FFinalistSouthampton · 2027
Renal & urology
AKT · Renal & urology/Fluid, electrolyte & acid-base

Hypovolaemia & volume depletion

Loss of extracellular fluid volume — haemorrhagic or non-haemorrhagic

Overview

A reduced circulating volume, from bleeding, gastrointestinal losses, renal losses, burns, third-spacing or simply not drinking. It is the commonest reversible cause of an FY1 bleep about tachycardia, hypotension or a rising creatinine, and assessing it correctly is a daily task rather than an exam curiosity.

Recognise

  • Thirst, reduced urine output (<0.5 mL/kg/h), dizziness on standing
  • Tachycardia, then a POSTURAL drop, then supine hypotension — in that order; a normal lying blood pressure does not exclude significant depletion
  • Capillary refill >2 s, cool peripheries, dry mucous membranes, reduced skin turgor, low JVP
  • Young patients compensate hard and then decompensate abruptly; older patients and those on beta-blockers may never mount a tachycardia
  • Rising urea out of proportion to creatinine (a urea:creatinine ratio suggesting pre-renal), concentrated urine, metabolic acidosis with a raised lactate

Red flags

  • Hypotension with a lactate >2 mmol/L, altered consciousness or anuria → shock; escalate immediately and treat alongside the search for the cause

Differentials & how to tell them apart

Sepsis (distributive shock)Warm peripheries with a wide pulse pressure and an infection source — fluid alone will not fix it
Cardiogenic shockRAISED JVP, crackles and a third heart sound — fluid makes this worse; the JVP is the discriminator
AnaphylaxisRapid onset after a trigger with urticaria, angioedema or wheeze — needs IM adrenaline, not just fluid
Adrenal crisisHypotension with hyponatraemia, hyperkalaemia and hypoglycaemia, poorly responsive to fluid until hydrocortisone is given
Diabetes insipidus / hyperglycaemiaMassive dilute urine output despite depletion — polyuria continues rather than shutting down

Investigations

Bedside first: observations including a POSTURAL blood pressure, capillary refill, fluid balance chart and daily weights (the most underused measure on the ward). Bloods: U&E and creatinine, FBC, lactate, glucose, CRP; group and save or crossmatch if bleeding is possible. Urine output measurement, catheterising only if it changes management. Point-of-care ultrasound for IVC collapsibility where available.

Management

A 500 mL balanced crystalloid bolus over under 15 minutes, then reassess — and find the cause

  1. 1ABCDE. Fluid challenge: 500 mL balanced crystalloid over <15 min (250 mL if elderly, frail or in heart failure). REASSESS after every bolus — pulse, blood pressure, capillary refill, JVP, urine output, lactate.
  2. 2Repeat boluses as needed while looking for and treating the cause. NICE CG174 separates the four Rs: Resuscitation, Routine maintenance, Replacement and Redistribution — maintenance is roughly 25–30 mL/kg/day water with about 1 mmol/kg/day each of sodium, potassium and chloride, and 50–100 g/day glucose.
  3. 3Escalate rather than continuing to bolus blindly.Gate: More than 2000 mL given without sustained response → senior review and critical care; consider a non-hypovolaemic cause of shock
Balanced crystalloid (Hartmann's, Plasma-Lyte) — First choice for resuscitation. 500 mL over less than 15 min, then reassess — 250 mL in heart failure, frailty or the elderly
0.9% sodium chloride — Acceptable, but large volumes cause a hyperchloraemic metabolic acidosis
Blood products — If losses are haemorrhagic, replace blood with blood — crystalloid only buys time and dilutes clotting factors

Key points

Look at the JVP before prescribing fluid — it is the fastest way to avoid pouring litres into a failing heart. And beware the reflex maintenance prescription: over-prescribed 0.9% saline causes hyperchloraemic acidosis and oedema, while over-prescribed 5% dextrose causes hyponatraemia. Weigh the patient daily; it beats every other measure of fluid balance on a busy ward.

Monitor & prognosis

Hourly observations while resuscitating, strict fluid balance, daily weights and daily U&E.

Excellent when recognised early. Delay converts a reversible pre-renal picture into established acute tubular necrosis.

Source: NICE CG174 — intravenous fluid therapy in adults in hospital