Fluid and Electrolyte Therapy
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Direct answer
Daily maintenance needs in an adult run at roughly 30 to 40 mL per kg of water, 1 to 2 mmol per kg of sodium and about 1 mmol per kg of potassium, with the 4-2-1 rule converting this into an hourly rate — 4 mL per kg for the first 10 kg, 2 mL per kg for the next 10, and 1 mL per kg beyond that. Ringer lactate is the preferred crystalloid for surgical resuscitation and perioperative use. The two emergencies to master are hyperkalaemia with its ECG changes, treated first with intravenous calcium, and symptomatic hyponatraemia, corrected slowly to avoid osmotic demyelination.
What you must remember
- Body water is roughly 60 per cent of body weight in men and 50 per cent in women, split two-thirds intracellular and one-third extracellular.
- The 4-2-1 rule converts maintenance to an hourly rate — 4, 2 and 1 mL per kg across successive 10 kg blocks — so a 70 kg patient receives 110 mL per hour, plus losses from vomiting, drains, fever and third-space sequestration.
- Fluid choices: normal saline risks hyperchloraemic acidosis in volume; Ringer lactate, a balanced crystalloid, is the surgical workhorse; five per cent dextrose is free water without electrolytes.
- Resuscitation uses balanced crystalloid boluses of about 20 mL per kg with reassessment, and blood products rather than further crystalloid once haemorrhage is established.
- Hyperkalaemia above about 6.5 mmol per litre, or any level with ECG changes — peaked T waves, wide QRS, sine wave — is treated in order: calcium gluconate to stabilise the myocardium, insulin with dextrose, nebulised salbutamol, then removal by resins or dialysis.
- Hypokalaemia produces weakness, ileus and U waves, is corrected orally where possible and never by intravenous push, and needs magnesium correction when refractory.
- Symptomatic hyponatraemia with seizures warrants 3 per cent saline boluses, but correction must stay within about 8 to 10 mmol per litre in 24 hours to avoid osmotic demyelination.
Common confusion
Candidates confuse maintenance with resuscitation — the 4-2-1 rule supplies baseline needs, while shock takes boluses and blood. Saline versus Ringer lactate is a recurring discrimination, with Ringer lactate preferred in surgery and avoided mainly in severe hyperkalaemia, where its small potassium content matters. The hyperkalaemia sequence is another trap: calcium protects the heart but does not lower the level, and insulin-dextrose shifts but does not remove.
Exam-focused takeaway
FMGE fluid questions are arithmetic-plus-emergency questions. Expect to compute an hourly rate by 4-2-1, match the fluid to the purpose — Ringer lactate for surgery, dextrose for free water, blood for haemorrhage — and answer the hyperkalaemia ECG with calcium gluconate first, then shifting and removal steps. The hyponatraemia answer carries a speed limit: hypertonic saline for seizures, but no more than about 8 to 10 mmol per litre in a day.
Frequently asked questions
What is the 4-2-1 rule?
An hourly maintenance formula — 4 mL per kg for the first 10 kg, 2 mL per kg for the next 10, and 1 mL per kg thereafter.
Which crystalloid is preferred in surgical practice?
Ringer lactate, a balanced solution with lactate buffer, chosen over large volumes of normal saline that cause hyperchloraemic acidosis.
What is the first drug in hyperkalaemia with ECG changes?
Intravenous calcium gluconate to stabilise the myocardium, then insulin with dextrose, salbutamol and removal measures.
How fast should hyponatraemia be corrected?
By no more than about 8 to 10 mmol per litre in 24 hours, except bolus hypertonic saline to abort seizures, to avoid osmotic demyelination.
Why check magnesium in refractory hypokalaemia?
Hypomagnesaemia drives renal potassium loss, so repletion fails until magnesium is corrected.
What are third-space losses?
Fluid sequestered into the interstitium and bowel lumen after surgery or inflammation, counted among ongoing losses beyond maintenance.