovolaemia)
| Mild (3-5% body weight) | Moderate (5-10%) | Severe (above 10%) |
|---|---|---|
| Thirst | Reduced skin turgor | Tachycardia |
| Dry mouth | Dry mucous membranes | Hypotension |
| Concentrated urine | Reduced urine output | Oliguria/anuria |
| Normal observations | Postural hypotension | Confusion, shock |
Bedside assessment checklist:
- Mucous membranes (dry vs moist)
- Skin turgor (test over clavicle — less affected by age than dorsum of hand)
- Capillary refill time (normal under 2 seconds)
- Pulse (rate and volume)
- Blood pressure (lying and standing — postural drop)
- JVP height
- Urine output and colour
- Fluid balance chart (input vs output over 24 hours)
- Daily weight (most accurate method — 1 kg = approximately 1 litre)
Signs of Fluid Overload
🧠 Mnemonic
ROPE — fluid overload signs:
- R ales/crackles (bibasal — pulmonary oedema)
- O edema (peripheral pitting — ankles, sacrum in bedbound)
- P leural effusion (stony dull percussion)
- E levated JVP (above 3 cm)
Plus: third heart sound (S3 gallop), displaced apex, hyponatraemia (dilutional)
NICE CG174 — IV Fluid Prescribing Principles
1. Resuscitation
For haemodynamic instability:
- 500 mL Hartmann's (Compound Sodium Lactate) or 0.9% NaCl
- Given over 15 minutes
- Reassess after each bolus
- Repeat up to 2 litres total; beyond that seek senior review
⚠️ Red Flag
Hartmann's solution is now preferred over 0.9% NaCl for most resuscitation — excess 0.9% NaCl causes hyperchloraemic metabolic acidosis (due to high chloride load). Exceptions: head injury (Hartmann's contains some potassium — risk of ICP effects), hypochloraemia, metabolic alkalosis.
2. Routine Maintenance
Daily requirements for an average 70 kg adult:
- Water: 25-30 mL/kg/day (approximately 1750-2000 mL/day)
- Sodium: 1 mmol/kg/day (approximately 70 mmol/day)
- Potassium: 1 mmol/kg/day (approximately 70 mmol/day)
NICE-recommended standard regimen:
- 25-30 mL/kg/day = approximately 2.5 litres/day total
- Typical example: 1 litre Hartmann's + 2 litres glucose 5% + 20-40 mmol KCl in each bag
⚠️ Red Flag
Avoid 0.9% NaCl as routine maintenance. Each litre contains 154 mmol sodium — well above daily requirements. Routine use causes hypernatraemia and hyperchloraemia. Use in resuscitation only.
3. Replacement
For specific losses (vomiting, diarrhoea, fistula, drain output, stoma high output):
| Loss | Fluid type | Additional considerations |
|---|---|---|
| Nasogastric aspirate | 0.9% NaCl | Add KCl (gastric juice rich in K+ and Cl-) |
| Upper GI/diarrhoea | Hartmann's or 0.9% NaCl + KCl | Replace potassium aggressively |
| Burns | Hartmann's (Parkland formula) | 4 mL/kg/% burn in first 24 hours |
| Urinary losses | 0.9% NaCl | If large; match output |
Electrolyte Disturbances and Correction
Hypokalaemia
| Level | Severity | Management |
|---|---|---|
| 3.0-3.5 mmol/L | Mild | Oral potassium (Sando-K 2 tablets BD) — equivalent to 24 mmol/day |
| 2.5-3.0 mmol/L | Moderate | IV KCl 40 mmol over 4-8 hours (max 20 mmol/hour peripheral; 40 mmol/hour central) |
| Below 2.5 or symptomatic | Severe | HDU/cardiac monitoring; IV replacement with ECG monitoring; check and correct magnesium |
⚠️ Red Flag
Never give undiluted KCl IV — it causes cardiac arrest. KCl must always be diluted. Maximum safe peripheral infusion rate: 10-20 mmol/hour. Faster rates require central access and cardiac monitoring.
Always check and correct magnesium — hypomagnesaemia causes refractory hypokalaemia. Magnesium 2-4 g IV over 20-30 minutes if below 0.5 mmol/L.
Hypernatraemia (Na above 145 mmol/L)
Usually caused by free water deficit (diabetes insipidus, inadequate water intake, fever).
Correction: Replace free water deficit slowly — no faster than 10 mmol/L/day reduction in sodium to avoid cerebral oedema.
- Oral fluids first if patient can drink
- IV: glucose 5% (hypotonic, provides free water)
- Calculate deficit: free water deficit = 0.6 x weight x (Na/140 - 1)
Hyponatraemia (Na below 135 mmol/L)
| Level | Severity | Speed of correction |
|---|---|---|
| 130-134 mmol/L | Mild | Fluid restrict if euvolaemic (SIADH) |
| 120-130 mmol/L | Moderate | Treat cause; 1.8% NaCl if symptomatic |
| Below 120 or symptomatic (seizures, confusion) | Severe | 3% NaCl 150 mL over 20 min; HDU; target rise of 5 mmol/L in first hour |
⚠️ Red Flag
Overly rapid correction of hyponatraemia causes osmotic demyelination syndrome (ODS) — previously called central pontine myelinolysis. Irreversible. Maximum correction rate: 10 mmol/L in first 24 hours, 18 mmol/L in 48 hours (except emergency treatment of seizures).
Fluid Prescribing Errors to Avoid
| Error | Consequence | Prevention |
|---|---|---|
| 0.9% NaCl as maintenance | Hyperchloraemia, metabolic acidosis | Use Hartmann's for maintenance |
| KCl not prescribed in maintenance | Hypokalaemia | Always add potassium to maintenance bags |
| Continuing IV fluids when oral intake established | Fluid overload | Review daily — switch to oral as soon as possible |
| Large-volume fluid in cardiac failure | Pulmonary oedema | Use 250 mL boluses; reassess after each |
| Rapid sodium correction | ODS (hyponatraemia) or cerebral oedema (hypernatraemia) | No more than 10 mmol/L/day change |
Frequently Asked Questions
"What is the difference between crystalloids and colloids?"
Crystalloids are aqueous solutions of small molecules (saline, glucose, Hartmann's) that distribute throughout the extracellular space. Colloids contain large molecules (albumin, starch) intended to remain intravascular and expand plasma volume. Clinical evidence (SAFE, CRISTAL, 6S trials) has failed to show any benefit of colloids over crystalloids in resuscitation, and HES (hydroxyethyl starch) is associated with increased AKI and mortality. NICE recommends crystalloids (Hartmann's or 0.9% NaCl) for all resuscitation.
"Why does vomiting cause hypokalaemia and metabolic alkalosis?"
Gastric juice is rich in hydrochloric acid (HCl), sodium, potassium, and water. Vomiting causes: loss of H+ and Cl- (causing metabolic alkalosis from relative bicarbonate excess); loss of K+ (directly and via renal K+ wasting as kidneys exchange K+ for H+ to correct alkalosis); sodium and volume depletion. The secondary hyperaldosteronism from volume depletion further promotes urinary potassium loss. Replacement requires 0.9% NaCl with KCl.
"How do you assess whether someone is fluid overloaded versus dehydrated when they have both peripheral oedema and raised creatinine?"
This is a common clinical scenario — the patient may be peripherally oedematous (third-spacing) but intravascularly depleted. Assessment: JVP is the most reliable bedside marker of intravascular volume. A low or flat JVP with pitting oedema suggests third-space fluid with intravascular depletion (give fluid cautiously). Raised JVP with oedema suggests cardiac failure or poor venous return. Fluid balance charts, daily weights, and renal response to a cautious fluid challenge help clarify the picture.
"What is SIADH and how is it managed?"
Syndrome of inappropriate antidiuretic hormone secretion — ADH secretion continues despite low plasma osmolality, causing free water retention and dilutional hyponatraemia with concentrated urine. Causes: CNS pathology (stroke, SAH, meningitis), malignancy (small cell lung cancer), medications (SSRIs, carbamazepine, cyclophosphamide, NSAIDs), pulmonary disease. Diagnosis: hyponatraemia + low plasma osmolality + concentrated urine (urinary osmolality above 100 mosmol/kg). Management: treat the cause, fluid restriction to 1-1.5 L/day, demeclocycline or tolvaptan for persistent cases.
"When should you stop IV fluids on the ward?"
IV fluids should be stopped as soon as the patient can meet their fluid and electrolyte needs orally. Review IV fluids at each ward round — most patients should only need IV fluids for 24-48 hours post-operatively or during the acute phase of their illness. Prolonged IV fluid use increases the risk of fluid overload, electrolyte disturbance, and line complications (phlebitis, infection). Document the reason for continuation if fluids are continued beyond 48 hours.
Related Posts
- IV Fluids Prescribing OSCE — detailed prescribing practice and common scenarios
- Acute Kidney Injury OSCE — fluid assessment and resuscitation in the context of AKI
- Blood Results Interpretation OSCE — interpreting sodium, potassium, and osmolality