Fluids, Electrolytes, and Acid-Base for Urology
Urology runs on fluid and electrolyte physiology — from choosing a drip and correcting potassium to recognising TUR syndrome, diversion acidosis and post-obstructive diuresis.
The big picture
Think of body water in compartments (intracellular and extracellular, the latter split into interstitial and intravascular) and of resuscitation as refilling the intravascular space. The fluid you choose, the sodium and potassium you watch, and the acid-base pattern you read are all downstream of where the problem sits.
Match the fluid to the job, treat dangerous potassium first, and recognise the urology-specific syndromes — TUR hyponatraemia, diversion hyperchloraemic acidosis, and post-obstructive diuresis.
Physiology engine
- Crystalloid choice: balanced solutions (e.g. Hartmann's/Ringer's) resemble plasma; large volumes of 0.9% saline cause hyperchloraemic metabolic acidosis. Avoid potassium-containing fluids in hyperkalaemia or anuria.
- Resuscitation: rapid boluses of balanced crystalloid to refill the circulation, reassessed against blood pressure, heart rate and urine output.
- Maintenance: a smaller daily volume with modest sodium and added potassium for the patient who cannot drink — over-prescribing causes oedema and dilutional hyponatraemia.
- Sodium reflects water balance: hyponatraemia is usually a water problem (dilution); correct symptomatic acute hyponatraemia carefully (hypertonic saline only for seizures/coma) to avoid osmotic demyelination, and never correct chronic hyponatraemia too fast.
- Potassium is the immediately dangerous ion: hyperkalaemia threatens the heart. Stabilise the myocardium with calcium, shift potassium into cells (insulin-dextrose, salbutamol), then remove it (and treat the cause, often obstruction/AKI).
- Acid-base: a high-anion-gap metabolic acidosis suggests lactate (sepsis/shock) or uraemia; a normal-gap (hyperchloraemic) acidosis suggests saline excess or a bowel urinary diversion.
- Large-volume 0.9% saline → hyperchloraemic metabolic acidosis; prefer balanced crystalloid.
- Hyperkalaemia: calcium first (cardioprotection), then shift, then remove.
- Post-obstructive diuresis can exceed 200 mL/hr — replace fluid/electrolytes and monitor.
Mechanism pathway
Tap any step to see why it happens.
Summary tables
Which fluid for which job
| Job | Fluid | Caution |
|---|---|---|
| Resuscitation | Balanced crystalloid boluses | Reassess after each bolus |
| Maintenance | Smaller volume, modest sodium + potassium | Over-prescription → oedema/hyponatraemia |
| Replacement | Match ongoing losses | Account for diuresis/drain losses |
| Hyperkalaemia / anuria | Avoid potassium-containing fluids | Treat the cause |
Urology-specific electrolyte syndromes
| Syndrome | Setting | Pattern | Action |
|---|---|---|---|
| TUR syndrome | Monopolar TURP (glycine) | Dilutional hyponatraemia + overload | Stop, supportive, cautious Na correction; use bipolar/saline |
| Diversion acidosis | Ileal conduit / neobladder | Hyperchloraemic (normal-gap) acidosis ± hypokalaemia | Alkali/potassium replacement; minimise contact time |
| Post-obstructive diuresis | After relieving obstruction | Large diuresis, electrolyte loss | Replace fluid/electrolytes, monitor weight/obs/renal function |
Memory hooks
Three jobs: resuscitate, maintain, replace.
Saline excess → hyperchloraemic acidosis; choose balanced.
HyperK: Calcium → shift (insulin/salbutamol) → remove.
TUR = glycine = dilutional hyponatraemia.
Conduit = hyperchloraemic acidosis; obstruction relieved = post-obstructive diuresis.
Board traps
Confused, hyponatraemic patient mid-monopolar TURP — TUR syndrome from glycine.
Ileal conduit with a normal-gap (hyperchloraemic) metabolic acidosis — diversion physiology.
Brisk diuresis after catheterising a chronically obstructed bladder — post-obstructive diuresis, replace losses.
Obstructive AKI with peaked T waves — treat hyperkalaemia (calcium first) AND relieve obstruction.
Clinical cases
Thirty minutes into a monopolar TURP, a patient becomes confused and hypertensive with a serum sodium of 119 mmol/L.
What is the cause and the immediate management?
A man with chronic high-pressure retention is catheterised, draining 1.6 L. Over the next hours he passes 250 mL/hr of urine and his blood pressure drifts down.
What is happening and what do you do?